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)

Thnx!!
<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.

No problem! It's completely understandable to have questions and seek clarification.

In this case, when we're calculating the heat of reaction, we need to consider the energy changes associated with breaking bonds in the reactants and forming new bonds in the products.

The change in heat of formation for Cu^2+ is given as +71.7 kJ/mol, meaning that 71.7 kJ of energy is required to form one mole of Cu^2+ ions from its elements in their standard states.

On the other hand, the change in heat of formation for Zn^2+ is given as -153.9 kJ/mol. This means that the formation of one mole of Zn^2+ ions from its elements releases 153.9 kJ of energy.

Now, in the reaction Zn(s) + Cu^2+(aq) -> Zn^2+(aq) + Cu(s), we're forming Zn^2+ ions and Cu(s) from Zn(s) and Cu^2+ ions. So, we need to pay attention to the changes in heat of formation for each reactant and product.

Since the heats of formation for Zn(s) and Cu(s) are both zero, we do not need to consider them in the calculation. However, we need to subtract the sum of the product heats of formation from the sum of the reactant heats of formation.

In this case, we have:
Reactants: Zn(s) and Cu^2+(aq)
Products: Zn^2+(aq) and Cu(s)

So, the heat of reaction can be calculated by:
Heat of reaction = (Heat of formation of Zn^2+(aq)) + (Heat of formation of Cu(s)) - (Heat of formation of Cu^2+(aq))
= 0 + 0 - 153.9 + 71.7
= -82.2 kJ/mol

The negative sign indicates that the reaction is exothermic, meaning it releases energy.

I hope this helps clarify the reasoning behind the calculation. Don't hesitate to ask if you have any further questions!