Equilibrium

March 28, 2007

Chemical Reactions

Reactants Products

A + 3B 2C

But the reverse can also happen

2C 3B + A

So we can describe the reaction as equilibrium

A + 3B 2C

Forward Reaction

Reverse Reaction

Equilibrium rate of forward reaction = rate of reverse reaction

Equilibrium - Concentration

At equilibrium, the concentration of product and reactants stays constant

Equilibrium Visualization

http://www.dlt.ncssm.edu/TIGER/chem5.htm

Law of Mass Action

• Once a reaction has reached equilibrium, the relative concentration of products remains constant

• We call this constant K, the equilibrium constant

Example: N2 + 3H2 2NH3

322

23

]][[

][

HN

NHK

me q u i l i b r i ua t r e a c t a n t s o fi o n c o n c e n t r a t

me q u i l i b r i ua t p r o d u c t s o fi o n c o n c e n t r a tK

Equilibrium Expression

• Things that appear in the equilibrium expression– Concentration of solutions– Pressure of gases– Reaction coefficient

• Things that do NOT appear– Pure liquids– Pure solids– Units

Learning CheckDoes this graph represent a K > 1, K< 1, or K =1 ?

Greater number of reactants – K < 1

Learning Check

Determine the Equilibrium Expression (K) for each of the following reactions

CaCO3 (s) CaO (s) + CO2(g)

2NO2(g) N2O4(g)

H2CO3(aq) CO2(g) + H2O(l)

][ 2COK

22

42

][

][

NO

ONK

][

][

32

2

COH

COK

Calculating the Equilibrium Constant

Calculate the equilbrium constant if the equilibrium concentrations of NO2 and

N2O4 are 2.0 mol/L.

2NO2(g) N2O4(g)

5 0.02

2

][

][

][reac tan ts

[p ro d u c ts]K

222

42 NO

ON

Equilibrium Position• At constant temperature….reaction can only have

one equilibrium constant but many equilibrium positions

N2 + 3H2 2NH3 K = 640 (25 °C)

• Equilibrium concentration of each product can be…

675455

8004010

9003030

NH3 (mol/L)H2 (mol/L)N2 (mol/L)

Value of Equilibrium Constant

• If K >>>>1, forward reaction is favored– Large concentration of products

• If K <<<<1, reverse reaction predominates– Large concentration of reactants

• If K = 1, reverse reaction and forward reaction equal

– Equal concentration of reactants

A + 2B 2C + D

2

2

]][[

][][

BA

DCK

Changing the Equilibrium Constant

• Change the temperature• Change the reaction coefficients

• N2 + 3H2 2NH3

2N2 + 6H2 4NH3

6 4 0]][[

][3

22

23

1 HN

NHK

?][][

][6

22

2

43

2 HN

NHK

Relationship between K1 and K2

K2 = K12 K2 = (640)2 = 4.096 x 105

Changing the Equilibrium Constant

• Change the temperature• Change the reaction coefficients

• N2 + 3H2 2NH3

2NH3 N2 + 3H2

6 4 0]][[

][3

22

23

1 HN

NHK

?][

]][[2

3

322

3 NH

HNK

Relationship between K1 and K3

K3 = 1/K1 K3 =1/ 640 = 0.00156

Learning Check

• Determine the value of the equilibrium constant for the following reaction

• 2NO2 N2O4

½ N2O4 NO2

2 5.0K

?K

22 5.0

11

][

][K

12/1

42

22

KON

NO

Disturbing the Equilibrium

Heat as a Reactant/ Product

• UO2(s) + 4HF(g) UF4(g) + 2H2O(g

FeSCN2+ Equilibrium

• KSCN + Fe(NO3)3 FeSCN2+ + KNO3

• SCN- + Fe3+ FeSCN2+

• Fe3+ + HPO42- FeHPO4

+

• Ag+ + SCN- AgSCN

• Ag+ + Cl- AgCl

Spectator ions….ignore