7. Chemical Energetics

Question 1

Define enthalpy change of reaction?

Answer 1

The energy change (flow of thermal energy) that occurs in a chemical reaction.

Question 2

Define enthalpy change of formation of a compound.

Answer 2

The enthalpy change when 1 mole of compound is formed from its constituent elements in their standard states at 298K and 1 bar.

Question 3

What type of energy does chemical energy possess?

Answer 3

Potential and kinetic energy

Question 4

Why is the 2 type of energies in chemical energy present?

Answer 4

Potential energy: It is due to electrostatic attraction between particles, it is usually the main component of the total energy.

Kinetic energy: Presence of translational, rotational, and vibrational kinetic energies in liquid and gases

Question 5

Endothermic reaction what happens to the surrounding temperature? Energy content of reactants VS products? What happens to the potential energy of the system (reacting chemicals)?

Answer 5

• Surrounding temperature decreases as heat is absorbed during the reaction
• Energy content of products > reactants
• Potential energy of the system increases

Question 6

Exothermic reaction what happens to the surrounding temperature? Energy content of reactants VS products? What happens to the potential energy of the system (reacting chemicals)?

Answer 6

• Surrounding temperature increases as heat is released from the reaction
• Energy content of the product is lower than the reactants
• Potential energy of the system decreases

Question 7

H2(g) + 1/2O2(g) —> H20(g) deltaH = -241.8 kJ mol-1
Explain why the enthalpy change of the above reaction is less exothermic than -285.9 kJ mol-1?

Answer 7

deltaH is less exothermic as heat is absorbed to convert liquid water to water vapour. The difference of +44.1 kJ mol-1 represents the enthalpy change of vaporization of water. H20(l) -> H20(g)

Question 8

Formula to measure heat change in a reaction?

Answer 8

q = mcΔT
heat change in solution = mass of solution (water) x specific heat capacity x change in temperature of the water/solution

Question 9

If heat capacity (C) of the solution is given instead of specific heat capacity (c), what is the other formula used to determine heat (enthalpy) change in a reaction?

Answer 9

q = CΔT
heat change of solution = heat capacity of solution x change in temperature of the water/solution

Question 10

To determine the enthalpy change of reaction, what is the formula used for endothermic and exothermic reactions, when heat change (q) is present?

Answer 10

Endothermic reaction: ΔH = + q/n
Exothermic reaction: ΔH = - q/n

Question 11

Calculations using average bond energy, why is the discrepancies when finding ΔHr compared to other means e.g experiments, Hess’s law.

Answer 11

The bond energies given in the data booklet are the average bond energies of that particular covalent bond in different types of molecules. These values may not apply to the specific molecules in this reaction.

Question 12

Why halogens at the start of the energy cycle are not given a state symbol?

Answer 12

Fluorine and chlorine are gases, bromine and iodine are solid at standard conditions 298K and 1 bar.

Question 13

Define standard enthalpy change of combustion.

Answer 13

The enthalpy change when 1 mole of a substance is completely burnt in excess oxygen at standard conditions.

Question 14

Define standard enthalpy change of neutralisation.

Answer 14

The enthalpy change when 1 mole of water is formed in the neutralisation reaction between an acid and a base, at standard conditions.

Question 15

Define bond dissociation energy, B.D.E.

Answer 15

Bond dissociation energy is the energy required to break 1 mole of a particular covalent bond in a specific molecule in the gaseous state.

Question 16

Define bond energy / B.E. / average bond enthalpy.

Answer 16

Bond energy is the average energy required to break 1 mole of covalent bond in gaseous state.

Question 17

Define standard enthalpy change of atomisation.

Answer 17

The enthalpy change when 1 mole of gaseous atoms is formed from the element at standard state.

Question 18

Define lattice energy / L.E.

Answer 18

The enthalpy change when 1 mole of solid ionic compound is formed from its constituent gaseous ions.

Question 19

Define ionisation energy, I.E.

Answer 19

The enthalpy change when 1 mole of electrons is removed from 1 mole of gaseous atoms to form 1 mole of singly charged gaseous cations.

Question 20

Define electron affinity, E.A.

Answer 20

Electron affinity is the enthalpy change when 1 mole of electrons is added to 1 mole of gaseous atoms to form 1 mole of singly charged gaseous anions.

Question 21

Define standard enthalpy change of hydration.

Answer 21

The enthalpy change when 1 mole of free gaseous ions is dissolved in an infinite volume of water at standard conditions.

Question 22

Define standard enthalpy change of solution.

Answer 22

The enthalpy change when 1 mole of solute is completely dissolved in an infinite volume of solvent at standard conditions.

Question 23

Define standard enthalpy change of vaporization.

Answer 23

The enthalpy change that must be added to a liquid substance to transform a quantity of that substance into a gas.