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# Enthalpy changes (unit 2 energetics) watch

1. Using average bond enthalpies.
I have always done reactants - products.
But in this question, it's products - reactants and I don't understand why.
Is it something to do with the fact that I'm given standard enthalpy of formation data and not standard enthalpy of combustion data?

2. (Original post by RoseBrilliante)
Using average bond enthalpies.
I have always done reactants - products.
But in this question, it's products - reactants and I don't understand why.
Is it something to do with the fact that I'm given standard enthalpy of formation data and not standard enthalpy of combustion data?

Yes, there isn't only one way to calculate enthalpy changes of reaction. Check out this video on Hess' law below:

3. I think with these questions you need a method that you can apply every time. Mine is as follows:
1) Write down the balanced equation
2) Find out the enthalpy change of all the reactants (take positive value)
3) Find out enthalpy change of all the products (take positive value)
4) Reactants - products = enthalpy change of the reaction
I think it's easier to keep all the values positive, because when you do reactants - products it will sort the sign out for you since you're measuring the energy in the bonds of the reactants and the energy in the bonds of the products. If the energy in the bonds of the reactants is higher, then you'll get a positive value and thus and endothermic reactions. This makes sense because an endothermic reactions occurs when you break strong bonds and form weak bonds. Whereas when the energy in the bonds of the reactants is lower then you'll end up with a negative value, which works because an exothermic reaction is when you break weak bonds and form strong bonds.

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Updated: May 21, 2013
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