Using Le Chatelier's Principle
Using Le Chatelier's Principle
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:
-
Identify the disturbance or stress on the system.
\(\color{blue}{\text{For example. there is an }\textbf{increase}\text{ in the concentration of reactant}}\).
-
Use Le Chatelier's principle to decide how the system will respond.
\(\color{blue}{\text{Le Chatelier predicts a shift to }\textbf{decrease}\text{ the concentration of reactant}}\).
-
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.
\(\color{blue}{\text{The }\textbf{forward reaction will be favoured}\text{. The equilibrium will shift }\textbf{to the right}}\).
-
Where appropriate, link equilibrium shift to any observed change in the system.
\(\color{blue}{\text{This might result in a }\textbf{colour change}}\).
Example: Using Le Chatelier's Principle
Question
Table salt is added to the (purple) solution in equilibrium:
\(\underset{\color{blue}{\text{blue}}}{\underbrace{{\color{blue}{{\text{CoCl}}_{4}^{2-}}}}} + 6{\text{H}}_{2}{\text{O}} \leftrightharpoons \underset{\color{red}{\text{pink}}}{\underbrace{{\color{red}{{\text{Co(H}}_{2}{\text{O)}}_{6}^{2+}}}}} + 4{\text{Cl}}^{-}\)
-
Use Le Chatelier's principle to predict the change in equilibrium position.
-
What would be observed?
Step 1: Identify the disturbance or stress on the system
Adding \(\text{NaCl}\) produces \(\text{Na}^{+}\) ions and \(\text{Cl}^{-}\) ions as the salt dissolves. Looking at the given equilibrium \(\text{Cl}^{-}\) is in the equation and the disturbance is the increase in concentration of the \(\text{Cl}^{-}\) ion.
Step 2: 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 \(\text{Cl}^{-}\) ions.
Step 3: 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 \(\text{Cl}^{-}\) ions and hence the rate of the reverse reaction will increase. The reverse reaction is favoured and the equilibrium will shift to the left.
Step 4: What would the colour change be due to this equilibrium shift?
The solution will appear more blue as more blue \(\text{CoCl}_{4}^{2-}\) ions are formed.
This lesson is part of:
Chemical Equilibrium