Dynamic Equilibrium & Le Chatelier's Principle (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching chemistry, and this video is about dynamic equilibrium and Le Chatelier's principle.

The two are covered together because Le Chatelier's principle describes what happens to a system in dynamic equilibrium when a change is made to it.

A reversible reaction in a closed system settles into a dynamic equilibrium, where the forward and backward reactions run at the same rate and the concentrations of the reactants and products stay constant. Le Chatelier's principle is the rule for what that equilibrium does when a condition is changed: the position of the equilibrium moves to counteract the change.

We start with reversible reactions and what dynamic equilibrium means, then the closed and open systems that decide whether an equilibrium can be reached at all. From there we take the principle itself, and then the conditions it applies to: concentration, pressure and temperature, along with the effect of a catalyst.

Some reactions go to completion: the reactants are used up to form the products, and the reaction stops when all of the reactants are used up. In a reversible reaction the products can react to reform the original reactants, and that is written with two opposing half arrows instead of a single arrow.

In a dynamic equilibrium the forward and backward reactions are still occurring. The rate of the forward reaction is the same as the rate of the backward reaction, so the concentrations of the reactants and products are constant. Once equilibrium has been reached those concentrations do not change, and that holds whether the equilibrium was approached using the reactants or using the products.

This balance between the forward and backward reactions is the system that Le Chatelier's principle acts on.

A closed system is one in which none of the reactants or products escape from the reaction mixture. In an open system, matter and energy can be lost to the surroundings.

When a reaction takes place entirely in solution, equilibrium can be reached in open flasks, because a negligible amount of material is lost through evaporation. If the reaction involves gases, equilibrium can only be reached in a closed system. In a closed system calcium carbonate is in equilibrium with calcium oxide and carbon dioxide, but in an open system the carbon dioxide is lost and the calcium carbonate keeps decomposing until the reaction goes to completion.

Without a closed system there is no equilibrium position for a change in conditions to shift.

The position of the equilibrium refers to the relative amounts of products and reactants in an equilibrium mixture. When the position of equilibrium shifts to the left, the concentration of reactants increases. When it shifts to the right, the concentration of products increases.

Le Chatelier's principle says that if a change is made to a system in dynamic equilibrium, the position of the equilibrium moves to counteract this change. The dynamic equilibrium described so far is exactly the system this principle applies to.

The principle is used to predict changes to the position of equilibrium when there are changes in temperature, pressure or concentration.

Changing the concentration of a reactant or a product shifts the position of the equilibrium. Changes in pressure only affect reactions where the reactants or products are gases. Changing the temperature shifts the position as well.

A catalyst behaves differently. A catalyst is a substance that increases the rate of a chemical reaction, and it increases the rate of the forward and reverse reactions equally, so it only causes a reaction to reach equilibrium faster. It has no effect on the position of the equilibrium once that equilibrium is reached.

A common misconception is to think that the concentrations of the reactants and products are equal at dynamic equilibrium. They are not equal, but they do remain constant, which is to say the concentrations are not changing. The concentrations will change as the reaction progresses, but only until the equilibrium is reached.

At dynamic equilibrium the forward and backward reactions run at the same rate, and the concentrations of the reactants and products stay constant.

That equilibrium needs a closed system, and a reaction involving gases can only reach equilibrium in one.

Le Chatelier's principle predicts which way the position of the equilibrium moves when concentration, pressure or temperature is changed, and a catalyst does not move it.

Whatever change is made, the position of the equilibrium moves to counteract it.

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