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
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 chemical equation with the concentrations of beneath each substance
- Step 3: Write the equilibrium constant for this reaction in terms of concentration
- Step 4: Substitute the equilibrium concentrations into the expression
- Step 5: Deduce the correct units for Kc
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
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 Kc
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
Answer
- Step 1: Write the equilibrium constant for the reaction in terms of partial pressures
- Step 2: Substitute the equilibrium concentrations into the expression
= 9.1 x 10-6
- Step 3: Deduce the correct units of Kp
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
- 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
- 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
- Step 5: Substitute the values into the equilibrium expression
= 54.7
- Step 6: Deduce the correct units for Kp
All units cancel out
Therefore, Kp = 54.7
- Other questions related to equilibrium expressions may involve calculating quantities present at equilibrium given appropriate data
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