Free Exothermic and Endothermic Reactions MCQs with Answers

1. In an exothermic reaction the enthalpy change is

• A. negative, because the products have less enthalpy than the reactants
• B. positive, because heat is released
• C. always zero
• D. negative, because the products have more enthalpy than the reactants

Explanation: Energy leaves the system as heat, so the products end up at a lower enthalpy and the difference, products minus reactants, is negative. The sign convention is taken from the point of view of the system, which is why released heat carries a minus sign even though the surroundings get warmer. Combustion and neutralisation are the standard exothermic examples.

2. Which of the following is an endothermic process?

• A. Combustion of methane
• B. Photosynthesis
• C. Neutralisation of an acid by an alkali
• D. Condensation of steam

Explanation: Photosynthesis absorbs light energy to build glucose from carbon dioxide and water, so the products store more energy than the reactants and the enthalpy change is positive. Combustion and neutralisation both release heat, and condensation releases the latent heat that was absorbed during evaporation. A useful check is that an endothermic reaction feels cold and usually needs a continuous energy supply.

3. A reaction mixture in a beaker becomes noticeably cold. This shows that the reaction is

• A. exothermic, since heat leaves the system
• B. at equilibrium
• C. endothermic, since heat is absorbed from the surroundings
• D. catalysed

Explanation: The system draws energy from its surroundings, and since the beaker and the solution are those surroundings their temperature falls. Dissolving ammonium chloride or ammonium nitrate in water is the usual classroom demonstration, and it is the principle behind an instant cold pack. An exothermic reaction would make the beaker warm instead.

4. 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.

5. 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.

6. 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.

7. 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.

8. Majority of reactions taking place at ordinary temperatures with ssH are

• A. Endothermic
• B. Exothermic
• C. Thermally unstable
• D. Reversible

Explanation: The symbol in the original stem is the enthalpy change, which did not print cleanly in the paper. Reactions that proceed readily at ordinary temperature without a continuous supply of heat are overwhelmingly those that release energy, that is exothermic ones with a negative enthalpy change. Endothermic reactions generally need heating to be maintained, which is why combustion and neutralisation happen spontaneously while thermal decomposition does not.