1. Hess's Law
Emma Stewart
Chem 12A - C

2. thermochemical studies 1839
Background
Germain Henri Hess ( )
thermochemical studies 1839
other work

3. Hess's Law:
“The enthalpy evolved or absorbed in a chemical process is the same whether the process takes place in one or several steps.”
Overall heat change reliant on initial and final conditions only
Enthalpy change is not dependent on the number or order of steps in the reactions
, used when ∆H is provided for several steps, while solving for the total change

4. Using Hess's Law
Goal: to manipulate the equations given in order to solve for the sum of their enthalpy changes
When a reaction is multiplied by a factor, the same must be done to its ∆H
When using the reverse of a reaction, the opposite sign must be used for its ∆H

5. Using Hess's Law
ex1: Calculate the ∆H for the reaction: C(s) +O2(g) -> CO2(g),
Given: C(s) +½O2(g) -> CO(g) ∆H=-110.5kJ
CO(g)+½O2(g) -> CO2(g) ∆H=-283.0kJ
C(s)+O2(g) -> CO2(g) ∆H=-293.5kJ

6. Using Hess's Law ex2: Calculate the ∆H for the reaction:
CaCO3(s)+2HCl(aq) -> CaCl2(aq)+H2O(l)+CO2(g),
Given: CaCO3(s) -> CaO(s)+CO2(g) ∆H=175kJ
CaO(s)+H2O(l) -> Ca(OH)2(s) ∆H=-67kJ
Ca(OH)2(s) +2HCl -> CaCl2(aq)+2H2O(l) ∆H=-198kJ
ex2: Calculate the ∆H for the reaction:
CaCO3(s)+2HCl(aq) -> CaCl2(aq)+H2O(l)+CO2(g),
Given: CaCO3(s) -> CaO(s)+CO2(g) ∆H=175kJ
Ca(OH)2(s) -> CaO(s)+H2O(l) ∆H=67kJ
Ca(OH)2(s) +2HCl -> CaCl2(aq)+2H2O(l) ∆H=-198kJ
CaCO3(s)+2HCl(aq)-> CaCl2(aq)+H2O(l)+CO2(g), ∆H=-90kJ

7. Notes on Hess's Law Enthalpy is affected by changes in state
Thermochemical reactions can be used to express the conversion of substances from one state to another (with the appropriate ∆H value)
The uses of Hess’s Law are not limited to two-or-three steps problems

8. Works Cited
Mr. Therrien’s Blue Booklet