Heat of Reaction
The heat of a reaction is the NET amount of heat that is taken in or given off by a reaction. The simplest way to understand this is to look at a simple example.
Every reaction that occurs takes in heat and gives off heat. For instance, if you think about a fire, you certainly know that the fire will give off heat, but you also know that forests and pieces of paper don't burst into flame randomly. In order to get a piece of paper to burn, the paper must be heated. This can be done with a match or a magnifying glass, or by adding it to an existing fire. What matters is not how the paper gets heat, but that the reaction cannot occur without that heat. The heat that must be added to make the reaction occur is called the activation energy.
One simple way to express this idea (that all reactions both take in and give off heat) is to show it graphically. For an exothermic reaction (one that gives off heat), the graph looks like this.
What this is showing can be broken down into a few simple ideas.
1. Everything has some energy (nothing is at 0 on the energy scale). So, the reactants have some energy and the products have some energy (but a different amount)
2. Energy must be put in to make the reaction happen. This is the energy shown in red on the left side and is called the activation energy.
3. Energy is released by the production of the products. This is shown in purple on the right side of the reaction. There is no special name for this energy.
4. The difference between the energy put in (activation energy) and the energy released is the heat of the reaction. This is indicated by the green arrow in the middle. Specifically, we subtract the energy out from the energy in, resulting in a negative number (exothermic)
For an endothermic reaction, the graph is similar, but the products have more energy than the reactants. As a result, more heat is put in than comes out. When the energy out is subtracted from the larger energy in value, the result is positive (endothermic).
That still leaves the question: Why do all reactions require energy to start and why do some give off energy (overall) and some take in energy (overall). The answers can be found by looking at bond energies.