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### Standard reaction enthalpies

Posted: Sat Jan 20, 2018 4:57 pm
I am confused about how to calculate the standard reaction enthalpies. For example, in 8.63, part a asks to calculate the standard reaction enthalpy for the final stage in the production of nitric acid. 3NO2(g)+H2O(l) --> 2HNO3(aq) + NO(g).
What are the steps to finding the answer?

### Re: Standard reaction enthalpies

Posted: Sat Jan 20, 2018 5:30 pm
For this question, you can use the standard enthalpies of formation from Appendix 2A to calculate the standard reaction enthalpy, which is one of the methods to calculating a standard reaction enthalpy.
Imagine that to carry out the reaction we convert the reactants into the elements in their most stable form, then from those elements we form the products. I would suggest looking at Figure 8.31 on page 295 of the textbook to visualize it. This line of thinking gives us this equation:

$\Delta H^{\circ}=\Sigma n\Delta H^{\circ}_{f}(products)-\Sigma n\Delta H^{\circ}_{f}(reactants)$

So in this problem you would look up the standard enthalpies of formation for $NO(g)$, $HNO_{3}(aq)$, $H_{2}O(l)$, and $NO_{2}$ from Appendix 2A and use the equation above.
Hope this helps!