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

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Presentation transcript:

Hess's Law Emma Stewart Chem 12A - C

thermochemical studies 1839 Background Germain Henri Hess (1802-1850) thermochemical studies 1839 other work

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

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

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

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

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

Works Cited http://www.science.uwaterloo.ca/~cchieh/cact/c120/hess.html http://hrsbstaff.ednet.ns.ca/mchiasso/Chem_12/Powerpoint_pdf/hess’s%20law.ppt http://www2.ucdsb.on.ca/tiss/stretton/CHEM2/enthal08.htm http://www.chemistry.co.nz/hess_law.htm http://gci.wrdsb.on.ca/academics/science/chemistry/Chemistry%204UI_files/Unit%203,%20Lesson%2006,%20Introduction%20to%20State%20Functions%20and%20Hess's%20Law.pdf Mr. Therrien’s Blue Booklet