Hess

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Hesss Law

• Thermochemistry

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Hesss Law

• Germaine Henri Hess (1802-1850) is important
  primarily for his thermochemical studies.
• Hesss Law states that the heat evolved or
  absorbed in a chemical process is the same
  whether the process takes place in one or in
  several steps.
• ? H overall reaction ?H step 1 ?H step 2 ?H
  step 3 etc.

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Hesss Law

• This is another method for calculating the
  enthalpy of a reaction
• Most reactions occur in more than one step.
• Hess's law can be applied to calculate enthalpies
  of reactions that are difficult to measure.
• We focus on the OVERALL reaction based on the
  balanced chemical equation, but there could be
  many steps involved to reach the final outcome.

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Rule 1

• If you reverse an equation the ?H must switch
  signs.
• Na (s) ½ Cl2(g) ?NaCl (s) ?H -411 kJ
• NaCl (s) ? Na (s) ½ Cl2(g) ?H 411 kJ

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Rule 2

• If you need to multiply or divide any part of the
  equation, the whole equation must follow the same
  operation, including ?H.
• Na (s) ½ Cl2(g) ?NaCl (s) ?H -411 kJ
• 2 Na (s) Cl2(g) ?2 NaCl (s) ?H -822 kJ

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Key idea

• Very much like when using the elimination method
  when you are solving systems of algebraic
  equations
• The same exact compounds on either side of the
  arrow cancel
• 2CO (g )? 2C (s ) O2(g )
• 2C (s ) 2O2 (g ) ?2CO2 (g )

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You must

• Include and pay attention to states of compounds
• (g), (l), (s)
• ie. H2O(l) and H2O (g) do NOT cancel each other

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Example

• Using Hesss law calculate the enthalpy change
  for
• CO2(g) H2(g) ? H2O(l) CO(g)
• if given the following intermediate reactions
• H2(g) ½O2(g) ?H2O(l) ?H 285.9 kJ
• CO(g) ½O2 (g) ?O2(g) ?H 393.5 kJ

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Try it

• Pg 681 11 ? 14
• Work on handout