PAG 3 - Enthalpy Determination (3.2.1e)

Question 1

Describe the experimental procedure for determining enthalpy change of combustion

Answer 1

Calorimetry

1. Add a combustible liquid (e.g. methanol) to a spirit burner and weight both together
2. Add a known volume/mass of water to a beaker
3. Record the initial temperature
4. Light the spirit burner and place under the beaker
5. After a few minutes, extinguish the flame and immediately weigh the maximum temperature
6. Re-weigh the spirit burner (assuming wick hasn’t been burnt)

Question 2

What are possible reasons for inaccurate enthalpy change of combustion value?

Answer 2

• Heat loss to surroundings other than water
• Incomplete combution of combustible liquid
• Evaporation of methanol from the wick (spirit burner must be weighed immediately to prevent inaccurate spirit burner mass)
• Non-standard conditions

Question 3

How do you experimentally calculate enthalpy change of reaction?

Answer 3

Use a thermometer, polystyrene cup

1. Weigh the mass of the reactants
2. Add the reaction mixture to the cup, measure the inital temperature
3. Keep stirring, and record the temperature when it stops changing
4. Use q=mc(delta)T

Calculation

• Once q is worked out, you must work out n
• Work out the number of moles of both reactants, use the limiting reactant and calculate enthalpy change