Chemical equilibrium is the state in a reversible reaction where the rate of the forward reaction equals the rate of the backward reaction, and the concentrations of reactants and products remain constant.
The position of equilibrium can be influenced by several external factors. These factors determine whether equilibrium shifts to the right (forward direction) or to the left (backward direction) according to Le Chatelier’s Principle. These factor include:
1. Change in concentration
2. Change in temperature
3. Change in pressure
4. Addition of catalyst
5. Change in volume
6. Addition of Inert Gas
1. Change in Concentration
Changing the concentration of reactants or products causes the equilibrium to shift in a direction that reduces that change.
How change in concentration Effects equilibrium state
Increasing reactant concentration or addition of more reactants ? equilibrium shifts right to form more products.
Decreasing reactant concentration or removal of reactants ? equilibrium shifts left to replace what was removed.
Increasing product concentration ? equilibrium shifts left.
Decreasing product concentration ? equilibrium shifts right.
Example
Consider a chemical reaction below which is at equilibrium:
H2 + I2 ↔ 2HI
Addition more reactant for instance hydrogen, the equilibrium system will shift to the right to favour the production of more hydrogen iodide. If the product is removed, the reaction will move to the right too. But if any of the reactants is removed, the reaction will move to the left and more product is added , the reaction will move to the left.
2. Change in Temperature
Temperature affects endothermic and exothermic reactions differently. Generally, for endothermic reaction, increase in temperature will favour forward reaction while decrease in temperature will favour backward reaction.
How temperature affects equilibrium of a system
Increasing temperature favors the endothermic direction.
Decreasing temperature favors the exothermic direction.
Example
For an exothermic reaction:
H2(g) + Cl2(g) ↔ 2HCl(g) ΔH = -774kJ
The forward reaction leads to production of hydrogen chloride while backward reaction leads to decomposition of hydrogen chloride. If the temperature of the system is increased, the reaction will move back, increase in decomposition of hydrogen chloride. Decrease in temperature will favour the production of hydrogen chloride( move forward.
3. Change in Pressure (for gases)
Pressure affects equilibrium involving gaseous reactants and products.
How pressure affects a system at equilibrium
Increasing pressure ? equilibrium shifts to the side with fewer moles of gas.
Decreasing pressure ? shifts to the side with more moles of gas.
For example, in the production of ammonia
N2 + 3H2 ↔ 2NH3
Left: 4 moles of gas
Right: 2 moles of gas
Increasing pressure shifts equilibrium right.
Decreasing pressure shifts it left.
Note: If both sides have equal moles of gas, pressure has no effect.
4. Effect of Catalyst
A catalyst does NOT shift equilibrium.
How catalyst affects a system at equilibrium
Increases the rate of forward and backward reactions equally.
Helps the system reach equilibrium faster, but does not change equilibrium concentrations.
Example
Adding an iron catalyst to the Haber process speeds up the reaction but does not change the final yield of ammonia.
5. Change in Volume (inverse of pressure)
Since pressure and volume are inversely related:
How volume affects system at equilibrium
Decreasing volume ? equilibrium shifts to the side with fewer moles of gas.
Increasing volume ? shifts to the side with more moles of gas.
Example
In the reaction below:
2CO(g) + O2(g) ↔ 2CO2(g)
If volume increases, equilibrium shifts left because that side has more gas moles.
6. Addition of Inert Gas
At constant volume, adding an inert gas does nothing to equilibrium.
At constant pressure, adding an inert gas increases volume, so equilibrium shifts toward the side with more moles of gas.
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