CIE AS Chemistry (9701) exams from 2022

Revision Notes

1.7.3 Equilibrium Constant

Equilibrium Constant: Concentrations

Equilibrium expression & constant

  • The equilibrium expression is an expression that links the equilibrium constant, Kc, to the concentrations of reactants and products at equilibrium taking the stoichiometry of the equation into account
  • So, for a given reaction:

aA + bB ⇌ cC + dD

the Kc is defined as follows:

Equilibria Equilibrium Expression, downloadable AS & A Level Chemistry revision notes

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

 

  • Solids are ignored in equilibrium expressions
  • The Kc of a reaction is specific and only changes if the temperature of the reaction changes

Worked example: Deducing equilibrium expressions

Equilibria Worked example - Deducing equilibrium expressions, downloadable AS & A Level Chemistry revision notes

Answer

Equilibrium Constant Concentrations Worked Example equations, downloadable AS & A Level Chemistry revision notes

Mole Fraction & Partial Pressure

Partial pressure

  • For reactions involving mixtures of gases, the equilibrium constant Kp is used as it is easier to measure the pressure than the concentration for gases
  • The partial pressure of a gas is the pressure that the gas would have if it was in the container all by itself
  • The total pressure is the sum of the partial pressure

Equilibria Partial Pressure Equation, downloadable AS & A Level Chemistry revision notes

Equation to calculate the total pressure in a mixture of gases

 

Equilibria Partial Pressures, downloadable AS & A Level Chemistry revision notes

The diagram shows that each gas in the mixture has a partial pressure contributing towards the total pressure

Mole fraction

  • The mole fraction of a gas is the ratio of moles of a particular gas to the total number of moles of gas present

Equilibria Mole Fraction Equation, downloadable AS & A Level Chemistry revision notes

Equation to calculate the mole fraction of a particular gas in a gas mixture

 

  • To calculate the partial pressures of each gas the following relationship can be used:

Equilibria Mole Fraction and Partial Pressure, downloadable AS & A Level Chemistry revision notes

Equation to calculate the partial pressure of a particular gas in a gas mixture

The sum of the mole fractions should add up to 1.00, while the sum of the partial pressures should add up to the total pressure.

Equilibrium Constant: Partial Pressures

Equilibrium expressions involving partial pressures

  • Equilibrium expressions in terms of partial pressures are written similarly to those involving concentrations with a few differences:

 

Equilibria Comparison Equilibrium Expressions, downloadable AS & A Level Chemistry revision notes

The diagram shows a comparison of writing equilibrium expressions in terms of partial pressures and concentrations

Worked example: Deducing equilibrium expressions of gaseous reactions

Equilibria Worked example - Deducing equilibrium expressions of gaseous reactions, downloadable AS & A Level Chemistry revision notes

Answer

Equilibrium Constant Partial Pressures Worked Example equations, downloadable AS & A Level Chemistry revision notes

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