Adding, releasing, and absorbing energy

Energy is required to break a bond between atoms. For instance, look at the sample equation A-B + C → C-A + B. The collision between C and A-B has to transfer enough energy to break the A-B bond, allowing the C-A bond to form.

Sometimes, even if there is a collision, not enough kinetic energy is available to be transferred — the molecules aren’t moving fast enough. You can help the situation somewhat by heating the mixture of reactants. The temperature is a measure of the average kinetic energy of the molecules; raising the temperature increases the kinetic energy available to break bonds during collisions.

The energy you have to supply to get a reaction going is called the activation energy (Ea) of the reaction. Note that even though you’ve added energy, the energy of the products isn’t always higher than the energy of the reactants — heat may be released during the reaction. This section describes two types of reactions, exothermic and endothermic, in which heat is either released or absorbed.