EquilibriumTopic 7

7.2.1 THE EQUILIBRIUM CONSTANT (Kc or K)7.2.1 THE EQUILIBRIUM CONSTANT (Kc or K)

For any type of chemical equilibrium of the type

a A + b B c C + d D

the following is a CONSTANT (at a given T)

K =[C]c [D]d

[A]a [B]b

conc. of products

conc. of reactantsequilibrium constant

If Kc is known, then we can predict concs. of products or reactants.

Determining KcDetermining Kc

2 NOCl(g) 2 NO(g) + Cl2(g)

[NOCl] [NO] [Cl2]

Initial 2.00 0 0

Change -0.66 +0.66+0.33

Equilibrium 1.34 0.660.33

K = [NO]2[Cl2 ]

[NOCl]2

K = [NO]2[Cl2 ]

[NOCl]2 =

(0.66)2(0.33)

(1.34)2 = 0.080

Chemical Equilibrium from the labFe3+ + SCN- FeSCN2+

Chemical Equilibrium from the labFe3+ + SCN- FeSCN2+

After a period of time, the concentrations of reactants and products are constant.

The forward and reverse reactions continue after equilibrium is attained.

Writing and Manipulating K ExpressionsWriting and Manipulating K Expressions

Solids and liquids NEVER appear in equilibrium expressions.

S(s) + O2(g)

SO2(g)

K = [SO2 ]

[O2 ]

Writing and Manipulating K ExpressionsWriting and Manipulating K Expressions

Solids and liquids NEVER appear in equilibrium expressions.

NH3(aq) + H2O(liq)

NH4+(aq) + OH-(aq)

K = [NH4

+ ][OH- ]

[NH3 ]

The Meaning of KThe Meaning of K

1.Can tell if a reaction is product-favored or reactant-favored.

For N2(g) + 3 H2(g) 2 NH3(g)

Kc = [NH3 ]2

[N2 ][H2 ]3 = 3.5 x 108

Conc. of products is much greater than that of reactants at equilibrium. The reaction is strongly

product-favored.

The Meaning of KThe Meaning of K

For AgCl(s) Ag+(aq) + Cl-(aq)

Kc = [Ag+] [Cl-] = 1.8 x 10-5

Conc. of products is much less than that of reactants at equilibrium.

The reaction with small K is strongly

reactant-favored.

Ag+(aq) + Cl-(aq)

AgCl(s)is product-favored.

Ag+(aq) + Cl-(aq)

AgCl(s)is product-favored.

Product- or Reactant Favored

Product-favoredK > 1

Reactant-favoredK < 1

EQUILIBRIUM AND EXTERNAL EFFECTS

Temperature, catalysts, and changes in concentration affect equilibria.

The outcome is governed by

LE CHATELIER’S PRINCIPLE “...if a system at equilibrium is

disturbed, the system tends to shift its equilibrium position to counter the effect of the disturbance.”

Le Chatelier’s Principle

Change in temperature change in Kc therefore change in P or concentrations at

equilibrium Use a catalyst: reaction comes more

quickly to equilibrium. Kc not changed. Add or take away reactant or product:

Kc does not change Reaction adjusts to new equilibrium “position”

7.2.5 Apply the concepts of kinetics and equilibrium to industrial processes. A good example of this is the production of

ammonia.

Fertilizers and many plastics are derived from ammonia.

High pressure is used to increase yield.

The reaction is exothermic so a lower temperature is used to increase yield.

A catalyst is used to speed up the reaction, but it doesn’t increase the yield.

N2(g) + 3 H2(g) 2 NH3(g) + heat

K = 3.5 x 108 at 298 K

Haber-Bosch Process for NH3