A clear visual guide to using mean bond enthalpies to estimate the enthalpy change of a reaction, using the combustion of ethanol as a worked example.

The resource introduces the key relationship:

ΔH ≈ Σ(bond enthalpies of bonds broken) − Σ(bond enthalpies of bonds formed)

or, more simply:

BONDS BROKEN − BONDS FORMED

Students are guided through the calculation step by step:

Draw and interpret the displayed structures of the reactants and products.
Count every bond present in the reacting molecules.
Calculate the total energy required to break bonds in the reactants.
Calculate the total energy released when new bonds form in the products.
Apply the bond enthalpy equation to estimate ΔH.
Recognise why an exothermic reaction produces a negative enthalpy change.

Using the combustion of ethanol, the calculation gives an estimated value of:

ΔH ≈ −1276 kJ mol⁻¹

The resource then explores an important question:

Why are they called mean bond enthalpies?

The strength of a particular type of bond is affected by its chemical environment. For example, an O–H bond does not have exactly the same bond enthalpy in water, methanol and ethanol.

Tabulated bond enthalpies are therefore mean (average) values obtained from bonds in different gaseous molecules.

This leads to an important conclusion:

Calculations using mean bond enthalpies provide estimates of ΔH rather than exact values.

The calculated value is compared with an experimental/data-book value to demonstrate this difference.

The resource also highlights the main advantage and limitation of the method:

Advantage – Speed: a relatively small table of readily available mean bond enthalpies can be used to estimate enthalpy changes for a very large number of reactions.

Limitation – Approximation: because the values are averages, they do not precisely represent the actual bond enthalpies in the particular molecules involved.

An additional A-level precision point is included: mean bond enthalpies refer to bonds in gaseous species, whereas experimental standard enthalpies may involve substances in different physical states.

Suitable for A-level Chemistry and equivalent courses covering energetics, bond enthalpies, enthalpy calculations and evaluation of thermochemical data.

Ideal for classroom teaching, worked examples, revision and independent study.