Hess

1
Hesss Law

• 17.4

2

• Hesss Law
• If a series of reactions are added together, the
  enthalpy change for the net reaction will be the
  sum of the enthalpy changes for the individual
  steps

3

• Example
• Smog is a problem in large cities. It is composed
  of NO2 formed from N2 and O2
• N2 2O2 ? 2NO2
• the formation of NO2 takes 2 steps

4

• N2(g) O2(g) ? 2NO(g) DH 181 kJ
• 2NO(g) O2(g) ? 2NO2(g) DH -113 kJ
• N2(g) 2O2(g) ? 2NO2(g)
• DHnet DH1 DH2
• DHnet (181kJ) (-113kJ) 68kJ
• The reaction is ?

5

• Rules for manipulating reactions
• If you multiply coefficients in the reaction by
  some number, then the enthalpy change must also
  be multiplied by that number.
• If an equation is reversed, then the sign of
  enthalpy change is also reversed.

6

• Example
• If
• C(s) H2O(g) ? H2(g) CO(g)
• DH 113kJ
• Then the reverse reaction is
• H2(g) CO(g) ? C(s) H2O(g)
• DH -113kJ

7

• The production of nitrogen dioxide occurs in two
  steps. Use the thermochemical equations to
  determine the heat of reaction for
• N2O4(g) ? 2NO2(g)