Hess

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Hesss Law
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Hesss Law
In going from a particular set of reactants to a
particular set of products, the change in
enthalpy is the same whether the reaction takes
place in one step or a series of steps.
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• Consider the combustion of methane, CH4, to
  produce gaseous CO2 and liquid H2O.
• CH4 (g) 2O2 (g) ? CO2 (g) 2H2O (l) ?H1
  -890 kJ
• where ?H1 is the enthalpy change for the
  reaction.
• This reaction can be thought of as occurring in
  two steps.
• In the first step, methane is combusted to
  produce water vapor
• CH4 (g) 2O2 (g) ? CO2 (g) 2H2O (g)
• In the second step, water vapor condenses from
  the gas phase to the liquid phase
• 2H2O (g) ? 2H2O (l)

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• Each of these reactions is associated with a
  specific enthalpy change

CH4 (g) 2O2 (g) ? CO2 (g) 2H2O (g) ?H1
-802 kJ

• 2H2O (g) ? 2H2O (l) ?H2 -88 kJ
• Combining these equations gives the following
• CH4 (g) 2O2 (g) 2H2O (g) ? CO2 (g) 2H2O (g)
  2H2O (l)
• We can cancel what is the same on both sides of
  the reaction and that gives the net reaction, or
  overall, reaction.

CH4 (g) 2O2 (g) ? CO2 (g) 2H2O (l) ?H1 ?H2
?H3 -802 kJ (-88 kJ) -890 kJ
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• This energy relationship can be shown in an
  enthalpy diagram.