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Posted by on Sunday, February 21, 2010 at 7:44am.

∆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!

  • Chemistry - , Sunday, February 21, 2010 at 1:34pm

    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.

  • Chemistry - , Sunday, February 21, 2010 at 6:36pm

    -637 kJ/mol.

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

  • Chemistry - , Monday, February 22, 2010 at 1:38pm

    thanks Dr. bob.

  • Chemistry - , Monday, May 17, 2010 at 6:45pm

    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.

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