Le Chatelier’s principle:
It states that a change in any of the factors that determine the equilibrium conditions of a system will cause the system to change in such a manner so as to reduce or to counteract the effect of the change. This is applicable to all physical and chemical equilibria.
1. Effect of Concentration Change:
- When the concentration of any of the reactants or products in a reaction at equilibrium is changed, the composition of the equilibrium mixture changes so as to minimize the effect of concentration changes
- If reactants are added, then the net reaction proceeds in the direction that consumes the added substance, i.e., products are formed and vice versa
- If a reactant is removed, then the net reaction proceeds in the direction that replenishes the removed substance, i.e., reactant is formed. If a product is removed, then the net reaction proceeds in the direction of forming products.
2. Effect of Pressure Change:
- A change in pressure or volume will result in an attempt to restore equilibrium by creating more or less moles of gas.
- For example, if the pressure in a system increases, or the volume decreases, the equilibrium will shift to favor the side of the reaction that involves fewer moles of gas.
- Similarly, if the volume of a system increases, or the pressure decreases, the production of additional moles of gas will be favored.
- Consider the reaction of nitrogen gas with hydrogen gas to form ammonia:
N2+3H2 ⇌ 2NH3 ΔH = −92kJ mol−1
- Note the number of moles of gas on the left-hand side and the number of moles of gas on the right-hand side. When the volume of the system is changed, the partial pressures of the gases change.
- If we were to decrease pressure by increasing volume, the equilibrium of the above reaction would shift to the left, because the reactant side has greater number of moles than the product side. The system tries to counteract the decrease in partial pressure of gas molecules by shifting to the side that exerts greater pressure.
- Similarly, if we were to increase pressure by decreasing volume, the equilibrium would shift to the right, counteracting the pressure increase by shifting to the side with fewer moles of gas that exert less pressure.
- Lastly, for a gas-phase reaction in which the number of moles of gas on both sides of the equation are equal, the system will be unaffected by changes in pressure, since Δn = 0.
- In applying Le Chatelier’s principle to a heterogeneous equilibrium the effect of pressure changes on solids and liquids can be ignored because the volume (and concentration) of a solution/liquid is nearly independent of pressure.
3. Effect of Inert Gas Addition:
- If the volume is kept constant and an inert gas such as argon is added which does not take part in the reaction, the equilibrium remain undisturbed.
- It is because the addition of an inert gas at constant volume does not change the partial pressures or the molar concentrations of the substance involved in the reaction.
- The reaction quotient changes only if the added gas is a reactant or product involved in the reaction.
4. Effect of Temperature Change:
- When a change in temperature occurs, the value of equilibrium constant, Kc is changed. In general, the temperature dependence of the equilibrium constant depends on the sign of ΔH for the reaction.
- The equilibrium constant for an exothermic reaction (negative ΔH) decreases as the temperature increases.
- The equilibrium constant for an endothermic reaction (positive ΔH) increases as the temperature increases.
- Temperature changes affect the equilibrium constant and rates of reactions.
- If we raise the temperature on an endothermic reaction, the equilibrium will shift the right. Conversely, lowering the temperature on an endothermic reaction will shift the equilibrium to the left.
- For an exothermic reaction, increasing the temperature will shift the equilibrium to the left, while decreasing the temperature will shift the equilibrium to the right.
5. Effect of a Catalyst:
- A catalyst increases the rate of the chemical reaction by making available a new low energy pathway for the conversion of reactants to products.
- It increases the rate of forward and reverse reactions that pass through the same transition state and does not affect equilibrium.
- Catalyst lowers the activation energy for the forward and reverse reactions by exactly the same amount.
- Catalyst does not affect the equilibrium composition of a reaction mixture.