How do you determine if ΔH° is positive or negative?

Question

2Al2O3(s) → 4Al(s) + 3O2(g).

I think ΔH° is 3340 kJ/mol for 2Al2O3(s) but..

What confuses me is that since it is on the product side, I think it would be positive. The enthalpies for Al and O2 are both zero.

So... 0 - 3340 = -3340, or it is 3340 because its on the product side?

Answer 1

Path is lengthy and confusing

its straight.

When heat is lost delta H standard is negative but when it gains it is positive.
Now made your calculations and get your answer.

Answer 2

Your first answer is close to correct. For any equation, you can calculate ΔH for the reaction by taking the (molar enthalpies of formation of the products) - (the molar enthalpies of formation of the reactants). You need to multiply each one by its coefficient from the balanced equation.