Le Chatelier’s principle
Le Chatelier’s Principle states that If a chemical system at equilibrium experiences a change in concentration, temperature or total pressure the equilibrium will shift in order to minimise that change.
Le Chatelier’s Principle helps to predict what a change in temperature, concentration or pressure will have on the position of the equilibrium in a chemical reaction. This is very important, particularly in industrial applications, where yields must be accurately predicted and maximised.
There are number of factors can inﬂuence the equilibrium of a reaction. These are:
The eﬀect of concentration on equilibrium
If the concentration of a substance is increased, the equilibrium will shift so that this concentration decreases. So for example, if the concentration of a reactant was increased, the equilibrium would shift in the direction of the reaction that uses up the reactants, so that the reactant concentration decreases and equilibrium is restored. In the reaction between nitrogen and hydrogen to produce ammonia:
N2(g) + 3H2(g) ⇔ 2NH3(g)
- If the nitrogen or hydrogen concentration was increased, Le Chatelier’s principle predicts that equilibrium will shift to favour the forward reaction so that the excess nitrogen and hydrogen are used up to produce ammonia. Equilibrium shifts to the right.
- If the nitrogen or hydrogen concentration was decreased, the reverse reaction would be favoured so that some of the ammonia would change back to nitrogen and hydrogen to restore equilibrium.
- The same would be true if the concentration of the product (NH3) was changed. If [NH3] decreases, the forward reaction is favoured and if [NH3] increases, the reverse reaction is favoured.
The eﬀect of temperature on equilibrium
If the temperature of a reaction mixture is increased, the equilibrium will shift to decrease the temperature. So it will favour the reaction which will use up heat energy, in other words the endothermic reaction. The opposite is true if the temperature is decreased. In this case, the reaction that produces heat energy will be favoured, in other words, the exothermic reaction.
The reaction shown below is exothermic (shown by the negative value for ∆ H). This means that the forward reaction, where nitrogen and hydrogen react to form ammonia, gives oﬀ heat. In the reverse reaction, where ammonia is broken down into hydrogen and nitrogen gas, heat is used up and so this reaction is endothermic.
N2(g) + 3H2(g) ⇔ 2NH3(g) and ∆H =−92kJ
An increase in temperature favours the reaction that is endothermic (the reverse reaction) because it uses up energy. If the temperature is increased, then the yield of ammonia (NH3) decreases.
A decrease in temperature favours the reaction that is exothermic (the forward reaction) because it produces energy. Therefore, if the temperature is decreased, then the yield of NH3 increases
The eﬀect of pressure on equilibrium
In the case of gases, we refer to pressure instead of concentration. Similar principles apply as those that were described before for concentration. When the pressure of a system increases, there are more particles in a particular space. The equilibrium will shift in a direction that reduces the number of gas particles so that the pressure is also reduced. To predict what will happen in a reaction, we need to look at the number of moles of gas that are in the reactants and products. Look at the example below:
e.g. 2SO2(g) + O2(g) ⇔ 2SO3(g)
In this reaction, two moles of product are formed for every three moles of reactants. If we increase the pressure on the closed system, the equilibrium will shift to the right because the forward reaction reduces the number of moles of gas that are present. This means that the yield of SO3 will increase. The opposite will apply if the pressure on the system decreases, the equilibrium will shift to the left, and the concentration of SO2 and O2 will increase.
Note: The following rules will help in predicting the changes that take place in equilibrium reactions:
- If the forward reaction that forms the product is endothermic, then an increase in temperature will favour this reaction and the yield of product will increase. Lowering the temperature will decrease the product yield.
- If the forward reaction that forms the product is exothermic, then a decrease in temperature will favour this reaction and the product yield will increase. Increasing the temperature will decrease the product yield.
- Increasing the pressure favours the side of the equilibrium with the least number of gas molecules. This is shown in the balanced symbol equation. This rule applies in reactions with one or more gaseous reactants or products.
- Decreasing the pressure favours the side of the equilibrium with the most number of gas molecules. This rule applies in reactions with one or more gaseous reactants or products.
- If the concentration of a reactant (on the left) is increased, then some of it must change to the products (on the right) for equilibrium to be maintained. The equilibrium position will shift to the right.
- If the concentration of a reactant (on the left) is decreased, then some of the products (on the right) must change back to reactants for equilibrium to be maintained. The equilibrium position will shift to the left.
- A catalyst does not aﬀect the equilibrium position of a reaction. It only inﬂuences the rate of the reaction, in other words, how quickly equilibrium is reached.
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