# Working with Hess's Law.

Working with Hess's Law

It is often possible to calculate H for a reaction from listed H values of other reactions (i.e. you can avoid having to do an experiment)

Enthalpy is a state function

It depends only upon the initial and final state of the reactants/products and not on the specific pathway taken to get from the reactants to the products

Whether one can arrive at the products via either a single step or multi-step mechanism is unimportant as far as the enthalpy of reaction is concerned - they should be equal

Consider the combustion reaction of methane to form CO2 and liquid H2O

CH4(g) + 2O2(g) -> CO2(g) + 2H2O(l)

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) H = -802 kJ

2H2O(g) -> 2H2O(l) H = -88 Kj

(Note: under conditions of standard temperature and pressure the liquid state of water is the normal state. Thus, the gas would be expected to condense. This is an exothermic process under these conditions. In a related process, it should get warmer when it rains)

Combining these equations yields the following:

CH4(g)+2O2(g)+2H2O(g) -> CO2(g)+2H2O(g)+2H2O(l)

H = (-802) kJ + (-88) kJ= -890 kJ

Hess's Law

if a reaction is carried out in a series of steps, H for the reaction will be equal ...