Question

how can products and reactants have different amounts of energy without violating the law of conservation of energy

Answer 1

In order to have a endothermic reaction, heat can be added to the  products to help it overcome the energy barrier. The energy is then used in order to fuel the reaction. In an exothermic reaction, energy is dispelled, usually in the form of heat.

Answer 2

The energy that lost isn't really lost. The amount of energy is constant throughout the universe; in other words the amount of energy in the universe will remain constant but energy can be transfer from one source to another.