I have the following reactions:

$$\begin{align} \ce{Mg(s) + 2HCl(aq) &-> MgCl2(aq) + H2(g)}\\ \ce{MgO(s) + 2HCl(aq) &-> MgCl2(aq) + H2O(aq)}\\ \ce{H2(g) + 1/2O2(g) &-> H2O(l)}\\ \ce{Mg(s) + 1/2O2(g) &-> MgO(s)} \end{align}$$

I also have $\Delta H$ in $\mathrm{kJ/mol}$ for the first three reactions.

Out of this information, I need to find $\Delta H^\circ_\mathrm{f}{\left(\ce{MgO(s)}\right)}$.

I’m pretty sure I have to use Hess’ law but I don’t know how to apply it here. I’ve tried using the second reaction as my “main” reaction and then I can use the third and fourth reaction as formation reactions for two of the compounds of the main reaction but after that I’m not sure how to proceed.


You are trying to find out the enthalpy of formation. The enthalpy of formation is the energy change when one mole of a substance is formed from its constituent elements in their standard state.

The last equation matches this, so that should be your 'main' or 'target' equation. From there you should be able to use Hess' law to calculate the enthalpy change of the formation of magnesium oxide.

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  • $\begingroup$ I know that, but the problem is that I don't know how to apply Hess' law to get from the first three to the last equation. $\endgroup$ – Joshua Apr 23 '14 at 13:22
  • $\begingroup$ Nevermind, I've found the answer, although it's not exactly the most trustworthy site to find an answer, the chemistry checks out I think: answers.yahoo.com/question/index?qid=20091124125122AATHGgL $\endgroup$ – Joshua Apr 23 '14 at 14:22

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