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)

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