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# Enthalpy of formation :S watch

1. In the nelson thornes, AQA As level text book on chapter 7.5 theres a summary question with a picture, asking me to calculate the enthalpy change for the reaction by thermochemical cycles via enthalpy formation values and enthalpy combustion values.

i am truly confused how to do this, so if any of you have the nelson thornes text book and could help me, i'd be ever so happy. - I don't have the summary answers either my teacher took them out to ensure nobody was cheating. so if anyone could help please please.
2. I don't do your particular board, but, if given the formation enathlpies, then the enthalpy change = Enthalpy change of formation of products - Enthalpy change of formation of reactants.
If given the combustion enthalpies, then the enthalpy change = Enthalpy change of combustion of reactants - Enthalpy change of combustion of products.
That might help.
3. (Original post by Lalanita)
In the nelson thornes, AQA As level text book on chapter 7.5 theres a summary question with a picture, asking me to calculate the enthalpy change for the reaction by thermochemical cycles via enthalpy formation values and enthalpy combustion values.

i am truly confused how to do this, so if any of you have the nelson thornes text book and could help me, i'd be ever so happy. - I don't have the summary answers either my teacher took them out to ensure nobody was cheating. so if anyone could help please please.
if you know what enthalpy of formation is, you can write and eqn for it, then underneath it, you can join them with combustion, ie your reactants and products burn completely in oxygen to give carbon dioxide and water.

stoichiometry needs to be correct, ie balanced eqn, then you calculate the enthalpy change for the reaction with the help of that hess cycle. You have to multiply your standard values given in kj/mol by number of mol consumed by your balanced eqn as appropriate.

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