IB Chemistry: Le Chatelier's Principle FAQ

You need to predict, with a stated reason, which way an equilibrium shifts when concentration, pressure, volume or temperature changes — and say whether Kc itself changes. At HL, that shift also feeds into ICE tables and Kc calculations. Examiners mark down answers that say "it shifts" without naming a direction and a cause.

If a system at dynamic equilibrium is disturbed, it shifts in whatever direction partly cancels out that disturbance, settling into a new equilibrium. It's a qualitative rule of thumb, not a formula — it tells you direction, never magnitude — and it only applies once a system has genuinely reached equilibrium, not while a reaction is still running to completion.

Yes — all three are examinable disturbances, along with volume changes in gaseous systems. Concentration, pressure and volume changes shift the position of equilibrium but leave Kc unchanged at constant temperature. Only a temperature change actually alters the value of Kc itself, because Kc is temperature-dependent — a distinction Paper 2 tests directly.

Both SL and HL students study Le Chatelier's principle — the qualitative reasoning is shared across the whole course. What's HL-only is the quantitative side: writing Kc expressions, building ICE tables and calculating equilibrium concentrations. According to the current DP Chemistry guide (first assessed 2025), that quantitative treatment sits in the HL-only additional content of Reactivity 2.3.

The biggest one I see marking scripts: writing that a change "speeds up the reaction" as the reason for a shift — but Le Chatelier's principle is about position, not rate, and rate isn't even part of the reasoning. A close second is forgetting to state whether Kc changes when temperature is the disturbance.

Run through four steps every time: (1) name exactly what's been disturbed, (2) state the direction of shift, (3) give the mechanistic reason — moles of gas, or exothermic/endothermic direction, and (4) say what happens to Kc. Step 4 is the one students forget most often, and it's usually worth its own mark.

No — a catalyst speeds up the forward and reverse reactions equally, so equilibrium is reached faster, but its position and Kc stay exactly the same. Students who confuse "faster to equilibrium" with "more product at equilibrium" lose easy marks here — a catalyst changes the route, never the destination.

Le Chatelier's principle predicts direction; Kc and Kp calculations give you the actual numbers behind that prediction. At HL, you're expected to calculate new equilibrium concentrations after a disturbance using an ICE table and confirm the system genuinely moved the way Le Chatelier's principle said it would — the qualitative and quantitative answers have to agree.

It sits within Reactivity 2.3, part of the Reactivity 2 theme ("How much, how fast and how far?") in the current DP Chemistry guide, first examined 2025. SL students cover the qualitative principle; HL students add the quantitative Kc and Kp treatment within that same sub-topic.

Both. Paper 1 (multiple-choice, including data-based items) tests quick recognition of shift direction, while Paper 2 (extended response) wants the full reasoned explanation, plus Kc consequences at HL. Since the 2025 DP Sciences update dropped the separate Paper 3, equilibrium-based practical and data questions now sit inside Paper 2.

Yes, it's a genuinely popular IA choice. Investigating how temperature or concentration shifts a coloured equilibrium — cobalt(II) chloride, iron(III) thiocyanate, or the dichromate–chromate system — using colorimetry gives clean, quantifiable data that's easy to graph and analyse.

Le Chatelier's principle is a qualitative shortcut telling you which way equilibrium moves. Kc is the actual quantitative ratio of products to reactants at equilibrium, and its value only changes with temperature. Students often blur the two — shifting the position via concentration or pressure changes does not mean Kc has changed at all.

Compared with Kc calculations, it's one of the more accessible parts of equilibrium — no maths, just logical reasoning about direction. Where students actually lose marks isn't the concept, it's precision: naming the real disturbance, giving a mechanistic reason, and stating the Kc consequence. Working through past-paper wording fixes this within a couple of sessions.

It's a small but recurring topic — expect it across most SL and HL sittings, often linked to energetics, kinetics or acid-base equilibria questions. It rarely carries more than a few marks alone, but sloppy reasoning here is one of the more common, entirely avoidable reasons an otherwise strong student drops a grade boundary.