**Unformatted text preview: **Equilibrium Law and the
Equilibrium Constant
Chapter 7.2 • Equilibrium law is the mathematical description of a
chemical system at equilibrium
• The equilibrium constant (K) is the numerical value
defining the equilibrium law fora given system
K is a unitless
quantity
K is constant for
a given reaction
at a given
temperature
Example: 4NH3 (g) + 7O2 (g) 4NO2 (g) + 6H2O (g) The equation
must be
correctly
balanced 4 6 NO2 H2 O Keq 4
7
NH3 O2 Practice
• Write equilibrium law equations for these reactions:
a) 2O3 (g) 3O2 (g)
b) H2 (g) + F2 (g) 2HF(g) Results for three experiments for the reaction:
2NH3 (g) at 500 C N2 (g) + 3H2 (g)
expt Initial Concentrations Equilibrium Concentrations [ NH 3 ]2
K
[ N 2 ] [ H 2 ]3 [N2] [H2] [NH3] [N2] [H2] [NH3] I 1.000 1.000 0 0.921 0.763 0.157 0.0602 II 0 0 1.00 0.399 1.197 0.203 0.0602 III 2.00 1.00 3.00 2.59 2.77 1.82 0.0602 N2O4(g) ↔ 2NO2(g) The Equilibrium Constant Varies with
Temperature Practice 4SO2 (g) + O2 (g) 2SO3(g)
Experiment 1
Initial
Equilibrium
[SO2 ] = 2.00M
[SO2 ] = 1.50M
[O2 ] = 1.50M
[O2 ] = 1.25M
[SO3 ] = 3.00M
[SO3] = 3.50M Equilibrium constant for Experiment 1 =
Experiment 2
Initial
Equilibrium
[SO2 ] = 0.500M
[SO2 ] = 0.590M
[O2 ] = 0.00M
[O2 ] = 0.045M
[SO2 ] = 0.350M
[SO3 ] = 0.260M
Equilibrium constant for Experiment 2 = Heterogeneous Equilibria
• A heterogeneous equilibrium system is one in which the
reactants and products are present in at least two different
states, such as gases and solids
• If pure solids or pure liquids are involved in a chemical
equilibrium system, their concentrations are not included in
the equilibrium law equation for the reaction system
CaCO3 (s) CO2 (g) + Calcium carbonate carbon dioxide CaO(s)
lime Practice
• Write equilibrium law equations for these
reactions:
a) H2 (g) + O2 (g)
b) NH4Cl (s) H2O (l)
NH3 (g) + HCl(g) The Magnitude of K
• The magnitude of the equilibrium constant, K, tells us
whether the equilibrium position favours products or
reactants If K=1
If K>1
If K<1 K(forward) and K(reverse)
2O3(g) 3O2(g) HOMEWORK
Required Reading:
p. 429-436
(remember to supplement your notes!) Questions:
p. 431 #1-3
p. 434 #1
p. 436 #1-6 ...

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- Fall '13
- Longsworth
- Chemical reaction, Equilibrium Law, NO2 H2