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Hess

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Hess s Law Definition of Hess s Law If a reaction is carried out in a series of steps, H is the sum of all the enthalpy changes CH4(g) + O2(g) CO2(g) + 2H2O ... – PowerPoint PPT presentation

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Title: Hess


1
Hesss Law
2
Definition of Hesss Law
  • If a reaction is carried out in a series of
    steps, ?H is the sum of all the enthalpy changes
  • CH4(g) O2(g) ? CO2(g) 2H2O(g) ?H-802
  • 2H2O(g) ? 2H2O(l) ?H-88
  • --------------------------------------------------
    ----------
  • CH4(g) O2(g) ? CO2(g) H2O(l) ?H-890

3
Example
  • The heat of combustion of C to CO2 is 393.5
    kJ/mole and the heat of combustion of CO to CO2
    is 283.0 kJ/mole. Calculate the heat of
    combustion of C to CO.

4
Example 2
  • Carbon occurs in two forms graphite and diamond.
    The heats of combustion are shown
  • C(graphite) O2 ? CO2 ?H -393.5kJ
  • C(diamond) O2 ? CO2 ?H -395.4kJ
  • Calculate the ?H for the conversion from
    graphite to diamond.

5
Example 3
  • Calculate the ?H for
  • 2C(s) H2(g) ? C2H2(g)
    given that
  • C2H2(g) 2.5O2 ? 2CO2 H2O(l) ?H-1299.6
  • C O2 ? CO2 ?H-393.5
  • H2(g) .5O2(g) ? H2O(l) ?H-285.9

6
Whats It Mean Practically
  • Can never obtain more or less energy form a
    chemical reaction by changing the method or
    number of steps in the reaction
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