Edexcel A Level Chemistry:复习笔记5.1.2 Equilbrium 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 
  • [CH3COOH] 0.470 mol dm-3
  • [C2H5OH] 0.070 mol dm-3
  • [CH3COOC2H5] 0.364 mol dm-3
  • [H2O] 0.364 mol dm-3
• 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

Kc =

• Step 4: Substitute the equilibrium concentrations into the expression

Kc

• Step 5: Deduce the correct units for Kc

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

Kc = 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

Kp = 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

Answer

• • 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

Kp = 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

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