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Hess's Law

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

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Hess's Law

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

  2. Background Germain Henri Hess (1802-1850) 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 Ca(OH)2(s) -> CaO(s)+H2O(l) ∆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 CaO(s)+H2O(l) -> Ca(OH)2(s)∆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 • 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

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