6.2 - Reversible Reactions & Dynamic Equilibrium

What is a reversible reaction?

Occurs when the products of a reaction can react backwards to produce the original reactants.

When is dynamic equilibrium reached?

In a closed system, when the forward and reverse reactions occur at the same rate and the concentrations of reactants and products remain constant.

Describe Le Chatelier’s principle

If a system is at equilibrium and a change is made to any of the conditions, then the system responds to counteract change and restore the equilibrium.

Describe the effect of changing the concentration of reactant and product on the position of the equilibrium.

• If the concentration of one of the reactants or products is changed, the system is no longer at equilibrium and the concentrations of all the substances will change until equilibrium is reached again.

• If the concentration of a reactant is increased, more products will be formed until equilibrium is reached again.

• If the concentration of a product is decreased, more reactants will react until equilibrium is reached again.

Describe the effect of changing temperature on the position of the equilibrium.

If the temperature of a system at equilibrium is increased:

• The relative amount of products at equilibrium increases for an endothermic reaction.

• The relative amount of products at equilibrium decreases for an exothermic reaction.

Describe the effect of changing pressure on the position of the equilibrium.

• This applies to equilibria that involve gases.

• An increase in pressure causes the equilibrium position to shift towards the side with the smaller number of molecules as shown by the symbol equation for that reaction.

• A decrease in pressure causes the equilibrium position to shift towards the side with the larger number of molecules as shown by the symbol equation for that reaction.

• Pressure has no effect on the reactions where the numbers of gas molecules are equal on both sides of the equation.

Describe the effect of a catalyst on the position of the equilibrium.

• No effect, it just speeds up both forward and backward reactions equally e.g. equilibrium is achieved faster.