Write equilibrium expressions in terms of concentration
Construct Kc expressions from balanced equations, using coefficients as powers and omitting pure solids and liquids.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Equilibrium constants apply at a fixed temperature; include only gaseous or aqueous species in the relevant expressions. For acid–base work, distinguish strength from concentration. Written practice is self-checked
Before you begin
- Balance equations, interpret reversible arrows and use concentration in mol dm⁻³.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
In a Kc expression, what does a balanced-equation coefficient become?
What you will learn
- Deduce a Kc expression from a balanced equation
- Use equilibrium concentrations and stoichiometric coefficients
- Omit pure solids and liquids from the expression
Coefficients become powers in Kc
For aA(aq) + bB(aq) ⇌ cC(aq) + dD(aq), Kc = [C]^c[D]^d / ([A]^a[B]^b), using concentrations at equilibrium.
The coefficient in the balanced equation becomes the power of that species' concentration. A coefficient of one is usually not written as an exponent.
Only relevant aqueous and gaseous concentrations appear
Pure solids and pure liquids are omitted from Kc because their concentrations are effectively constant in this expression. Include aqueous species and gases; use equilibrium, not initial, concentrations.
Kc is tied to a particular balanced reaction and temperature. If the equation is reversed, the expression is inverted; if coefficients are scaled, the powers and constant change accordingly.
Pause and explain
Which species are omitted from Kc?
Worked example
Write Kc for N₂(g) + 3H₂(g) ⇌ 2NH₃(g).
Show the worked solution
- Place product concentrations in the numerator and reactants in the denominator.
- Use coefficient 2 as the power of NH₃ and 3 as the power of H₂.
- Use equilibrium concentrations.
Answer Kc = [NH₃]² / ([N₂][H₂]³).
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Write Kc for H₂(g) + I₂(g) ⇌ 2HI(g).
Give me a hint
Use the coefficient of HI as its exponent.
Compare my reasoning
- HI is the product, so it is in the numerator.
- H₂ and I₂ are reactants, so they are in the denominator.
- Square the HI concentration.
Kc = [HI]² / ([H₂][I₂]).
Look for these in your work
- Products and reactants placed correctly
- Coefficient becomes exponent
- Equilibrium concentrations understood
Write Kc for CaCO₃(s) ⇌ CaO(s) + CO₂(g).
Give me a hint
Omit the two pure solids.
Compare my reasoning
- CaCO₃ and CaO are pure solids.
- Only CO₂ remains in the expression.
- Use its equilibrium concentration.
Kc = [CO₂].
Look for these in your work
- Solids omitted
- Gas included
- Expression simplified
For 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), a student writes Kc = [SO₃]/([SO₂][O₂]). Correct the expression and explain the errors.
Give me a hint
Use products over reactants and preserve each coefficient as a power.
Compare my reasoning
- SO₃ coefficient 2 gives [SO₃]².
- SO₂ coefficient 2 gives [SO₂]².
- O₂ has coefficient one; keep it to the first power.
Kc = [SO₃]² / ([SO₂]²[O₂]). The student omitted coefficient powers for SO₃ and SO₂.
Look for these in your work
- All coefficients applied
- Numerator and denominator correct
- Errors explained
Common mistakes
- Using coefficients as multipliers instead of powers
- Putting products in the denominator
- Including pure solids in Kc
- Using starting concentrations
What can you explain now?
In a Kc expression, what does a balanced-equation coefficient become?
Compare with the explanation
A power on the concentration
Stoichiometric coefficients become exponents.
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Make it stick
How do the balanced equation and physical states determine each term in Kc?
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