Free Thermochemistry and Energetics of Chemical Reactions MCQs with Answers

• 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.

• 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…

• 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.

• A. everything in the universe
• B. the container only
• C. the energy of the reaction
• D. the part of the universe under study, separated from the surroundings by a boundary

Explanation: The system is whatever the chemist chooses to study, usually the reacting substances, and everything else is the surroundings, with the…

• A. the path taken to reach the present state
• B. the present state of the system only, not on how it was reached
• C. the time taken for the change
• D. the catalyst used

Explanation: Enthalpy, internal energy, entropy, pressure, volume and temperature are all state functions, so the change in any of them depends only on…

• A. heat supplied to the system only
• B. work done by the system only
• C. total of all kinetic and potential energies of the particles in the system
• D. energy lost to the surroundings

Explanation: Internal energy sums the translational, rotational and vibrational energy of the particles together with the potential energy stored in…

• A. the conservation of energy
• B. the conservation of mass
• C. the increase of entropy
• D. the constancy of temperature

Explanation: Energy can be converted from one form to another but can neither be created nor destroyed, which is expressed as the change in internal…

• A. internal energy
• B. enthalpy
• C. entropy
• D. free energy

Explanation: Enthalpy was defined precisely so that its change measures the heat of a process at constant pressure, which is the condition of almost…

• A. always negative
• B. always positive
• C. zero by definition
• D. equal to its atomic mass

Explanation: Forming an element from itself involves no change, so the value is defined as zero and this provides the reference point from which all…

• A. depends on the number of steps taken
• B. is the same whether the reaction occurs in one step or several
• C. is always negative
• D. depends on the catalyst used

Explanation: Because enthalpy is a state function, the total change depends only on the initial and final states, so a reaction that cannot be measured…

• 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…

• 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…

• 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…

• 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.

• 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…

• 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…

• 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…

• 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…

• 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…

• 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…