Chemistry - A Molecular Science

Le Châtelier’s Principle:

Systems at equilibrium respond to stress in such a way as to

minimize the effect of the stress

.^

We conclude the following based on Le Châtelier’s principle:

(^) •
The addition of a substance to an equilibrium mixture causes the reaction to proceed so as to reduce the amount
of the added substance.
(^) •
The removal of a substance from an equilibr
ium mixture causes the reaction to proceed
so as to replace some of the removed substance.
To drive reactions more to completion, pr
oducts are often removed from the reaction as
they are formed, or a large excess of one r
eactant is used. The following applies Le
Châtelier’s principle to the exothermic reaction that we have been studying:
CH
I + OH 3
1-^ U
CH
OH + I 3
1-^
ΔH < 0

• Removing CH

I results in the reaction of more CH 3

OH + I 3

1- to replace some of the

removed CH

I thus producing more OH 3

1-.

• Adding CH

OH also results in more reaction with I 3

1- to produce more CH

I + OH 3

1- in order

to remove some of the additional CH

OH. 3

• Removing CH

OH causes further reaction between CH 3

I and OH 3

1- to replace the

removed CH

OH. Additional I 3

1- and heat are also produced.

-^

Because the reaction is exothermic, heat is a product and we can write,

CH

3 I + OH

1-^

U

CH

3 OH + I

1- +

heat

Heat is a product of the reaction, so cooling (re

moving heat from) the system reacts so as

replace some of the heat, which also produces CH

OH + I 3

1- and consumes CH

I + OH 3

1-.

In summary,

the amount of product in an exothermic reaction can be increased by

reducing temperature, adding more reactant, or by removing other products

.

Example 9.12

Use the reaction N

(g) + 3H 2

(g) 2

U

2NH

(g) 3

ΔH

o = -90 kJ to predict the effect of each

of the following on the equilibrium concentration of hydrogen:

a) Increasing the temperature

The reaction is exothermic, so heat is a pr

oduct (reactants are at higher energy in the

reaction diagram). Therefore, increasing th

e temperature drives the reaction back to the

left by decreasing the value of K. Thus, [H

] increases. 2

b) Removing N

(^2)
The reaction must replace some of the N
, so NH 2
reacts, which increases [H 3
]. 2
c) Adding NH
(^3)
Some of the added NH
reacts to produce more H 3
and N 2
, so [H 2
] increases. 2
Chapter 9 Reaction Energetics
© by
North
Carolina
State
University