Required Practical 2 - Enthalpy Change

Question 1

Enthalpy Change of Combustion

Answer 1

• Weight fuel before
• Burn fuel to heat water in a flame calorimeter
• Measure temperature change
• Reweigh fuel
• Calculate number of moles of fuel burnt

Question 2

Suggest why a flame calorimeter is better than an ordinary calorimeter

Answer 2

• Flame is enclosed so all heat transferred to water

- Burns in pure oxygen so ensures complete combustion

Question 3

Suggest ways to reduce energy loss when using a calorimeter

Answer 3

• mineral wool around it
• lid
• draught screen

Question 4

Suggest why good insulators like polystyrene cups are used as calorimeters

Answer 4

Have a low specific heat capacity to reduce energy loss to surroundings

Question 5

Enthalpy Change of Solution

Answer 5

• Record temperatures of solution before at regular intervals for around 5 minutes
• Start clock when solutions are combined
• Measure temperature at regular intervals (30 sec)
• Plot a graph of temperature against time
• Draw a line of best fit and extrapolate back to time solutions were combined to calculate temperature change

Question 6

Suggest sources of error when calculating enthalpy changes

Answer 6

• heat transfer from surroundings
• assumes all solutions have the heat capacity of water
• neglecting the specific heat capacity of the calorimeter
• reaction or dissolving may be incomplete or slow

Question 7

Explain why it is important that the reaction mixture is stirred before recording each temperature

Answer 7

Allow all solution to reach same temperature

Question 8

Suggest a source of error when calculating enthalpy changes apart from due to heat loss, apparatus or human error

Answer 8

Incomplete combustion / reaction

Question 9

Suggest why initial temperature is recorded at regular intervals before the reactants are combined

Answer 9

Accurate initial temperature (hence temperature change)