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Equilibrium Constants

Chemistry: AQA A Level, topic 1.6.2

Equilibrium Constants

It's great to know all about equilibria, but it's even better to be able to do some maths with it.

Equilibrium constant: Kc

Equilibrium constant: Kc

  • The equilibrium constant is for a particular reaction at a particular temperature.
  • An equilibrium constant greater than one indicates that the forward reaction dominates.
  • An equilibrium constant less than one indicates that the backward reaction dominates.
Maths

Maths

  • For the reaction aA + bB ⇌\rightleftharpoons cC + dD:
  • Kc=[D]d[C]c[A]a[B]bK_c = \frac{[D]^d[C]^c}{[A]^a[B]^b}
  • The square brackets indicate concentrations.
  • The indices are the stoichiometries of the components.
    • These are the numbers before each species in the reaction equation - represented above by lower case letters.
Calculations

Calculations

  • We may be given the value of Kc so we will have to sub the values in for each concentration, this will give us a value for Kc.
  • Make sure to sub the units of each concentration into this calculation to make sure the Kc value has the correct units.
  • We can also be given the Kc value and may have to rearrange the equation to find a concentration.

Test yourself on Equilibrium Constants

  • If Kc is greater than one, what does it indicate?

  • When does the backward reaction dominate?

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