In the article on ‘Measuring the change in enthalpy of a reaction (Part 2)’, the change in enthalpy of a reaction is determined via calorimetry. Enthalpy change of reactions can also be estimated using bond enthalpy data.
Rewriting eq2 in terms of enthalpy,
where ΔH is the change in enthalpy of the reaction, ΔHbf is the bond enthalpy of formation (ΔHbf < 0) and ΔHbb is the bond enthalpy of breaking (ΔHbb > 0).
Since both ΔHbb and ΔHbf are taken from the same bond enthalpy data set, which has all numbers quoted as positive values, eq6 becomes
or to be specific, if there are multiple bonds broken and multiple bonds formed:
In other words,
The change in enthalpy of a reaction is the difference between the sum of the magnitude of bond enthalpies of all bonds broken and the sum of the magnitude of bond enthalpies of all bonds formed.
Question
With reference to the bond enthalpy data in the previous article,
a) what is the change in enthalpy for the reaction:
b) what is the change in enthalpy for the hydrogenation of propene?
Answer
a) Using eq8, ΔH = 0 – (416 x 4) = -1664 kJmol-1.
b) 1 mole of π (C – C) bond and one mole of H – H bond are broken and two moles of C – H bonds are formed. Using the bond enthalpy data, the bond enthalpy of one mole of π (C – C) bond can be estimated from the difference between one mole of C = C bond and one mole of σ (C – C) bond. Thus, using eq8,