1. Hess’s Law

2. Hess’s Law
“In going from a particular set of reactants to a particular set of products, the change in enthalpy is the same whether the reaction takes place in one step or a series of steps.”

3. Consider the combustion of methane, CH4, to produce gaseous CO2 and liquid H2O.
CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (l) ∆H1 = -890 kJ
where ∆H1 is the enthalpy change for the reaction.
This reaction can be thought of as occurring in two steps.
In the first step, methane is combusted to produce water vapor:
CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (g)
In the second step, water vapor condenses from the gas phase to the liquid phase:
2H2O (g) → 2H2O (l)

4. Each of these reactions is associated with a specific enthalpy change:
CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (g) ∆H1 = -802 kJ
2H2O (g) → 2H2O (l) ∆H2 = -88 kJ
Combining these equations gives the following:
CH4 (g) + 2O2 (g) + 2H2O (g) → CO2 (g) + 2H2O (g) + 2H2O (l)
We can cancel what is the same on both sides of the reaction and that gives the net reaction, or overall, reaction.
CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (l)
∆H1 = ∆H2 + ∆H3 = -802 kJ + (-88 kJ) = -890 kJ

5. This energy relationship can be shown in an enthalpy diagram.

6. Characteristics of Enthalpy Changes
If a reaction is reversed, the sign of ∆H is also reversed.
The magnitude of ∆H is directly proportional to the quantities of reactants and products in a reaction. If the coefficients in a balanced equation are multiplied by an integer, the value of ∆H is multiplied by the same integer.
Formulas canceled from both sides of an equation must be for the substance in identical physical states.
Use the final reaction as a guide when using steps 1 and 2.