Hess

1
Hess Law
2
Limitations of Calorimetry

• Calorimetry is an accurate technique for
  determining enthalpy changes of a system.
• But there are limitations
• How do chemists deal with chemical systems that
  cannot be analysed using calorimetry?

3
Hess Law of Summation

• For any reaction that can be written in a series
  of steps, the enthalpy change of the reaction
  will be the same as the sum of the enthalpy
  changes for the steps
• The net changes of a system are independent of
  the way in which the system changes from the
  initial state to the final state
• If a set of reactions occur in different steps
  but the initial reactants and final products are
  the same, the overall enthalpy change will be the
  same as well

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Hess Law of Summation
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Hess Law of Summation
?H1 ?H2 ?H3
6
Manipulating Thermochemical Equations

• Ensure all equations are balanced
• Write the equations so that the arrows line up.
• Compare the given equations to the target
  equation
• All equations need to add together to give the
  target equation
• Flip equations to place reactants and products
  on the correct sides
• Remember to also flip the sign for the enthalpy
  value
• Multiply molar coefficients by an integer or
  fraction as required
• Remember to also multiply the enthalpy value
• Add up reactants and products canceling out
  substances that appear on both sides (watch your
  states of matter!)
• Add the enthalpy values

7
Examples
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Practice

• Hess Law Worksheet
• P. 326-329 1-5
• P. 330 1-4