12.3 Hess

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12.3 Hesss law

• 12.3 Hesss Law

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Hesss law

• Proposed by a Swiss chemist (G.H. Hess) in the
  early 1800s.
• Essentially, it is a the sum of the parts equal
  the whole idea.
• Hesss Law If a series of reactions are added
  together, the enthalpy change for the net
  reaction will be the sun of the enthalpy changes
  for the individual steps

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Hesss law An example

• Smog that is seen over large cities is caused by
  NO2 (nitrogen dioxide). It forms from nitrogen
  and oxygen.
• N2 O2 ? 2NO2
• Look at the two following steps and their
  enthalpy changes

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Hesss law An example

• N2 O2 ? 2NO ?H1 181 kJ
• 2 NO O2 ? 2NO2 ?H2 -113 kJ
• What happens if you combine the two, putting all
  reactants on one side and all products on the
  other?
• N2 O2 2 NO O2 ? 2NO2 2NO
• Combine your O2s
• N2 2 O2 2 NO ? 2NO2 2NO

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Hesss law An example

• N2 2 O2 2 NO ? 2NO2 2NO
• Now, mark out anything that is the same on both
  sides
• N2 2 O2 ? 2NO2
• Look back at your original equation from slide
  3. Voila!
• Now, since all we did was add the two equations
  together, we can simple add their enthalpy
  changes to get the overall enthalpy change for
  the system.

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Hesss law An example

• ?Hnet ?H1 ?H2
• ?Hnet (181 kJ) (-113 kJ)
• ?Hnet 68 kJ

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Applying Hesss law

• Sometimes you have to manipulate the step
  equations. If so, you have to do the same thing
  to the enthalpy changes (?H)
• Rules
• If you multiply or divide the coefficients by a
  factor, you do the same thing to the ?H
• If you reverse an equation, you change the sign
  of the ?H. (When a reaction absorbs heat in one
  direction, it produces it in the reverse
  reaction.)

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Applying Hesss law

• Sample problem 2 on page 390
• Practice problems 3 and 4 on page 390