Equilibrium
Dynamic equilibrium occurs in reversible reactions when the rate of the forward reaction equals the rate of the backward reaction in a closed system. Le Chatelier's principle is used to predict how the position of equilibrium shifts in response to changes in concentration, pressure, or temperature. The equilibrium constant, Kc (for concentrations) or Kp (for partial pressures), gives a quantitative measure of the position of equilibrium at a specific temperature.
Full topic guide: the detailed syllabus page with worked examples and common mistakes lives at studyvector.co.uk/a-level/chemistry/physical-chemistry/equilibrium.
Topic preview: Equilibrium
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Topic explanation
Dynamic equilibrium occurs in reversible reactions when the rate of the forward reaction equals the rate of the backward reaction in a closed system. Le Chatelier's principle is used to predict how the position of equilibrium shifts in response to changes in concentration, pressure, or temperature. The equilibrium constant, Kc (for concentrations) or Kp (for partial pressures), gives a quantitative measure of the position of equilibrium at a specific temperature.
Equilibrium is easiest to revise when it is treated as a precise exam behaviour, not a loose note-taking category. In A-Level Chemistry, the goal is to recognise how the topic appears in a question, identify the command word, and decide what evidence, method, or vocabulary earns marks. StudyVector keeps this page tied to AQA · Edexcel · OCR language where coverage is available, then routes practice towards the same topic so revision moves from explanation into retrieval.
A strong revision session starts with a short recall check. Write down the rule, definition, process, or method linked to Equilibrium before looking at any notes. Then answer one exam-style prompt and compare your answer with the mark-scheme logic: did you make a clear point, support it with the right step, and avoid drifting into a nearby topic? This matters because many lost marks come from almost-correct answers that do not match the expected structure.
Use this guide as the first layer: understand the topic, look at the worked examples, complete the mini quiz, then move into full practice. The full StudyVector practice loop is designed to capture whether mistakes are caused by knowledge, method, language, or timing. That distinction is important. If the error is factual, you need reteaching. If the error is method-based, you need a worked retry. If the error is wording, you need command-word calibration. That is how Equilibrium becomes a controlled revision target rather than another page in a folder.
Lost marks → repair task
Why marks are usually lost here
These are the error patterns StudyVector looks for after an attempt. The goal is not a generic explanation; it is one repair move and one follow-up question.
Unit, formula, or method slip
Examiner move: Select the correct method and keep units, substitutions, signs, and rounding visible.
Repair drill: Redo the calculation or method line slowly, naming the formula before substituting values.
Missing chain of reasoning
Examiner move: Show the link between point, method, evidence, and conclusion instead of jumping to the final line.
Repair drill: Write the missing because/therefore step, then retry one isomorphic question.
Command-word miss
Examiner move: Answer the action in the command word before adding extra detail.
Repair drill: 60-second rewrite: start the answer with explain, compare, evaluate, state, or calculate in mind.
Mini quiz
Use these checks before full practice. They test topic recognition, exam technique, and whether you can connect the explanation to a marked response.
1. What should you check first when a Equilibrium question appears in A-Level Chemistry?
- A.The command word and the exact topic focus
- B.The longest paragraph in your notes
- C.A memorised answer from a different topic
2. Which revision action gives the strongest evidence that Equilibrium is improving?
- A.Rereading the explanation twice
- B.Answering a timed exam-style question and reviewing lost marks
- C.Highlighting every key phrase in the topic notes
Sample questions
Topic-specific public question previews are still being reviewed. We keep them off public pages until the topic match is safe.
Exam tips
- Read the command word carefully — "explain" needs reasons; "state" expects a short fact.
- For Equilibrium, show structured working even when you are practising multiple choice — it builds accuracy under time pressure.
- Mark yourself against the mark scheme style: one clear point per mark, in logical order.
- Come back to this topic after a day or two; short spaced reviews beat one long cram.
Worked examples
Example 1
Modelled exam response
For the reaction N2(g) + 3H2(g) <=> 2NH3(g), the initial concentrations are [N2]=1.0 M, [H2]=2.0 M, and [NH3]=0 M. At equilibrium, [N2]=0.8 M. Calculate Kc. Step 1: Use stoichiometry to find equilibrium concentrations. Change in [N2] = 1.0 - 0.8 = 0.2 M. Change in [H2] = 3 * 0.2 = 0.6 M. Change in [NH3] = 2 * 0.2 = 0.4 M. Equilibrium [H2] = 2.0 - 0.6 = 1.4 M. Equilibrium [NH3] = 0.4 M. Step 2: Write the Kc expression: Kc = [NH3]^2 / ([N2][H2]^3). Step 3: Substitute equilibrium concentrations: Kc = (0.4)^2 / (0.8 * (1.4)^3) = 0.16 / (0.8 * 2.744) = 0.073 dm^6 mol^-2.
Example 2
Identify the task before answering
Question type: a Equilibrium prompt asks for a clear response in A-Level Chemistry. Step 1: underline the command word. Step 2: name the exact part of Equilibrium being tested. Step 3: decide whether the mark scheme wants a definition, method, explanation, comparison, or calculation. Why it works: most weak answers fail before the content starts because they answer the topic generally rather than the exact exam task.
Example 3
Turn feedback into a repair task
Suppose your answer shows partial understanding but loses marks for precision. First, rewrite the missing mark as a short target: "I need to state the mechanism, unit, reason, or evidence explicitly." Then answer one similar question without notes. Finally, compare the second attempt with the first and check whether the same mark was recovered. Why it works: Equilibrium improves faster when feedback creates a specific retry, not another passive reading session.
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Common mistakes
- Stating that the concentrations of reactants and products are equal at equilibrium. At equilibrium, it is the rates of the forward and reverse reactions that are equal, not necessarily the concentrations.
- Incorrectly applying Le Chatelier's principle, especially with temperature. An increase in temperature favours the endothermic direction, not necessarily the forward or backward reaction.
- Forgetting that catalysts do not affect the position of equilibrium. Catalysts increase the rate of both the forward and backward reactions equally, so the equilibrium is reached faster, but the yield of products remains unchanged.
Exam board notes
All boards thoroughly test Le Chatelier's principle and Kc calculations. AQA and Edexcel often include Kp calculations, requiring students to be comfortable with partial pressures and mole fractions. OCR may place more emphasis on the industrial applications of equilibrium, such as the Haber process for ammonia or the Contact process for sulfuric acid, and the compromises made between rate, yield, and cost.
FAQs
What is the difference between Kc and Kp?
Kc is the equilibrium constant in terms of molar concentrations of reactants and products. Kp is the equilibrium constant in terms of the partial pressures of gaseous reactants and products. Kp is only used for reactions involving gases.
How does changing pressure affect the equilibrium position?
Changing the pressure only affects equilibria involving gases where there is a different number of moles of gas on each side of the equation. An increase in pressure will shift the equilibrium to the side with fewer moles of gas to counteract the change.
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