Now Hess's law has to do with the rearrangement of thermochemical equations to help us find the overall enthalpy of reaction. Recall that a thermochemical equation is a chemical equation that includes an enthalpy of reaction. We're going to say the thermochemical equation and its enthalpy of reaction, which is ΔH_{rnxn}, are directly proportional. This means that any change to the original equation will cause the same change within your enthalpy of reaction.

Let's say we have an original thermochemical equation, where we have 2 moles of magnesium as solid plus O_{2} gas, and it produces 2 moles of magnesium oxide. The enthalpy associated with this is -1204 kilojoules. I can do different things, different rearrangements to this thermochemical equation that will affect it, but also affect my ΔH of reaction. So, let's say we multiply it. Here we have our original equation, and let's say we multiply it by 2. Now, if I multiply it by 2, the coefficients now become 4 for magnesium, 2 for oxygen, and 4 for magnesium oxide. If I multiply the equation by 2, then that means I also have to multiply my ΔH by 2. So this original value of -1204 kilojoules gets multiplied by 2, and now my new enthalpy of the reaction is -2408 kilojoules.

Now let's say instead of multiplying, I divide it. So, my original equation again, now I divide it by 2. Dividing it by 2 changes my coefficients now to 1 for magnesium solid, 0.5 for O_{2}, and 1 for magnesium oxide. Again, whatever I do to the equation, I must do the same thing to my ΔH. So -1204 gets divided by 2, and it becomes -602 kilojoules.

Finally, the last thing I can do to my thermochemical equation is I can reverse it. What was a product can become a reactant, so that's 2 MgO solid. And what was a reactant can now become products, so 2 Mg solid plus ½ O_{2}. Now when I reverse the reaction, what that does is it reverses the sign of ΔH. So, it was -1204, now it's going to become +1204. So just realize that if I reverse the reaction, I reverse the sign of ΔH. So, just remember, these are the 3 things I can do to a thermochemical equation, and they have a direct impact on my ΔH of reaction for that particular thermochemical equation.