Hess’s Law

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.”

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)

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

This energy relationship can be shown in an enthalpy diagram.

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.