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Equilibrium Constants

Chemistry: IB Diploma Higher Level, topic 5.7.2

Equilibrium Constants

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

Equilibrium Constant: Keq

Equilibrium Constant: Keq

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

Math

  • For the reaction aA + bB ⇌\rightleftharpoons cC + dD:
  • Keq=[D]d[C]c[A]a[B]bK_{eq} = \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 Keq so we will have to sub the values in for each concentration, this will give us a value for Keq.
  • Make sure to sub the units of each concentration into this calculation to make sure the Keq value has the correct units.
  • We can also be given the Keq value and may have to rearrange the equation to find a concentration.
States

States

  • When writing the Keq expression, remember that solids and liquids are not incluced.
  • For the reaction aA(s) + bB(aq) ⇌\rightleftharpoons cC(aq) + dD(l):
  • Keq=[C]c[B]bK_{eq} = \frac{[C]^c}{[B]^b}
  • The AP Chemistry exam will try and trick you out with this!!
Equilibrium constant and free energy

Equilibrium constant and free energy

  • We can relate the equilibrium constant for a reaction to its free energy change. The equation that relates the two is:
  • ΔG = -RTln(K)
  • The more negative the ΔG value, the larger the value for the equilibrium constant.
  • The more positive the ΔG value, the smaller the equilibrium constant.

Test yourself on Equilibrium Constants

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

  • When does the backward reaction dominate?

Practise this topic for free