APCE Thermodynamics & Thermochemistry

Question 1

What does the first law of thermodynamics state?

Accepted Answer

Energy is conserved in all processes.

Answer

The first law of thermodynamics, also known as the law of conservation of energy, states that energy cannot be created or destroyed in an isolated system. It can only be transferred or transformed from one form to another. This means the total energy of the universe remains constant.

Question 2

What is enthalpy (ΔH)?

Accepted Answer

The heat content of a system.

Answer

Enthalpy (ΔH) is a thermodynamic property that represents the total heat content of a system at constant pressure. While it is often used to describe the heat released or absorbed during a chemical reaction (enthalpy change), it fundamentally refers to the internal energy of the system plus the product of its pressure and volume.

Question 3

What is the difference between exothermic and endothermic reactions?

Accepted Answer

Exothermic reactions release heat, and endothermic reactions absorb heat.

Answer

Exothermic reactions are chemical processes that release energy, typically in the form of heat, into the surroundings, causing the temperature of the surroundings to rise (ΔH < 0). Conversely, endothermic reactions absorb energy from the surroundings, usually as heat, leading to a decrease in the surroundings' temperature (ΔH > 0).

Question 4

What is Gibbs free energy (ΔG)?

Accepted Answer

The energy available to do work at constant temperature and pressure.

Answer

Gibbs free energy (ΔG) is a thermodynamic potential that measures the "useful" or process-initiating energy obtainable from an isothermal, isobaric thermodynamic system. It determines the spontaneity of a process: a negative ΔG indicates a spontaneous reaction, while a positive ΔG indicates a non-spontaneous reaction under those conditions.

Question 5

What is the standard enthalpy of formation (ΔHf°)?

Accepted Answer

The heat change when one mole of a compound is formed from its elements in their standard states.

Answer

The standard enthalpy of formation (ΔHf°) is the enthalpy change that occurs when one mole of a compound is formed from its constituent elements, with all substances in their most stable physical states under standard conditions (1 atm pressure, 298 K). This value is a fundamental thermodynamic property used to calculate enthalpy changes for other reactions.

AP Practice Test

AP Practice Test

APCE Thermodynamics & Thermochemistry