What is a reversible reaction?
Products can react to remake reactants — both directions happen at once.
Most reactions you see in school are 'one-way' — burning, neutralisation, rusting. But MANY reactions in industry and biology are reversible:
Launching your learning experience…
Detailed notes on Chemical equilibrium for IB MYP Science, covering key concepts, explanations, examples, and exam-focused revision points.
The things students keep getting wrong in this sub-topic, answered.
Many reactions can go BOTH ways. This MYP Sciences note covers reversible reactions, the ⇌ symbol, what 'dynamic equilibrium' really means, and how Le Chatelier's principle lets you predict which side will be favoured when conditions change.
Mapped to the IB MYP Sciences subject guide (2026 onwards).
Products can react to remake reactants — both directions happen at once.
Most reactions you see in school are 'one-way' — burning, neutralisation, rusting. But MANY reactions in industry and biology are reversible:
Forward and backward rates equalise; concentrations stop changing.
As a reversible reaction proceeds in a sealed container:
This is dynamic equilibrium — 'dynamic' because the reactions don't stop (particles keep reacting both ways) but the bulk concentrations remain constant.
Two requirements:
When you disturb a system at equilibrium, it shifts to UNDO the change.
Le Chatelier's principle: if a change is made to a system at equilibrium, the equilibrium position shifts to OPPOSE that change.
Concentration. Add more of a reactant → forward shift (toward products). Remove product → also forward shift (system makes more).
Temperature. Heating shifts toward the ENDOTHERMIC direction (absorbs the extra heat). Cooling shifts toward EXOTHERMIC.
Pressure (gases only). Increasing pressure shifts toward the side with FEWER gas molecules (smaller volume).
Catalyst. Speeds up forward AND backward equally. Equilibrium is reached FASTER but at the SAME position — no shift.
Worked example: Haber process (N₂ + 3H₂ ⇌ 2NH₃, exothermic):
Verbatim phrases and definitions MYP criterion-A markschemes credit.
Criterion A asks for Le Chatelier predictions and definitions. Criterion D often discusses the Haber process — yield/rate trade-off and economic decisions.
Sources: IB MYP Sciences guide (IBO, official subject guide). Last reviewed 2026-05-25.
Step-by-step solutions to past-paper-style questions on reversible reaction and equilibrium, written exactly the way a tutor would explain them at the board.
Question
For N₂ + 3H₂ ⇌ 2NH₃, predict the shift if more N₂ is added.
Step-by-step solution
Step 1
Adding N₂ INCREASES its concentration — a disturbance.
Step 2
Le Chatelier: the system shifts to OPPOSE the change — i.e. reduce the extra N₂.
Step 3
Forward reaction is favoured: more N₂ + H₂ → NH₃. Equilibrium shifts to the RIGHT.
Answer
Equilibrium shifts to the right (more NH₃ formed).
Question
The forward reaction N₂ + 3H₂ ⇌ 2NH₃ is EXOTHERMIC. Predict the effect on equilibrium of (a) increasing temperature, (b) decreasing temperature.
Step-by-step solution
Step 1
Forward = exothermic (releases heat). Backward = endothermic (absorbs heat).
Step 2
(a) Increase T: shift in endothermic direction (backward) to absorb the extra heat. Less NH₃ formed.
Step 3
(b) Decrease T: shift exothermic (forward). More NH₃ formed.
Answer
(a) Backward shift, less NH₃. (b) Forward shift, more NH₃.
Question
Lower temperature gives more ammonia in the Haber process. Why does industry use ~450 °C instead of much lower?
Step-by-step solution
Step 1
Lower T gives a better equilibrium yield (more NH₃) — but slows the reaction down enormously.
Step 2
At very low T, the reaction takes too long to be economically useful, even with a catalyst.
Step 3
450 °C is a COMPROMISE: reasonable yield AND fast enough rate. Engineers choose the temperature that gives the best balance.
Answer
It's a compromise — low T gives more yield but the reaction is too slow. 450 °C balances yield against rate.
Definitions to memorise and the exact keywords mark schemes credit for reversible reaction and equilibrium answers — sharpened from recent examiner reports for the 2026 IB MYP Sciences sitting.
A reaction that can proceed in both forward and backward directions; shown by ⇌.
A state where the forward and backward reactions proceed at equal rates so concentrations remain constant.
If a system at equilibrium is disturbed, the equilibrium shifts to oppose the change.
Industrial process making ammonia: N₂ + 3H₂ ⇌ 2NH₃, typically at ~450 °C, ~200 atm, with iron catalyst.
The traps other students keep falling into on reversible reaction and equilibrium questions — taken from recent IB MYP Sciences examiner reports and mark schemes — and how to avoid them.
Why it happens
Concentrations are constant.
How to avoid it
Equilibrium is DYNAMIC — both forward and backward reactions continue. Their rates are equal so there's no NET change, but individual particles keep reacting.
Why it happens
Catalysts speed reactions up.
How to avoid it
A catalyst speeds up forward AND backward equally. Equilibrium is reached FASTER but the position doesn't change.
Why it happens
Equal RATES sound like equal AMOUNTS.
How to avoid it
Equilibrium means equal RATES — not equal concentrations. The actual position depends on the reaction.
All rights reserved
©2026 Tutopiya
All resources on this platform are independently created by Tutopiya and have no endorsement from the International Baccalaureate Organization.
The ⇌ arrow means the reaction can go BOTH ways. Hydrogen and nitrogen react to form ammonia, but ammonia ALSO breaks apart into hydrogen and nitrogen. Both happen at once, in the same flask, with the same particles.
Why this matters: in a sealed container, neither side ever 'wins'. Both processes continue side by side. Eventually the system reaches equilibrium: forward rate = backward rate.