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I can't figure this question out for the life of me. If anyone could help me, that'd be great!

Given change of heat of formation for Cu^2+ (aq)=+71.7kJ/mol and that for Zn^2+(aq)= -153.9kJ/mole; find the heat of reaction for:
Zn(s) + Cu^2+(aq) -> Zn^2+(aq) + Cu(s)

<3 l

Subtract the sum of the the product heats of formation from the sum of the reactant heats of formation. The heats of formation of Zn(s)and Cu(s) are zero. Therefore
Heat of reaction = 71.7 -153.9 = -82.2 kJ/mole
(The minus sign means the reaction is endothermic)

What I don't understand is, why would you subract it? If Zn^2+=153.9kJ/mol and Cu^2+=+71.7kJ/mol then how would you get -82.2kJ if Zn^2+ is negative? Sorry for acting so dumb! Haha
Thanks again
<3 l

your question is not dumb. I had difficulty justifying it myself. A positive heat of formation implies that energy was required to make the the substance from elements in their standard state. That is the convention that is used. If I am wrong about this, or another expert can provide a clearer answer, some one else might respond.

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