Energy Changes in Chemical Reactions
Every chemical reaction involves breaking old bonds (which needs energy) and forming new bonds (which releases energy). The overall energy change depends on which process requires more energy.
Exothermic reactions happen when more energy is released making new bonds than is needed to break the old ones. You'll notice the temperature rising because energy transfers from the chemicals to the surroundings. Common examples include combustion, displacement reactions, and neutralisation.
Endothermic reactions are less common but equally important. Here, more energy is absorbed breaking bonds than is released forming new ones. The temperature drops as energy is absorbed from the surroundings into the chemicals. Think photosynthesis and thermal decomposition.
Key Point: The enthalpy change (ΔH) tells you the energy change per mole - it's negative for exothermic reactions and positive for endothermic ones.
