AQA A Level chemistry - AS Unit 2: Section 3.2.1 Energetics

3.2.1 Energetics - Bond enthalpies

Students should:

be able to determine mean bond enthalpies from given data
be able to use mean bond enthalpies to calculate a value of ΔH for simple reaction

Bond enthalpy and bond dissociation enthalpy

Bond dissociation enthalpy is the enthalpy change when one mole of specific bonds are broken.

X-Y(g) X(g) + Y(g) : ΔH(dissociation).

Bond enthalpy (mean bond energy) is the average value of a particular type of bond which has been measured over a range of molecules.

Example:

CH₄ has four C-H bonds, and so will have four different bond dissociation enthalpies corresponding to the following bonds breaking:

CH₄ CH₃ + H

CH₃ CH₂ + H

CH₂ CH + H

CH C + H

the C-H bond enthalpy is the average value of the four bond dissociation enthalpies.

Bond energies (enthalpies) can be used to calculate unknown enthalpy changes in reactions where only a few bonds are being formed/broken.

Bonds broken (left hand side) - bonds formed (right hand side) = enthalpy change for the reaction.
(all bond values positive)