Free Thermochemistry and Energetics of Chemical Reactions MCQs with Answers

11. Given that C + O2 gives CO2 has an enthalpy change of minus 393 kJ per mole, and CO + half O2 gives CO2 has an enthalpy change of minus 283 kJ per mole, the enthalpy of formation of carbon monoxide is

• A. minus 676 kJ per mole
• B. plus 110 kJ per mole
• C. minus 110 kJ per mole
• D. minus 283 kJ per mole

Explanation: Forming carbon monoxide and then burning it must total the same as burning carbon directly, so the unknown equals minus 393 minus the value of minus 283, giving minus 110 kJ per mole. Adding the two values instead of subtracting gives minus 676, which is the trap. This calculation is the standard textbook illustration of Hess's law.

12. The enthalpy of neutralisation of a strong acid by a strong alkali is almost constant at about minus 57 kJ per mole because

• A. all acids contain the same number of hydrogen atoms
• B. the salts formed have similar masses
• C. the reaction is always carried out at the same concentration
• D. in every case the same reaction occurs, hydrogen ions combining with hydroxide ions to form water

Explanation: Strong acids and strong alkalis are fully ionised, so the spectator ions take no part and the only chemical change is the formation of water from its ions, which releases the same energy whatever salt is produced. With a weak acid the value is smaller, because some energy is absorbed in ionising the acid first. This constancy is strong evidence for the ionic theory of neutralisation.

13. The standard enthalpy of combustion is the enthalpy change when

• A. one mole of a substance is burnt completely in excess oxygen under standard conditions
• B. one gram of a substance is burnt
• C. one mole of a substance is formed from its elements
• D. one mole of a substance is dissolved in water

Explanation: The definition specifies one mole, complete combustion and excess oxygen, and combustion values are always negative because burning releases energy. Incomplete combustion would give a different and smaller value, which is why excess oxygen is stipulated. Formation from elements is a separate quantity that may be positive or negative.

14. Bond breaking and bond formation are respectively

• A. both exothermic
• B. both endothermic
• C. exothermic and endothermic
• D. endothermic and exothermic

Explanation: Energy must be supplied to pull bonded atoms apart, so breaking is endothermic, while forming a bond releases energy and is exothermic. Whether the overall reaction is exothermic depends on which total is larger, so a reaction releasing more energy in making new bonds than it spends breaking old ones is exothermic. This is the reasoning behind calculating enthalpy change from bond energies.

15. In a bomb calorimeter, the heat of reaction measured corresponds to the change in

• A. enthalpy, since the pressure is constant
• B. internal energy, since the volume is constant
• C. entropy
• D. free energy

Explanation: A bomb calorimeter is a sealed rigid vessel, so the volume cannot change, no expansion work is done and all the energy released appears as heat, which equals the change in internal energy. A reaction in an open vessel is at constant pressure and gives the enthalpy change instead. The two differ only by the work done pushing back the atmosphere.

16. When 1 mole of methane burns, the energy released is used to heat water. This illustrates that

• A. energy is created during combustion
• B. energy is destroyed during combustion
• C. energy is transferred from the system to the surroundings but the total remains constant
• D. the enthalpy of the system increases

Explanation: The chemical energy stored in the bonds of methane and oxygen is converted into heat, which passes to the water, so nothing is created or destroyed and the first law holds. The system loses enthalpy, which is why the value is negative, while the surroundings gain the same amount. Every combustion process is an example of this transfer.

17. The mathematical statement of the first law of thermodynamics is

• A. change in internal energy equals heat added plus work done on the system
• B. change in internal energy equals heat added minus temperature
• C. enthalpy equals internal energy divided by pressure
• D. heat equals work in every process

Explanation: The internal energy of a system can be raised either by heating it or by doing work on it, and the equation simply accounts for both routes. When the system does work on the surroundings, for example by expanding against the atmosphere, that term becomes negative. The sign conventions must be stated clearly, since they are the commonest source of error.

18. Which of the following is NOT a state function?

• A. Enthalpy
• B. Internal energy
• C. Work
• D. Temperature

Explanation: The amount of work done in going from one state to another depends on the route taken, for example whether a gas expands quickly or slowly, so work is a path function. Enthalpy, internal energy and temperature depend only on the current condition of the system. Heat is the other common path function, which is why heat and work are written as small quantities rather than as changes in a property.

19. The dissolving of ammonium nitrate in water lowers the temperature of the solution, which means that

• A. the process is exothermic
• B. no energy change occurs
• C. the lattice energy is zero
• D. the energy absorbed in breaking the lattice exceeds the energy released on hydrating the ions

Explanation: Dissolving involves two steps with opposite signs: separating the ions from the lattice costs energy while surrounding them with water molecules releases it, and the overall sign depends on which is larger. For ammonium nitrate the lattice term wins, so the process is endothermic and the solution cools, which is what an instant cold pack exploits. For calcium chloride the balance goes the other way and the solution warms.

20. An energy profile diagram for an endothermic reaction shows

• A. products at a higher energy level than reactants, with an activation barrier in between
• B. products at a lower energy level than reactants
• C. no activation energy
• D. reactants and products at the same level

Explanation: In an endothermic reaction energy is absorbed overall, so the products lie above the reactants and the enthalpy change is positive, but the curve must still rise to a peak because the activation barrier has to be crossed. The barrier for the reverse, exothermic direction is correspondingly smaller. Reading the enthalpy change as the difference between the two flat levels, not as the height of the peak, is the key skill.