Henry Le Chatelier (1850 -1936) was a chemist and a mining engineer who spent his time studying flames to prevent mine explosions.

He proposed a Law of Mobile equilibrium or Le Chatelier’s principle

The Principle states that When a system at equilibrium is subjected to a stress, the system will adjust so as to relieve the stress.

Similar to Newton’s third law; for every action force there is equal but opposite reaction force. For equilibrium, whatever change is made to a system at equilibrium, the system responds to reverse that change.

At equilibrium, rate of forward and reverse reaction is the same**Remember K is calculated by the ratio of products over reactants**

What happens if we add more reactants to this equation?

What happens if we add more reactants to this equation?

The equilibrium will be imbalanced because there will be more concentration of A or B.The “STRESS” is applied on to the reactant side.K value would be less than 1.

What happens if we add more reactants to this equation?

To alleviate this stress, the products have to increase to make it back into equilibrium.The products will increase until Kc is reachedSo, the effect would be to increase the concentration of the products

What happens if we add more reactants to this equation?

Therefore, adding reactants will cause ‘stress’ on the reactant side of the equilibrium and the equilibrium responds by increasing the concentration of the products.

What happens if we add more PRODUCTS to this equation?

What happens if we add more PRODUCTS to this equation?

What happens if we add more PRODUCTS to this equation?

The stress will be put on the products side.The K value would now be greater than 1How will the reaction respond?

What happens if we add more PRODUCTS to this equation?

To minimize the stress, the concentration of the reactants will increase. The reactants will increase until K = 1

What happens if we add more PRODUCTS to this equation?

To minimize the stress, the concentration of the reactants will increase using up more products to produce the reactants.

What happens if we remove some PRODUCTS to this equation?

What happens if we take out some PRODUCTS to this equation?

What happens if we take out some PRODUCTS to this equation?

The stress is now placed on the reactants side as there will be too little products formed.. To counteract or alleviate this stress, more products will be produced by having more reactants react.

What happens if we take out some PRODUCTS to this equation?

The concentration of the products increase until Kc is reachedFinally the reaction will reach equilibrium.

Iron thiocyanate equilibrium example

RedPale Yellow

Iron (III) and thiocyanate solution are in equilibrium.

The position of equilibrium can be determined from the colour of the solution.

If there are more FeSCN2+, the solution will be red,If equilibrium lies to the left, it will be pale yellow.

Iron thiocyanate equilibrium example

RedPale Yellow

Iron thiocyanate equilibrium example

RedPale Yellow

What happens when I add more of the Iron (III) or Thiocyanate?

Iron thiocyanate equilibrium example

RedPale Yellow

What happens when I add NaOH to the test tubes?The NaOH will dissociate into Na+ and OH-, the OH- will react with Fe3+ and form a precipitate. Thus, the concentration of Fe3+ will decrease.

Where will this shift the equlibrium?

Towards the reactants side, leaving a yellow colour.

When a system is either increased or decreased, it causes stress on the equilibrium.The system will attempt to relieve that stress by either replacing the lost heat or consume the added heat.There are two types of reactions involving heat;Exothermic and endothermic

Endothermic: A + B + heat C + DExothermic: A + B C + D + heat

If heat is added to an endothermic reaction, this will add stress to the reactant side.To counteract this response, the equilibrium will favor the products

If heat is added to an endothermic reaction, this will add stress to the reactant side.To counteract this response, the equilibrium will favor the products

If heat is taken out from an endothermic reaction, this will add stress to the product side.

To counteract this stress, more reactants would be formed to reach equilibrium.

In exothermic reaction, the heat is given off once the reactants react with each other.

In exothermic reaction, the heat is given off once the reactants react with each other. If heat is added to an exothermic reaction, the rate of the reactants will react faster and more products will form. The stress will be placed on the products.

In exothermic reaction, the heat is given off once the reactants react with each other. If heat is added to an exothermic reaction, the rate of the reactants will react faster and more products will form. The stress will be placed on the products.

In exothermic reaction, the heat is given off once the reactants react with each other. To counteract, the REACTANTS increases to balance the equilibrium. Thus, favoring the reactants.

If heat is taken OUT from an exothermic reaction…From the kinetic theory, the molecules in the reactants move slower, thus, less products will form.

To balance, the concentration of the products will increase, or the forward reaction is favored.

• *Note: temperature is the only factor that can change the equilibrium constant, Kc.*

• Example: The conversion of dinitrogen tetroxide to nitrogen dioxide is reversible and temperature dependent.

• N2O2(g) 2NO2(g)

• OR• N2O2(g) + heat 2NO2(g)

• What happens to the equilibrium if heat is added to the system?

• What happens to the equilibrium if the container holding the system is cooled?

colourless brown

Pressure is caused by the molecules in the gas colliding against the walls of the vessel.

Gases will always go from high pressure to low pressure to seek equilibrium.

If you have fewer molecules, there is lower amounts of pressure compared to a system with more molecules.

1 CO(g) + 2 H2 (g) <---> CH3OH (g)

If the pressure in the reaction was to be raised (or volume decreased), the pressure will be higher in the reactants than products, so the products are favored.

1 C(s) + 2 H2 (g) <---> CH4(g)If the pressure in the reaction was to be raised, there is 2 molecules on

the reactant side, and 1 on the products. The stress is on the reactant side, so the products will be favored.

What would happen to the 1 CaCO3(s) <---> CaO(s) + CO2(g)

equilibrium if the pressure was raised?

There are no molecules that would be affected by pressure change on the left, but just 1 molecule on the right. The reaction will shift to the left.

In summary, for pressure, the reaction will favor the side with the less pressure or the smaller number of molecules.

If the number of molecules of reactants and products are the same, there will not be any effects.

Example questions

• N2(g) + 3 H2(g)   2 NH3(g)a) What is the effect on the equilibrium if the size

of the container is cut in half, but the number of particles and temperature remain unchanged?

b) What is the effect on equilibrium if the reaction chamber is increased in volume, while keeping temperature and total number of particles constant?

•

Applied to the effect of adding a catalyst • Recall: addition of a speeds up a reaction by reducing Ea.

• Since a catalyst lowers the Ea of both the forward and reverse reaction by the same amount, the addition of a catalyst to a system at equilibrium will have no effect.

• However, if a catalyst is added to a system that is not at equilibrium, the system will reach equilibrium quicker since the forward and reverse rates are increased.

1. An increase in concentration on one side of an equation favors or drives the reaction to the opposite side. adding reactants favors productsadding products favors reactants

2. An increase in temperature favors or drives an endothermic reaction forward to products.

3. An increase in temperature drives an exothermic reaction backwards to reactants.

4. An increase in pressure drives a reaction toward the side with fewer molecules (moles) of gas. Increased pressure "forces" the reaction into a smaller volume. The gas volume is smaller with fewer gas molecules.

5. Adding a catalyst does not alter the relative amounts of reactant and product. Forward reaction happens more easily and so does the reverse reaction. Equilibrium is only reached faster.