# Hess's Law Manipulating Equations

I am having trouble with a question in my homework. It states:

Given the following thermochemical equations

2KCl(s) + 3O2(g) .......2KClO3(s) Change in H = 78.0kJ

P4(s) + 6Cl2(g)..........4PCl3(g) Change in H = -1148.0kJ

P4(s) + 2O2(g) + 6Cl2(g).........4POCl3(g) Change in H = -2168.8kJ

Calculate change in H for the reaction

KClO3(s) + 3PCl3(g)..........3POCl3 + KCl(s)

I know I am suppossed to combine the 3 thermochemical equations to come out with the above reaction. My problem is that I've tried every possible combination and still can't come up with this recaction. Could you help get me started with this as I have become completely frustrated.

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#### Solution Preview

In order to solve this try to arrange the given reactions to give the desired reaction. You can multiply by any number or switch them around to change the sign.

(Reverse reaction #1 and divide it by 2. This gives one mole of ...

#### Solution Summary

The solution explains how to approach the problem and then takes the reader through the steps clearly to manipulate the given Hess's Law equations and find H for the reaction described.