The principle of Le Chatelier is a general rule that predicts the effect that the various factors affecting the chemical balance. In particular, it helps to predict in which direction a reaction that is in equilibrium will move when that equilibrium is disturbed by an external agent.
This principle states that:
“When a system that is in equilibrium is subjected to a disturbance or an external stress, the system reacts in the direction that allows it to partially counteract said disturbance.”
By disturbance it is understood either:
- A change in the concentration of any of the reagents.
- A change in the concentration of any of the products.
- Changes in pressure.
- Changes in volume.
- A change in temperature
When changes such as those mentioned above occur, one of the two reactions, the direct or the inverse, is favored over the other, so the system reacts in that direction by moving towards a new state of equilibrium.
This is similar to the adjustments a tightrope walker makes. When gravity pulls him from one side, the tightrope walker reacts by moving to the opposite side. We say that his balance has shifted, since he is still in balance (he does not fall), but now he has a different posture.
Next, we will see how Le Châtelier’s principle is used to predict the direction in which a system will react when undergoing different types of stresses or disturbances such as: changes in concentration, volume , pressure and temperature.
Effect of changes in concentration
Suppose we have the following equilibrium reaction:
If we change the concentration of any of the species involved (A, B, C or D) to this reaction, Le Châtelier’s principle predicts that the system will react to counteract this change, either by consuming the excess added or by replacing the quantity eliminated. There can be 4 different situations:
1. Increased concentration of reagents
If we increase the concentration of a reagent, for example, A, according to Le Châtelier’s principle, the system will react to consume the added excess. This means that it will react in the direct direction (from left to right), since in this direction A is consumed.
In this case, it is said that the direct reaction is favored and that the equilibrium shifted towards the products, since in the new equilibrium the concentration of products is greater than that in the original equilibrium.
2. Increased concentration of products
If we increase the concentration of a product, for example C, the system will react in reverse to consume the excess C added (that is, from right to left).
In this case it is said that the reverse reaction is favored and that the equilibrium shifts towards the reactants.
3. Decreased concentration of reagents
Decreasing the concentration has the opposite effect of increasing said concentration. If we remove A from the middle, thus lowering its concentration, the system will react to the left (in reverse) to counteract the change. The equilibrium shifts towards the reactants.
4. Decreased concentration of products
If we decrease the concentration of C or D, the system will react in a forward direction to replace the decreased concentration (that is, from left to right). The equilibrium shifts to the products.
Determine the effect of adding more thiocyanate ions (SCN – ) at the following chemical equilibrium, knowing that the product on the right is intense red and iron (III) is yellow.
Solution: In this case, we are adding thiocyanate which is a reactant, so the equilibrium must be shifted to the right, increasing the concentration of the complex and, therefore, the intensity of the red color in the solution.
Effect of changes in volume and pressure
Changes in volume and pressure do not significantly affect reactions in equilibrium in the liquid or solid state , but they can affect those in the gaseous state . This is because, for gases, concentration is proportional to pressure, as predicted by the ideal gas law.
Increasing the pressure of a gas while keeping the temperature constant is equivalent to decreasing its volume, so the effect of both disturbances will be the same.
If we increase the total pressure to a reaction in equilibrium involving gases (or decrease its volume), the system will react by trying to reduce said pressure again, so the equilibrium will move towards where there are fewer molecules of gaseous particles. .
If the same number of gas molecules are consumed and produced in the reaction, then changes in volume and pressure will not affect equilibrium.
On the other hand, if we only modify the partial pressure of some of the gases, the effect is the same as that of increasing or decreasing the concentration of that species.
Given the decomposition reaction of dinitrogen tetroxide at equilibrium:
Determine the effect of reducing the volume by increasing the pressure.
Solution: If we increase the total pressure of the system or reduce its volume, the equilibrium shifts towards the reactants, since, in that direction, there is a net decrease in gaseous molecules (2 are consumed and 1 is produced), which makes it possible to counteract the increased pressure.
Effect of changes in temperature
The effect of temperature on chemical equilibrium is different from the other factors we have seen so far. In the above cases, a new equilibrium is obtained after the disturbance, but the same equilibrium constant is maintained. However, if the temperature changes, the equilibrium constant will change.
To know how the equilibrium constant changes with temperature, we need to know the sign of the reaction enthalpy:
- If a reaction releases heat, that is, it is exothermic, its enthalpy is negative, and the equilibrium constant decreases with increasing temperature. In these cases, the equilibrium shifts in the opposite direction towards the reactants.
- If a reaction absorbs heat, that is, it is endothermic, its enthalpy is positive and just the opposite happens.
Remembering the effect of temperature is easy if we consider heat as a reactant that occurs in the case of exothermic reactions and is consumed in the case of endothermic ones. Increasing the temperature would be like “adding” heat to the system and cooling them would be equivalent to removing it.
So, if a reaction is exothermic and the temperature is increased, it would be like adding a reaction product, so the equilibrium shifts in the opposite direction, towards the reactants, and if it cools the opposite happens.
On the other hand, if a reaction is endothermic and the temperature is increased, it would be like adding a reagent, so the equilibrium shifts towards the products, while a decrease in temperature has the opposite effect.
The decomposition reaction of N 2 O 4 has a reaction enthalpy of +58.0 kJ / mol. In what direction will equilibrium shift if the temperature is lowered?
Solution: Since the enthalpy is positive, the reaction is endothermic. For this reason, cooling it will favor the reverse reaction, and the equilibrium will shift towards the reactants, that is, towards the formation of more N 2 O 4 .
Effect of catalysts, inhibitors and inert substances
Catalysts and inhibitors affect the rate at which reactions occur, but they affect both direct and indirect reactions equally. For this reason, they do not affect the steady state.
On the other hand, the addition of an inert substance such as a gas that does not react with any of the reactants or products, will not affect the direct reaction or the reverse in any way, so it will not have any effect on the equilibrium either.
Application of the Le Châtelier Principle
A good understanding of this principle is very useful, since it allows us to manipulate the equilibrium state of a reaction at our convenience.
In those cases in which we are interested in obtaining more product from a reaction to improve performance, we use all possible tools to favor the direct reaction and shift the balance towards the products.
- Add large amounts of reagents, if these are inexpensive.
- Withdraw product as the reaction progresses, in order to keep the system in a constant state of tension, always trying to reach equilibrium by reacting in a direct direction.
- Increase the temperature in the case of endothermic reactions.
- Increase the pressure in those reactions in which more gas molecules are consumed than are produced.
We may also want to minimize the amount of a product that is formed by an unwanted reaction. In these cases we do the opposite.
Example of application of the Le Châtelier Principle
In organic synthesis, the Le Châtelier principle is constantly used to improve reaction yields.
For example, in the dehydration reactions of alcohols, desiccants are added to the medium that absorb the water formed during the reaction. This reduces the concentration of said product, which shifts the balance towards the products.