Chemical Equilibrium
8.0 Factors Affecting Equilibrium (Le-Chatelier's Principle)
8.0 Factors Affecting Equilibrium (Le-Chatelier's Principle)
It states that "when a system in equilibrium is subjected to a change in temperature, pressure or concentration of a reacting species, the system reacts in a way that partially offsets the change while reaching a new state of equilibrium".
Factors influencing equilibrium are
1. Effect of Concentration
- If concentration of reactant increased, reaction shifts in forward direction.
- If concentration of reactant decreased, reaction shifts in backward direction.
- If concentration of product increased, reaction shifts in backward direction.
- If concentration of reactant decreased, reaction shifts in forward direction.
Note: If any of the reactant or product is in solid state, its addition does not alter the original equilibrium.
2. Effect of Pressure
- An Increase in pressure by decreasing volume will shift the reaction to the side having fewer number of moles of the gas while a decrease of pressure by increasing volume will shift the reaction to the side having more number of moles of the gas.
- Effect of pressure depends on the ${\Delta {n_g}}$ of reaction i.e., $$\Delta {n_g} = {n_{{p_{(g)}}}} - {n_{{R_{(g)}}}}$$ Now, three condition arises: $$\begin{equation} \begin{aligned} 1.{\text{ }}\Delta {n_g} = 0;{\text{no effect of pressure}} \\ \\ {\text{2}}{\text{. }}\Delta {n_g} = + ve;\,\,{\text{Since, number of moles on reactant side is less}}{\text{. }} \\ {\text{ If pressure increase by decreasing volume, reaction takes place in backward direction or vice - versa}}{\text{.}} \\ \\ {\text{3}}{\text{. }}\Delta {n_g} = - ve;\;{\text{Since, number of moles on product side is less}}{\text{. }} \\ {\text{ If pressure increase by decreasing volume, reaction takes place in forward direction or vice - versa}}{\text{.}} \\\end{aligned} \end{equation} $$
3. Effect of Inert Gas
- Effect of inert gas is studied either at constant pressure or constant volume at equilibrium.
| Inert gas | ${\Delta {n_g} = 0}$ | ${\Delta {n_g} = - ve}$ | ${\Delta {n_g} = + ve}$ |
| At constant $V$ | No effect | No effect | No effect |
| At constant $P$ | No effect | $ \leftarrow $ backward | $ \to $ forward |
4. Effect of Catalyst
- Addition of catalyst decreases the activation energy and increases the rate of forward and backward reaction equally so that the equilibrium is achieved in lesser time.
5. Effect of Temperature
- Temperature of a system can be increased by adding heat (endothermic reaction) and can be decreased by taking out heat from the system. Due to this increase or decrease in temperature, reaction shifts in forward/backward direction along with changing the value of equilibrium constant.
- If the reaction is endothermic, increase in temperature shift reaction in forward direction while decrease in temperature shift reaction in backward direction.
- If the reaction is exothermic, increase in temperature shift reaction in backward direction while decrease in temperature shift reaction in forward direction.
The equilibrium constants both ${K_c}$ and ${K_p}$ are effected by temperature. The equation predicting this effect is called Vant Hoff's equation represented as
$$\log \frac{{{K_2}}}{{{K_1}}} = \frac{{\Delta {H^ \circ }}}{{2.303R}}\left( {\frac{1}{{{T_1}}} - \frac{1}{{{T_2}}}} \right)$$
where ${\Delta {H^ \circ }}$ is standard enthaply of reaction,
${R = 8.314J{K^{ - 1}}}$
${{T_1}}$ and ${{T_2}}$ are temperatures in Kelvin
${{K_1}}$ and ${{K_2}}$ are equilibrium constants at above temperature respectively.
- If ${T_2} > {T_1}$ and ${\Delta {H^ \circ }}$ is positive (endothermic reaction), then ${K_2} > {K_1}$
- If ${T_2} > {T_1}$ and ${\Delta {H^ \circ }}$ is negative (exothermic reaction), then ${K_2} < {K_1}$
