Question 1

Process-path constraints vocabulary: An isobaric process is one carried out at constant __________.

Accepted Answer

Introduction: Thermodynamic analyses often classify processes by which property is held constant: isothermal (T), isobaric (P), isochoric (V), isentropic (S), or isenthalpic (H). Correctly identifying these terms is foundational for applying the first and second laws to real equipment like compressors, turbines, valves, and heat exchangers. Given Data / Assumptions: Steady or unsteady processes may be idealized with one property held constant. No specific working fluid is required to define the term. Concept / Approach: “Isobaric” literally means “equal pressure.” In an isobaric process, the system pressure remains constant throughout, while other properties such as temperature, volume, enthalpy, and entropy may change according to the path and heat/work interactions. Examples include boiling/condensing at constant pressure and many mixing operations open to atmosphere. Step-by-Step Solution: Match the prefix: iso (same) + baric (pressure).Conclude the held property is pressure.Select “pressure” as the correct completion. Verification / Alternative check: Property diagrams (P–V, T–S) show horizontal lines for isobaric paths on P–V coordinates. Why Other Options Are Wrong: Temperature → isothermal; volume → isochoric/isochoric; entropy → isentropic; enthalpy → isenthalpic (e.g., throttling). Common Pitfalls: Confusing isentropic (adiabatic and reversible) with isothermal; they are distinct idealizations. Final Answer: pressure

Question 2

Corresponding states principle: For most gases, the compressibility factor Z shows near-universal behavior at the same values of which pair of variables?

Accepted Answer

Introduction: The law of corresponding states provides a powerful way to generalize real-gas behavior. By scaling variables relative to their critical properties, diverse gases collapse onto similar Z = f(Tr, Pr) curves, enabling estimation of P–V–T properties without gas-specific equations in preliminary design. Given Data / Assumptions: Reduced temperature Tr = T / Tc (absolute units). Reduced pressure Pr = P / Pc. Compressibility factor Z = PV / RT for the gas. Concept / Approach: When properties are expressed in reduced form with respect to critical constants, many gases exhibit similar deviations from ideality. Generalized compressibility charts plot Z versus Pr for various Tr, allowing quick estimates of real-gas properties using only Tc and Pc of the gas along with P and T of the state point. Step-by-Step Solution: Recognize that “corresponding states” requires reduced variables.Identify the standard pair: (Pr, Tr).Conclude that Z is approximately the same for many gases at the same Pr and Tr. Verification / Alternative check: Generalized charts and Lee–Kesler correlations explicitly use Pr and Tr to correlate Z across substances. Why Other Options Are Wrong: Pressure and temperature (absolute) do not collapse behaviors across different gases. Critical pressure and temperature are constants of the substance, not state variables to match between gases. Common Pitfalls: Using Celsius instead of Kelvin for reduced temperature; always use absolute scales. Final Answer: reduced pressure and reduced temperature

Question 3

Determining latent heat from vapor-pressure data: If the vapor pressures at two temperatures are known for a solid in equilibrium with its liquid, which relation allows you to calculate the latent heat of fusion?

Accepted Answer

Introduction: Phase-equilibrium thermodynamics connects temperature, pressure, and latent heats. When experimental vapor-pressure data are available at two temperatures for a solid–liquid system, the appropriate relation can be used to back-calculate the latent heat of fusion, essential for solidification and melting process design. Given Data / Assumptions: Two equilibrium points (P1, T1) and (P2, T2) for the same substance. Solid–liquid equilibrium considered via the vapor-pressure curve (near melting). Latent heat is temperature-insensitive over the interval (engineering approximation). Concept / Approach: The Clapeyron–Clausius equation in integrated form relates slopes of coexistence curves to latent heats: d ln P / d(1/T) = − ΔH / R for phase changes involving vapor. With two data points, ΔH (here, fusion or sublimation-related depending on the path) can be estimated. For melting, one often uses the Clapeyron form involving ΔV and ΔH; with vapor-pressure data, the Clausius–Clapeyron approximation is common. Step-by-Step Solution: Write Clausius–Clapeyron: ln(P2/P1) = −ΔH/R * (1/T2 − 1/T1).Solve for ΔH using known P1, P2, T1, T2.Interpret ΔH as the latent heat relevant to the phase transition considered. Verification / Alternative check: Consistency can be checked by computing ΔH from multiple temperature intervals; results should agree within experimental uncertainty. Why Other Options Are Wrong: Maxwell, Van Laar, Gibbs–Duhem, Nernst: Important relations, but not used directly to extract latent heat from vapor-pressure vs temperature data. Common Pitfalls: Using Celsius instead of Kelvin; failing to ensure pressure units cancel in the logarithm. Final Answer: Clapeyron–Clausius equation

Question 4

Fugacity and activity: identifying the incorrect real-gas statement: Choose the WRONG statement regarding activity coefficients, fugacity, and the fugacity coefficient for ideal gases and mixtures.

Accepted Answer

Introduction: Fugacity and activity are corrected pressures and concentrations that account for nonideal behavior in gases and solutions. They appear in equilibrium expressions and chemical potential formulations. Distinguishing correct from incorrect statements avoids mistakes in reaction and phase-equilibrium calculations. Given Data / Assumptions: Ideal-gas limit is approached as pressure → 0 or via idealized models. Definitions: fugacity f, fugacity coefficient φ = f / P. Activity coefficients γ apply to solutions and are dimensionless. Concept / Approach: For an ideal gas, fugacity equals pressure (f = P), hence the fugacity coefficient φ = f / P = 1, not zero. In ideal-gas mixtures, each component’s fugacity equals its partial pressure yi P. Activity coefficients are dimensionless measures of deviation from ideal-solution behavior. Therefore, any statement claiming φ = 0 for an ideal gas is incorrect. Step-by-Step Solution: Recall: ideal gas → f = P, φ = 1.Mixtures: fi = yi P for ideal-gas mixture.Activities in solutions use γ and dimensionless conventions.Conclude that the incorrect statement is “fugacity coefficient is zero for an ideal gas.” Verification / Alternative check: Taking the limit as P → 0 for any real gas, φ → 1, confirming the ideal-gas reference state. Why Other Options Are Wrong: Options (a), (b), (c), (e) are correct statements consistent with standard thermodynamics. Common Pitfalls: Confusing φ (dimensionless) with compressibility factor Z; both → 1 in the ideal limit but represent different corrections. Final Answer: The fugacity co-efficient is zero for an ideal gas.

Question 5

Thermometry and equilibrium: Measurement and definition of temperature are fundamentally enabled by which law of thermodynamics?

Accepted Answer

Introduction: Before we can write energy or entropy balances, we must be able to define and measure temperature. The foundational principle behind thermometry is the recognition of thermal equilibrium as an equivalence relation, which is formalized by the zeroth law of thermodynamics. Given Data / Assumptions: Thermometers reach thermal equilibrium with the system they measure. No heat sources or sinks disturb the equilibrium during measurement. Thermal contact does not significantly alter the system state. Concept / Approach: The zeroth law states: If body A is in thermal equilibrium with body B, and body B is in thermal equilibrium with body C, then A and C are in thermal equilibrium with each other. This transitive property justifies the use of a thermometer (body B) to compare and assign consistent temperature values to other systems (A and C). Without the zeroth law, temperature would lack a consistent operational definition. Step-by-Step Solution: Identify the measurement problem: assigning a scalar quantity (temperature) consistently.Invoke the zeroth law: defines equivalence classes of thermal states.Conclude that thermometers can be calibrated and used to measure temperature reliably. Verification / Alternative check: Temperature scales (Kelvin, Celsius, Fahrenheit) are built upon fixed points (e.g., triple point of water) and rely on equilibrium comparisons, directly referencing the zeroth law. Why Other Options Are Wrong: First law: energy conservation; does not define temperature. Second law: directionality of processes; defines absolute temperature scale indirectly but not measurement per se. Third law: behavior near absolute zero. Common Pitfalls: Assuming the first law alone explains temperature; it does not establish equilibrium comparability. Final Answer: zeroth

Question 6

Thermodynamic work in chemical reactions: No boundary (PΔV) work is done by the system when a reaction occurs under which constant condition?

Accepted Answer

Introduction / Context: In classical thermodynamics for closed systems doing only boundary work, mechanical work arises from volume change against an external pressure. Many reaction systems are designed to eliminate this contribution so that energy accounting focuses on heat effects and changes in internal energy or enthalpy. Understanding when the PΔV term vanishes is fundamental. Given Data / Assumptions: Single, closed reacting system with only pressure–volume (boundary) work considered. No electrical, shaft, surface, or other non-expansion work included. Process constraints can be constant volume, constant temperature, or constant pressure. Concept / Approach: Boundary work for a quasi-static step is W = ∫ P_ext dV. If the process is conducted at constant volume, dV = 0 at every instant, so W = 0 regardless of pressure or temperature changes. At constant pressure, work generally does not vanish if volume changes; at constant temperature, work may be nonzero (e.g., ideal-gas expansions/compressions). Step-by-Step Solution: Write boundary work: W = ∫ P_ext dV.Impose constraint: constant volume → dV = 0 for the entire process.Therefore, W = 0 exactly; no mechanical work is performed by the system.Other constraints (constant T or constant P) do not inherently force dV = 0. Verification / Alternative check: Bomb calorimetry is conducted at constant volume specifically so that the measured heat equals the change in internal energy (ΔU) of reaction for the system, confirming that boundary work is zero in such setups. Why Other Options Are Wrong: Constant temperature: systems can expand or contract, giving nonzero work. Constant pressure: volume usually changes during reaction, so work is typically nonzero. 'None of these' contradicts the constant-volume case. Common Pitfalls: Confusing constant pressure (useful for ΔH) with zero work; they are not equivalent. Ignoring non-expansion work paths not covered in this basic boundary-work discussion. Final Answer: volume

Question 7

Comparing refrigeration cycles: Among the listed cycles operating between the same temperature limits, which refrigeration cycle has the maximum possible coefficient of performance (COP)?

Accepted Answer

Introduction / Context: The coefficient of performance (COP) measures refrigeration effectiveness: COP_R = Q_L / W_in. For any two thermal reservoirs, the second law sets an upper bound on COP. Identifying the theoretically best cycle helps benchmark real systems and diagnose inefficiencies. Given Data / Assumptions: All cycles considered operate between the same cold (T_L) and hot (T_H) temperature limits. Ideal, reversible limits are admissible for comparison. No constraints on working fluid other than cycle definition. Concept / Approach: The Carnot refrigeration cycle is fully reversible and hence ideal. For given T_L and T_H, its COP is COP_Carnot = T_L / (T_H − T_L) (temperatures in Kelvin). No real cycle can exceed this bound. Practical vapor-compression cycles approach but never surpass it; air (Bell–Coleman) cycles are typically far below due to gas compression/expansion irreversibilities. Step-by-Step Solution: Define the reversible benchmark: COP_Carnot = T_L / (T_H − T_L).Any irreversibility reduces COP from the Carnot limit.Among the listed options, only the Carnot cycle is fully reversible by definition.Therefore, select 'Carnot cycle' as having the maximum COP. Verification / Alternative check: Compare with idealized vapor-compression: even with a reversible expander replacing the throttling valve, additional irreversibilities (heat transfer with finite ΔT, non-isentropic compression, real fluid effects) keep COP lower than the Carnot limit. Why Other Options Are Wrong: Air cycle: lower COP for the same temperature span. Vapor-compression (ordinary or with expander): better than throttling-only in the latter case but still below Carnot. Common Pitfalls: Confusing 'best practical' with 'theoretical maximum'—the question asks the latter. Using Celsius instead of Kelvin in COP_Carnot formula. Final Answer: Carnot cycle

Question 8

Work from gas expansion over a fixed pressure range: For a given initial state and the same final pressure, which reversible path yields the maximum work from the expanding gas?

Accepted Answer

Introduction / Context: Mechanical work from a gas depends on both endpoints and the path on a P–V diagram. For ideal gases, comparing common reversible paths (isothermal, adiabatic/isentropic) reveals which produces the greatest area under the curve (integral of P dV) for a specified pressure drop. Given Data / Assumptions: Ideal-gas behavior for clarity of comparison. Reversible (quasi-static) paths to avoid additional entropy production. Initial state fixed; final pressure is specified and identical across paths. Concept / Approach: Reversible isothermal expansion follows P = nRT/V with T constant, producing a relatively high pressure at any volume compared with a reversible adiabatic whose pressure falls faster with increasing volume (P ∝ V^−γ). The larger average pressure during isothermal expansion yields greater work over the same pressure range. Step-by-Step Solution: Work (reversible) = ∫ P dV.Isothermal: P_iso = nRT/V → W_iso = nRT ln(V2/V1).Adiabatic/isentropic: P_ad drops more steeply; area under curve is smaller to the same final pressure.Therefore, reversible isothermal expansion gives maximum work for ideal gases across the same pressure drop. Verification / Alternative check: Graphical P–V comparison shows the isothermal path lies above the adiabatic between the same endpoints in pressure, confirming a larger integral of P dV. Why Other Options Are Wrong: Adiabatic and isentropic (same for ideal gases) yield less work than isothermal for the specified condition. 'None of these' contradicts known results. Common Pitfalls: Comparing at the same final volume rather than same final pressure; the result depends on constraint. Ignoring real-gas deviations; however, the teaching result holds for ideal-gas comparisons. Final Answer: isothermal

Question 9

Throttle (Joule–Thomson) expansion: During Joule–Thomson expansion of real gases through a valve or porous plug, which thermodynamic property remains constant?

Accepted Answer

Introduction / Context: Joule–Thomson (J–T) expansion is a throttling process widely used in refrigeration, natural gas processing, and cryogenics. Characterizing the conserved quantity helps predict temperature change (cooling or heating) across the restriction and informs selection of pre-cooling and pressure levels. Given Data / Assumptions: Steady-flow throttling with large pressure drop across a valve/porous plug. No shaft work; negligible changes in kinetic and potential energies. Adiabatic surroundings (no heat exchange). Concept / Approach: The steady-flow energy equation reduces to h1 ≈ h2 for a throttling device under the listed assumptions; hence the process is isenthalpic. Temperature does not in general remain constant; the change is determined by the J–T coefficient μ_JT = (∂T/∂P)_H. Entropy increases due to irreversibility; the process is not isentropic. Step-by-Step Solution: Apply steady-flow energy balance: h1 + ke1 + pe1 + q − w = h2 + ke2 + pe2.Set q ≈ 0, w = 0, and ke, pe changes negligible → h1 ≈ h2.Therefore, enthalpy remains constant during J–T expansion.Temperature change sign depends on μ_JT and inlet state relative to inversion curve. Verification / Alternative check: Real-gas property charts (h–P diagrams) display vertical throttling lines, visually confirming constant enthalpy paths. Why Other Options Are Wrong: Entropy remains constant only for reversible adiabatic (isentropic) expansions, not for throttling. Temperature remaining constant would imply μ_JT = 0, which is not generally true except at specific conditions. Common Pitfalls: Assuming all expansions are isentropic; throttling is highly irreversible. Ignoring inversion temperature; some gases warm on J–T expansion above inversion. Final Answer: enthalpy remains constant

Chemical Engineering Thermodynamics Questions