Final answer:
Chemical reactions absorb or release heat due to the energy required for breaking bonds (endothermic) and the energy released by forming new bonds (exothermic). The overall heat change is described by enthalpy change (ΔH). The Law of Conservation of Energy ensures that all energy changes are accounted for in the system or its surroundings.
Step-by-step explanation:
Almost all chemical reactions absorb or release heat due to the energy changes associated with breaking and forming bonds. During a chemical reaction, old bonds are broken in the reactants, and new bonds are formed in the products. Breaking bonds is an endothermic process, which requires energy, while forming new bonds is an exothermic process, releasing energy. Whether a reaction is overall endothermic or exothermic depends on the balance between the energy absorbed to break bonds and the energy released from forming new ones.
A standard example of an endothermic reaction is photosynthesis, where plants absorb sunlight to convert carbon dioxide and water into glucose. Conversely, an example of an exothermic reaction is the combustion of methane in a kitchen burner, which releases energy as heat. The energy involved in chemical reactions is often quantified as the change in enthalpy (ΔH), also known as the heat of reaction, which is the difference between the total heat absorbed and released.
According to the Law of Conservation of Energy, energy cannot be created or destroyed in an isolated system. The energy released or absorbed in a chemical reaction will correspondingly affect the surroundings -- heating up or cooling down, respectively. This is observed when using a calorimeter to measure the thermal energy change during a reaction.