Revise: Chemical energyBond enthalpies

Energy is required to break a covalent bond between two atoms to overcome the attractive force. Bond breaking is an endothermic process.

The opposite is true if we want to make new bonds. Energy is released when new chemical bonds are formed, so bond making is an exothermic process.

Breaking or making the same chemical bond will require the same energy to be put in or released.

The values for bond enthalpies are found in the data book. For some bonds, the mean bond enthalpy is quoted.

This is to give an average value to work from since the precise enthalpy value for a bond may be different in different molecules.

For example, the energy needed to break a carbon to carbon single bond (C-C) in ethane (C₂H₆) will be different to the energy needed to break a carbon to carbon single bond in decane (C₁₀H₂₂).

The bond enthalpies quoted in the data book are the energies required to break one mole of a particular bond between a pair of atoms in the gaseous state.

We can use these bond enthalpies to approximately calculate the enthalpy change for a given reaction.