Chapter 8: Unit 7. Le Chatelier’s Principle
Le Châtelier’s Principle
The chemical equilibrium can be disturbed by an external condition which can affect the rate of the forward and the rate of reverse reaction equality and the chemical equilibrium is no longer held. In order to re-establish the chemical equilibrium, the chemical reaction has to shift to side of the reaction which has the least stress from adding this external condition.
This shifting to the least stressed side of the reaction and re-establishing the chemical equilibrium is known as Le Châtelier’s Principle.
A video of You Tube illustrates the Le Châtelier’s Principle as well as the effects of the
shifting the chemical equilibrium by external conditions or stresses.
An animation made by the State University of New York illustrates the Le Châtelier’s
Principle:
The external conditions can be a change in:
A. Changing the concentration of reactants or the products by adding or removing
B. Changing the heat of the reaction by cooling or heating
C. Changing the volume of the reaction system
D. Changing the pressure of the reaction system
E. Effect of Catalysts on Chemical Equilibrium Reactions
Now the external conditions can be closely discussed.
The videos from Khan Academy explain these factors in some details:
https://www.khanacademy.org/science/chemistry/chemical-equilibrium/factors-that-affect-chemical-equilibrium/v/le-chatelier-s-principle
A. Changing the concentration of reactants or the products by adding or removing
Let us consider the chemical equilibrium below:
NH4HS(s)⇔ NH3(g) + H2S(g) Kp = 0.108 at 25oC
Kp = 0.108 which means the equilibrium constant is considering only the gaseous phases. The Kp expression will be written as follows:
Kp = [ NH3(g) ] x [ H2S(g) ]
The solid NH4HS is discarded from the equilibrium constant expression.
Now let us consider the cases of increasing and decreasing the products:
a. Increasing the concentration of NH3(g) or H2S(g) by adding them externally:
In case of increasing one of the products concentration, the right side (the product side of the chemical reaction) will be more stressed. In order to re-establish the chemical equilibrium, the chemical equilibrium will shift to the side that is least stressed, namely; to the left side of the chemical reaction (to the reactant’s side).
Evidence of this shift can be observed by the increase in the amount of the solid NH4HS.
b. Decreasing the concentration of NH3(g) or H2S(g) by removing them from the chemical reaction:
In case of decreasing one of the products concentration by removing them out of the chemical reaction, the right side (the product side of the chemical reaction) will be less stressed and the left side which is the reactant’s side will be more stressed. In order to re-establish the chemical equilibrium, the chemical equilibrium will shift to the side that is least stressed, namely; to the right side of the chemical reaction (to the products side).
Evidence of this shift can be observed by the decrease in the amount of the solid NH4HS.
Let us look at another chemical equilibrium in which the reactant and the product have colors which makes it very easy to follow and observe:
Co(H2O)62+(aq) + 4 Cl-(aq) + Heat⇔ CoCl42-(aq) + 6 H2O(g)
The Co(H2O)62+ complex is pink, and the CoCl42- complex is blue.
Let us consider the cases related to the concentrations of the reactants and the products:
a. Increasing the concentration of Co(H2O)62+(aq) or Cl-(aq) by adding them externally:
Increasing the concentrations of the reactants will put stress on the side of the reactants side and the chemical equilibrium will adjust itself by shifting to the side with the least stress, namely; to the right side of the chemical reaction (to the products side).
Evidence of this shift can be observed by the color change from the pink color to the blue color.
b. Decreasing the concentration of Co(H2O)62+(aq) or Cl-(aq) by removing them from the chemical reaction:
Decreasing the concentrations of the reactants will put stress on the side of the products side and the chemical equilibrium will adjust itself by shifting to the side with the least stress, namely; to the left side of the chemical reaction (to the reactants side).
Evidence of this shift can be observed by the color change from the blue color to the pink color.
c. Increasing the concentration of CoCl42-(aq) or H2O(g) by adding them externally:
Increasing the concentrations of the products will put stress on the side of the products side and the chemical equilibrium will adjust itself by shifting to the side with the least stress, namely; to the left side of the chemical reaction (to the reactants side).
Evidence of this shift can be observed by the color change from the blue color to the pink color.
d. Decreasing the concentration of CoCl42-(aq) or H2O(g) by removing them from the chemical reaction:
Decreasing the concentrations of the products will put stress on the side of the reactants side and the chemical equilibrium will adjust itself by shifting to the side with the least stress, namely; to the right side of the chemical reaction (to the products side).
Evidence of this shift can be observed by the color change from the pink color to the blue color.
B. Changing the heat of the reaction by cooling or heating
The endothermic reaction of the previous example will be considered:
Co(H2O)62+(aq) + 4 Cl-(aq) + Heat ⇔CoCl42-(aq) + 6 H2O(g)
a. Increasing the heat by heating the chemical equilibrium system externally:
In an endothermic reaction, the heat is taken into the reaction system and it appears on the left side of chemical reaction. Thus increasing the heat will put more stress on the left side of the chemical equilibrium (reactants side) and the chemical equilibrium will adjust itself by shifting to the least stressed side which is the right side (products side).
Evidence of this shift can be observed by the color change from the pink color to the blue color.
b. Decreasing the heat by cooling the chemical equilibrium system externally:
If the heat is decreased or removed by cooling the reaction system, then the products side will be more stressed and the chemical equilibrium will adjust itself by shifting to the side that is least stressed. It will shift to the left to side of the reactants.
Evidence of this shift can be observed by the color change from the blue color to the pink color.
A video You tube shows the effect of the temperature on the chemical equilibrium:
Co(H2O)62+(aq) + 4 Cl-(aq) + Heat⇔CoCl42-(aq) + 6 H2O(g)
C. Changing the volume of the reaction system
Co(H2O)62+(aq) + 4 Cl-(aq) + Heat⇔ CoCl42-(aq) + 6 H2O(g)
Increasing the volume of the reaction system of the chemical equilibrium will lead to a decrease in the pressure exercised on the system (Boyle’s Law). In such case, the chemical equilibrium will adjust itself by re-establishing the chemical equilibrium and it will shift to side that has the largest sum of the moles present.
It is very important to note that, one should consider only gaseous and aqueous phases of the reactants and the products. Liquid and solid phases of the reactants and the products are considered and they are discarded form the equilibrium constant expression.
The sum of the number of moles of the reactants is 1 + 4 = 5 moles.
The sum of the number of moles of the products is 1 + 6 = 7 moles.
Increasing the volume of the chemical equilibrium system will cause the chemical equilibrium to shift to the side of the chemical equilibrium with the most number of moles. The shift will be to the products’ side since it has the large sum of number of moles.
Decreasing the volume means increasing the pressure on the chemical equilibrium system. This will cause the chemical equilibrium to shift to the side with least sum of number of moles present. In this chemical equilibrium, decreasing the volume (increasing the pressure) will cause the chemical equilibrium to shift to the reactants’ side (the least sum of number of moles).
Evidence of this shift can be observed by the color change from the blue color to the pink color.
D. Changing the pressure of the reaction system
The effect of changing the pressure of the reaction system is quite the opposite of changing the volume of the reaction system (Boyle’s Law).
Increasing the pressure (decreasing the volume) will cause the chemical equilibrium system to shift to the side with the least sum of number of moles. In the reaction above, the shift will be to reactants side to the left with the least sum of number of moles (to the left).
Decreasing the pressure (increasing the volume) will cause the chemical equilibrium system to shift to the side with the largest sum of number of moles to the products’ side (to the right).
E. Effect of Catalysts on Chemical Equilibrium Reactions
Adding a catalyst to the chemical equilibrium reaction increases the rate of forward and reverse
reactions at the time with the same magnitude.
The chemical equilibrium is reached much faster than without the catalyst. However, the chemical
Equilibrium position does not change.
The rate of forward and reverse reactions at equilibrium are still equal but both rates are larger than the rates of forward and reverse without a catalyst.