Bond enthalpies and mean bond enthalpies

Connector:

1. What type of energy change happens when a chemical bond is:

i. Broken endothermic

ii. Made exothermic

2. How is the enthalpy change of a chemical reaction calculated? Write an equation to show this. What are the units of enthalpy change?

ΔH = H (products) – H (reactants)

units kJ mol-1 (kJ/mol)

Crowe2010

Using bond enthalpy data

Bond enthalpy

Specific to a particular bond in a diatomic molecule e.g H2, HBr

H2 (g) 2H (g) bond dissociation energy = +436 Kj/mol

HBr (g) H (g) + Br (g) bond dissociation energy = +366 Kj/mol

Mean bond enthalpy

e.g. methane

As H atoms are removed from the methane molecule the environment of the others left behind changes and so the strength of the bond changes.

So, here we have to rely on the mean bond enthalpy.

ΔH = + 1662 kJ mol-1

So, mean bond enthalpy = +1662/4 = + 415.5 kJ mol-1

Calculations involving bond enthalpies

Method:

1. Construct a Hess cycle, showing all the bonds to be broken, and the products of the broken bonds.

2. Put in the bond enthalpies

• What is needed to complete the Hess cycle? (Hint: What is the definition of the heat of formation? What is the definition of the heat of atomisation?)

• Put in the given values.

• Now find two routes around the cycle without going against the flow of an arrow.