Applying the Equilibrium Law (DP IB Chemistry: HL)

Le Châtelier's principle

• Le Châtelier's principle says that if a change is made to a system at dynamic equilibrium, the position of the equilibrium moves to minimise this change
• The principle can be used to predict changes to the position of equilibrium when there are changes in temperature, pressure or concentration

Explaining Le Châtelier’s Principle

• The equilibrium law can explain and quantify the effect of changes in concentration at a particular temperature
• These explanations are based on the idea that K_c is not affected by a change in concentration
  • Remember that the position of equilibrium is affected by a change in concentration:

Effects of Concentration Table

• K_c for a general reaction such aA + bB ⇌ cC + dD is:

Equilibrium expression linking the equilibrium concentration of reactants and products at equilibrium

• If the concentration of A increases
  • The position of equilibrium shifts to the right as the forward reaction works to remove excess A
  • The concentrations of C and D increase to minimise this change
  • The concentration of B decreases because it is being used up to minimise the change
  • Therefore, the value of K_c remains unchanged

• If the concentration of A decreases
  • The position of equilibrium shifts to the left as the backward reaction works to replace A
  • The concentrations of C and D decrease to minimise this change
  • The concentration of B increases because it is also being produced when C and D react
  • Therefore, the value of K_c remains unchanged

• The Haber Process is represented by the following chemical equation:

N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g)

• An increase in the amount of nitrogen will cause the following:
  • The equilibrium to shift to the right
  • An increase in the amount of ammonia
  • A decrease in the amount of hydrogen
  • K_c will remain unchanged

Graph showing the effects of adding nitrogen on the concentration of reactants and products in the Haber Process

• An increase in the amount of nitrogen causes the rate of the forward reaction to increase
• This means that more ammonia is produced, causing the rate of the backward reaction to increase
• This process of increasing forward and backward reactions continues until a new equilibrium is established
• The rate at this newly established equilibrium will be higher than the original rate

Graph showing the effects of adding nitrogen on the rate of reaction in the Haber Process

• Similar points about concentrations and rates can be made for the addition of hydrogen or the removal of ammonia
  • Regardless, the value of K_c remains unchanged
  • Only changes in temperature affect K_c

Calculations involving K_c

• In the equilibrium expression, each term inside a square bracket represents the concentration of that chemical in mol dm^-3
• Therefore, the units of K_c depend on the equilibrium expression
• Some questions give the number of moles of each of the reactants and products at equilibrium together with the volume of the reaction mixture
• The concentrations of the reactants and products can then be calculated from the number of moles and total volume

Equation to calculate concentration from number of moles and volume

Answer:

Step 1: Calculate the concentrations of the reactants and products:

Step 2: Write out the balanced symbol equation with the concentrations of each chemical underneath:

Step 3: Write out the equilibrium constant for the reaction:

Step 4: Substitute the equilibrium concentrations into the expression and calculate the answer:

Step 5: Deduce the correct units for K_c:

• • All units cancel out
  • Therefore, K_c = 4.03
  • Note that the smallest number of significant figures used in the question is 3, so the final answer should also be given to 3 significant figures

• Some questions give the initial and equilibrium concentrations of the reactants but not the products
• An initial, change and equilibrium (ICE) table should be used to determine the equilibrium concentration of the products using the molar ratio of reactants and products in the stoichiometric equation

Answer:

Step 1: Write out the balanced chemical equation with the concentrations of beneath each substance using an initial, change and equilibrium table:

Step 2: Calculate the concentrations of the reactants and products:

Step 3: Write the equilibrium constant for this reaction in terms of concentration:

Step 4: Substitute the equilibrium concentrations into the expression:

= 0.28

Step 5: Deduce the correct units for K_c:

• • All units cancel out
  • Therefore, K_c = 0.28