Le Chatelier's Principle
A change in temperature.
We know that an equilibrium
constant can only be changed when there is a change in temperature. But
how does it change? Lets look at exothermic and endothermic reactions.
Energy + A + B 
C + D -------- endothermic reaction
A + B C + D + Energy ---------- exothermic reaction.
Increasing the temperature
translates into an increase in the available energy. Using Le Chatelier's
Principle we can see that with an increase in temperature endothermic
reactions will drive forwards, while exothermic reactions will favour
the reverse reaction.
If an endothermic reaction proceeds in the forward direction, with
an increase in temperature, then the equilibrium expression given below
will increase in value as the concentration of products increases while
the concentration of reactants decreases. A new higher value for K is achieved
The opposite will happen if the system is cooled. Endothermic reactions will proceed in the reverse direction when cooled. Therefore the equilibrium constant of an endothermic reaction decreases as the system is cooled.
| Change to the system | .Reaction
type |
Value
of K |
Reaction
will proceed |
Concentration
of products |
Concentration
of reactants |
| Temperature increase | Endothermic | increase | right
|
increase
|
decrease
|
| Temperature decrease | Endothermic | decrease | left
|
decrease
|
increase
|
| Temperature decrease | Exothermic | increase | right
|
increase
|
decrease
|
| Temperature increase | Exothermic | decrease | left
|
decrease |
increase
|
Hide
solutions |
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The effect of temperature on the equilibrium between nitrogen dioxide (NO2 ), brown and colourless, dinitrogen tetroxide (N2O4) is demonstrated in the experiment shown on the left.
An amount of nitrogen dioxide (NO2 ), is placed in a test tube, sealed and allowed to reach equilibrium according to the equation below.
2NO2 (g) N2O4 (g) ----- exothermic
The test tube is placed in hot water and then in cold water. Notice how the equilibrium shifts to the left, producing more brown NO2 gas when placed in the hot water and moves to the right, producing more colorless N2O4 gas when placed in cold water.
Complete the table below.
| Change to the system | .Reaction
type |
Value
of K |
Reaction
will proceed |
Concentration
of products |
Concentration
of reactants |
| Temperature increase | Endothermic | ||||
| Temperature decrease | Endothermic | ||||
| Temperature decrease | Exothermic | ||||
| Temperature increase | Exothermic |
Solution
Addition of a catalyst
Addition of a catalyst does not influence the equilibrium position and has no effect on the equilibrium constant. Catalysts increase the rate of the reaction and may help the reaction to reach equilibrium faster. Catalysts increase the rate of the forward and reverse reactions equally and therefore are unable to impact on changing the equilibrium.
Conduct the following experiments.
Become familiar with the experiment before you attempt it in class.
Investigation of an equilibrium
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