1. Bond enthalpies and mean bond enthalpies
Connector:
What type of energy change happens when a chemical bond is:
Broken endothermic
Made exothermic
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

2. Using bond enthalpy data

3. Bond enthalpy
Specific to a particular bond in a diatomic molecule e.g H2, HBr
H2 (g) H (g) bond dissociation energy = +436 Kj/mol
HBr (g) H (g) + Br (g) bond dissociation energy = +366 Kj/mol

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

5. Calculations involving bond enthalpies

6. Method:
Construct a Hess cycle, showing all the bonds to be broken, and the products of the broken bonds.
Put in the bond enthalpies

8. What is needed to complete the Hess cycle
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.