Question

∆H⁰rxn, for the reaction of Ca(s) + 2H+(aq) -> Ca2+(aq) + H2(g) is -544 kJ/mol.

∆H⁰rxn, for the reaction of CaO(s) + 2H+(aq) -> Ca2+(aq) + H2O(l) is -193 kJ/mol.

Based on these results, what is the ∆Hf for CaO(s) [in kJ/mol] if the heat of formation of water is -286 kJ/mol?

Can someone please help me with this. I calculated all the given information above; however, I have no idea how to connect the dots and answer this question. Thank you!

Answer 1

Add equation 1 to the reverse of equation 2 to equation 3. Then add the delta Hs; remember to reverse the sign if you reverse the reaction. The final reaction will be
Ca(s) + 1/2 O2 --> CaO(s) Notice that everything else cancels.
Delta H for the desired reaction will be -544 + 193 - 286 = ?? kJ/mol.

Answer 2

-637 kJ/mol.

okay, I understand now... thanks Dr. Bob!

Answer 3

thanks Dr. bob.

Answer 4

I don't understand why you add the water heat of formation. Can someone please explain this, it doesn't make any sense to me.