Chemical Equilibria
Dynamic equilibrium, Kc, Kp and composition calculations.
Before you begin
This topic combines explanation questions on Le Chatelier’s principle with calculations using Kc, Kp and equilibrium composition. The lessons build the dynamic model, write K expressions, calculate composition, then apply all three to the Haber process.
Be comfortable with:
- Stoichiometry (A Level): n = cV and unit discipline.
- Dalton’s Law and Partial Pressures: partial pressures for Kₚ.
- Enthalpy Changes and Energy Profiles: exothermic and endothermic language.
Learning goals
- Dynamic Equilibrium and Le Chatelier
- Equilibrium Constants (Kc and Kp)
- Equilibrium Composition Calculations
- Haber Process (Case Study)
Syllabus statements covered
- explain, in terms of rates of the forward and reverse reactions, what is meant by a reversible reaction and dynamic equilibrium
- state Le Chatelier’s Principle and apply it to deduce qualitatively (from appropriate information) the effects of changes in concentration, pressure or temperature, on a system at equilibrium
- deduce whether changes in concentration, pressure or temperature or the presence of a catalyst affect the value of the equilibrium constant for a reaction
- deduce expressions for equilibrium constants in terms of concentrations, Kc, and partial pressures, Kp [treatment of the relationship between Kp and Kc is not required]
- calculate the values of equilibrium constants in terms of concentrations or partial pressures from appropriate data
- calculate the quantities present at equilibrium, given appropriate data (such calculations will not require the solving of quadratic equations)
- describe and explain the conditions used in the Haber process, as an example of the importance of an understanding of chemical equilibrium in the chemical industry
Lessons
Work through them in order.
- Dynamic Equilibrium and Le Chatelier’s PrinciplePredict shifts caused by concentration, pressure and temperature changes.
- Equilibrium Constants Kc and KpWrite Kc and Kp expressions and interpret their magnitude.
- Equilibrium Composition CalculationsUse ICE tables, approximations and sanity checks.
- Haber Process Case StudyWeigh yield against rate and cost to explain the conditions used in the Haber process.
Practise and check
Recommended nextChemical Equilibria topic check
Or choose
Topic reference
Quick Reference
| Question type | Do this first | What to show |
|---|---|---|
| “predict the shift” | identify the disturbance | Le Chatelier + name the species affected |
| “does K change?” | check if temperature changed | “only temperature changes K” |
| “write K_c / Kₚ” | write balanced equation | correct powers; exclude solids/liquids |
| “find equilibrium amounts” | set an ICE table | substitute into K and solve for x |
| “pressure change” | compare total moles of gas each side | pressure affects equilibrium only if gas moles differ (ignore solids/liquids) |
| “temperature change” | identify exo/endo direction | increasing T favours the endothermic direction (and changes K) |
| “catalyst effect” | separate rate vs position | catalyst changes rate to reach equilibrium; does not change position or K |
| “ICE sanity check” | check constraints | all equilibrium amounts ≥ 0; justify/check any approximation (for example, x≪ the initial amount) |
What You Must Memorise
- Dynamic equilibrium: forward rate = reverse rate (concentrations constant, not necessarily equal).
- Le Chatelier: equilibrium shifts to oppose a disturbance (disturbance → shift → named species increases/decreases).
- Only temperature changes K (concentration/pressure/catalyst do not change K).
- Writing K_c/Kₚ: products over reactants, powers from coefficients, exclude pure solids/liquids.
- ICE tables: use coefficients (e.g. -2x), and don’t use moles directly in K_c (use concentration/partial pressure).
- Haber trade-off trio: equilibrium yield vs rate vs economics/safety.
Common Exam Traps
- Writing K_c incorrectly (including solids/liquids, wrong powers, wrong species).
- Changing K_c because concentration changes (only temperature changes K).
- Mixing “rate” language with “equilibrium position” language.
- Pressure questions: forgetting to count total moles of gas on each side first.
- ICE tables: forgetting coefficient changes (e.g. -2x for 2) or using moles directly in K_c.
- Kₚ mistakes: using concentration instead of partial pressure (or mixing the two).
- Kₚ unit slips: inconsistent pressure units in the same calculation.
- Vague Le Chatelier: writing “shifts right” with no named species increasing/decreasing.