Ask a GCSE or A-Level student to state Le Chatelier’s principle and most can recite it fluently: a system at equilibrium responds to a change by shifting to partially counteract it. Ask the same student a slightly reworded exam question about the same reaction, and the mark often disappears anyway. The principle isn’t the problem — a specific misconception about what “shifting” actually means usually is.
The misconception: thinking equilibrium shifts mean the reaction stops and restarts
A lot of students picture equilibrium as a reaction that was running forward, then stopped, then started running backward instead when conditions changed. That mental model makes several exam questions almost impossible to answer correctly, because in reality both the forward and reverse reactions are happening constantly, in both directions, the whole time — equilibrium shifting just means their relative rates change temporarily until a new balance is reached.
Where this shows up in exam marks
This misconception is invisible right up until a question asks something like “explain, in terms of rate, why increasing pressure increases the yield of ammonia in the Haber process.” A student who pictures equilibrium as “stop one direction, start the other” will describe the wrong mechanism entirely, even while correctly stating that the yield increases. The examiner is marking the reasoning, not just the final direction of shift — which is exactly where the mark is lost.
Equilibrium never stops and restarts in the other direction. Both reactions run constantly, in both directions, for as long as the system exists — “shifting” just means their relative rates change.
Why this misconception survives revision
Textbook diagrams showing a single arrow “shifting right” reinforce the wrong mental model, because a static diagram can’t show two things happening simultaneously at different rates. Most self-study from a textbook or video doesn’t catch this, because the student’s answer to “which way does it shift” is correct — the number is right, the reasoning underneath it is wrong, and nothing in a multiple-choice self-test reveals that.
What actually fixes it
The fix is usually a short, specific conversation: being asked to explain why a shift happens, out loud, in terms of rates rather than direction, and being corrected the moment the “stop and restart” language creeps in. That’s a five-minute fix with the right question asked at the right moment — but it has to be asked by someone listening for it, which a video or a textbook can’t do.
If equilibrium-style “explain why” questions are consistently where marks go missing despite knowing the right final answer, that specific gap — reasoning, not recall — is worth flagging directly in a tutoring session rather than assuming more content revision will close it.
Try it yourself
Explain, in terms of rate, why increasing the temperature of an exothermic equilibrium reaction decreases the yield of product.
Show the worked approach
- State what temperature actually changes: it increases the rate of BOTH the forward and reverse reactions - never just one.
- Identify which direction is endothermic - by definition, the reverse of an exothermic reaction is endothermic.
- Reason about which rate increases more: increasing temperature increases the rate of the endothermic (here, reverse) reaction more than the exothermic one.
- Conclude correctly: because the reverse rate increases more, the system shifts left (toward reactants) until a new equilibrium is reached - decreasing product yield, exactly as Le Chatelier's principle predicts, but now explained through genuine rate reasoning rather than just quoting the rule.
What to focus on: Naming which rate (forward or reverse) increases more, and why - this is the exact reasoning step a "state only" answer skips.
Common mistake: Saying the reaction "moves backward" or "reverses" as if the forward reaction switches off - both reactions are still happening simultaneously, just at different relative rates.
Frequently asked questions
Do GCSE students need to understand this, or is it A-Level only?
GCSE Triple Science touches Le Chatelier's principle at a simpler level and can still lose marks to this same misconception, but it's tested much more rigorously at A-Level, where "explain in terms of rate" questions are common and this exact gap shows up most.
What's the actual correct way to explain a pressure shift in the Haber process?
Increasing pressure increases the concentration of all gases, which increases the forward AND reverse reaction rates - but because there are fewer gas molecules on the product side, the forward rate increases more, so the system shifts right until a new, higher-yield equilibrium is reached. Both directions are still happening the whole time.
How do I know if I have this misconception without realising it?
A quick test: can you explain why equilibrium shifts in terms of the forward and reverse rates changing relative to each other, rather than one reaction "starting" or "stopping"? If the second phrasing feels more natural, that's the misconception talking.
Does this affect Kc/Kp calculations too?
Not directly - the arithmetic of a Kc calculation works the same either way. It shows up specifically on "explain why" questions, which is exactly why it can hide undetected through weeks of calculation-only revision.
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