Enthalpy & Bond Energies
- During a reaction, enthalpy changes take place because bonds are being broken and formed
- Energy (in the form of heat) is needed to overcome attractive forces between atoms
- Bond breaking is therefore endothermic
- Energy is released from the reaction to the surroundings (in the form of heat) when new bonds are formed
- Bond forming is therefore exothermic
Making and breaking bonds
Breaking bonds requires energy from the surroundings and making bonds releases energy to the surroundings
- If more energy is required to break bonds than energy is released when new bonds are formed, the reaction is endothermic
- If more energy is released when new bonds are formed than energy is required to break bonds, the reaction is exothermic
- In reality, only some bonds in the reactants are broken and then new ones are formed