Required Practical 2 - Enthalpy Change
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Enthalpy Change of Combustion
• Weight fuel before • Burn fuel to heat water in a flame calorimeter • Measure temperature change • Reweigh fuel • Calculate number of moles of fuel burnt
Suggest why a flame calorimeter is better than an ordinary calorimeter
• Flame is enclosed so all heat transferred to water - Burns in pure oxygen so ensures complete combustion
Suggest ways to reduce energy loss when using a calorimeter
• mineral wool around it • lid • draught screen
Suggest why good insulators like polystyrene cups are used as calorimeters
Have a low specific heat capacity to reduce energy loss to surroundings
Enthalpy Change of Solution
• 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
Suggest sources of error when calculating enthalpy changes
• 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
Explain why it is important that the reaction mixture is stirred before recording each temperature
Allow all solution to reach same temperature
Suggest a source of error when calculating enthalpy changes apart from due to heat loss, apparatus or human error
Incomplete combustion / reaction
Suggest why initial temperature is recorded at regular intervals before the reactants are combined
Accurate initial temperature (hence temperature change)