Hess' Law of Heat Summation

Hess' Law of Heat Summation states that the enthalpy of a net reaction is equal to the sum of the individual steps.  The reason this works is that enthalpy is a state function.  All that matters is where you start and where you end; it is path independent.

One way of using this concept is to add up several equations, and by canceling out the extraneous substances.  Think of this process like putting a jig-saw puzzle together.  Try to find what matches best to start with, then add the other reactions as needed.  Remember that you final equation must look EXACTLY like the reaction you were given.  The states and coefficients must be the same.

 In this example you are given 3 reactions to put together- a,b, and c.  You want your final answer to look EXACTLY like the original equation.  To do this, you need to start with a, reverse b (remember to make the negative positive), then multiply c by 1/2 and add.  Remember that whatever you do to the reaction you must do the same thing to the ∆Hr.