Hess's Law.

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Hesss Law Germain Henri Hess. Hesss Law In going from a particular set of reactants to a particular set of products, the change in enthalpy is the same.

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Presentation transcript:

Hess's Law

Hess’s law Hess’s Law states that the heat of a whole reaction is equivalent to the sum of it’s steps. For example: C + O2  CO2 The book tells us that this can occur as 2 steps C + ½O2  CO H = – 110.5 kJ CO + ½O2  CO2 H = – 283.0 kJ I.e. C + O2  CO2 H = – 393.5 kJ Hess’s law allows us to add equations. We add all reactants, products, & H values.

Example using Hess’ Law Determine the enthalpy of formation of pentane, C5H12, using the information provided. Overall rxn 5C + 6H2 → C5H12 C5H12 + 8O2 →5CO2 + 6H2O -3535.6kj C + O2 → CO2 -393.5kj H2 + ½ O2 → H2O -285.8 kj

1. FLIP Since C5H12 is a product … reverse the equation and the sign of the ∆H value 5CO2 + 6H2O → C5H12 + 8 O2 + 3535.6kj MULTIPLY Multiply 5 (C + O2 → CO2) 5(-393.5kj) So 5C + 5O2 → 5CO2 Because 5CO2 and 5C overall) Multiply 6(H2 + ½ O2 → H2O) 6(-285.8kj) So 6H2 + 3O2 → 6H2O Because need 6H2 and 6H2O overall

5C + 6H2 → C5H12 -147.3 kj ADD Now Cancel like terms 5CO2 + 6H2O → C5H12 + 8O2 + 3535.6kj 5C + 5O2 → 5CO2 5(-393.5kj) 6H2 + 3O2 → 6H2O 6(-285.8kj) 5C + 6H2 → C5H12 -147.3 kj