Can anyone tell me how to differentiate between these two problems? I feel like i'm missing part of my brain or something! I'm never sure whether i'm suppose to add up the enthalpies, or subtract the products from the reactants (no switching signs eithers).

That is a great question because I always seem to screw up these type of problems. Can anyone tell me how to differentiate between these two problems? I feel like i'm missing part of my brain or something! I'm never sure whether i'm suppose to add up the enthalpies, or subtract the products from the reactants (no switching signs eithers).

Hess's Law questions will have three reactions with their values, followed by a "mother reaction" that stems from the first three. Then use the info to get the heat of formation of the mother.

Can anyone tell me how to differentiate between these two problems? I feel like i'm missing part of my brain or something! I'm never sure whether i'm suppose to add up the enthalpies, or subtract the products from the reactants (no switching signs eithers).

welll the way I see it, whenever they give you several reactions its a Hess's Law problem:

Rxn 1 Hf = 10
Rxn 2 Hf = -20
Rxn 3 Hf = 5
Rxn 4 Hf = ?

Then you have to manipulate the 3 above reactions so that after adding Rxn 1, 2, and 3, you get rxn 4. This is where you just add up the Hf's

Heat of formation is when they usually have only 1 rxn:
H2S + 3/2O2 => H2O + SO2 Hf = -562kj
Then they give you Hf H2O, Hf SO2 and they ask you to find Hf of H2S.
(notice that you do not need Hf of O2)

In this case you do the (Hf of products) - (Hf of reactants) and solve!

hope that makes a LITTLE sense...

Hess's Law questions will have three reactions with their values, followed by a "mother reaction" that stems from the first three. Then use the info to get the heat of formation of the mother.
Of course, there can be more or less then 3 ;)

:thumbup:

thanks, that makes everything clearer. i realized i wasn't looking at what the values meant, i just was ready to plug and chug. thanks again guys

yea the guys ont hsi forum are great. By the way. Just a suggestion... Try to learn why you are doing certain things instead of just plugging. It will give you more confidence if something else comes up that you are not used to or the problem is worded differently.