Energetics exam qs

Question 1

What is the meaning of standard enthalpy change of combustion ?

Answer 1

Enthalpy change when one mole of a substance burns completely in
oxygen with all substances in their standard states

Question 2

The enthalpy of combustion determined experimentally is less exothermic
than that calculated using enthalpies of formation.
Give one possible reason for this, other than heat loss.

Answer 2

Incomplete combustion ( has a lower enthalpy of combustion)
Evaporation of fuel
Experiment not completed under standard
condition

( 1 max)

Question 3

(a) Define the term enthalpy change

Answer 3

heat energy change at a constant pressure

Question 4

Suggest two reasons why the value of enthalpy of combustion from this
experiment is less exothermic than a data book value

Answer 4

• heat loss to surroundings
• incomplete combustion

Question 5

d) Suggest one addition to this apparatus that would improve the accuracy of
the enthalpy value obtained.

Answer 5

using a lid on copper calorimeter ( to reduce heat loss)