Question 1

In inorganic chemistry, what is the oxidation number of sulfur in hydrogen sulfide (H2S)?

Accepted Answer

Introduction / Context: Oxidation numbers are a formal way of tracking electron gain and loss in chemical compounds. They help in balancing redox reactions, understanding oxidation states of elements and predicting reaction behaviour. This problem focuses on determining the oxidation number of sulfur in hydrogen sulfide, a simple but important inorganic compound with formula H2S. Given Data / Assumptions: - The compound is hydrogen sulfide with formula H2S. - Hydrogen is bonded to sulfur, and there is no overall charge on the molecule. - We use standard oxidation number rules, including the usual oxidation state of hydrogen in covalent compounds. Concept / Approach: In most covalent compounds where hydrogen is bonded to non metals, hydrogen has an oxidation number of +1. The sum of oxidation numbers of all atoms in a neutral molecule must equal zero. Therefore, if we assign +1 to each hydrogen and let x represent the oxidation number of sulfur, we can solve a simple algebraic equation to find x. This method is widely applicable to many binary compounds and is an essential basic skill in inorganic chemistry. Step-by-Step Solution: Step 1: Write the chemical formula H2S and note that the molecule is neutral (overall charge is 0). Step 2: Use the rule that hydrogen usually has an oxidation number of +1 in compounds with non metals. Step 3: Assign +1 to each hydrogen atom. With two hydrogen atoms, the total oxidation contribution from hydrogen is 2 * (+1) = +2. Step 4: Let the oxidation number of sulfur be x. The sum of oxidation numbers must equal zero for a neutral compound, so we write the equation: 2 + x = 0. Step 5: Solve the equation: x = -2. Step 6: Conclude that the oxidation number of sulfur in hydrogen sulfide is -2. Verification / Alternative check: You can cross check this by considering related compounds of sulfur. In sulfide ions (S^2-), sulfur has an oxidation number of -2. Hydrogen sulfide can be thought of as the molecular form of hydrogen ions (H+) combining with sulfide ions (S^2-), giving an overall neutral molecule. This supports the result that sulfur is in the -2 oxidation state in H2S. Furthermore, in many metal sulfides like FeS or ZnS, sulfur is also in the -2 state, reinforcing the consistency of this assignment. Why Other Options Are Wrong: 2: A positive value of +2 for sulfur would make the sum of oxidation numbers equal +4, which cannot be correct for a neutral molecule H2S. 1: An oxidation number of +1 for sulfur would give a total of +3 (+2 from hydrogen and +1 from sulfur), again inconsistent with overall neutrality. -1: If sulfur were -1, the total sum would be +1, not zero, so this is also not possible. Common Pitfalls: Students sometimes forget to multiply the oxidation number of hydrogen by the number of hydrogen atoms and therefore get incorrect totals. Another mistake is to confuse peroxides or metal hydrides, where hydrogen oxidation states can differ, with typical covalent hydrides like H2S. To avoid such errors, always apply the rule that hydrogen is usually +1 with non metals, multiply by its subscript and ensure the total oxidation number equals the overall charge of the species. Final Answer: The oxidation number of sulfur in H2S is -2.

Question 2

When cutting and peeling onions in the kitchen causes irritation and tears in the eyes, this effect is mainly due to the presence of which type of element in onion compounds?

Accepted Answer

Introduction / Context: This everyday chemistry question connects real life experience in the kitchen with chemical concepts. Most people have experienced tearing and eye irritation while chopping onions. Understanding which element in onion compounds is responsible for this effect helps you see how organic molecules, enzymes and volatile compounds influence our senses and health. Given Data / Assumptions: - The situation involves cutting and peeling onions, which leads to tears in the eyes. - Several possible components are listed: amino acids, sulfur, carbon and fat. - We consider the main tear inducing factor and not minor nutritional components. Concept / Approach: Onions contain sulfur containing amino acid derivatives. When an onion is cut, enzymes are released that act on these sulfur compounds to produce volatile, irritating molecules. One well known compound formed is syn propanethial S oxide, a volatile sulfur containing compound. When this vapor reaches the eyes, it reacts with moisture to form mild sulfuric acid, causing irritation and tearing as the body attempts to wash the irritant away. Therefore, sulfur in these volatile compounds is the key element responsible for tears. Step-by-Step Solution: Step 1: Recognise that onions belong to the Allium family and are rich in organosulfur compounds rather than in fats. Step 2: When onions are cut, cell walls are broken and enzymes mix with sulfur containing precursors such as sulfoxides. Step 3: Enzymatic reactions convert these precursors into volatile sulfur compounds like syn propanethial S oxide. Step 4: These volatile molecules evaporate and reach the eyes, where they react with the moisture on the eye surface to form small amounts of sulfuric acid. Step 5: The mild acid irritates nerve endings in the eyes, which triggers tearing as a protective response. Step 6: Conclude that it is the sulfur element in these volatile compounds that is primarily responsible for tears. Verification / Alternative check: Support for this explanation comes from simple experiments. Cooling onions in a refrigerator before cutting them slows the enzyme activity and reduces the release of sulfur vapors, which results in fewer tears. Similarly, cutting onions under running water or near a fan carries away the volatile sulfur compounds before they reach the eyes. These practical tips show that controlling sulfur vapors, not amino acids in general, fats or carbon content, reduces irritation. Carbon is present in nearly all organic molecules, but it is not specifically responsible for the tear inducing effect. Why Other Options Are Wrong: Amino acid as the primary tear causing component: while sulfur containing amino acid derivatives are precursors, it is the volatile sulfur products, not generic amino acids, that cause tearing. Carbon: carbon is a basic element in all organic compounds but does not specifically produce eye irritation in this context. Fat: onions are low in fat, and fats are not volatile irritants, so they are not responsible for the tearing effect. Common Pitfalls: Learners might focus on the fact that onions contain amino acids and assume these alone cause tears, or they may give very general answers like "organic compounds" without identifying sulfur. Another pitfall is forgetting that volatile, gas forming compounds are most likely to irritate the eyes. Remember that when a question asks for the key element behind onion tears, the correct answer is sulfur in volatile sulfur containing molecules. Final Answer: Tears during onion cutting are mainly due to sulfur present in volatile sulfur containing compounds.

Question 3

For the diatomic carbon molecule C2 in its ground state, how is it classified magnetically: paramagnetic, diamagnetic, ferromagnetic, or undetermined?

Accepted Answer

Introduction / Context: This question tests your understanding of molecular orbital theory and how it is used to predict magnetic properties of diatomic molecules. Magnetism in molecules depends on the presence or absence of unpaired electrons. Paramagnetic substances have one or more unpaired electrons, while diamagnetic substances have all electrons paired. The C2 molecule is a classic example used in examinations to check whether students can correctly fill molecular orbitals and infer magnetic behaviour. Given Data / Assumptions: - The species considered is the neutral C2 molecule in its ground state. - Each carbon atom has atomic number 6, with an electron configuration of 1s2 2s2 2p2. - We apply basic molecular orbital theory for homonuclear diatomic molecules of second period elements. - We focus on valence electrons for determining magnetic behaviour. Concept / Approach: Magnetic properties depend on unpaired electrons in molecular orbitals. To determine whether C2 is paramagnetic or diamagnetic, we must count the total number of valence electrons and fill molecular orbitals according to their energy ordering, following the Pauli principle and Hunds rule. If all electrons are paired, C2 is diamagnetic; if any are unpaired, it is paramagnetic. For lighter diatomic molecules like B2, C2 and N2, the typical order is sigma 2s, sigma 2s*, pi 2p, sigma 2p, though conventions may differ. The important result is that C2 with 8 valence electrons fills bonding and antibonding orbitals in such a way that all electrons are paired. Step-by-Step Solution: Step 1: Determine the total valence electrons in C2. Each carbon atom contributes 4 valence electrons (2s2 2p2), so C2 has 8 valence electrons in total. Step 2: List the relevant molecular orbitals in approximate energy order for lighter homonuclear diatomic molecules: sigma 2s, sigma 2s*, pi 2p, sigma 2p. Step 3: Fill sigma 2s with 2 electrons (paired), then sigma 2s* with 2 electrons (paired), taking up 4 of the 8 valence electrons. Step 4: Place the remaining 4 electrons in the pi 2p orbitals. The two degenerate pi 2p orbitals together can hold 4 electrons. They fill as pairs, giving two pairs of electrons with no unpaired electrons left. Step 5: Confirm that no electrons occupy sigma 2p in the ground state for C2; the 8 valence electrons are already accommodated in lower energy orbitals. Step 6: Since all electrons in the filled molecular orbitals are paired, C2 is diamagnetic. Verification / Alternative check: Another way to check is to recall exam standard results for simple diatomic molecules. B2 (10 total electrons, 6 valence) is typically paramagnetic because it has unpaired electrons in pi 2p orbitals. C2 (12 total electrons, 8 valence) is diamagnetic because all valence electrons are paired. N2 (14 total electrons, 10 valence) is also diamagnetic. Remembering this sequence and the corresponding bond orders reinforces that C2 does not have unpaired electrons. Therefore, C2 is diamagnetic in its ground state. Why Other Options Are Wrong: Paramagnetic: This would require at least one unpaired electron in a molecular orbital, which is not the case for C2 in the ground state. Ferromagnetic: Ferromagnetism is associated with bulk materials like iron, not simple diatomic molecules in the gas phase under standard conditions. Cannot be determined from any theory: Molecular orbital theory does allow us to predict the magnetic nature of C2, so this option is incorrect. Common Pitfalls: A common mistake is to misapply the MO energy order for diatomic molecules and incorrectly place electrons in sigma 2p before fully filling pi 2p in the case of lighter elements. This can lead to incorrect predictions about unpaired electrons. Another pitfall is confusing C2 with O2, which is classically paramagnetic. Carefully distinguishing between these species and memorising standard results for B2, C2, N2 and O2 can help avoid these errors. Final Answer: The C2 molecule in its ground state is diamagnetic because all its electrons are paired in molecular orbitals.

Question 4

In water chemistry, permanent hardness of water is mainly due to the presence of which dissolved salt among the following?

Accepted Answer

Introduction / Context: Hardness of water is an important concept in environmental chemistry and everyday life, influencing soap consumption, scale formation in boilers and the effectiveness of detergents. Hardness is classified into temporary and permanent hardness depending on the nature of the dissolved salts that cause it. This question asks you to identify the salt responsible for permanent hardness of water from a list of common calcium and magnesium compounds. Given Data / Assumptions: - The question refers specifically to permanent hardness, not temporary hardness. - Options include calcium sulphate, calcium bicarbonate, sodium bicarbonate and magnesium bicarbonate. - We assume standard definitions: temporary hardness is typically due to bicarbonates of calcium and magnesium; permanent hardness is due to sulphates and chlorides of calcium and magnesium. Concept / Approach: Temporary hardness arises mainly from the presence of calcium bicarbonate and magnesium bicarbonate in water. It can be removed by boiling, because bicarbonates decompose to form insoluble carbonates that precipitate. Permanent hardness is caused by salts such as calcium sulphate, magnesium sulphate and the chlorides of these metals. These salts do not decompose upon boiling and require chemical treatment, such as using washing soda or ion exchange, to be removed. Therefore, among the given options, calcium sulphate is the salt that contributes to permanent hardness. Step-by-Step Solution: Step 1: Recall that bicarbonates of calcium and magnesium are mainly responsible for temporary hardness and can be removed by boiling. Step 2: Identify which options are bicarbonates: calcium bicarbonate, sodium bicarbonate and magnesium bicarbonate all have the HCO3^- ion. Step 3: Calcium sulphate (CaSO4) is a sulphate salt of calcium, not a bicarbonate, and is known to cause permanent hardness. Step 4: Since the question asks specifically about permanent hardness, eliminate bicarbonate options and select calcium sulphate. Verification / Alternative check: To verify, consider the effect of boiling water containing each of these salts. Water with calcium bicarbonate will lose hardness after boiling because calcium carbonate precipitates, reducing hardness. This is why temporary hardness can be treated simply by boiling. In contrast, boiling water containing calcium sulphate does not significantly remove the dissolved CaSO4, and hardness remains. Industrial water treatment plants often remove permanent hardness using chemical softening or ion exchange methods, supporting the classification of calcium sulphate as a permanent hardness causing salt. Why Other Options Are Wrong: Calcium bicarbonate: This contributes mainly to temporary hardness, which can be removed by boiling. Sodium bicarbonate: While it affects alkalinity, sodium salts typically do not contribute significantly to hardness because hardness is associated with calcium and magnesium ions. Magnesium bicarbonate: Like calcium bicarbonate, this causes temporary hardness that can be removed by boiling, not permanent hardness. Common Pitfalls: Learners sometimes memorise lists of salts without distinguishing between bicarbonates versus sulphates and chlorides. Another pitfall is to think that any calcium salt must cause permanent hardness. In reality, the anion plays a crucial role: bicarbonates are associated with temporary hardness, while sulphates and chlorides are associated with permanent hardness. A clear mental table of these categories will help you answer many water chemistry questions correctly. Final Answer: Permanent hardness of water is mainly due to the presence of calcium sulphate (CaSO4).

Question 5

In atomic and molecular chemistry, species that contain the same total number of electrons are collectively referred to by which term?

Accepted Answer

Introduction / Context: This question belongs to the basic terminology of atomic structure and nuclear chemistry. It asks you to identify the correct term for species that have the same number of electrons. Distinguishing between isotopes, isobars, isotones and isoelectronic species is important because these terms describe different relationships between atoms or ions and are frequently tested in competitive examinations. Given Data / Assumptions: - The focus is on species with the same total number of electrons. - The options include isotopes, isobars, isoelectronic and isotones. - We assume standard textbook definitions: isotopes have same atomic number, isobars have same mass number, isotones have same number of neutrons and isoelectronic species have the same number of electrons. Concept / Approach: Isoelectronic species are defined as atoms, ions or molecules that contain the same number of electrons. For example, N3-, O2-, F-, Ne, Na+ and Mg2+ are all isoelectronic with 10 electrons. Isotopes, on the other hand, are atoms of the same element with the same atomic number but different mass numbers. Isobars are atoms of different elements that have the same mass number. Isotones have the same number of neutrons but different numbers of protons. Since the question explicitly refers to the same number of electrons, the correct term is isoelectronic. Step-by-Step Solution: Step 1: Recall the definition of isoelectronic species: entities that possess the same total number of electrons, even if they are different elements or ions. Step 2: Recall the definition of isotopes: same atomic number, different mass numbers, hence same number of protons and electrons but different neutrons. Step 3: Recall the definition of isobars: same mass number, different atomic numbers, so their electron counts usually differ. Step 4: Recall the definition of isotones: same number of neutrons, different number of protons and typically electrons. Step 5: Since the question is specifically about species containing the same number of electrons, select the term isoelectronic. Verification / Alternative check: To verify, consider the set of species O2-, F-, Ne and Na+. Oxygen has atomic number 8, so O2- has 10 electrons. Fluorine has 9 electrons, so F- has 10 electrons. Neon has 10 electrons, and Na+ (11 - 1) also has 10 electrons. These different atoms and ions are isoelectronic. They are not isotopes, because they have different atomic numbers. They are not isobars, because their mass numbers differ, and they are not isotones, because their neutron counts differ. This example reinforces that the correct term for same electron count is isoelectronic. Why Other Options Are Wrong: Isotopes: refer to same atomic number and different mass numbers, emphasising protons and neutrons rather than electrons alone. Isobars: refer to the same mass number, not the same number of electrons. Isotones: refer to the same number of neutrons, independent of electron count. Common Pitfalls: A common mistake is to confuse isoelectronic with isotopes because both sets can share similar electronic structures. However, isotopes are restricted to the same element and involve neutron differences, whereas isoelectronic species can come from different elements and charge states. Another pitfall is to mix up the prefixes iso, bar and tone. To avoid confusion, remember that isoelectronic focuses specifically on electrons, while the other terms are primarily about nuclear properties. Final Answer: Species containing the same number of electrons are called isoelectronic species.

Question 6

What is the correct ground state electron configuration of the cobalt(II) ion, Co2+, written using noble gas core notation?

Accepted Answer

Introduction / Context: This question tests your understanding of electron configurations of transition metal ions. Transition metals often lose electrons from the 4s orbital before the 3d orbital when forming cations. Recognising this pattern is crucial for writing correct electron configurations, predicting magnetic behaviour and understanding coordination chemistry. Here, you are asked to write the configuration of the cobalt(II) ion, Co2+, in noble gas core notation. Given Data / Assumptions: - The element is cobalt (Co) with atomic number 27. - We must write the configuration for the Co2+ ion, which has lost two electrons compared with the neutral atom. - We express the answer in noble gas core notation using [Ar] for the core, since argon has atomic number 18. - We use the rule that 4s electrons are removed before 3d electrons when forming transition metal cations. Concept / Approach: First, we determine the electron configuration of the neutral cobalt atom. Then we remove two electrons to obtain Co2+. For neutral Co (Z = 27), the configuration is [Ar] 3d7 4s2, because electrons fill 4s before 3d but 3d actually lies slightly lower in energy once filled. When cobalt forms the 2+ ion, it loses two electrons from the 4s orbital first, not from 3d. Therefore, Co2+ has configuration [Ar] 3d7 with no electrons remaining in the 4s orbital. Step-by-Step Solution: Step 1: Write the ground state electron configuration of neutral cobalt. Starting from [Ar] (18 electrons), we add 9 more electrons: 4s2 and 3d7. So Co: [Ar] 3d7 4s2. Step 2: Identify that we need the configuration of Co2+, which has 2 fewer electrons than neutral cobalt. Step 3: Apply the rule for transition metal cations: electrons are removed first from the 4s subshell, then from 3d. Step 4: Remove the two 4s electrons from [Ar] 3d7 4s2. This leaves [Ar] 3d7 as the configuration for Co2+. Step 5: Confirm that we have accounted for all 25 electrons remaining (27 - 2 = 25) in the configuration [Ar] 3d7. Verification / Alternative check: To verify, count electrons explicitly. [Ar] represents 18 electrons. The 3d7 subshell adds 7 electrons, giving a total of 25 electrons for Co2+, which matches 27 - 2. If we had chosen [Ar] 3d6, that would represent only 24 electrons and thus be suitable for Co3+, not Co2+. Options that still include 4s electrons, such as [Ar] 3d5 4s2 or [Ar] 3d7 4s1, would correspond to different electron counts and would contradict the rule that 4s electrons are removed first when forming cations. Why Other Options Are Wrong: [Ar] 3d6: This configuration corresponds to cobalt(III), Co3+, where three electrons have been removed, not two. [Ar] 3d5 4s2: This still has 7 valence electrons beyond argon (5 in 3d and 2 in 4s), giving 25 electrons, but it is not consistent with actual energy ordering and removal rules; Co2+ does not retain 4s2 in its ground state. Common Pitfalls: Students often forget that when transition metals form cations, 4s electrons are removed before 3d electrons, because 4s becomes higher in energy in the presence of a filled 3d subshell. Another common mistake is miscounting the total number of electrons after ion formation. To avoid these errors, always write the neutral atom configuration carefully, then explicitly remove electrons from the 4s subshell first, and finally recount to confirm the correct number of electrons. Final Answer: The correct ground state electron configuration of the cobalt(II) ion, Co2+, is [Ar] 3d7.

Question 7

In industrial fat processing, which gas is commonly used to convert vegetable oils into semi solid vegetable ghee or vanaspati by hydrogenation?

Accepted Answer

Introduction / Context: This question relates to applied chemistry and the food industry. Vegetable oils can be chemically changed into semi solid fats called vanaspati or vegetable ghee through a process called hydrogenation. Knowing which gas is used in this process helps connect organic chemistry concepts like addition reactions to everyday food products. Given Data / Assumptions: - The process mentioned is making vegetables and vanaspati from vegetable oils, which refers to hydrogenation of unsaturated oils. - The options list gases: hydrogen, nitrogen, argon and helium. - We assume a typical industrial process where unsaturated double bonds in fatty acid chains are converted to single bonds. Concept / Approach: Vegetable oils are largely composed of triglycerides containing unsaturated fatty acids with carbon carbon double bonds. During hydrogenation, hydrogen gas is passed over the heated oil in the presence of a metal catalyst such as nickel. Hydrogen adds across the double bonds, converting them into single bonds and making the fat more saturated. This increases melting point and turns the liquid oil into a semi solid fat. Inert gases like nitrogen, argon and helium do not participate in the required addition reaction and therefore are not used as the reactive gas in hydrogenation. Step-by-Step Solution: Step 1: Recognise that the process of converting liquid vegetable oils into semi solid vanaspati is called hydrogenation. Step 2: Recall that hydrogenation literally means addition of hydrogen across carbon carbon double bonds in unsaturated fatty acid chains. Step 3: Note that hydrogen gas (H2) is the reactive gas that provides the hydrogen atoms needed for this addition reaction. Step 4: Identify that nitrogen, argon and helium are largely inert under these conditions and are not used to saturate double bonds in oils. Step 5: Conclude that hydrogen gas is the correct choice. Verification / Alternative check: In organic chemistry, typical hydrogenation reactions are written as RCH=CHR + H2 leading to RCH2 CH2R in the presence of catalysts like Ni, Pt or Pd. Industrial production of margarine and vanaspati uses exactly this type of reaction on large triglyceride molecules. The name "hydrogenated vegetable oil" appears on many food labels, directly indicating hydrogen as the reacting gas. Nitrogen, argon and helium are sometimes used as inert atmospheres in chemical processes but they do not add to double bonds in this context. Why Other Options Are Wrong: Nitrogen: Although relatively inert and sometimes used to blanket tanks, nitrogen does not add across double bonds to convert oils into fats. Argon and Helium: Both are noble gases, chemically inert under normal conditions and unsuitable as reagents for hydrogenation; they are used as protective atmospheres, not as reactive gases. Common Pitfalls: A possible misunderstanding is to confuse gases used for inert atmospheres or packaging with gases that actively react in chemical processes. Students may also see the mention of vanaspati and think only of food processing without linking it to the underlying organic reaction. To avoid this, remember that hydrogenation of oils is a classic alkene addition reaction, and hydrogen is the necessary reactant gas. Final Answer: The gas used to convert vegetable oils into vanaspati by hydrogenation is hydrogen.

Question 8

In metallurgical processes, gold and silver are commonly extracted from their low grade ores by which of the following methods?

Accepted Answer

Introduction / Context: This question belongs to the topic of metallurgy in chemistry. Different metals are extracted from their ores using specific processes based on their reactivity and ore type. Gold and silver are precious metals often found in low grade ores. Their extraction method is distinctive and is frequently tested in examinations. Knowing the correct method helps you understand how selective dissolution and complex formation can be used in metallurgy. Given Data / Assumptions: - The metals considered are gold and silver. - The ores are often low in metal content and require a process that can selectively dissolve the metal. - Options include smelting, roasting, leaching and a broad hydrometallurgical approach. - We assume familiarity with the cyanide process, also called MacArthur Forrest process, used for these metals. Concept / Approach: Gold and silver are typically extracted using leaching with a dilute solution of sodium cyanide or potassium cyanide in the presence of air. The metals form soluble complexes such as [Au(CN)2]^- or [Ag(CN)2]^-. These complexes are then treated to recover the metal, often by displacement with zinc. This method is a form of hydrometallurgy but is specifically known as the cyanide leaching process. Smelting and roasting are more suitable for sulphide ores of base metals like copper, zinc and iron, not for gold and silver in low grade ores. Step-by-Step Solution: Step 1: Recall that gold and silver occur in native form and in ores that may not respond efficiently to high temperature processes alone. Step 2: Remember that cyanide leaching is the standard industrial method for extracting gold and silver from crushed ore by forming soluble cyanide complexes. Step 3: Leaching involves selectively dissolving the desired metal into solution, a characteristic hydrometallurgical technique. Step 4: Smelting involves melting and reduction at high temperatures and is not the primary modern method for these precious metals from low grade ores. Step 5: Roasting is mainly used to convert sulphide ores to oxides or to remove volatile impurities and is not itself the extraction step for gold and silver. Step 6: Therefore, the correct choice is leaching by the cyanide process. Verification / Alternative check: To verify, you can recall the key reactions for gold extraction. In the cyanide process, gold reacts with sodium cyanide, oxygen and water to form a soluble complex: 4Au + 8NaCN + O2 + 2H2O produces 4Na[Au(CN)2] and 4NaOH. A similar reaction occurs for silver. These complexes are then treated with zinc dust, which displaces gold or silver from solution. This sequence of dissolution and recovery is a hallmark of leaching. Smelting and roasting, in contrast, do not rely on forming such complexes and are used in different contexts. Why Other Options Are Wrong: Smelting: Involves high temperature reduction but is not typically used in the first step to extract gold and silver from low grade ores. Roasting: Mainly serves as a preparatory step for sulphide ores, not as the primary extraction process for gold and silver. Hydrometallurgy without leaching: Hydrometallurgy is a broad term that includes leaching, but the most specific and correct answer among the options is leaching by the cyanide process. Common Pitfalls: Students sometimes select hydrometallurgy as a general term without specifying leaching or confuse roasting and smelting because they both involve heat. Another pitfall is forgetting that gold and silver are noble metals and are usually extracted by processes that exploit their ability to form stable complexes, not by simple high temperature reduction. Always link gold and silver extraction with the cyanide leaching process in exam contexts. Final Answer: Gold and silver are commonly extracted from their ores by leaching using the cyanide process.

Question 9

According to the kinetic theory of gases, how does the average kinetic energy of gas molecules change with temperature?

Accepted Answer

Introduction / Context: This question belongs to physical chemistry and the kinetic theory of gases. The theory connects microscopic motion of gas molecules with macroscopic properties like temperature and pressure. Understanding how average kinetic energy depends on temperature is essential for explaining phenomena such as diffusion, pressure changes and gas laws. Given Data / Assumptions: - The focus is on average kinetic energy of gas molecules, not on individual instantaneous speeds. - Temperature is measured on an absolute scale, typically in kelvin (K). - We assume the gas behaves ideally so that kinetic theory relationships apply. Concept / Approach: According to kinetic theory, the average translational kinetic energy of an ideal gas molecule is directly proportional to the absolute temperature. The formula is usually written as average kinetic energy per molecule equals (3/2) k T, where k is Boltzmann constant and T is temperature in kelvin. Because of this direct proportionality, when temperature increases, average kinetic energy also increases; when temperature decreases, average kinetic energy decreases. Temperature is therefore a measure of the average kinetic energy of gas molecules. Step-by-Step Solution: Step 1: Recall the expression for average kinetic energy of a gas molecule: KE(avg) = (3/2) k T, where k is Boltzmann constant and T is absolute temperature. Step 2: Note that k is a constant and (3/2) is a constant factor, so KE(avg) is directly proportional to T. Step 3: If T increases, the product (3/2) k T increases, so average kinetic energy must increase. Step 4: If T decreases, the average kinetic energy decreases accordingly. Step 5: Therefore, the correct description is that average kinetic energy increases with increase of temperature. Verification / Alternative check: This relationship is consistent with everyday observations. When a gas is heated, its molecules move faster, pressure tends to increase at constant volume and the gas expands at constant pressure. All these behaviours are explained by higher kinetic energy at higher temperatures. Conversely, cooling a gas slows molecular motion and reduces kinetic energy. None of these observations support the idea that kinetic energy remains constant or moves in the opposite direction to temperature. Why Other Options Are Wrong: Unaffected by temperature: This would imply no connection between microscopic motion and temperature, contradicting kinetic theory and experimental evidence. Decreases as temperature increases: This is opposite to the theoretical and observed relationship; heating cannot cause molecules to slow down on average. Increases as temperature decreases: Also opposite to reality; cooling slows particles rather than making them more energetic. Common Pitfalls: Some students mix up average kinetic energy with total internal energy or confuse the effect of volume changes with temperature changes. Another pitfall is thinking in terms of Celsius and forgetting that absolute zero on the Kelvin scale corresponds to zero average kinetic energy. To avoid confusion, always associate higher absolute temperature with higher average kinetic energy for ideal gases. Final Answer: The average kinetic energy of gas molecules increases as temperature increases.

Question 10

Among the following haloacetic acids, which one is the strongest acid: ClCH2COOH, BrCH2COOH, FCH2COOH or ICH2COOH?

Accepted Answer

Introduction / Context: This organic chemistry question examines your understanding of how substituents influence the acidity of carboxylic acids. Haloacetic acids are acetic acid derivatives where a hydrogen on the alpha carbon is replaced by a halogen. The electron withdrawing power of the halogen through the inductive effect stabilises the conjugate base and changes acid strength. Recognising which substituent makes the strongest acid is a classic exam problem. Given Data / Assumptions: - Four haloacetic acids are given: fluoroacetic, chloroacetic, bromoacetic and iodoacetic acids. - All have the general formula XCH2COOH where X is a halogen (F, Cl, Br, I). - We assume that solvent and other conditions are comparable, so we focus on inductive effects of the halogens. Concept / Approach: The acidity of a carboxylic acid depends on the stability of its conjugate base (carboxylate anion). Electron withdrawing substituents near the carboxyl group increase acidity by pulling electron density away, stabilising the negative charge. Halogens exert a strong -I (negative inductive) effect, and the strength of this effect decreases down the group: F > Cl > Br > I. Therefore, the acid with fluorine attached, FCH2COOH, should have the strongest -I effect and the most stabilised conjugate base, making it the strongest acid among the options. Step-by-Step Solution: Step 1: Identify the substituent in each acid: F in fluoroacetic acid, Cl in chloroacetic acid, Br in bromoacetic acid and I in iodoacetic acid. Step 2: Recall the electron withdrawing ability of halogens via the inductive effect: fluorine, being the most electronegative, has the strongest -I effect, followed by chlorine, bromine and iodine. Step 3: Recognise that a stronger -I effect stabilises the carboxylate anion more effectively, increasing the acid strength. Step 4: Arrange the acids in decreasing order of acidity based on inductive effect: FCH2COOH > ClCH2COOH > BrCH2COOH > ICH2COOH. Step 5: Conclude that FCH2COOH (fluoroacetic acid) is the strongest acid among the given options. Verification / Alternative check: You can confirm this reasoning by remembering standard pKa trends for haloacetic acids. Fluoroacetic acid has the lowest pKa value (strongest acid), chloroacetic acid is slightly weaker, bromoacetic weaker still and iodoacetic the weakest in this series. This sequence matches the electronegativity trend of the halogens. The closer and more electronegative the substituent is to the carboxyl group, the stronger its inductive effect and the greater the acid strength. Why Other Options Are Wrong: ClCH2COOH: Chloroacetic acid is quite strong but weaker than fluoroacetic acid because chlorine is less electronegative than fluorine. BrCH2COOH: Bromoacetic acid has a weaker -I effect than both fluoro and chloro derivatives, thus it is less acidic. ICH2COOH: Iodoacetic acid has the weakest electron withdrawing effect among the four and is therefore the weakest acid of the series. Common Pitfalls: A common mistake is to think that heavier halogens always make stronger acids because they are larger or more polarisable. While polarisability plays a role in some systems, in this simple alpha substituted carboxylic acid series, the dominant factor is electronegativity and inductive effect. Another pitfall is to forget that the substituent is on the alpha carbon, which makes the inductive effect particularly strong compared with more distant positions. Final Answer: The strongest acid among the given haloacetic acids is FCH2COOH (fluoroacetic acid).

Question 11

Which Noble Gas has the highest First Ionization Energy?

Accepted Answer

Helium has the highest First Ionization Energy in the Noble Gases group. Ionization Energy :: Ionisation energy is the energy required to remove one mole of electrons from one mole of gaseous atoms. Atoms are more stable when they have full valence shells. Noble Gases have a very stable electron configuration, therefore, they have the highest amount of ionization energy within their periods. It decreases down a family because electrons farther from the nucleus are easier to remove. Ionization Energy levels of Noble Gases group in Descending Order :: He, Ne, Ar, Kr, Xe, Rn

Question 12

All of the following bases are constituents of a typical RNA molecule except which one: thymine, adenine, uracil or guanine?

Accepted Answer

Introduction / Context: This question lies at the intersection of chemistry and biology, focusing on the nitrogenous bases present in nucleic acids. RNA (ribonucleic acid) and DNA (deoxyribonucleic acid) differ slightly in their base composition. Knowing which bases occur in RNA is essential for understanding transcription, translation and genetic coding. Given Data / Assumptions: - The bases mentioned are thymine, adenine, uracil and guanine. - We are asked to identify the base that is not normally found in RNA. - We assume standard cellular RNA, such as mRNA, tRNA and rRNA, with canonical bases. Concept / Approach: RNA typically contains four main nitrogenous bases: adenine (A), guanine (G), cytosine (C) and uracil (U). In DNA, thymine (T) is present instead of uracil. Adenine pairs with uracil in RNA and with thymine in DNA. Therefore, thymine is characteristic of DNA and is generally absent from RNA, which uses uracil in its place. Among the given options, adenine, uracil and guanine are all standard RNA bases; thymine is the exception. Step-by-Step Solution: Step 1: Recall the four standard bases in RNA: adenine, guanine, cytosine and uracil. Step 2: Compare each option with this list. Adenine is present in RNA; guanine is present in RNA; uracil is present in RNA. Step 3: Thymine is not part of the standard RNA base set. It is the base that appears in DNA instead of uracil. Step 4: Conclude that thymine is the base that is not normally found in RNA. Verification / Alternative check: In DNA, the base pairings are A T and G C, whereas in RNA, they are A U and G C. This substitution of uracil for thymine is a well known distinction between the two nucleic acids. Biochemical diagrams of RNA nucleotides show ribose sugar, phosphate and bases A, G, C or U. Thymine containing nucleotides are associated with deoxyribose sugar and DNA. These widely accepted facts confirm that thymine is the correct answer. Why Other Options Are Wrong: Adenine: A purine base found in both DNA and RNA; it participates in base pairing and coding in RNA. Uracil: A pyrimidine base unique to RNA in the canonical nucleic acids; it replaces thymine.Guanine: Another purine base present in both DNA and RNA. Common Pitfalls: A frequent mistake is to confuse the base sets of DNA and RNA, especially under exam pressure. Some students may recall the four bases but forget which nucleic acid uses uracil and which uses thymine. A helpful memory tip is that thymine and deoxyribose both contain the letter T (through association with DNA), while uracil appears only in RNA. Keeping this distinction clear ensures you can answer similar questions quickly. Final Answer: The base that is not a standard constituent of RNA is thymine.

Question 13

In the context of atomic structure, which of the following is not considered a fundamental subatomic particle: neutron, ion, electron or proton?

Accepted Answer

Introduction / Context: This question tests your understanding of basic atomic structure and terminology. Atoms are built from fundamental subatomic particles, and many chemical concepts are based on how these particles interact. Distinguishing between truly subatomic particles and composite entities like ions helps clarify the language of chemistry and physics. Given Data / Assumptions: - The options are neutron, ion, electron and proton. - We must identify which option is not itself a subatomic particle. - We assume the standard model where atoms are composed of protons, neutrons and electrons. Concept / Approach: Subatomic particles are particles smaller than an atom and include protons, neutrons and electrons as the primary constituents of ordinary matter. An ion, by contrast, is not a fundamental particle; it is an atom or group of atoms that has gained or lost one or more electrons and therefore carries a net charge. The ion as a whole is bigger than the atom and consists of many subatomic particles, but it is not itself a subatomic particle. Step-by-Step Solution: Step 1: Recall that protons, neutrons and electrons are the basic building blocks of atoms and are therefore subatomic particles. Step 2: A proton is a positively charged particle found in the nucleus. Step 3: A neutron is an electrically neutral particle in the nucleus. Step 4: An electron is a negatively charged particle located in regions of space around the nucleus. Step 5: An ion is a charged atom or a group of atoms that has gained or lost electrons; it is made of subatomic particles but is not itself a single subatomic particle. Step 6: Therefore, ion is the correct answer to the question. Verification / Alternative check: Consider common examples. A sodium ion, Na+, has 11 protons, 11 neutrons (for the most common isotope) and 10 electrons. Clearly, Na+ is a collection of many protons, neutrons and electrons. Similarly, a sulphate ion, SO4^2-, is an entire polyatomic group containing multiple atoms. In both cases, the word ion describes a charged aggregate, not a fundamental subatomic particle. In contrast, protons, neutrons and electrons cannot be broken down within normal chemical reactions and are therefore considered subatomic in the context of chemistry. Why Other Options Are Wrong: Neutron: A fundamental subatomic particle with no electric charge, found in atomic nuclei. Electron: A fundamental subatomic particle with a negative charge, essential in chemical bonding and electricity. Proton: A fundamental subatomic particle with a positive charge, defining the atomic number and identity of an element. Common Pitfalls: Students sometimes treat any small entity in chemistry as a subatomic particle without distinguishing fundamental particles from composite ones like ions or molecules. Another pitfall is to focus only on charge and assume that anything charged, such as an ion, must be subatomic. To avoid confusion, remember that subatomic particles are the basic components of atoms, whereas ions are atoms or groups of atoms that have acquired a net charge. Final Answer: The entity that is not a fundamental subatomic particle is the ion.

Question 14

In particle physics, the charge on a positron is equal in magnitude and sign to the charge on which of the following particles?

Accepted Answer

Introduction / Context: This question tests your knowledge of elementary particles and their electric charges. A positron is the antiparticle of the electron and plays an important role in nuclear decay processes and medical imaging techniques such as positron emission tomography. Understanding how its charge compares with that of familiar particles like the proton helps clarify basic charge relationships. Given Data / Assumptions: - The particle under consideration is the positron, often denoted e+. - Options include proton, electron, alpha particle and neutron. - We focus on the magnitude and sign of the electric charge on each particle. Concept / Approach: The positron is defined as the antimatter counterpart of the electron. It has the same mass as an electron but the opposite electric charge. The electron has a charge of -1 in elementary charge units, whereas the positron has a charge of +1. The proton also has a charge of +1 in the same units. An alpha particle (a helium nucleus) has a charge of +2, and a neutron has zero charge. Therefore, in terms of magnitude and sign, the charge on a positron matches the charge on a proton. Step-by-Step Solution: Step 1: Recall that the electron has a charge of -1 (negative elementary charge). Step 2: As the antiparticle of the electron, the positron has the same mass as the electron but a charge of +1. Step 3: A proton, the positively charged particle in the nucleus, also has a charge of +1. Step 4: An alpha particle, being a helium nucleus containing two protons and two neutrons, has a total charge of +2. Step 5: A neutron has no net charge; its charge is 0. Step 6: Therefore, the positron and proton share the same charge magnitude and sign, making proton the correct answer. Verification / Alternative check: Charge conservation in nuclear reactions provides an additional check. In beta plus decay, a proton in the nucleus converts into a neutron and emits a positron and a neutrino. The proton (charge +1) changes to a neutron (charge 0) plus a positron (charge +1), so the total charge before and after remains +1. This reaction would not balance correctly if the positron had any charge other than +1. Since the proton also has +1, this confirms that their charges are equal in magnitude and sign. Why Other Options Are Wrong: Electron: Has a charge of -1, opposite in sign to the positron, which is +1. Alpha particle: Has a charge of +2, double the charge of a positron; therefore, they are not equal. Neutron: Has zero charge, so its charge cannot equal the non zero charge of a positron. Common Pitfalls: A common confusion is to think that a positron must have the same charge as an alpha particle because both are positive. However, it is essential to compare numerical values, not just the sign. Another pitfall is to assume that the positron, as a type of electron, must have a small or fractional charge. In fact, all these particles carry integer multiples of the elementary charge. Remember that a positron and a proton both carry a single positive elementary charge, +1. Final Answer: The charge on a positron is equal to the charge on a proton.

Question 15

For the hydrogen electrode reaction 2H+ + 2e- ⇌ H2(g), the reduction potential becomes negative under which of the following conditions of hydrogen gas pressure (PH2) and hydrogen ion concentration [H+]?  (1) PH2 = 2 atm, [H+] = 2.0 M  (2) PH2 = 1 a…

Accepted Answer

Introduction / Context: This electrochemistry question asks you to apply the Nernst equation to the standard hydrogen electrode reaction. The standard hydrogen electrode is defined to have a reduction potential of 0.00 V when hydrogen gas is at 1 atm and the hydrogen ion concentration is 1.0 M at 25°C. Deviations from these conditions cause the electrode potential to become positive or negative. Understanding this helps you predict the behaviour of electrodes under non standard conditions. Given Data / Assumptions: - Half cell reaction: 2H+ + 2e- ⇌ H2(g). - Standard electrode potential E° is defined as 0.00 V for PH2 = 1 atm and [H+] = 1.0 M. - Four combinations of PH2 (in atm) and [H+] (in mol L^-1) are given. - Temperature is assumed to be 25°C (298 K) so that the usual form of the Nernst equation applies. Concept / Approach: The Nernst equation for this half cell at 25°C is: E = E° - (0.0591 / 2) log( PH2 / [H+]^2 ). Since E° = 0 for the standard hydrogen electrode, the expression simplifies to: E = -0.02955 log( PH2 / [H+]^2 ). The sign of E depends on the sign of the logarithm term. If the argument PH2 / [H+]^2 is greater than 1, log of that argument is positive, E is negative. If the argument is less than 1, log is negative and E is positive. If the argument equals 1, log is zero and E is zero. Step-by-Step Solution: Step 1: Write the simplified Nernst equation: E = -0.02955 log( PH2 / [H+]^2 ). Step 2: Evaluate the ratio PH2 / [H+]^2 for each condition. Condition 1: PH2 = 2 atm, [H+] = 2.0 M. Ratio = 2 / (2^2) = 2 / 4 = 0.5. Condition 2: PH2 = 1 atm, [H+] = 2.0 M. Ratio = 1 / (2^2) = 1 / 4 = 0.25. Condition 3: PH2 = 1 atm, [H+] = 1.0 M. Ratio = 1 / (1^2) = 1. Condition 4: PH2 = 2 atm, [H+] = 1.0 M. Ratio = 2 / (1^2) = 2. Step 3: Check whether each ratio is greater than, equal to or less than 1. Condition 1: 0.5 < 1, so log(0.5) is negative and E becomes positive. Condition 2: 0.25 < 1, so log(0.25) is negative and E is again positive. Condition 3: ratio is 1, so log(1) = 0 and E = 0 (standard conditions). Condition 4: 2 > 1, so log(2) is positive, making E negative. Step 4: Therefore, only condition 4 yields a negative electrode potential. Verification / Alternative check: You can also reason qualitatively. Increasing PH2 while holding [H+] constant favours the reverse reaction (formation of H+ from H2), which lowers the reduction tendency and makes the reduction potential more negative. Conversely, increasing [H+] while keeping PH2 fixed increases the reduction tendency and leads to a more positive potential. In condition 4, PH2 is higher than standard while [H+] is at standard, so you expect the electrode to be less willing to reduce H+ and thus to have a negative potential relative to the standard hydrogen electrode. Why Other Options Are Wrong: Condition 1: Both PH2 and [H+] are above standard, but [H+] is squared in the Nernst expression, making the denominator larger and the ratio less than 1, so E is positive, not negative. Condition 2: Higher [H+] and standard PH2 give a ratio less than 1, again yielding a positive potential. Condition 3: Standard conditions by definition give E = 0 V. Common Pitfalls: A common error is to forget the stoichiometric coefficient of H+ in the Nernst equation and fail to square [H+]. Another pitfall is to try to decide the sign of E simply by comparing PH2 and [H+] separately rather than calculating the ratio. To avoid these mistakes, always write the correct Nernst expression for the reaction and evaluate the argument of the logarithm carefully. Final Answer: The reduction potential of the hydrogen half cell becomes negative under condition 4 only (PH2 = 2 atm and [H+] = 1.0 M).

Question 16

In stoichiometric calculations, what is the mass of 1 mole of carbon dioxide (CO2) expressed in grams?

Accepted Answer

Introduction / Context: This fundamental chemistry question checks your ability to calculate molar mass from a chemical formula. Molar mass is essential for converting between moles and grams in stoichiometry, gas law calculations and solution chemistry. Carbon dioxide is a common compound that appears in many exam problems and real world contexts such as respiration and combustion. Given Data / Assumptions: - The formula of carbon dioxide is CO2. - We use approximate atomic masses: carbon ≈ 12 u, oxygen ≈ 16 u. - The question asks for the mass of 1 mole of CO2 in grams, which equals the molar mass. Concept / Approach: Molar mass of a compound is found by summing the atomic masses of all atoms in its formula. For CO2, this means adding the mass of one carbon atom and two oxygen atoms. Once we have the molar mass in atomic mass units (u), the numerical value in grams per mole is the same. Therefore, the mass of 1 mole of CO2 is simply its molar mass expressed in grams. Step-by-Step Solution: Step 1: Identify the atoms in CO2: one carbon atom and two oxygen atoms. Step 2: Use approximate atomic masses: carbon = 12, oxygen = 16. Step 3: Calculate the contribution from carbon: 1 * 12 = 12. Step 4: Calculate the contribution from oxygen: 2 * 16 = 32. Step 5: Add these values to obtain molar mass: 12 + 32 = 44. Step 6: Conclude that the molar mass of CO2 is 44 g mol^-1, so 1 mole of CO2 has a mass of 44 grams. Verification / Alternative check: This value is widely used in many chemistry and physics problems, such as calculating the mass of CO2 produced in combustion reactions or determining the number of moles in a given mass of dry ice. If you recall that oxygen gas (O2) has a molar mass of 32 g mol^-1 and that CO2 contains one carbon (12) plus two oxygens (32), the total of 44 is straightforward and consistent. Any significantly different value would contradict well established molar mass tables. Why Other Options Are Wrong: 32 grams: This is the molar mass of O2, not CO2; it counts only two oxygens and omits the carbon contribution. 16 grams: Represents approximately the mass of one oxygen atom per mole, far too low for CO2. 12 grams: Represents the molar mass of elemental carbon, not carbon dioxide. Common Pitfalls: One common mistake is to forget to multiply the atomic mass of oxygen by 2 in CO2 and incorrectly use 16 instead of 32 for the oxygen part. Another error is to confuse CO2 with CO or O2 when recalling numbers from memory. To avoid such problems, always write the formula clearly, list each element with its subscript and multiply atomic masses by the appropriate subscripts before summing. Final Answer: The mass of 1 mole of carbon dioxide (CO2) is 44 grams.

Question 17

In plant physiology, which one of the following crops is known to have the highest photosynthetic efficiency and activity under typical field conditions: sugarcane, wheat, cotton or rice?

Accepted Answer

Introduction / Context: This question links basic plant biology with agriculture. Different crops use different photosynthetic pathways, such as C3 and C4, which affect their efficiency under high light, temperature and low carbon dioxide conditions. Understanding which crops have higher photosynthetic efficiency is important in agronomy, crop selection and questions related to food security and bioenergy. Given Data / Assumptions: - The crops listed are sugarcane, wheat, cotton and rice. - We are asked which crop has the highest photosynthetic activity, which essentially refers to photosynthetic efficiency in converting light energy into biomass. - We assume typical tropical or subtropical field conditions where high light intensity and warm temperatures favour certain photosynthetic pathways. Concept / Approach: Plants are broadly classified as C3 or C4 based on their carbon fixation pathway. C4 plants, such as sugarcane and maize, possess a special mechanism that concentrates CO2 in bundle sheath cells, minimising photorespiration and improving efficiency under high light and temperature. C3 plants, like wheat, rice and cotton, are more prone to photorespiration under such conditions, reducing their effective photosynthetic efficiency. Among the given options, sugarcane is a well known C4 crop with very high photosynthetic efficiency, which is why it is often used for sugar and biofuel production. Step-by-Step Solution: Step 1: Classify the crops by photosynthetic pathway: sugarcane is a C4 plant, while wheat, cotton and rice are primarily C3 plants. Step 2: Recall that C4 plants have higher photosynthetic efficiency under high light, high temperature and low CO2 conditions due to their CO2 concentrating mechanism. Step 3: Recognise that sugarcane is often cited in textbooks as an example of a crop with very high photosynthetic activity and productivity. Step 4: Wheat, rice and cotton are important crops but do not match sugarcane in maximum photosynthetic efficiency per unit leaf area. Step 5: Therefore, among the options, sugarcane is the correct choice for highest photosynthetic activity. Verification / Alternative check: Agricultural data and plant physiology references show that C4 crops such as sugarcane and maize typically achieve higher yields of biomass per unit of intercepted light compared with C3 cereals under warm, sunny conditions. Sugarcane in particular is used as a model crop for high productivity and is a major source of sucrose and ethanol. Wheat and rice, despite being staple food grains, are C3 plants and have lower maximum photosynthetic rates under similar conditions. Cotton is mainly grown for fibre and is also a C3 plant without the C4 advantage. Why Other Options Are Wrong: Wheat: A C3 cereal crop; its photosynthetic efficiency is lower than that of C4 crops like sugarcane under high light and temperature. Cotton: A C3 plant grown for its fibre, not known for the highest photosynthetic efficiency compared with C4 crops. Rice: Another C3 staple cereal, very important for food but not as efficient as sugarcane in photosynthesis under the specified conditions. Common Pitfalls: Students may associate high productivity only with cereals like rice and wheat because of their importance in diets, overlooking the role of sugarcane as a highly productive biomass and sugar crop. Another pitfall is to forget or confuse which crops are C3 versus C4. To avoid such errors, remember that sugarcane, maize and sorghum are classical C4 crops with high photosynthetic efficiency, while wheat and rice are C3 cereals. Final Answer: The crop with the highest photosynthetic activity among the options is sugarcane.

Question 18

What is the key difference between the nuclear fission chain reaction in a power producing nuclear reactor and that in an atomic bomb?

Accepted Answer

Introduction / Context: Nuclear energy and nuclear weapons both rely on the process of nuclear fission, in which heavy nuclei such as uranium 235 or plutonium 239 split into smaller fragments and release a large amount of energy. In both a nuclear power reactor and an atomic bomb, a chain reaction involving fission events can occur. However, the way this chain reaction is managed and its rate of progress are very different. This question tests your understanding of that key difference between a controlled reactor used for peaceful power generation and an atomic bomb used as a weapon. Given Data / Assumptions: A nuclear reactor is a device designed to sustain a fission chain reaction in a safe, steady, and controlled manner to produce heat and electricity. An atomic bomb is a weapon that uses a very rapid, uncontrolled fission chain reaction to release energy in a fraction of a second. Both systems use fissile materials and neutrons to propagate a chain reaction. We are comparing the behaviour of the chain reaction in each case. Concept / Approach: The central idea is the concept of a chain reaction. In a chain reaction, neutrons released from one fission event cause additional fissions, which release more neutrons, and so on. In a nuclear reactor, the design aims to keep the chain reaction critical or slightly sub critical so that the power level remains stable. Control rods, moderators, coolant, and engineered safety systems absorb excess neutrons and remove heat. In an atomic bomb, the geometry and mass of fissile material are arranged so that, when brought together rapidly, the system becomes prompt supercritical. No control mechanisms slow the reaction, so fission events grow explosively in a very short time. Step-by-Step Solution: Step 1: Recall that both nuclear reactors and atomic bombs use the same basic physical process of nuclear fission and can support a chain reaction. Step 2: In a power reactor, engineers insert control rods made of neutron absorbing materials like boron or cadmium and use moderators and coolant to manage neutron energies and reaction rate. Step 3: This arrangement makes the chain reaction controlled, meaning the number of fissions per unit time is kept nearly constant so that energy is released steadily. Step 4: In an atomic bomb, the fissile material is assembled into a supercritical mass almost instantaneously, and there are no control rods to absorb neutrons. Step 5: As a result, the chain reaction becomes uncontrolled and proceeds extremely rapidly, releasing enormous energy in an explosion rather than steady heat. Verification / Alternative check: Think about how long each device operates. A nuclear power reactor can run at a constant power level for months or years, which is possible only if the reaction is controlled. Operators can insert or withdraw control rods to adjust the power output. In contrast, an atomic bomb releases almost all of its energy within microseconds to milliseconds. There is no mechanism to slow down or stop the reaction once initiated. Therefore, the essential distinction is that the reactor has a controlled chain reaction, while the bomb has an uncontrolled chain reaction. Why Other Options Are Wrong: The statement that no chain reaction takes place in a nuclear reactor but does in an atomic bomb is wrong because reactors clearly rely on a sustained chain reaction. The claim that the chain reaction in a nuclear reactor is completely uncontrolled is incorrect because control is the main feature that allows safe operation. The statement that no chain reaction takes place in an atomic bomb is also wrong since the bomb destructive power comes from an extremely rapid chain reaction. Therefore, only the option that describes a controlled chain reaction in a reactor and an uncontrolled one in a bomb is correct. Common Pitfalls: Learners sometimes mistakenly believe that reactors do not use chain reactions because they associate chain reactions only with explosions. Others may think that both systems are simply scaled versions of the same device. The main point is that in reactors, design and control systems keep the reaction rate stable, while in bombs the design deliberately avoids control so that the reaction runs away. Remember the keywords controlled for reactors and uncontrolled for bombs to avoid confusion in similar questions. Final Answer: The key difference is that the fission chain reaction in a nuclear reactor is carefully controlled, while in an atomic bomb it is uncontrolled and explosive.

Question 19

Positive ions, also called cations, are formed from neutral atoms by the loss of which type of subatomic particles?

Accepted Answer

Introduction / Context: Atoms are made up of three main types of subatomic particles, namely protons, neutrons, and electrons. When atoms gain or lose charged particles, they can form ions. Positive ions are called cations, and negative ions are called anions. This question asks which particle must be lost when a neutral atom changes into a positively charged ion. Understanding this process is essential in basic chemistry, especially in topics like ionic bonding, electrolysis, and redox reactions. Given Data / Assumptions: A neutral atom has equal numbers of protons and electrons so that the total charge is zero. Positive ions have a net positive charge. Available subatomic particles in the options are neutrons, protons, nuclear charge, and electrons. We assume that the nucleus remains intact in ordinary chemical processes. Concept / Approach: The overall charge of an atom is determined by the balance between positively charged protons in the nucleus and negatively charged electrons in shells around the nucleus. Neutrons carry no charge. In most chemical changes, the nucleus and its protons remain unchanged, while electrons can be transferred or shared. To form a positive ion from a neutral atom, the atom must lose negative charge so that positive charge dominates. That means electrons are removed. Losing electrons produces a cation with a net positive charge equal to the number of electrons lost. Step-by-Step Solution: Step 1: Consider a neutral atom with Z protons and Z electrons, so total charge is zero. Step 2: To obtain a net positive charge, the atom must have fewer negative charges than positive charges. Step 3: Chemical changes usually involve the movement of electrons, not protons or neutrons, which remain locked in the nucleus. Step 4: If the atom loses one or more electrons, it will still have the same number of protons, but fewer electrons. Step 5: The resulting species has excess positive charge and is called a positive ion or cation. Therefore, loss of electrons is responsible. Verification / Alternative check: Take a simple example of sodium. A neutral sodium atom has 11 protons and 11 electrons. If it loses one electron to form Na plus, it now has 11 protons and only 10 electrons. The net charge is plus one because positive charges exceed negative charges by one unit. This process does not involve any change in the number of neutrons or protons. Similar reasoning applies to other common cations such as Ca two plus or Al three plus, where the atom has lost two or three electrons respectively. This confirms that the correct answer is electrons. Why Other Options Are Wrong: Neutrons carry no electric charge, so gaining or losing neutrons changes the mass and isotope of an atom, not its charge; option neutrons is wrong. Protons are positively charged, but chemical reactions do not usually remove them from the nucleus, and losing protons would change the element identity; so option protons is incorrect for normal ion formation. The phrase nuclear charge is vague and not a specific particle; in chemical ion formation the nucleus remains unchanged, so this option is also wrong. Only loss of electrons explains the formation of a positive ion from a neutral atom. Common Pitfalls: Some learners confuse the roles of protons and electrons and may think that a positive ion is created by gaining protons. Others may assume that any change in particles inside the atom can produce charge. To avoid such mistakes, remember that in ordinary chemistry the nucleus stays the same and electrons are the particles that move. Positive ions form when an atom loses electrons, while negative ions form when an atom gains electrons. Final Answer: Positive ions or cations are formed from neutral atoms by the loss of electrons.

Question 20

The presence or absence of a hydroxyl group on which numbered carbon atom of the sugar unit differentiates RNA from DNA in nucleic acids?

Accepted Answer

Introduction / Context: Deoxyribonucleic acid and ribonucleic acid are the two main types of nucleic acids found in living organisms. Both contain a sugar, a phosphate group, and nitrogenous bases, but the sugar component is slightly different in DNA and RNA. This difference in the sugar structure has important consequences for the stability and function of these molecules. The question asks on which carbon atom of the sugar the presence or absence of a hydroxyl group distinguishes RNA from DNA. Given Data / Assumptions: DNA contains the sugar deoxyribose. RNA contains the sugar ribose. Both sugars are five carbon pentose sugars. We are concerned with which numbered carbon in the sugar ring bears a hydroxyl group in RNA that is missing in DNA. Concept / Approach: In ribose, the sugar found in RNA, the ring carbons are labelled 1 prime, 2 prime, 3 prime, 4 prime, and the side chain carbon is 5 prime. Ribose has hydroxyl groups attached at the 2 prime and 3 prime positions. In deoxyribose, the sugar in DNA, the hydroxyl group at the 2 prime position is replaced by a hydrogen atom, so DNA is described as deoxy at the second carbon. This missing hydroxyl group at the second carbon makes DNA more chemically stable and less reactive than RNA. Therefore, the key difference is at the 2 prime carbon of the sugar ring. Step-by-Step Solution: Step 1: Recall that ribose and deoxyribose are both pentose sugars present in RNA and DNA respectively. Step 2: Note that ribose has hydroxyl groups at both the 2 prime and 3 prime carbons on the sugar ring. Step 3: Remember that deoxyribose lacks the hydroxyl group at the 2 prime position and instead has a hydrogen atom there. Step 4: Understand that this absence of the hydroxyl group at the 2 prime carbon is why the molecule is called deoxyribose, meaning ribose that has lost an oxygen. Step 5: Conclude that the presence or absence of a hydroxyl group at the second carbon atom differentiates RNA from DNA. Verification / Alternative check: Most biochemistry diagrams of DNA and RNA show the sugar backbone explicitly. In RNA, the sugar is labelled as beta D ribose with OH groups on the 2 prime and 3 prime positions. In DNA, the sugar is beta D deoxyribose, where the 2 prime position has only a hydrogen. When polymer chains are compared, RNA appears less stable and more prone to hydrolysis because the 2 prime OH can participate in intramolecular reactions. This structural detail is emphasized in textbooks as the key chemical difference between the sugars of RNA and DNA, confirming that the second carbon is correct. Why Other Options Are Wrong: The first carbon atom is bonded to the nitrogenous base and carries a hydroxyl group in both ribose and deoxyribose, so it does not differentiate RNA and DNA. The third carbon atom has a hydroxyl group in both sugars and is involved in phosphodiester bond formation, but again this is common to both. The fourth carbon is part of the ring structure but is not the position where the deoxy difference appears. Therefore, none of these positions distinguish DNA from RNA; only the second carbon does. Common Pitfalls: A frequent mistake is to think that any hydroxyl group present in one and not in the other must be on the third carbon, because the three prime end of DNA is often mentioned in replication and transcription. Another confusion arises from mixing up carbon numbering in the sugar ring. To avoid this, remember the phrase deoxy at two prime for DNA and keep in mind that the critical missing hydroxyl group is at the second carbon in deoxyribose. Final Answer: The presence or absence of a hydroxyl group on the second carbon atom, the 2 prime carbon, of the sugar differentiates RNA from DNA.

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