Reversible Reactions

1
Reversible Reactions

• Section 19.2
• Pages 539 - 544

2

• What is a completion reaction?

• A reaction where the amount of reactants
  decrease and the amount of products increase
  until there are no reactants left
• Cannot be changed back to the original reactants

3

• What is a reversible reaction?

• A reaction where the amount of reactants and
  products formed and reformed occur simultaneously
• Can be changed back to original reactants

4
when the formation of products equal the
reformation of reactants

• Equilibrium

• LeChateliers Principle If a stress is
  applied to a system at equilibrium, the reaction
  will shift in a way to relieve the stress

5
What Affects Equilibrium?

• Temperature

• Concentration

• Pressure

6

• N2O4 heat ?? 2NO2
• (light) (dark)
• Which way will the reaction shift if
• The reaction is heated?
• The reaction is cooled?
• The concentration of NO2 is increased?
• The pressure is increased?

Shift towards the products
Shift towards the reactants
Shift towards the reactants
Shift towards the reactants
If pressure is increased, the reaction will
shift toward the side with the fewer number of
moles of a gas
7
Equilibrium Constants (pg 545)

• Equilibrium Constant Keq
• Ratio of products concentrations to reactant
  concentrations

Example H2 I2 ?? 2HI
HI2
Keq
H x I
8
Equilibrium Constants (pg 545)

• If Keq gt 1, products are favored at equilibrium
• If Keq lt 1, reactants are favored at equilibrium

9
Lets Practice!

• The following reaction is contained in a flask at
  300oC. At equilibrium, hydrogen contains 0.15
  mol, nitrogen contains 0.25 mol and ammonia
  contains 0.10 mol. Balance the reaction and
  calculate the Keq.
• N2 H2 ?? NH3

NH32
3
2
Keq
N2 x H23
0.102
Keq 12 Products are favored
Keq
0.25 x 0.153