Yesterday we defined an exothermic reaction as one where energy is released, because the total energy stored in the bonds decreases.
This leads us to two new concepts: activation energy and stability.

On of the noticeable things about most reactions is that they only go one way. You can turn methane and oxygen into carbon dioxide and water very easily, but turning them back is very difficult. In fact, if you leave methane and oxygen in contact, without setting light to the mixture, they will still slowly degrade into carbon dioxide and water. The products are more stable than the reactants.
They are more stable because they store less energy. That’s what “more stable” means. Atoms are lazy creatures who do not want to be more energetic than they have to be. They’d rather be cool than hot, they’d rather be water than hydrogen and oxygen. The universe moves down the energy gradient.

Imagine you are an atom. You want to be as close to sea level as possible. You’re sitting happily in your little valley, until something pushes you up the slope. There yu are, balanced on the ridge. On one side is the valley you started from; on the other is a much deeper valley, one where the floor is closer to sea level. So that’s the way you roll down, into the lower valley. If you started from the lower valley, you’d still want to roll back into it, because you have lower energy on that side. That’s more or less why it’s hard to turn products back into reactants: even if you heat them up enough to start breaking bonds, the ones that reform will correspond to the products.

The ridge between the two valleys is the energy required to start breaking bonds so that new ones can form, and it’s called the activation energy. No matter how much energy is stored in the bonds and how much more stable the products would be, the reaction can’t start until something supplies the activation energy to get it going. TNT is much safer than nitroglycerin, because the activation energy for TNT is much higher. You have to deliberately detonate TNT, whereas a sharp knock can set off nitroglycerin. The stability of the products is almost identical, but the activation energy is different.