MYP Chemistry: Reversible Reactions & Equilibrium — Answered

A reversible reaction is one that can run forwards and backwards under the same conditions — reactants form products, and products can reform reactants. You write it with a double arrow (⇌) instead of a single one. Classic MYP examples: hydrated copper sulfate turning anhydrous on heating, then reforming with water.

Dynamic equilibrium is the point in a reversible reaction where the forward and reverse reactions happen at the same rate, so the overall amounts of reactants and products stop changing. It's not static — both reactions keep going. I tell every class I teach: equilibrium means balanced rates, not a stopped reaction.

Use the reversible arrow ⇌ between reactants and products instead of a normal one-way arrow (→). For example, N2(g) + 3H2(g) ⇌ 2NH3(g) shows nitrogen and hydrogen forming ammonia, while ammonia simultaneously decomposes back into nitrogen and hydrogen — both directions captured in one equation.

Changing concentration, temperature or pressure pushes a system away from equilibrium, and the reaction responds by favouring whichever direction counteracts that change. Add more reactant and the forward reaction speeds up to use it; raise the temperature and the equilibrium shifts toward the endothermic direction. MYP treats this qualitatively.

Start by identifying the command term. If it's 'state' or 'describe', give the fact in one or two lines. If it's 'explain', you must give a reason — why the rates are equal, or why a shift happens — not just describe what occurs. Always refer back to the specific reaction given in the question.

MYP Chemistry equilibrium questions mostly use state, describe, explain, predict and deduce — each demanding a different depth of response. 'State' wants a single fact; 'explain' wants a reason with a mechanism; 'predict' wants a direction of shift justified using particle behaviour or energy changes, not a guess.

Name the change, state the direction the equilibrium shifts, then justify it as the system counteracting that change. Example: 'Increasing temperature shifts the equilibrium toward the endothermic reaction, because the system absorbs the extra heat energy, reducing the disturbance.' That change–direction–reason structure earns full marks consistently.

The mistake I see every year: students write that equilibrium means 'the reaction has stopped.' It hasn't — forward and reverse reactions are still both happening, just at matching rates. The second common error is treating a catalyst as something that shifts equilibrium; it only helps the system reach it faster.

Reversible reactions and equilibrium are usually assessed under Criterion A (Knowing and Understanding) for definitions and explanations, and Criterion C (Processing and Evaluating) when you're interpreting data or graphs from an equilibrium investigation — the cobalt chloride colour-change experiment is a common one. Your school's unit outline confirms exactly which criteria apply.

MYP gives you the qualitative picture: reversible reactions, dynamic equilibrium, and shifting conditions. DP Chemistry formalises this with Le Chatelier's principle and the equilibrium constant, Kc. According to the IB Chemistry guide, first examined in 2025, this sits in Reactivity 1.4, 'Equilibrium,' where you calculate Kc values and construct ICE tables.

The best preparation is short, regular practice on command-term-based questions rather than re-reading notes — ask your child to explain, in their own words, why a reaction reaches equilibrium and what happens if a condition changes. Past unit tests and topical worksheets that mirror MYP assessment criteria beat generic revision guides here.

Look for resources organised by MYP command term and assessment criteria, not just topic summaries — that's what actually maps to how your child gets marked. Dedicated MYP Chemistry revision notes and topical worksheets with graded practice questions let you see exactly where marks are being lost, and fix that before the unit test.

Not conceptually hard, but easy to answer badly. The idea itself is simple — two reactions balancing out — but MYP questions reward specific, reasoned answers, and students lose marks for vague statements like 'it just moves the equilibrium.' Practising the change–direction–reason structure usually fixes this within a couple of sessions.