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Using Le Chatelier’s Principle (ESCNV)

Using Le Chatelier’s Principle (ESCNV)

When a system is in chemical equilibrium, and there has been a change in conditions (e.g. concentration, pressure, temperature) the following steps are suggested:

  1.  Identify the disturbance or stress on the system. For example, there is an increase in the concentration of reactantFor example, there is an increase in the concentration of reactant.
  2.  Use Le Chatelier’s principle to decide how the system will respond.Le Chatelier predicts a shift to decrease the concentration of reactantLe Chatelier predicts a shift to decrease the concentration of reactant.
  3. Look at the given equation and decide whether the rate of the forward reaction or the rate of the reverse reaction is increased. State the shift in equilibrium.The forward reaction will be favoured. The equilibrium will shift to the rightThe forward reaction will be favoured. The equilibrium will shift to the right.
  4.  Where appropriate, link equilibrium shift to any observed change in the system. This might result in a colour changeThis might result in a colour change.

WORKED EXAMPLE 6: USING LE CHATELIER’S PRINCIPLE

Table salt is added to the (purple) solution in equilibrium:

CoCl2−4blue+6H2O⇋Co(H2O)2+6pink+4Cl−CoCl42−⏟blue+6H2O⇋Co(H2O)62+⏟pink+4Cl−

  1.  Use Le Chatelier’s principle to predict the change in equilibrium position.
  2.  What would be observed?

Identify the disturbance or stress on the system

Adding NaClNaCl produces Na+Na+ ions and Cl−Cl− ions as the salt dissolves. Looking at the given equilibrium Cl−Cl− is in the equation and the disturbance is the increase in concentration of the Cl−Cl− ion.

Use Le Chatelier’s principle to decide how the system will respond

By Le Chatelier’s principle, the equilibrium position will shift to reduce the concentration of Cl−Cl− ions.

Decide whether the rate of the forward reaction or the rate of the reverse reaction is increased and state the resulting shift in equilibrium

The reverse reaction uses Cl−Cl− ions and hence the rate of the reverse reaction will increase. The reverse reaction is favoured and the equilibrium will shift to the left.

What would the colour change be due to this equilibrium shift?

The solution will appear more blue as more blue CoCl2−4CoCl42− ions are formed.