9.2 Measuring Enthalpy Changes
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What does 0K equate to in Celsius?
-273oC
What is specific heat capacity?
Energy required to raise the temperature of 1g of substance by 1K
What does a small SHC mean?
They are good conductors of heat
Water SHC
4.18 J/g/K
Symbol for heat energy
q
Equation for energy change
q = mcΔt
What is combustion?
Reaction of a substance with oxygen
Spirit burner experiment
Measure out 150cm3 of water and pour into beaker Record initial temperature of water Get methanol spirit burner and weigh it Place spirit burner underneath the beaker and light it, stir water with thermometer After around 3 minutes, put out flame, record max temperature of water Reweigh spirit burner with methanol
How to find enthalpy change of combustion for alcohol
Calculate energy change of the water in kJ Calculate mols of alcohol burnt Divide energy change by mols to find ΔHc
Why is experimental ΔH different to the actual value?
Heat loss to the surroundings other than the water Incomplete combustion of methanol Evaporation of methanol from the wick Non standard conditions
How to minimise errors from heat loss and incomplete combustion
Use draught screens and input oxygen gas
How to determine enthalpy change of reactions
Many reactions are between two aqueous solutions Enthalpy change can be measured using polystyrene cups Cups are cheap, waterproof and lightweight, provide insulation
Working out ΔH for a solid and solution
Measure change in temperature of solution Use density and volume to work out mass of solution Calculate energy change, SHC similar to water Calculate mol of limiting reactant Divide energy change by mol