In a laboratory experiment, 1.16g of an organic liquid fuel was completely burned in oxygen. The heat formed during this combuston raised the temperature of 100g of water from 295.3K to 357.8K. Calculate the standard enthalpy change of combustion of the fuel. Its Mr is 58.
1. First off, you need to calculate the amount of heat given out by the fuel using q=mc^T.
q=mc^T= 100x 4.18 x (357.8-295.3) = 26125 J = 26.125 kJ
2. Next you need to find out how many moles o the fuel produced this heat. It's back to the old moles=mass/Mr equation.
n=1.16/58=0.02 moles of fuel
3. The standard enthalpy of combustion involves 1 mole of fuel. So the heat produced by 1 mole of fuel = -26.125/0.02 = -1306 kJ mol-1 ( It's negative because combustion is an exothermic reaction)
The actual enthalpy change of combustion is -1615 kJ mol-1 - loads of heat has been lost and not measured. E.g. it's like a fair bit would escape through the copper calorimeter and also the fuel might not combust properly.
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