1. Hess’s Law

2. Definition of Hess’s Law
If a reaction is carried out in a series of steps, ΔH is the sum of all the enthalpy changes
CH4(g) + O2(g) → CO2(g) + 2H2O(g) ΔH=-802
2H2O(g) → 2H2O(l) ΔH=-88
CH4(g) + O2(g) → CO2(g) + H2O(l) ΔH=-890

3. Example
The heat of combustion of C to CO2 is –393.5 kJ/mole and the heat of combustion of CO to CO2 is –283.0 kJ/mole. Calculate the heat of combustion of C to CO.

4. Example 2
Carbon occurs in two forms graphite and diamond. The heats of combustion are shown:
C(graphite) + O2 → CO2 ΔH = kJ
C(diamond) + O2 → CO2 ΔH =-395.4kJ
Calculate the ΔH for the conversion from graphite to diamond.

5. Example 3 Calculate the ΔH for 2C(s) + H2(g) → C2H2(g) given that
C2H2(g) + 2.5O2 → 2CO2 + H2O(l) ΔH=
C + O2 → CO ΔH=-393.5
H2(g) + .5O2(g) → H2O(l) ΔH=-285.9

6. What’s It Mean Practically
Can never obtain more or less energy form a chemical reaction by changing the method or number of steps in the reaction