Please can someone explain to me how to do these!? I thought I understood what we were doing when we did them before the summer holidays last year but we are doing exam questions in revision of the chapter now and I'm hopelessly confused! The book is no use to me, I can't see any similarities between the questions we're getting and the examples given.
I'm not too sure about heats of formation either! but for heat of combustion, basically your given a combustion equation and you need to find the delta H value, i.e. the amount of heat given off or taken in by the combustion reaction.
you're then given three formation equations and their delta H values. you need to manipulate these three equations so that you can make the combustion one.
this is done by reversing the equation or multiplying everything in the equation by a number. remember that if you do this, you also need to do the same to the delta H value (if you reverse the equation, change to delta value from positive to negative or vice versa.).
now you have three new equations. go back to your original combustion equation and see what you need from each side. all of these components should be found in the three formation equations. circle what you need, cross off what you don't.
now you have the combustion equation sorted, take the delta values from the formation equations and add them together. this is the final answer!
hope this helps :)