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Hess's Law
Objectives:
1. Explain how Hess's law can be used to determine enthalpy change for a chemical reaction.
2. Explain the two rules associated with Hess's law.
Notes: (12-3)
When predicting the enthalpy (difference in heat) change for single and multiple step reactions, Hess realized that the amount of enthalpy in the final product was the same for both multiple and single step versions of the same reaction. Because of this Hess formulated his law that states:
If a series of reactions are added up, the enthalpy change for the multiple reactions will be the sum of the single reaction.
Rules for solving enthalpy equations.
 | If the equations contain coefficients, the enthalpy change for the reaction is multiplied by the same factor. |
| When a reaction is reversed, the sign of the enthalpy also reverses. |