Exam QuestionsRevision NotesVideo TutorialsPast Papers

Equilibrium Constant Kc

01Equilibrium Constant Kc

Kc expressions

This section covers how to construct Kc expressions from balanced equations using equilibrium concentrations and stoichiometric coefficients.

Constructing the expression

The equilibrium constant KcK_c relates the equilibrium concentrations of the substances in a reversible reaction. A concentration written as [X][\mathrm{X}] means the equilibrium concentration of species X in mol dm−3\mathrm{mol\,dm^{-3}}.

For the general homogeneous equilibrium:

aA+bB⇌cC+dDa\mathrm{A}+b\mathrm{B}\rightleftharpoons c\mathrm{C}+d\mathrm{D}

the corresponding expression is:

Kc=[C]c[D]d[A]a[B]bK_c=\frac{[\mathrm{C}]^c[\mathrm{D}]^d}{[\mathrm{A}]^a[\mathrm{B}]^b}
  • Product concentration terms are placed in the numerator.
  • Reactant concentration terms are placed in the denominator.
  • Each concentration is raised to the power of its stoichiometric coefficient in the balanced equation.
  • The concentrations used must be the values at equilibrium.

Examples

Ammonia equilibrium

NX2(g)+3 HX2(g)⇌2 NHX3(g)\ce{N2(g) + 3H2(g) <=> 2NH3(g)}

The coefficient 2 becomes the power of [NHX3][\ce{NH3}], while the coefficient 3 becomes the power of [HX2][\ce{H2}]:

Kc=[NHX3]2[NX2][HX2]3K_c=\frac{[\ce{NH3}]^2}{[\ce{N2}][\ce{H2}]^3}

Sulfur trioxide equilibrium

2 SOX2(g)+OX2(g)⇌2 SOX3(g)\ce{2SO2(g) + O2(g) <=> 2SO3(g)}

The expression therefore contains squared concentration terms for SOX2\ce{SO2} and SOX3\ce{SO3}:

Kc=[SOX3]2[SOX2]2[OX2]K_c=\frac{[\ce{SO3}]^2}{[\ce{SO2}]^2[\ce{O2}]}

The form of KcK_c is tied to the balanced equation: changing the stoichiometric equation changes the corresponding equilibrium constant expression.

Exam Tip: Write the balanced equation first. Use its coefficients as powers in the KcK_c expression and check that every concentration used is an equilibrium concentration.

Create a free account to continue

Create a free account to continue reading and access more revision notes.