Energy Changes: Bond Energies

Energy changes in chemical reactions are due to the breaking and forming of chemical bonds.
Energy is required to break bonds, this is known as the bond energy. It is an endothermic process.
Energy is released when new bonds are formed, making it an exothermic process. The amount of energy released depends on the strength of the bonds.
A chemical reaction will only occur if the total energy released when the new bonds are formed is greater than the total energy required to break the initial bonds.
Breaking bonds (endothermic reactions) means the system takes in energy from the surroundings, cooling the surroundings and decreasing the overall heat energy. The temperature of the system decreases.
Forming bonds (exothermic reactions) means the system releases energy, warming the surroundings and increasing the overall heat energy. The temperature of the system increases.
The calculation of the overall energy change in a reaction involves: Energy Change = Energy needed to break bonds - Energy released when new bonds form.
If the energy change value is positive, the reaction is endothermic. If it’s negative, the reaction is exothermic.
Enthalpy diagrams can be used to represent energy changes in reactions. The vertical axis represents energy, while the horizontal axis symbolises the course of reaction.
During a chemical reaction, if the energy released by forming new bonds is more than the energy absorbed in breaking the old bonds, the excess energy is generally given out to the surroundings, usually as heat–a principle behind hand-warmers and instant cold packs.
For an exothermic reaction, the energy level of the reactants is higher than that of the products. This difference is the energy released to the surroundings.
For an endothermic reaction, the energy level of the products is higher than that of the reactants. This difference is the energy absorbed from the surroundings.
Bond energies are measured in kilojoules per mole (kJ/mol).
Average bond energies can be used to calculate the energy change in a reaction. This is a useful simplification but doesn’t always reflect the actual complexity of most molecules.
When a bond is formed, energy is released. Therefore, the energy is always released when bond formation takes place.
Each type of bond (e.g., O-H, C-H, O=O, etc.) has a specific bond energy associated with it, which is the average energy needed to break that type of bond over a range of compounds. This makes it possible to estimate overall energy changes for reactions even if they haven’t been studied in detail experimentally.