CIE AS Chemistry (9701) exams from 2022

Revision Notes

1.7.4 Equilibrium Constant Calculations

Equilibrium Constant: Calculations

Calculations involving Kc

  • In the equilibrium expression each figure within a square bracket represents the concentration in mol dm-3
  • The units of Kc therefore depend on the form of 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

 

Equilibria Equation for concentration, downloadable AS & A Level Chemistry revision notes

Equation to calculate concentration from number of moles and volume

Worked example: Calculating Kc of ethanoic acid

Equilibria Worked example - Calculating Kc of ethanoic acid, downloadable AS & A Level Chemistry revision notes

Answer

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

Equilibrium Constant Calculations WE Step 1 equation 1, downloadable AS & A Level Chemistry revision notes

  • Step 2: Write out the balanced chemical equation with the concentrations of beneath each substance

Equilibrium Constant Calculations WE Step 1 equation 2

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

Equilibrium Constant Calculations WE Step 1 equation 3

  • Step 4: Substitute the equilibrium concentrations into the expression

Equilibrium Constant Calculations WE Step 1 equation 4

  • Step 5: Deduce the correct units for Kc

Equilibrium Constant Calculations WE Step 1 equation 5

All units cancel out

Therefore, Kc = 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 products
  • An initial, change and equilibrium table should be used to determine the equilibrium concentration of the products using the molar ratio of reactants and products in the stoichiometric equation

Worked example: Calculating Kc of ethyl ethanoate

Equilibria Worked example - Calculating Kc of ethyl ethanoate, downloadable AS & A Level Chemistry revision notes

Answer

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

Equilibria Calculating Kc of ethyl ethanoate table

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

Equilibrium Constant Calculations WE Step 2 equation

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

Equilibrium Constant Calculations WE Step 3 equation

  • Step 4: Substitute the equilibrium concentrations into the expression

Equilibrium Constant Calculations WE Step 4 equation

= 0.28

  • Step 5: Deduce the correct units for Kc

Equilibrium Constant Calculations WE Step 5 equation

All units cancel out

Therefore, Kc = 0.288

Calculations involving Kp

  • In the equilibrium expression the p represent the partial pressure of the reactants and products in Pa
  • The units of Kp therefore depend on the form of the equilibrium expression

Worked example: Calculating Kp of a gaseous reaction

Equilibria Worked example - Calculating Kp of a gaseous reaction, downloadable AS & A Level Chemistry revision notes

Answer

  • Step 1: Write the equilibrium constant for the reaction in terms of partial pressures

Equilibrium Constant Calculations WE 3 Step 1 equation

  • Step 2: Substitute the equilibrium concentrations into the expression

Equilibrium Constant Calculations WE 3 Step 2 equation

= 9.1 x 10-6

  • Step 3: Deduce the correct units of Kp

Equilibrium Constant Calculations WE 3 Step 3 equation

The units of Kp are Pa-1

Therefore, Kp = 9.1 x 10-6 Pa-1

  • Some questions only give the number of moles of gases present and the total pressure
  • The number of moles of each gas should be used to first calculate the mole fractions
  • The mole fractions are then used to calculate the partial pressures
  • The values of the partial pressures are then substituted in the equilibrium expression

Worked example: Calculating Kp of hydrogen iodide equilibrium reaction

Equilibria Worked example - Calculating Kp of hydrogen iodide equilibrium reaction, downloadable AS & A Level Chemistry revision notes

  • Step 1: Calculate the total number of moles

Total number of moles = 1.71 x 10-3 + 2.91 x 10-3 + 1.65 x 10-2

= 2.112 x 10-2

  • Step 2: Calculate the mole fraction of each gas

Equilibrium Constant Calculations WE 4 Step 2 equation

  • Step 3: Calculate the partial pressure of each gas

H2 = 0.0810 x 100 = 8.10 kPa

I2 = 0.1378 x 100 = 13.78 kPa

HI = 0.7813 x 100 = 78.13 kPa

  • Step 4: Write the equilibrium constant in terms of partial pressure

Equilibrium Constant Calculations WE 4 Step 4 equation

  • Step 5: Substitute the values into the equilibrium expression

Equilibrium Constant Calculations WE 4 Step 5 equation

= 54.7

  • Step 6: Deduce the correct units for Kp

Equilibrium Constant Calculations WE 4 Step 6 equation

All units cancel out

Therefore, Kp = 54.7

  • Other questions related to equilibrium expressions may involve calculating quantities present at equilibrium given appropriate data

Worked example: Calculating partial pressures

Equilibria Worked example - Calculating partial pressures, downloadable AS & A Level Chemistry revision notes

Answer

There are equal volumes of reactants A and B in a 1:1 molar ratio.
This means their partial pressures will be the same.
B therefore also has an equilibrium partial pressure of 0.5

Total pressure = sum of equilibrium (Σ) partial pressures

Therefore, the sum of all the partial pressures must equal to 3 atm

0.5 + 0.5 + pc = 3 atm

pc = 2 atm

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