Thermochemical reactions are chemical reactions that involve the absorption or release of heat energy. These reactions are often represented by thermochemical equations, which show the change in enthalpy (ΔH) along with the reactants and products. The heat change can indicate whether a reaction is exothermic, releasing heat, or endothermic, absorbing heat. In these reactions, the energy changes are crucial for understanding how substances interact. For example, the combustion of fuels like methane releases heat, making it an exothermic reaction. Conversely, the process of photosynthesis in plants absorbs energy from sunlight, classifying it as an endothermic reaction.