Changing the position of equilibrium - Higher

This video looks at reversible reactions and dynamic equilibrium

The equilibrium position of a reversible reaction is a measure of the concentrations of the reactants and products at equilibrium. Using the Haber process as an example:

N2(g) + 3H2(g) ⇌ 2NH3(g)

The equilibrium position is:

  • to the left if the concentrations of N2 and H2 are greater than the concentration of NH23
  • to the right if the concentration of NH3 is greater than the concentrations of N2 and H2

The equilibrium position can be changed by altering the reaction conditions, such as by:

  • changing the pressure
  • changing the concentration
  • changing the temperature

Changing the pressure

In a reaction involving gases, if the pressure is increased, the equilibrium position moves in the direction of the fewest molecules of gas.

There are fewer molecules on the right hand side of the equation for the Haber process:

\begin{array}{rcl} N_{2}(g) + 3H_{2}(g) & \rightleftharpoons & 2NH_{3}(g) \\ 1 + 3 = 4~molecules && 2~molecules \end{array}

If the pressure is increased, the equilibrium position moves to the right.

Question

Calcium carbonate decomposes when it is heated:

CaCO3(s) ⇌ CaO(s) + CO2(g)

Predict the effect of increasing the pressure.

The equilibrium position will move to the left, in the direction of the fewest molecules of gas.

Changing the concentration

In a reaction involving solutions, if the concentration of a solute is increased, the equilibrium position moves in the direction away from this solute. For example, bismuth chloride reacts with water in a reversible reaction:

BiCl3(aq) + H2O(l) ⇌ BiOCl(s) + 2HCl(aq)

The concentration of hydrochloric acid can be increased by adding more hydrochloric acid. When this happens, the equilibrium position moves to the left, away from HCl(aq) in the equation.

Similarly, if the concentration of a solute is decreased, the equilibrium position moves in the direction towards this solute. For example, bromine reacts with water in a reversible reaction:

Br2(l) + H2O(l) ⇌ HOBr(aq) + HBr(aq)

If the concentration of hydrobromic acid (HBr) is decreased, for example by adding some sodium hydroxide solution, the equilibrium position moves to the right, towards HBr(aq) in the equation.

Question

Iron(III) ions react with thiocyanate ions, SCN-, in a reversible reaction:

Fe3+(aq) + SCN-(aq) ⇌ FeSCN2+(aq)

Predict the effect of adding more iron(III) ions.

The equilibrium position will move to the right, in the direction away from Fe3+ in the equation.

Changing the temperature

In a reversible reaction, if the reaction is exothermic in one direction, it is endothermic in the other direction. If the temperature is increased, the equilibrium position moves in the direction of the endothermic process. For example, sulfur dioxide reacts with oxygen in a reversible reaction:

2SO22(g) + O2(g) ⇌ 2SO2(g) (forward reaction is exothermic)

If the forward reaction is exothermic, the backward reaction must be endothermic. Therefore, if the temperature is increased, the equilibrium position moves to the left.

Question

Hydrogen can be manufactured by reacting carbon with steam:

C(s) + H2O(g) ⇌ H2(g) + CO(g) (forward reaction is endothermic)

Predict the effect of increasing the temperature, at constant pressure.

The equilibrium position will move to the right, in the direction of the endothermic reaction.

Summary of condition changes to the equilibrium position

ChangeEquilibrium
Pressure increasedPosition moves towards the fewest molecules of gas
Concentration of a reactant increasedPosition moves away from that reactant
Temperature increasedPosition moves in the direction of the endothermic reaction
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