Topic E: Chemical Energetics

Question 1

What is enthalpy

Answer 1

Enthalpy is a measure of the heat energy contained in a substance. It is stored in the chemical bonds and intermolecular forces as potential energy

Question 2

How is enthalpy related to stability

Answer 2

The higher the enthalpy of a substance, the less stable it is

Question 3

What is the formula for enthalpy change in terms of enthalpy of formation

Answer 3

ΔH = (Sum of enthalpy of formation of products) - (Sum of enthalpy of formation of reactants)

Question 4

What are the key factors affecting enthalpy change

Answer 4

Temperature
Pressure
Concentration of solutions
State of reactants and products

Question 5

What is standard enthalpy change of formation

Answer 5

Enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions of 298K and 100 kPa

Question 6

How does the standard enthalpy change of formation relate to stability

Answer 6

In general, the more negative the standard enthalpy change of formation for a compound, the more energetically stable it is

Question 7

What is standard enthalpy change of combustion

Answer 7

Enthalpy change when one mole of substance in its standard state is completely burned in excess oxygen under standard condition

Question 8

Why is standard enthalpy change of combustion always negative

Answer 8

Heat is always evolved during combustion

Question 9

What is standard enthalpy change of neutralisation

Answer 9

Enthalpy change when one mole of water is formed between the neutralisation of an acid and base under standard condition

Question 10

Why is standard enthalpy change of neutralisation always negative

Answer 10

Heat is always evolved during neutralisation

Question 11

What is the equation for the enthalpy change of an exothermic and endothermic reaction

Answer 11

Exothermic: ΔH = - (Q/n)
Endothermic: ΔH = + (Q/n)

n is the number of moles of the limiting reagant

Question 12

What is bond enthalpy

Answer 12

Energy needed to break one mole of bonds in the gaseous molecule, under standard conditions

Question 13

What is the formula for enthalpy change in terms of enthalpy of combustion

Answer 13

ΔH = (Sum of enthalpy of combustion of reactants) - (Sum of enthalpy of combustion of products)

Question 14

What is the formula for enthalpy change in terms of bond energy

Answer 14

ΔH = (Sum of bonds broken in reactants) - (Sum of bonds broken in products)
*Only valid for gases

Question 15

What is lattice energy

Answer 15

Standard enthalpy change when one mole of a solid ionic compound is separated into gaseous ions under standard conditions

Question 16

What is the relationship between enthalpy change in bond energy and enthalpy change of atomisation

Answer 16

Enthalpy change in bond energy = 2 * (enthalpy change of atomisation)

Question 17

What is first electron affinity

Answer 17

Standard enthalpy change when one mole of gaseous electrons is added to one mole of gaseous atoms to form one mole of singly negatively charged gaseous ions
e.g O (g) + e → O- (g)

Question 18

Why is first electron affinity exothermic

Answer 18

Energy is released due to the attraction formed between the gaseous atom and the electron

Question 19

What is second electron affinity

Answer 19

Standard enthalpy change when one mole of gaseous electrons is added to one mole of negatively charged gaseous ions to form one mole of doubly negatively charged gaseous ions
e.g O- (g) + e → O2- (g)

Question 20

Why is second electron affinity endothermic

Answer 20

Energy is absorbed to overcome the repulsion between the singly negatively charged ion and the electron

Question 21

What is first ionisation energy

Answer 21

The minimum energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of singly positively charged gaseous ion
e.g Na (g) → Na+ (g) + e

Question 22

Why is the first ionisation energy endothermic

Answer 22

Energy is absorbed to overcome the effective nuclear charge between positively charged nuclear and the outgoing electron

Question 23

What is the standard enthalpy change of atomisation

Answer 23

Standard enthalpy change when one mole of gaseous atoms is formed from the element in its standard states

Question 24

What does change in Gibbs free energy tell us about a reaction

Answer 24

If the change in Gibbs free energy (ΔG) is negative, the reaction is spontaneous
If ΔG is positive, the reaction is non-spontaneous

Question 25

What is the formula for ΔG

Answer 25

ΔG = ΔH - TΔS
ΔG = (Sum of gibbs free energy of products) - (Sum of gibbs free energy of reactants)

Question 26

What is the formula for change in entropy

Answer 26

ΔS = (Sum of entropy of products) - (Sum of entropy of reactants)

Question 27

What does a positive and negative value of ΔS mean?

Answer 27

Positive ΔS:
- The reaction is more feasible
- Final state is more disordered than the initial state

Negative ΔS:
- The reaction is not feasible
- Initial state is more disordered than the final state

Question 28

What are the factors affecting entropy of a chemical system

Answer 28

Change in temperature
Change in phase
Change in number of particles (especially gas molecules)
Mixing of particles