How can enthalpy of formation values be used to calculate reaction enthalpy?

Friday, May 18th, 2007

Think about it this way…
the reactants (left hand side) go on to make the products (right hand side)
Imagine that this reaction proceeds VIA the elements in their standard states, (according to Hess’s law we can go any way we want providing that we get to the desired products)
First of all, the reactants have to return to their elements. This process is the opposite of the enthalpy of formation, i.e. the negative or reverse of delta Hf = -delta Hf
The products are then formed from the elements – this is simply the enthalpy of formation, i.e. + deltaHf
So, if you sum these two processes to go from reactants to products you have:
Reaction enthalpy = – delta Hf(reactants) + deltaHf(products)
rearrange this to get:
Reaction enthalpy = deltaHf(products) – delta Hf(reactants)Идея за подаръкикони