Topic 8

Question 1

Standard conditions

Answer 1

101kPa or 1atm (and specified temp)

Question 2

Enthalpy change

Answer 2

The heat energy change measured at a constant pressure (units KJ/mol).
Note: remember +/-

Question 3

Enthalpy change of formation

Answer 3

When 1 mole of a compound is formed from its constituent elements in their standard states.

Question 4

Enthalpy change of combustion

Answer 4

When 1 mole of a compound undergoes complete combustion in oxygen.

Question 5

Enthalpy change of neutralisation

Answer 5

When a strong acid and strong base react to make 1 mole of water.

Question 6

Enthalpy change of vaporisation

Answer 6

When 1 mole of a liquid substance is converted to a gaseous state.

Question 7

Bond dissociation enthalpy

Answer 7

When 1 mole of a bond in a gaseous state is broken.

Question 8

Mean bond enthalpy

Answer 8

When 1 mole of a bond, averaged out over many different molecules, is broken.

Question 9

State Hess’s law

Answer 9

The enthalpy change for a reaction is independent of the route

Question 10

Describe the sources of error in experiments measuring the enthalpy change of reaction.

Answer 10

Generally:
Heat energy is lost to the surroundings (insulation, lid)
Heat capacity of beaker/cup not taken into account
Impure reactants
Inaccuracy of thermometer

If spirit burner is used:
Incomplete combustion of alcohol/saturated compounds particularly
Distance between beaker and burner too large (draught excluder)
Evaporation of hot alcohol

If reactants mixed:
Heat capacity not 4.18
Mass of solution not 50g