Gasoline (petrol) is useful as a fuel in car engines primarily because it stores which type of energy that can be converted into useful work?

Introduction / Context:
Fuels such as gasoline power car engines by storing energy that can be released and converted into mechanical work. Understanding the nature of this stored energy is a basic concept in physics and chemistry. This question asks you to identify which type of energy is stored in gasoline that makes it useful as a fuel.

Given Data / Assumptions:

• The substance is gasoline, also called petrol, used in internal combustion engines.
• We consider the energy stored in the fuel before combustion.
• Options include chemical potential energy, electrical potential energy, gravitational potential energy and kinetic energy.
• We assume normal use of gasoline in car engines where it is burned with air.

Concept / Approach:
Gasoline is a mixture of hydrocarbons. The energy it stores is in the form of chemical potential energy associated with the bonds between atoms in the molecules. When gasoline burns in the presence of oxygen in an internal combustion engine, chemical reactions occur that rearrange atoms into lower energy products such as carbon dioxide and water, releasing the difference in energy as heat. This heat increases pressure and temperature of gases, which then push pistons and perform mechanical work. The stored energy is therefore chemical in nature. Electrical potential energy is associated with electric charges in electric fields, gravitational potential energy is associated with height in a gravitational field, and kinetic energy is associated with motion. None of these describes the energy stored in gasoline before it is burned.

Step-by-Step Solution:
Step 1: Recognise that gasoline is a chemical fuel composed of hydrocarbons such as octane. Step 2: This fuel does not have significant motion before use, so kinetic energy is not the main stored form. Step 3: In a fuel tank, gasoline is not at a special height to have significant gravitational potential energy relative to its surroundings. Step 4: No electric field is storing energy in it as electrical potential energy in the usual sense of electric circuits. Step 5: Instead, the energy is stored in the chemical bonds between atoms in the hydrocarbon molecules. Step 6: During combustion, these bonds are broken and new bonds are formed in products, releasing energy as heat and work. Step 7: Therefore, the correct term for the stored energy in gasoline is chemical potential energy.

Verification / Alternative check:
Chemistry and thermodynamics textbooks describe fuels in terms of their calorific value, which is the amount of heat released by complete combustion. This heat arises from changes in chemical bond energies. Energy diagrams for combustion reactions show that reactants like gasoline and oxygen have higher chemical potential energy compared to products like carbon dioxide and water. The difference is released as energy that can be converted into mechanical work in an engine. These descriptions confirm that the form of stored energy in gasoline is chemical potential energy.

Why Other Options Are Wrong:
Electrical potential energy describes energy stored due to positions of charges in an electric field, which is not the primary form of energy in gasoline. Gravitational potential energy depends on height in a gravitational field; the energy content of gasoline does not mainly come from its position in gravity. Kinetic energy is the energy of motion and does not describe stored energy in stationary fuel before it is burned.

Common Pitfalls:
Some learners may think of energy in terms of motion or height because these are easy to visualise. However, for fuels, the relevant concept is chemical energy stored in bonds. It is also important not to confuse electrical energy used in electric vehicles with chemical energy used in conventional petrol engines. Remembering that combustion of fuels always involves conversion of chemical potential energy into heat and mechanical work will help you choose the correct answer quickly.

Final Answer:
Gasoline is useful in cars because it contains chemical potential energy that can be released and converted into mechanical work when it burns.