Chemistry » Chemical Equilibrium » Le Chatelier's Principle

# Effect of Temperature on Equilibrium

## The effect of temperature on equilibrium

If the temperature of a reaction mixture is changed, the equilibrium will shift to minimise that change.

• If the temperature is increased the equilibrium will shift to favour the reaction which will reduce the temperature. The endothermic reaction is favoured.

• If the temperature is decreased the equilibrium will shift to favour the reaction which will increase the temperature. The exothermic reaction is favoured.

For example the forward reaction shown below is exothermic (shown by the negative value for $$\Delta H$$). This means that the forward reaction, where nitrogen and hydrogen react to form ammonia, gives off heat, increasing the temperature (the forward reaction is exothermic). In the reverse reaction, where ammonia decomposes into hydrogen and nitrogen gas, heat is taken in by the reaction, cooling the vessel (the reverse reaction is endothermic).

#### Fact:

Remember that heat is released during an exothermic reaction. It is a product of the reaction.

$$\color{blue}{\text{N}_{2}\text{(g)}} + \color{blue}{\text{3H}_{2}\text{(g)}} \leftrightharpoons {\color{red}{\text{2NH}_{3}\text{(g)}}} \qquad \Delta{H} = -92$$ $$\text{kJ}$$

• An increase in temperature:

• Favours the endothermic reaction because it takes in energy (cools the container).

• The reverse reaction is endothermic, so the reverse reaction is favoured.

• The yield of ammonia $$(\text{NH}_{3})$$ will decrease.

• A decrease in temperature:

• Favours the exothermic reaction because it releases energy (warms the container).

• The forward reaction is exothermic, so the forward reaction is favoured.

• The yield of $$\text{NH}_{3}$$ will increase.

In the informal experiment on Le Chatelier’s principle, the solution should be purple to start. To achieve this $$\text{CoCl}_{2}$$ must be dissolved in ethanol and a few drops of water must be added. This solution is toxic, and all the usual laboratory precautions should be taken.

The hot water will make the solution a deep blue, the cold water will make the solution a pink/red colour. If necessary the test tube can be gently shaken to ensure mixing.

## Optional Experiment: Le Chatelier’s Principle

### Aim

To determine the effect of a change in concentration and temperature on chemical equilibrium

### Apparatus

• $$\text{0.2}$$ $$\text{mol.dm^{-3}}$$ purple $$\text{CoCl}_{2}$$ in ethanol solution, concentrated $$\text{HCl}$$, water

• test tube, tongs

• ice-bath, water-bath, hot-plate or bunsen burner

### Method

1. Place the water bath on the hot-plate and heat.

2. During each step observe and record the colour change that takes place.

3. Put $$\text{4}$$$$\text{5}$$ drops of $$\text{0.2}$$ $$\text{mol.dm^{-3}}$$ $$\text{CoCl}_{2}$$ solution into the test tube.

4. Add $$\text{10}$$$$\text{12}$$ drops of water.

5. Add $$\text{20}$$$$\text{25}$$ drops of concentrated $$\text{HCl}$$.

6. Place the test tube in the water-bath on the hot-plate (use tongs). Leave for $$\text{1}$$$$\text{2}$$ minutes. Record your observations.

7. Place the test tube in the ice-bath. Leave for $$\text{1}$$$$\text{2}$$ minutes. Record your observations.

The equation for the reaction that takes place is:

$\underset{\color{blue}{\text{blue}}}{\underbrace{{\color{blue}{{\text{CoCl}}_{4}^{2-}\text{(aq)}}}}} + 6{\text{H}}_{2}{\text{O(l)}} \leftrightharpoons \underset{\color{red}{\text{pink}}}{\underbrace{{\color{red}{{\text{Co(H}}_{2}{\text{O)}}_{6}^{2+}\text{(aq)}}}}} + 4{\text{Cl}}^{-}\text{(aq)}$

### Results

Complete your observations in the table below, noting the colour changes that take place, and also indicating whether the concentration of each of the ions in solution increases or decreases.

 Original colour Final colour $${\color{blue}{{\text{[CoCl}}_{4}^{2-}{\text{]}}}}$$ $${\color{red}{{\text{[Co(H}}_{2}{\text{O)}}_{6}^{2+}{\text{]}}}}$$ $${\color{red}{{\text{[Cl}}^{-}{\text{]}}}}$$ Add $$\text{HCl}$$ Add $$\text{H}_{2}\text{O}$$ Increase temp Decrease temp

### Conclusions

• Use Le Chatelier’s principle to explain the changes that you recorded in the table above.

• Draw a conclusion about the effect of a change in concentration of either the reactants or products on the equilibrium position.

• Also draw a conclusion about the effect of a change in temperature on the equilibrium position.