Hess's Law

1. 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.

2. Hess's Law • 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.

3. Rules for solving enthalpy equations • If the equations contain coefficients, the enthalpy change for the reaction is multiplied by the same factor. • Standard heats of formation are listed for the products (DHf) • to find the enthalpy of the reaction you must switch the sign of the reactants and add • 2CO + O2g 2CO2 • DHfCO = -110.5 kJ/mol; DHfCO2 = -393.5 kJ/mol • multiply the standard enthalpy of formations by their coefficients • (2mol)(DHfCO = -110.5 kJ/mol) = -221 kJ • (2mol)(DHfCO2 = -393.5 kJ/mol) = -787.0 kJ • change the sign of the reactant coefficient and add or simply subtract the reactants from the products • DHrxn = -787.0 kJ -(-221 kJ) = -787.0 kJ + 221 kJ = -566.0kJ

4. Reverse Reactions