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AP Chemistry Practice Quiz: Introduction to Enthalpy of Reaction

Written by AP Content Team, Verified for 2026 AP Exams, Last updated: May 2026

Test your understanding with short quizzes. This quiz has 10 questions to check your progress.

Question 1 of 10

A chemical reaction is observed to release thermal energy into its surroundings. Which of the following correctly describes this reaction?

All Questions (10)

A chemical reaction is observed to release thermal energy into its surroundings. Which of the following correctly describes this reaction?

A) The reaction is endothermic with a positive ΔH.

B) The reaction is exothermic with a positive ΔH.

C) The reaction is endothermic with a negative ΔH.

D) The reaction is exothermic with a negative ΔH.

Correct Answer: D

According to the provided content, an exothermic reaction is one in which thermal energy is transferred to the surroundings. The enthalpy change (ΔH) for an exothermic reaction is negative, indicating that heat energy is released.

If a chemical reaction causes the temperature of its container to decrease, what can be concluded about the process?

A) It is an exothermic process because kinetic energy was converted to potential energy.

B) It is an endothermic process because thermal energy was transferred from the surroundings.

C) It is an exothermic process because thermal energy was transferred to the surroundings.

D) It is an endothermic process because potential energy was converted to kinetic energy.

Correct Answer: B

A decrease in the temperature of the surroundings (the container) indicates that thermal energy is being transferred from the surroundings into the chemical system. This type of process is defined as endothermic.

The molar enthalpy of reaction for the combustion of ethanol is -1367 kJ/mol. How much heat (q) is released when 0.50 moles of ethanol are burned at constant pressure?

A) 684 kJ

B) 1367 kJ

C) 2734 kJ

D) 3418 kJ

Correct Answer: A

The relationship between heat (q), moles (n), and molar enthalpy (ΔH) is q = n * ΔH. In this case, q = (0.50 mol) * (-1367 kJ/mol) = -683.5 kJ. The negative sign indicates heat is released, so 684 kJ (rounded) is the amount of heat released.

What is the fundamental cause of the enthalpy change (ΔH) in a chemical reaction?

A) A change in the total mass of the system as reactants are converted to products.

B) The difference in chemical potential energy between the products and the reactants.

C) The transfer of thermal energy required to keep the system at constant pressure.

D) A change in the kinetic energy of the surroundings.

Correct Answer: B

The content states that the difference in chemical potential energy between products and reactants, which arises from breaking and forming chemical bonds, results in the overall energy change observed as a change in kinetic energy (temperature) and is quantified as the enthalpy change, ΔH.

A certain reaction has a positive enthalpy change (ΔH > 0). Which of the following must be true?

A) The chemical potential energy of the products is less than that of the reactants.

B) The reaction transfers thermal energy to the surroundings.

C) The reaction is endothermic.

D) The kinetic energy of the system increases.

Correct Answer: C

A positive ΔH signifies that heat energy is absorbed by the chemical reaction at constant pressure. A reaction that absorbs thermal energy from the surroundings is defined as endothermic.

The term 'enthalpy change of a reaction (ΔH)' specifically refers to the amount of heat energy released or absorbed under which condition?

A) Constant temperature

B) Constant volume

C) Constant pressure

D) Standard temperature and pressure (STP)

Correct Answer: C

The provided definition explicitly states that the enthalpy change of a reaction (ΔH) is the amount of heat energy released or absorbed by a chemical reaction at constant pressure.

In an endothermic reaction, chemical potential energy is converted into another form. How does this energy transformation manifest?

A) The potential energy of the products is lower than the reactants, causing an increase in the kinetic energy of the surroundings.

B) The potential energy of the products is higher than the reactants, which is achieved by converting kinetic energy from the surroundings into potential energy.

C) The potential energy of the reactants is converted directly into kinetic energy within the system, causing the system's temperature to rise.

D) The potential energy does not change, but kinetic energy is transferred from the system to the surroundings.

Correct Answer: B

In an endothermic reaction, energy is absorbed from the surroundings. This energy is used to form the higher-energy bonds of the products. Therefore, kinetic energy from the surroundings is converted into chemical potential energy in the system, resulting in a temperature drop in the surroundings.

If the molar enthalpy for the decomposition of a substance is +45 kJ/mol, what is the value of q when 3.0 moles of the substance decompose?

A) -135 kJ

B) +15 kJ

C) -15 kJ

D) +135 kJ

Correct Answer: D

Using the formula q = n * ΔH, we calculate q = (3.0 mol) * (+45 kJ/mol) = +135 kJ. The positive sign indicates that 135 kJ of heat is absorbed by the system.

Which statement correctly links the sign of ΔH to the relative chemical potential energies of reactants and products?

A) In an exothermic reaction (negative ΔH), the products have higher chemical potential energy than the reactants.

B) In an endothermic reaction (positive ΔH), the products have higher chemical potential energy than the reactants.

C) In an exothermic reaction (positive ΔH), the reactants have higher chemical potential energy than the products.

D) In an endothermic reaction (negative ΔH), the reactants have higher chemical potential energy than the products.

Correct Answer: B

A positive ΔH indicates an endothermic reaction where the system absorbs energy. This absorbed energy increases the chemical potential energy of the system, so the products are at a higher potential energy state than the reactants.

The temperature change observed during a chemical reaction is a macroscopic sign of what microscopic event?

A) The change in the phase of the reactants.

B) The change in pressure of the system.

C) The conversion between chemical potential energy and kinetic energy.

D) The change in the total number of atoms in the system.

Correct Answer: C

The content explains that the difference in chemical potential energy between products and reactants results in a change in kinetic energy. This change in the average kinetic energy of the molecules is what we measure as a temperature change.