Chemical Equilibrium

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Outline: Chapter 12 Gaseous Chemical Equilibrium

I. Introduction
A. Equilibrium between liquid and gaseous water:
H2O(l)«H2O(g)
1. Chemical reactions involving gases are carried out in closed containers resemble in many ways the H2O(l)«H2O(g) system.
2. The reactions are reversible; the reactions are not completely consumed.
3. At equilibrium both forward and reverse reactions take place at the same rate.
4. Where gaseous chemical equilibrium are involved more than one gaseous substance is present. For example:
aA(g) +bB(g) «cC(g) +dD(g)
The equilibrium mixture contains two gaseous products (C and D) and two gaseous reactions (A and B).This is called partial equilibrium.
B. Partial pressure
- The part of the total pressure in a gas mixture that can be attributed to a particular component. The partial pressure of a component may be found by multiplying the total pressure by the mole fraction. (each species that is Pc,PD,PA,and PB)
II. The N2O4-NO2 Equilibrium system
A. NO2 is formed by decomposition of the part of N2O4.
N2O4(g)«2NO2(g).

1.At first this is the only reaction taking place, as soon as the NO2 is formed the reverse reaction can occur:
2NO2(g)«N2O4(g)
2. The partial pressure of N2O4 drops and the forward reaction slows down. Conversely the partial pressure of NO2 increases so the rate of the reverse reaction increases. These rates become equal, and establish a dynamic chemical equilibrium.
N2O4(g)«2NO2(g)

B. Table 12.1 Establishment of Equilibrium in the system N2O4(g)«2NO2(g)
(@ 100°C)

Time (s) 0 20 40 60 80 100
PN2O4(atm) 1.00 0.60 0.35 0.22* 0.22 0.22
PNO2(atm) 0.00 0.80 1.30 1.56 1.56 1.56


3.Equilibrium constant K : A number of characteristic of an equilibrium system at a particular temperature.
III. The Equilibrium Constant Expression
A. Equilibrium constant expression can be written stating the condition that must be attained at equilibrium.
- the equilibrium ...