Thermochemistry Practice Questions
17 free Thermochemistry practice questions for the General Science. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.
Which of the following describes an exothermic reaction?
- A Heat is absorbed from the surroundings.
- B The enthalpy of the products is greater than the enthalpy of the reactants.
- C Heat is released into the surroundings.
- D The temperature of the reaction vessel decreases.
Correct answer: Heat is released into the surroundings.
In an exothermic reaction, energy is released because the bonds formed in the products are stronger than those broken in the reactants. This results in a negative change in enthalpy and a temperature increase in the surroundings.
The enthalpy change of a chemical reaction is most commonly expressed in which unit?
- A Joules per second
- B Kilojoules per mole
- C Kilocalories per second
- D Kelvin per mole
Correct answer: Kilojoules per mole
Enthalpy change is normally reported in kilojoules per mole (kJ/mol). This unit gives the heat energy absorbed or released per mole of a substance undergoing a chemical or physical change.
According to Hess's Law, the total enthalpy change for a chemical reaction is:
- A Dependent on the number of steps taken.
- B Zero for any reaction in a closed system.
- C Independent of the reaction pathway taken.
- D Equal to the change in entropy multiplied by temperature.
Correct answer: Independent of the reaction pathway taken.
Hess's Law states that enthalpy is a state function. Therefore, the total enthalpy change for a reaction is the sum of the enthalpy changes for each individual step in the process, regardless of the route.
Which piece of laboratory equipment is specifically designed to measure the heat of a chemical reaction at constant pressure?
- A Spectrometer
- B Calorimeter
- C Centrifuge
- D Manometer
Correct answer: Calorimeter
A calorimeter is used to measure the heat flow of a chemical reaction or physical change. By observing the temperature change of a known mass of water or solution, the heat evolved or absorbed can be calculated using the specific heat capacity formula.
A reaction with a positive enthalpy change is classified as which of the following?
- A Exothermic
- B Adiabatic
- C Thermoneutral
- D Endothermic
Correct answer: Endothermic
An endothermic reaction absorbs heat from its surroundings. The internal energy of the system increases, giving a positive value of the enthalpy change and typically causing the surrounding temperature to drop.
The amount of heat required to raise the temperature of one gram of a substance by one degree Celsius is known as its:
- A Molar heat capacity
- B Latent heat of vaporization
- C Specific heat capacity
- D Enthalpy of formation
Correct answer: Specific heat capacity
Specific heat capacity is an intensive property of a substance. For example, liquid water has a high specific heat capacity of about 4.18 joules per gram per degree Celsius, so it takes considerable energy to change its temperature.
What happens to the sign of the enthalpy change when a chemical equation is reversed?
- A It remains unchanged.
- B It becomes zero.
- C It is reversed.
- D It is halved.
Correct answer: It is reversed.
If a forward reaction is exothermic, the reverse reaction must be endothermic by the same magnitude. This is a direct consequence of the law of conservation of energy.
Which of the following processes is endothermic?
- A Combustion of methane
- B Freezing of liquid water
- C Condensation of steam
- D Sublimation of dry ice
Correct answer: Sublimation of dry ice
Sublimation involves a phase change from solid directly to gas, which requires an input of energy to overcome intermolecular forces. Therefore, it is an endothermic process, unlike combustion or condensation which release energy.
The standard enthalpy of formation of an element in its most stable state under standard conditions is defined as which of the following?
- A 100 kJ/mol
- B Dependent on the element's atomic number
- C Exactly zero
- D Equal to its bond dissociation energy
Correct answer: Exactly zero
By convention, the standard enthalpy of formation of a pure element in its reference state, such as oxygen gas or graphite, is exactly zero. This provides the baseline used to calculate the enthalpy changes of compounds.
A bomb calorimeter is primarily used to measure the heat of combustion at which condition?
- A Constant pressure
- B Constant temperature
- C Constant mass
- D Constant volume
Correct answer: Constant volume
A bomb calorimeter is a rigid, sealed container. Because the volume cannot change, the heat measured equals the change in internal energy rather than the change in enthalpy.
What defines the latent heat of fusion?
- A Heat required to turn a liquid into a gas.
- B Heat released when a gas turns into a solid.
- C Heat needed to melt a solid at its melting point.
- D Heat required to raise the temperature of a solid.
Correct answer: Heat needed to melt a solid at its melting point.
Latent heat of fusion is the energy required to change the state of a substance from solid to liquid without changing its temperature. During this phase transition, the energy is used to disrupt the crystal lattice rather than increase kinetic energy.
Bond enthalpy is always a positive value because:
- A Breaking bonds releases energy to the surroundings.
- B Forming bonds requires an input of energy.
- C Breaking bonds requires an input of energy.
- D It is a measure of the kinetic energy of molecules.
Correct answer: Breaking bonds requires an input of energy.
Breaking a chemical bond is an endothermic process, meaning energy must be absorbed to pull the atoms apart. Conversely, bond formation is always an exothermic process.
Which of the following is a 'State Function'?
- A Heat (q)
- B Work (w)
- C Enthalpy (H)
- D Distance traveled (d)
Correct answer: Enthalpy (H)
A state function is a property whose value depends only on the current state of the system, not on how it got there. Enthalpy, internal energy, and entropy are state functions, while heat and work are path-dependent.
The First Law of Thermodynamics is essentially a statement of:
- A The Law of Conservation of Mass
- B The Law of Conservation of Energy
- C The Law of Increasing Entropy
- D The Law of Conservation of Momentum
Correct answer: The Law of Conservation of Energy
The First Law states that energy cannot be created or destroyed, only transformed. In a chemical system, the change in internal energy is equal to the heat added to the system plus the work done on the system.
Which condition is specified for 'Standard State' in thermochemistry?
- A 0 K and 1 atm
- B 298.15 K and 1 atm
- C 273.15 K and 101.3 kPa
- D 373.15 K and 101.3 kPa
Correct answer: 298.15 K and 1 atm
Standard state usually refers to a pressure of 1 bar (or 1 atm) and a specified temperature, most commonly 25°C (298.15 K). For solutions, the standard concentration is 1 Molar.
Which of the following describes the enthalpy of neutralization?
- A The heat change when one mole of a compound is burned in oxygen.
- B The heat change when an acid and a base form one mole of water.
- C The heat required to dissolve one mole of salt in water.
- D The heat required to remove an electron from a gaseous atom.
Correct answer: The heat change when an acid and a base form one mole of water.
Enthalpy of neutralization is the heat evolved when one equivalent of an acid and a base react to form one mole of water. For strong acids and strong bases, this value is nearly constant at about -57.3 kJ/mol.
When steam condenses into liquid water, what happens to the surroundings?
- A The surroundings lose heat.
- B The surroundings gain heat.
- C The temperature of the surroundings stays constant.
- D The surroundings perform work on the system.
Correct answer: The surroundings gain heat.
Condensation is an exothermic phase change. As water molecules move from a high-energy gas phase to a lower-energy liquid phase, they release latent heat into the surrounding environment.