Hess's Law

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Hess's Law
Emma Stewart Chem 12A - C
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Background
Germain Henri Hess (1802-1850)
thermochemical studies 1839
other work
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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

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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

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Using Hess's Law
ex1 Calculate the ?H for the reaction C(s)
O2(g) -gt CO2(g), Given C(s) ½O2(g) -gt
CO(g) ?H-110.5kJ CO(g)½O2(g) -gt
CO2(g) ?H-283.0kJ
C(s)O2(g) -gt CO2(g) ?H-293.5kJ
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Using Hess's Law
ex2 Calculate the ?H for the reaction
CaCO3(s)2HCl(aq) -gt CaCl2(aq)H2O(l)CO2(g),
Given CaCO3(s) -gt CaO(s)CO2(g) ?H175kJ
Ca(OH)2(s) -gt CaO(s)H2O(l) ?H67kJ Ca(OH)2(s)
2HCl -gt CaCl2(aq)2H2O(l) ?H-198kJ
ex2 Calculate the ?H for the reaction
CaCO3(s)2HCl(aq) -gt CaCl2(aq)H2O(l)CO2(g),
Given CaCO3(s) -gt CaO(s)CO2(g) ?H175kJ
CaO(s)H2O(l) -gt Ca(OH)2(s)
?H-67kJ Ca(OH)2(s) 2HCl -gt CaCl2(aq)2H2O(l)
?H-198kJ
CaCO3(s)2HCl(aq)-gt CaCl2(aq)H2O(l)CO2(g),
?H-90kJ
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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 Hesss Law are not limited to
  two-or-three steps problems

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Works Cited

• http//www.science.uwaterloo.ca/cchieh/cact/c120/
  hess.html
• http//hrsbstaff.ednet.ns.ca/mchiasso/Chem_12/Powe
  rpoint_pdf/hesss20law.ppt
• http//www2.ucdsb.on.ca/tiss/stretton/CHEM2/enthal
  08.htm
• http//www.chemistry.co.nz/hess_law.htm
• http//gci.wrdsb.on.ca/academics/science/chemistry
  /Chemistry204UI_files/Unit203,20Lesson2006,20
  Introduction20to20State20Functions20and20Hess
  's20Law.pdf
• Mr. Therriens Blue Booklet