Effect of Changes in Conditions on Equilibrium Composition

Equilibria

What four factors can affect the position of equilibrium?

Concentration, pressure, temperature and the addition of a catalyst.

What happens to an equilibrium mixture if the conditions remain constant?

The concentrations of each component remain constant.

What is meant by the position of equilibrium moving to the right?

More products are formed and the equilibrium mixture contains a greater proportion of products.

What is meant by the position of equilibrium moving to the left?

More reactants are formed and the equilibrium mixture contains a greater proportion of reactants.

What happens to equilibrium if the concentration of a reactant is increased?

The equilibrium position moves to the right, forming more products.

What happens to equilibrium if the concentration of a reactant is decreased?

The equilibrium position moves to the left, forming more reactants.

What happens to equilibrium if the concentration of a product is increased?

The equilibrium position moves to the left, forming more reactants.

What happens to equilibrium if the concentration of a product is decreased?

The equilibrium position moves to the right, forming more products.

Why does increasing the concentration of a reactant move equilibrium to the right?

The rate of the forward reaction initially increases, so more products form until a new equilibrium is established.

What happens after the concentration of a reactant is increased in an equilibrium mixture?

The forward reaction rate increases, more products form, the backward reaction rate then increases, and eventually a new equilibrium is established.

When does changing pressure affect the position of equilibrium?

When the reversible reaction involves gases and there are different total numbers of moles of gas on the two sides of the equation.

How can the pressure of a gaseous equilibrium mixture be increased at constant temperature?

By decreasing the volume of the container.

How can the pressure of a gaseous equilibrium mixture be decreased at constant temperature?

By increasing the volume of the container.

What happens to equilibrium when pressure is increased?

The equilibrium position moves towards the side with fewer moles of gas.

What happens to equilibrium when pressure is decreased?

The equilibrium position moves towards the side with more moles of gas.

What happens if pressure is changed and there are equal numbers of moles of gas on both sides?

The position of equilibrium does not change.

Which substances should be counted when predicting the effect of pressure on equilibrium?

Only gaseous substances.

For N₂(g) + 3H₂(g) ⇌ 2NH₃(g), what happens when pressure is increased?

The equilibrium shifts to the right because there are 4 moles of gas on the left and only 2 on the right.

How does increasing pressure affect the equilibrium yield of ammonia in the Haber process?

It increases the equilibrium yield of ammonia.

How does temperature affect the rates of the forward and backward reactions?

Increasing temperature increases the rates of both reactions.

Which reaction is affected more by an increase in temperature?

The endothermic reaction increases in rate more than the exothermic reaction.

What happens to equilibrium when temperature is increased?

The equilibrium position moves in the endothermic direction.

What happens to equilibrium when temperature is decreased?

The equilibrium position moves in the exothermic direction.

If the forward reaction is exothermic, what happens when temperature increases?

The equilibrium position moves to the left.

If the forward reaction is exothermic, what happens when temperature decreases?

The equilibrium position moves to the right.

If the forward reaction is endothermic, what happens when temperature increases?

The equilibrium position moves to the right.

If the forward reaction is endothermic, what happens when temperature decreases?

The equilibrium position moves to the left.

What does a negative ΔH for the forward reaction mean?

The forward reaction is exothermic.

What does a positive ΔH for the forward reaction mean?

The forward reaction is endothermic.

Why does increasing temperature favour the endothermic direction?

The rate of the endothermic reaction increases more than the rate of the exothermic reaction.

What is an important exam rule when explaining the effect of temperature on equilibrium?

State that increasing temperature increases the rate of the endothermic reaction more than the rate of the exothermic reaction.

What effect does adding a catalyst have on the forward and backward reaction rates?

It increases both reaction rates by the same amount.

What effect does a catalyst have on the position of equilibrium?

None. The equilibrium position is unchanged.

Why does a catalyst not change the position of equilibrium?

It increases the rates of the forward and backward reactions equally.

What is the advantage of adding a catalyst to a reversible reaction?

Equilibrium is reached more quickly.

Does a catalyst increase the equilibrium yield of products?

No. It only reduces the time needed to reach equilibrium.

Can concentration, pressure and temperature always be used to predict equilibrium changes perfectly?

No. Qualitative predictions can have limitations, particularly when more than one condition changes at the same time.

Why can it be difficult to predict an equilibrium shift when both temperature and pressure change?

The two changes may favour equilibrium shifts in opposite directions and it may not be known which effect is greater.

What is Le Chatelier's principle used for?

It can be used to predict possible changes in the position of equilibrium when conditions are altered.

Does Le Chatelier's principle explain why equilibrium changes?

No. It is a useful prediction rule rather than a full explanation.

For 2NO₂(g) ⇌ N₂O₄(g), ΔH = −57.2 kJ mol⁻¹, what does increasing temperature favour?

The backward endothermic reaction, so equilibrium shifts to the left and more NO₂ forms.

For 2NO₂(g) ⇌ N₂O₄(g), what does increasing pressure favour?

The right-hand side because it contains fewer moles of gas.

For H₂(g) + I₂(g) ⇌ 2HI(g), what effect does increasing pressure have?

No effect on the position of equilibrium because there are 2 moles of gas on each side.