SPEC 4.6.2.4 – 4.6.2.5 • PAPER 2 • HIGHER TIER ONLY
Le Chatelier's Principle — Concentration
How equilibrium responds when you change the amount of something. The single principle, the four cases, and the rate-vs-position trap that costs students marks every series.
What You'll Learn
HIGHER TIER ONLY
All content in this lesson appears on the Higher Tier paper only (8462/2H). Foundation Tier students can skip it. Spec sub-points 4.6.2.4 (the principle) and 4.6.2.5 (concentration effects).
By the end of this lesson
State Le Chatelier's principle (spec 4.6.2.4), using the keyword "counteract".
Predict the shift direction for all four concentration changes (spec 4.6.2.5).
Write the full three-mark explanation: direction → reason → yield consequence.
Distinguish rate changes from equilibrium-position changes — the most-tested misconception.
Builds on Lesson 6.05
This lesson assumes you can already define dynamic equilibrium and use the ⇌ symbol. If "exactly the same rate" doesn't ring a bell, revisit 6.05 first.
Le Chatelier's Principle
The single rule that predicts what happens whenever you disturb a system at equilibrium. Stated by French chemist Henry Louis Le Chatelier in 1884.
Definition
Le Chatelier's Principle: if a system at equilibrium is subjected to a change in conditions, the position of equilibrium shifts to counteract the change.
What the AQA spec says (4.6.2.4)
"If a system is at equilibrium and a change is made to any of the conditions, then the system responds to counteract the change." The principle can be used to make qualitative predictions about how equilibria respond to changes.
Plain English
The system opposes whatever you do to it. Push it one way → it pushes back the other way. Add more reactant → it consumes some of it. Remove a product → it makes more.
A subtle but important point
The system partially counteracts the change — never fully reverses it. If you add 10 mol of reactant, the equilibrium shifts so that some (not all) of it gets consumed. You still end up with more reactant than before. Mark schemes accept "oppose" or "counteract" — both score the reasoning mark.
The Four Concentration Cases
The spec (4.6.2.5) gives two cases directly: "if [reactant] increases, more products will be formed" and "if [product] decreases, more reactants will react". The other two cases follow from applying Le Chatelier in the same way.
ChangeShiftsWhy (apply Le Chatelier)
Increase [reactant]RIGHT → (toward products)System uses up the extra reactant to oppose the increase
Decrease [reactant]← LEFT (toward reactants)System makes more reactant to oppose the decrease
Increase [product]← LEFT (toward reactants)System uses up the extra product to oppose the increase
Decrease [product]RIGHT → (toward products)System makes more product to oppose the decrease
The pattern
Add → consume.Remove → replace. The shift direction is whichever way uses up the species you added (or makes more of the one you removed). One rule, four applications.
Effect on Yield
"Yield" = how much product the reaction makes at equilibrium. The shift direction tells you immediately whether yield goes up or down.
Shift Right
More product formed → yield increases
Shift Left
Product converts back to reactant → yield decreases
No Shift
Position unchanged → yield unchanged
Industrial application
Continuously removing the product as it forms keeps the equilibrium shifting right (Le Chatelier — the system tries to replace the lost product). This is exactly how the Haber process and the Contact process maximise yield. You'll see this again in lessons 6.07 and 10.04.
The Three-Part Explanation
Every multi-mark Le Chatelier question rewards the same three-part structure. Miss one part and you drop the corresponding mark.
Mark 1 — Direction
State which way equilibrium shifts. "The position of equilibrium shifts to the right."
Mark 2 — Le Chatelier reason
Link the shift back to the principle. "By Le Chatelier's principle, the system opposes the increase in [X] by consuming some of it." Use the word "oppose" or "counteract".
Mark 3 — Yield consequence
Link the shift to the effect on yield. "More [product] is formed, so the yield of [product] increases."
Memorise the structure
DIRECTION → LE CHATELIER REASON → YIELD
PEE Worked Example — Haber Process
Typical 3-mark question · Spec 4.6.2.5 3 MARKS
Consider the Haber Process equilibrium:
N₂(g) + 3H₂(g) ⇌ 2NH₃(g)
Explain the effect of increasing the concentration of N₂ on the equilibrium position and the yield of NH₃.
P
Point — direction: Increasing [N₂] shifts the position of equilibrium to the right (toward NH₃).
E
Explain — Le Chatelier reason: By Le Chatelier's principle, the system opposes the increase in [N₂] by consuming some of it. The forward reaction is favoured, so more N₂ reacts with H₂ to form NH₃.
E
Effect — yield consequence: More NH₃ is formed at the new equilibrium position, so the yield of NH₃ increases.
MK
Mark allocation (typical 3-mark concentration explanation): 1 mk for direction; 1 mk for Le Chatelier reason ("oppose" / "counteract"); 1 mk for yield consequence.
Industrial Application — Continuous Removal
One of the most important applications of Le Chatelier's principle. The spec for 4.6.2.5 highlights this exact scenario: "if the concentration of a product is decreased, more reactants will react until equilibrium is reached again".
The strategy
Continuously remove product as it forms. By Le Chatelier, the equilibrium keeps shifting right to replace the missing product → more reactant gets converted → higher overall yield over time.
Examples in industry
Haber Process — NH₃ is liquefied and removed; the system keeps making more ammonia (covered fully in Lesson 6.07).
Continuous extraction — a product is dissolved or condensed off as it forms.
Distillation — the lower-boiling product evaporates off the reaction mixture.
Exam phrasing
If a question describes "the product is removed as it forms" → invoke Le Chatelier. The equilibrium shifts right to replace the removed product → yield is maximised.
Practice — Concentration Shift MCQ
Concentration-shift MCQ · Spec 4.6.2.4 + 4.6.2.5 1 MARK
An aqueous equilibrium between a pink-coloured complex (A) and a blue-coloured complex (B) is set up:
complex A (pink) + Cl⁻ ⇌ complex B (blue) + H₂O
At equilibrium, the mixture appears purple (a mix of both colours).
Which change would make the mixture more blue? Tick one box.
▢ Add more chloride ions (Cl⁻)
▢ Add more water
▢ Wait for an hour without disturbing the mixture
A
Answer: "Add more chloride ions" ✓
WHY
Blue = more complex B (the product). To make more B, equilibrium must shift right. By Le Chatelier, increasing [Cl⁻] (a reactant) shifts the system right to consume the extra Cl⁻ — producing more B. The mixture becomes more blue.
TRAP
"Add more water" is wrong: water is a product, so adding it would shift equilibrium left — making the mixture more pink, not more blue.
TRAP
"Wait for an hour" is wrong: the system is already at equilibrium. Time alone doesn't shift it. (From Lesson 6.05 — at equilibrium, both reactions continue at exactly the same rate; net change is zero.)
The Catalyst Trap — Rate vs Position
The single most-tested misconception about equilibrium. Students conflate the speed of reaching equilibrium with the position of equilibrium.
EXAMINER ALERT — Recurring across years
A catalyst speeds up the forward and reverse reactions equally. It changes how fast equilibrium is reached — but the position of equilibrium (and therefore the yield) is unchanged.
FactorAffects rate?Affects equilibrium position?
Catalyst✓ Yes (speeds up both forward & reverse equally)✗ No — position and yield unchanged
Concentration✓ Yes (more frequent collisions)✓ Yes — shifts toward / away from species changed
Temperature✓ Yes✓ Yes — covered in Lesson 6.07
Pressure (gases)✓ Yes✓ Sometimes — covered in Lesson 6.07
The phrase to remember
"a catalyst has no effect on the position of equilibrium"
Practice — Catalyst Effect
Catalyst-effect question · Spec 4.6.2.3 + 4.6.2.4 1 MARK
Methanol can be produced from carbon monoxide and hydrogen at high pressure:
CO(g) + 2H₂(g) ⇌ CH₃OH(g)
State the effect of adding a catalyst on the equilibrium position of this reaction.
A
Answer: "no effect (on equilibrium position)" — or equivalently "the equilibrium position stays the same".
WHY
A catalyst lowers the activation energy of both the forward and the reverse reactions equally. So both rates increase by the same factor — they remain equal at the same equilibrium position. The system reaches equilibrium faster, but at the same place.
TRAP
Don't write "shifts right" or "increases yield" — the question asks about equilibrium position, not rate. This is the trap the examiners are testing.
EXTEND
Even though a catalyst doesn't change yield, industries still use catalysts because they make the process economically viable — equilibrium is reached fast enough to recover product within useful timescales.
Examiner Alert — "Shifts Right" Without Reason
RECURRING EXAMINER COMPLAINT
Students state the direction of the shift correctly but skip the Le Chatelier reasoning. On 2- and 3-mark questions, that drops one or two marks every time.
✗ Loses Marks
"Equilibrium shifts to the right, so more product is made."
Direction stated, yield consequence stated — but no Le Chatelier link. On a 3-mark question, this scores 2/3 at best.
✓ Full Marks
"Equilibrium shifts to the right because, by Le Chatelier's principle, the system opposes the increase in [reactant] by consuming some of it. More product is made, so yield increases."
Direction + reason + consequence. All three marks secured.
Trigger word for the second mark
Use "oppose" or "counteract" in your answer. These are the spec/MS keywords that signal Le Chatelier reasoning. "The system opposes the increase in [X] by..."
Examiner Alert — Rate ≠ Yield
The most-tested misconception across the whole equilibrium topic. Students assume that anything which speeds up the reaction must increase the yield. That's wrong.
RATE AND POSITION ARE INDEPENDENT
Rate = how fast equilibrium is reached. Affected by: catalyst, temperature, concentration, surface area.
Equilibrium position = where the system settles (and therefore the yield). Affected by: concentration, temperature, pressure (for gases). NOT affected by catalyst.
✗ Wrong
"Adding a catalyst increases the rate, so it increases the yield."
Conflates rate with position. Catalyst doesn't shift equilibrium.
✓ Right
"A catalyst increases the rate of both forward and reverse reactions equally, so equilibrium is reached faster but the position — and therefore the yield — is unchanged."
Two effects clearly distinguished.
Quick test
If a question asks about "yield" or "amount of product at equilibrium", you're being asked about position. Catalysts and surface area don't matter. Only concentration, temperature, and pressure shift position.
Practice — Removing Product
Application question · Spec 4.6.2.5 2 MARKS
An industrial reaction reaches equilibrium according to the following equation:
X(g) + Y(g) ⇌ Z(g)
To improve efficiency, product Z is continuously condensed and removed from the reaction mixture as it forms.
Explain the effect of removing Z on the yield of Z.
M1
Direction + Le Chatelier reason: Removing Z decreases its concentration. By Le Chatelier's principle, the system opposes this decrease by shifting right to make more Z.
M2
Yield consequence: Continuous removal keeps shifting the equilibrium right, so more X and Y keep converting to Z. The yield of Z is increased over time.
TIP
Same logic as the Haber Process (Lesson 6.07). Whenever a question describes "the product is removed/condensed/extracted as it forms", invoke Le Chatelier with shift-right reasoning.
Common Errors — At-A-Glance
Run your answer past this table before moving on. If anything you wrote matches the left-hand column, fix it before submitting.
What students write ✗What earns marks ✓Why it matters
"Equilibrium shifts right" (no reason given)"Shifts right to oppose the increase in [X]"The Le Chatelier link is the second mark
"Catalyst increases yield""Catalyst increases rate; equilibrium position and yield unchanged"Rate ≠ position; recurring trap on every series
"Equilibrium fully reverses the change""Equilibrium counteracts the change (partially, not fully)"System never restores original conditions; only opposes the change
Naming the shift but not the yield effectAlways link: shift → "more/less product → yield ↑/↓"Yield consequence is the third mark
"Adding water shifts right" (when water IS a product)Check if the species is reactant or product first; then apply the ruleShift direction depends on which side the species is on
Three Things to Remember
Three habits that protect Le Chatelier marks across every concentration question.
1. Add → consume; Remove → replace
The shift direction is whichever way uses up the species you added (or makes more of the one you removed). One rule, four cases. Don't memorise four cases — apply one principle.
2. Three-part explanation: direction, reason, yield
Every multi-mark answer needs all three. Use "oppose" or "counteract" to trigger the Le Chatelier mark. Without the reason, you cap at 2/3.
3. Catalyst affects rate, NOT position
The single highest-frequency trap. If a question asks about yield or position and mentions a catalyst, the answer is "no effect". Catalysts get you there faster — they don't change where you end up.
Summary
Spec 4.6.2.4 — Le Chatelier's Principle
The spec wording: "If a system is at equilibrium and a change is made to any of the conditions, then the system responds to counteract the change ."
The system partially counteracts the change — it never fully reverses it
Trigger words for marks: " oppose " or " counteract "
Spec 4.6.2.5 — Concentration Effects
Increase [reactant] → shifts RIGHT → yield increases
Decrease [reactant] → shifts LEFT → yield decreases
Increase [product] → shifts LEFT → yield decreases
Decrease [product] → shifts RIGHT → yield increases (basis for industrial removal)
Rate vs Position — The Big Trap
Catalyst: ✓ rate ✗ position (no effect on yield)
Concentration: ✓ rate ✓ position (full Le Chatelier shifts)
Temperature, pressure: also affect both — but those are Lesson 6.07
The 3-mark template
DIRECTION → "oppose" → YIELD ↑/↓
Coming Next — Lesson 6.07 [HT only]
Temperature and pressure effects on equilibrium, plus the Haber Process compromise conditions. You'll need everything from this lesson plus the new factors. Spec 4.6.2.6 and 4.6.2.7.
Next: What You'll Learn