# Enthalpy AP Chemistry

Hello! My first time on here but having massive troubles with my AP Chemistry homework and need help. I have a question that gives four different reactions and I am asked to calculate the (delta)H for each of them. I am given no more information. I am not sure how to go about the problem...

I have the idea that for each reaction to add together the enthalpy values for the products and subtract from that the sum enthalpys for the reactants...but I do not know where to obtain or calculate these values.

I am online now and will be until this is finished - my MSN is forest_angel@hotmail.com if you'd rather do that than post on here - but I could REALLY use some help on this and I'd appreciate a response asap if anyone has ANY idea or ANY advice.

Thanks again!
-Cora

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Staff Emeritus
Gold Member
Hi Corabella, I am afraid I cannot be of too much value on Enthalphy. Did you try looking up formulas and try to apply them? For instance this one:

dH = dE + w = dE + PdV = dE + dnRT

Basically: the difference in heat equals the diffence in energy plus the pressure of the system times the volume.

But your idea sounded as if it might work, did you find the correct answer yet? Maybe you can teach me a few things :P

lavalamp
If you've been set this question, then don't you have a databook to get the values from?

The chances are that you are expected to use Haber cycles to calculate the answers, or if you are at a slightly higher level of Chemistry, then Born-Haber cycles. Either way, you'll need some values. If you don't have some then I'd go and bother my teacher about it.

daniel3777
your book should have an appendix with selected thermodynamic data in it. if i understand the question right, i think you may need the enthalpy of formation of the chemicals.

The short method of doing enthalpy of reactions is energy of formation for the products minus the energy of formation for the reactants.
dH = [product1 + product2] - [reactant1 + reactant2]

Of course you can always have more than 2 products and reactants. You can have as many as you want.

Another way is to have several reactions where you know the energy change in each reaction then you sort of combine the terms. It's a lot like solving for simultaneous equations in math. It's hard to give examples for this method. Post a problem and I could show you how it works.