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Hess' Law

Hess' Law states that:
The enthalpy change in an chemical reaction is equal to the sum of the energy changes in any sequence of reactions which start with the given reactants and end by producing the same products.

Hess' Law can be applied in two important ways:
  1. If a reaction can be written as a sequence of steps then ΔH for that reaction will be equal to the sum of the ΔH values from each of the steps;

    ΔHreaction = ∑ ΔHi

  2. If the heat formation ΔHf° for all the compounds in the reaction are known then we can calculate the heat of reaction as:

    ΔHreaction = ∑ ΔHf°products - ∑ ΔHf°reactants



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