Write equilibrium expressions in terms of partial pressure
Construct Kp expressions from gaseous reaction equations and use each gas's equilibrium partial pressure.
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
Which quantity is used in Kp?
What you will learn
- Deduce a Kp expression for a gaseous equilibrium
- Use partial pressures and stoichiometric coefficients
- Distinguish Kp calculations from a Kp–Kc conversion
Build Kp using gaseous species only
For aA(g) + bB(g) ⇌ cC(g) + dD(g), Kp = (pC)^c(pD)^d / ((pA)^a(pB)^b), using equilibrium partial pressures.
Use the same products-over-reactants structure and coefficient powers as Kc, but the quantities are partial pressures rather than concentrations. Pure solids and liquids do not appear.
Use the supplied pressure data directly
If partial pressures are not given, first calculate them from mole fractions and total pressure. Keep units consistent within the expression and follow the notation/data conventions supplied by the examination question.
This syllabus does not require the relationship between Kp and Kc. Do not use an unstated conversion formula; calculate Kp from partial pressures when requested.
Pause and explain
What power is used for a gas with coefficient 3 in the equation?
Worked example
Write Kp for N₂(g) + 3H₂(g) ⇌ 2NH₃(g).
Show the worked solution
- Use product partial pressure in the numerator.
- Square p(NH₃) and cube p(H₂).
- Use equilibrium partial pressures.
Answer Kp = p(NH₃)² / (p(N₂)p(H₂)³).
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Test this topic →From guided practice to a new situation
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 Kp for 2SO₂(g) + O₂(g) ⇌ 2SO₃(g).
Give me a hint
Place product partial pressures above reactants and apply powers.
Compare my reasoning
- SO₃ coefficient is two.
- SO₂ coefficient is two; O₂ coefficient is one.
- Use equilibrium partial pressures.
Kp = p(SO₃)² / (p(SO₂)²p(O₂)).
Look for these in your work
- Species placed correctly
- Powers correct
- Partial pressures used
For H₂(g) + I₂(g) ⇌ 2HI(g), equilibrium partial pressures are 0.20, 0.10 and 0.60 atm. Calculate Kp using the expression.
Give me a hint
Use p(HI)²/(p(H₂)p(I₂)).
Compare my reasoning
- Numerator = 0.60²=0.36.
- Denominator = 0.20×0.10=0.020.
- Divide numerator by denominator.
Kp = 18.
Look for these in your work
- Formula applied
- Partial pressures matched
- Calculation correct
A student substitutes mol dm⁻³ concentrations into Kp. Explain the correction and the limit on using Kc.
Give me a hint
Keep the concentration expression and partial-pressure expression distinct.
Compare my reasoning
- Concentrations belong in Kc.
- Kp uses equilibrium gas partial pressures.
- Calculate from partial pressures; the Kp–Kc relationship is outside this syllabus requirement.
Use partial pressures for Kp and concentrations for Kc. This syllabus does not require converting between Kp and Kc.
Look for these in your work
- Quantities distinguished
- Correct Kp basis stated
- Syllabus boundary respected
Common mistakes
- Using concentrations in Kp
- Forgetting powers from equation coefficients
- Including pure solids or liquids
- Using a Kp–Kc relation not required by this syllabus
What can you explain now?
Which quantity is used in Kp?
Compare with the explanation
Equilibrium partial pressures of gases
Kp is expressed using gas partial pressures.
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Make it stick
What is the one key difference between the forms of Kc and Kp?
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