According to Le chatelier’s principle, if stress is applied to the system at equilibrium, then the system adjusts itself to overcome applied stress in such a way that the system changes its position, to reduce the effect of that stress.
It is a general rule that describes the effect of changes in temperature, pressure, and concentration on chemical systems that are in equilibrium. This principle can be used to predict how a chemical system will respond to changes in these factors, and it can be used to design experiments to alter the equilibrium state of a system.
The Le-chatelier principle is applicable to reversible reactions. It is the application of the law of mass action. It was formulated by H.Le Chatelier and F Braun.
There are different parameters that can disturb the dynamic equilibrium state of the system. For example, change in concentration, temperature, volume, pressure, catalysts, etc. The system can not completely cancel out the effect of these changes but it minimizes the effects of changes.
The Chatelier principle helps to maintain the dynamic equilibrium of the system. It makes adjustments in such a way as to minimize the effect of disturbance.
The possible effects are:
- Changing the concentration of reactants or products
- Increasing or decreasing the pressure
- Changing the temperature
- Changing the volume
Application of Le Chatelier Principle
Effect of change in concentration
At a dynamic equilibrium state, the rate of the forward reaction is equal to the rate of the reverse reaction. The system has an equal concentration of reactant and product at equilibrium. When the concentration of the reactant or product is changed, then the equilibrium state of the system is disturbed. At that particular time, the system makes adjustments accordingly to minimize the effect of change.
If the concentration of one species is increased at an equilibrium state then the system readjusts in such a way as to decrease the concentration of that species. This will help to maintain dynamic equilibrium. This can also be explained by the common ion effect.
Effect of change in volume
Changes in volume can also influence the equilibrium of a gaseous chemical reaction. Their effect is similar to the effect of pressure change. If the reaction proceeds with an increase in the number of moles then the increase in volume shifts the equilibrium backward and vice versa.
Effect of change in pressure/volume
Change in pressure only affects the equilibrium state of gaseous substances having unequal moles of product and reactants. By principle, when the pressure is increasing in a gaseous equilibrium reaction, the equilibrium will shift in direction with less number of moles to decrease the pressure.
It does not affect the equilibria of solid substances or liquids or gases having equal moles of reactants and products.
For example, reactants and products have an equal number of moles so pressure change does not affect the equilibrium state.
Example of pressure change
This reaction proceeds with an increase in the number of moles. It means that the volume of the system is increasing. So, a decrease in pressure shift equilibrium toward a forward direction whereas, an increase in pressure shift equilibrium toward a backward direction.
Effect of change in temperature
Chemical equilibrium can also be disturbed by changing the temperature. It depends upon whether a chemical reaction is endothermic or exothermic.
An increase in temperature adds heat to the system whereas a decrease in temperature removes the heat from the system. In both of the case equilibrium of the system is disturbed.
In fact, an increase in temperature favors endothermic reactions, whereas, a decrease in temperature favors exothermic reactions.
For example, the above reaction proceeds with the release of heat energy. So this is an exothermic reaction. It increases the temperature of the system. The system moves in such a way as to decrease the temperature. Hence, the equilibrium will shift toward the reactant’s side.
Effect of catalyst
There are many chemical reactions that proceed very slowly on their own. They require a catalyst to speed them up. The amount of catalyst does not affect the equilibrium concentration of reactants and products. Rather, it helps to attain equilibrium in a lesser time by increasing the speed of forward and backward reactions.
In other words, the catalyst does not shift the position of equilibrium in any direction. It just lowers the activation energy of forward and backward reactions.
Industrial applications of the Le-chatelier principle
Haber process
Haber’s process is used for the manufacturing of ammonia from nitrogen and hydrogen gas.
In an equilibrium state, we have
Le chatelier principle and Haber’s process
Effect of change in concentration
If we increase the concentration of any reactant then equilibrium shifts in a forward direction. Similarly, if the concentration of products increases then the equilibrium will shift to the reactant side.
Effect of change in pressure/volume
This reaction proceeds with a decrease in volume. This means if we increase the pressure equilibrium will shift in the forward direction to maintain equilibrium.
Effect of pressure in ammonia synthesis
| Pressure (atm) | Percentage yield of Ammonia |
| 1-5 | 10-15 |
| 70-80 | 75-80 |
| 81-90 | 85 |
| 91-100 | 95 |
Effect of temperature change
This is an exothermic reaction in a forward direction. As we already discussed an increase in temperature favors endothermic reaction. So when we increase the temperature the equilibrium will shift in a backward direction. This will decrease the concentration of products. Whereas, the decreasing temperature will shift the equilibrium in a forward direction. But we can not decrease the temperature too much. Because at low temperatures rate of reaction slow down. Hence, the optimum temperature for the synthesis of ammonia is 450ºC.
Effect of temperature in ammonia synthesis
| Temperature (°C) | Percentage yield of Ammonia |
| 100-200 | 80-90 |
| 200-300 | 60-70 |
| 400-500 | 25-35 |
| 500-600 | 10-20 |
Le chatelier principle and contact process
The contact process is used for the synthesis of sulfur trioxide which is used for the synthesis of sulfuric acid.
In an equilibrium state, we have,
Effect of change in concentration
An increase in the concentration of reactants shifts the equilibrium in a forward direction whereas an increase in the concentration of product shifts the equilibrium in a backward direction.
Effect of change in a temperature
As it is an exothermic reaction so increases in temperature shift equilibrium in a backward direction whereas, a decrease in temperature shifts equilibrium in a forward direction and we can get a greater amount of product.
Effect of change in pressure/volume
The reaction proceeds with a decrease in volume. This means if we increase the pressure, equilibrium will shift in the forward direction to maintain equilibrium.
Brikeland eyde process
Birkland-eyed process is commonly used for the synthesis of nitric acid.
In the equilibrium state, we have,
Le chatelier principle and Birkeland Eyde process
Effect of change in concentration
An increase in the concentration of reactants shifts the equilibrium in a forward direction whereas, an increase in the concentration of product shifts the equilibrium in a backward direction.
Effect of change in temperature
This is an endothermic reaction so an increase in temperature shifts equilibrium in the forward direction and we get more nitric acid whereas, a decrease in temperature shifts equilibrium in the backward direction.
Effect of change in pressure
As the number of moles of reactant and product remains the same during a chemical reaction. So this reaction is not affected by any change in pressure.
Concepts Berg
What is meant by a dynamic equilibrium state?
In a reversible reaction when the two opposing reactions occur at the same rate then this is called a dynamic equilibrium state.
What is the common ion effect?
The common ion effect is the decrease in solubility of weak acid/ base by adding salts having common ions. Or simply, the common ion effect is the suppression of ionization of weak electrolytes by the addition of strong electrolytes having a common ion.
What are different parameters that can affect the state of equilibrium?
There are different parameters that can disturb the equilibrium state i.e,
- Change in concentration of reactant or product
- Increased or decreased pressure
- Change in temperature
- Changing the volume of a system
What are exothermic reactions?
A chemical reaction in which energy is released from the system to the surrounding in the form of light or heat.
What are endothermic reactions?
A chemical reaction in which energy is absorbed by the system from the surrounding in the form of light or heat.
Where we can apply the Le-chatelier principle?
We can apply the Le-chatelier principle where the equilibrium state is being disturbed by changes in reaction parameters like pressure, volume, temperature, concentration, etc.
How can we maximize the yield of ammonia by using the Le Chatelier principle?
By using the Le Chatelier principle we can maximize the yield of ammonia in different ways, i.e,
- Continual removal of ammonia from the product.
- Increasing the pressure
- Decreasing the temperature
- Decreasing the volume
How can we shift the chemical reaction in the forward direction?
The chemical reaction can be shifted in the forward direction in two ways,
- By adding more reactants
- By removing the products
References
- Essential of Physical Chemistry by B.S. Bahl, Arun Bahl, G.D. Tuli, and S Chand.
- Fourth edition of Chemistry The Molecular Science by Moree, Jur, and Stanitski.
- Tenth edition of Chemistry by Zumdahl and DeCoste.
- Second edition of Chemistry by Julia Burdge.
