Every chemical reaction involves some kind of measurable change in overall energy. Molecules either absorb energy from their surroundings or release energy into their surroundings. Chemists carefully track these energy changes using mathematical state functions like enthalpy.
Enthalpy simply measures the total heat content of a system at constant pressure. The first law of thermodynamics states that energy can never be created or destroyed. This rule is also known as the fundamental law of conservation of energy.
Energy can only transfer between a system and its environment or change its form. For example, chemical energy is a type of potential energy stored within molecular bonds. It depends directly on how the atoms and their negatively charged electrons are arranged.
During a reaction, old chemical bonds within the reactants must first break. Then, the individual atoms rearrange themselves and entirely new bonds quickly form. Breaking bonds always requires an input of energy from the surrounding environment.
Forming new bonds always releases energy back out into the surrounding environment. If the newly formed bonds are stronger, the overall reaction releases thermal energy. Students often confuse heat and energy as being the exact same thing.
Heat is simply the transfer of thermal energy due to a difference in temperature. Energy is a much broader concept that includes motion, position, and stored chemical bonds.