What causes energy changes in chemical reactions?

The difference between the energies of the reactants and products determines the energy change that happens as a result of the reaction.

Energy is stored in the chemical bonds between the constituent atoms of the fuel (wood, for example) in reactions like combustion. When the wood burns and forms carbon dioxide and water, part of the energy is released as heat and light, and the remaining portion is stored in the bonds of carbon dioxide and water.

The energy change is still just a measure of the difference between the energy of the reactants and the products in some reactions where the products end up with more energy than the reactants (like when we might roll a car uphill).

Energy changes in chemical reactions are caused by the breaking and forming of chemical bonds. When bonds are broken, energy is absorbed, and when new bonds are formed, energy is released. This exchange of energy results in either an endothermic reaction (where energy is absorbed) or an exothermic reaction (where energy is released).