Chapter 13: Chemical Equilibrium

1
Chapter 13 Chemical Equilibrium

• Concept All reactions are reversible at the
  molecular level (microscopic reversibility)
• Figure 13.1 Molecular Representation of the
  Reaction 2NO2(g) N2O4(g) Over Time in a Closed
  Vessel

2

• The equilibrium state
• There are no further changes in concentration no
  matter how long we wait. Both reactants and
  products may be present.
• The forward rate the backward rate.
• Dynamic process both reactions are still
  occurring, but at the same rate.
• Now, how can we describe this situation
  mathematically?

3
Figure 13.2Changes in Concentrations
CO(g) H2O(g) ? CO2 (g) H2(g)
4

• The Law of Mass Action a mathematical
  description of the equilibrium condition.
• Consider the following general reaction
• a A b B ? c C d D
• Ratef kf Aa Bb and Rateb kb
  Cc Dd
• At equilibrium, Ratef Rateb
• so, kf Aa Bb kb Cc Dd

5

• Now collect the constants
• This expression defines the equilibrium constant
  Kc and is known as the mass-action expression for
  the reaction.
• Kc is a constant for a given reaction at
  constant temperature.

6

• Ex. For the reaction N2(g) 3 H2(g) ? 2
  NH3(g)
• Some special points to observe about Kc
• The form of Kc depends upon the exact equation
  used.
• ex. For 2 N2(g) 6 H2(g) ? 4 NH3(g)
• What is the form and value of Kc?

7

• or what about 2 NH3(g) ? N2(g) 3 H2(g) ?
• Moral Always write the reaction before writing
  an equilibrium constant!

8

• 2) For reactions involving gases, partial
  pressures are proportional to molarity. Thus, we
  can write a new equilibrium constant Kp.
• Since P (n/V)RT, we can derive the following
• where ?n mols gas products mols gas
  reactants
• and R 0.08206 L atm/mol K

9

• Pure solids and liquids do not appear in the
  mass-action expression.
• Reason the concentration of a pure solid or
  liquid remains constant.
• Ex. For AgCl(s) ? Ag(aq) Cl(aq)
• Kc Ag Cl