Dynamic Equilibrium

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 3.19 — Dynamic equilibrium in a sealed container (bold C statement — Paper 2 only)
• 3.20 — The characteristics of a reaction at dynamic equilibrium (bold C statement — Paper 2 only)
• 3.21 — Why a catalyst does not affect the position of equilibrium (bold C statement — Paper 2 only)
• 3.22 — The effect of changing temperature or pressure on the position of equilibrium (bold C statement — Paper 2 only)

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

Run a reversible reaction in a SEALED container, so that nothing can escape, and something particular happens. At the start there are only reactants, so the forward reaction is fast and the reverse cannot happen at all. As products build up the reverse reaction speeds up, and as reactants are used up the forward reaction slows down. Eventually the two rates become EQUAL, and at that point the amounts of everything stop changing.

That state is DYNAMIC EQUILIBRIUM, and the word dynamic is the entire point. The reaction has NOT stopped. Both the forward and reverse reactions are still going at full speed — they are simply going at the SAME speed, so every molecule converted one way is matched by one converted back.

Think of it like an escalator with people walking down it

A down escalator with someone walking up it at exactly the escalator's speed produces a person who does not move relative to the ground. From outside, nothing is happening. From the walker's point of view a great deal is happening, continuously and strenuously, and if the escalator slowed they would immediately start rising. That is dynamic equilibrium. The constancy is not stillness but a balance between two active processes — which is exactly why changing the conditions moves it. A finished reaction could not respond to anything; a balanced one responds to everything.

Statement 3.20 — the characteristics of dynamic equilibrium

1. The RATE of the forward reaction EQUALS the rate of the reverse reaction.
2. The CONCENTRATIONS of reactants and products remain CONSTANT — but they are not necessarily equal to one another.
3. Both reactions are STILL HAPPENING; the reaction has not stopped.
4. It can only be reached in a CLOSED SYSTEM, where nothing enters or leaves.
5. The POSITION of equilibrium describes how much product there is relative to reactant — it lies to the right if there is mostly product, to the left if mostly reactant.

The second point is the one most often stated wrongly. Constant does not mean equal. An equilibrium mixture might be 95% products and 5% reactants and be perfectly at equilibrium, because what is constant is each amount over time, not the two amounts as a proportion of each other.

Now statement 3.21, which follows directly from the definition. A catalyst speeds up a reaction by lowering the activation energy — and it lowers it by the same amount in BOTH directions, because it is the same pathway travelled either way. So the forward and reverse reactions are both speeded up equally, and the point at which they become equal is unchanged.

Statement 3.21 — why a catalyst changes nothing at equilibrium

1. A catalyst lowers the activation energy of the FORWARD reaction.
2. It lowers the activation energy of the REVERSE reaction by exactly the same amount, since it is the same alternative pathway.
3. So both rates increase by the same factor.
4. Equilibrium is the point where the two rates are EQUAL, and multiplying both by the same factor leaves them equal at the same point.
5. So a catalyst makes equilibrium be REACHED SOONER and does not change WHERE it lies.
6. This is genuinely useful industrially: the same yield, in less time.

Statement 3.22 is the one that does change the position, and both effects follow from the same reasoning: if you disturb an equilibrium, it shifts in the direction that OPPOSES the change you made.

Statement 3.22 — temperature and pressure

1. RAISING THE TEMPERATURE shifts the equilibrium in the ENDOTHERMIC direction, because that direction absorbs the added heat and so opposes the rise.
2. LOWERING THE TEMPERATURE shifts it in the EXOTHERMIC direction, which releases heat and opposes the fall.
3. RAISING THE PRESSURE shifts the equilibrium towards the side with FEWER GAS MOLECULES, because fewer molecules exert less pressure and so oppose the increase.
4. LOWERING THE PRESSURE shifts it towards the side with MORE gas molecules.
5. Pressure has NO EFFECT if both sides have the same number of gas molecules, or if no gases are involved.
6. So the method is always the same: identify what the change did, and shift towards whichever side undoes it.

Applying both rules to one reaction

For N₂ + 3H₂ ⇌ 2NH₃, the forward reaction is exothermic. Predict the effect on the yield of ammonia of (a) raising the temperature, (b) raising the pressure, (c) adding a catalyst.

1. (a) The forward reaction is exothermic, so the REVERSE is endothermic. Raising the temperature shifts the equilibrium in the endothermic direction, which is backwards — so the yield of ammonia DECREASES.
2. (b) Count the gas molecules: 1 + 3 = 4 on the left, 2 on the right. The right has fewer.
3. Raising the pressure shifts the equilibrium towards the side with fewer gas molecules, which is the right — so the yield of ammonia INCREASES.
4. (c) A catalyst speeds both directions equally, so the position is UNCHANGED and the yield is the same. Equilibrium is simply reached sooner.
5. Note the tension in (a) and (b): a high pressure helps the yield and a high temperature harms it, which is why the industrial process is a compromise.

Answer: (a) yield falls, (b) yield rises, (c) yield unchanged but reached faster.

That tension is worth ending on. Raising the temperature reduces the yield and yet the process is run hot, because at a low temperature the reaction is far too slow to be useful however good the yield would eventually be. Position of equilibrium and rate are two different things, and an industrial process has to satisfy both.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

The vocabulary of equilibrium

Dynamic equilibrium

The state in which the forward and reverse reactions occur at equal rates, so concentrations remain constant.

Closed system

A container from which nothing can enter or leave. Equilibrium can only be reached in one.

Position of equilibrium

How much product there is relative to reactant. To the RIGHT means mostly products; to the LEFT means mostly reactants.

Dynamic

Both reactions are STILL HAPPENING. The reaction has not stopped — the two rates are simply equal.

Statement 3.20 — the characteristics

• The RATE of the forward reaction EQUALS the rate of the reverse reaction
• The CONCENTRATIONS of reactants and products stay CONSTANT
• Constant does NOT mean equal — a mixture can be 95% product and be at equilibrium
• Both reactions are STILL OCCURRING; nothing has stopped
• It requires a CLOSED SYSTEM, with nothing entering or leaving

Statement 3.22 — predicting a shift

1. Identify what the change has done to the system — added heat, or increased pressure.
2. The equilibrium shifts in the direction that OPPOSES that change.
3. For TEMPERATURE: heating shifts it the ENDOTHERMIC way; cooling shifts it the EXOTHERMIC way.
4. For PRESSURE: raising it shifts towards FEWER gas molecules; lowering it shifts towards MORE.
5. Count the gas molecules on each side from the balanced equation, ignoring solids and liquids.
6. If both sides have the same number of gas molecules, pressure has NO effect.

What shifts the position, and what does not

Model answer [3 marks]

State three characteristics of a reaction at dynamic equilibrium. [3]

The rate of the forward reaction is equal to the rate of the reverse reaction. The concentrations of the reactants and the products remain constant, although they are not necessarily equal to one another. Both the forward and reverse reactions are still taking place, so the reaction has not stopped, and the equilibrium can only be established in a closed system from which nothing escapes.

Model answer [4 marks]

Explain why adding a catalyst does not change the position of an equilibrium. [4]

A catalyst works by providing an alternative pathway with a lower activation energy, and it lowers the activation energy of the forward and the reverse reactions by exactly the same amount because it is the same pathway in both directions. Both the forward and the reverse rates are therefore increased by the same factor. Equilibrium is the point at which the two rates are equal, and multiplying both rates by the same factor leaves them equal at exactly the same composition. The catalyst therefore causes equilibrium to be reached more quickly without changing where it lies.

Model answer [3 marks]

For N₂ + 3H₂ ⇌ 2NH₃, in which the forward reaction is exothermic, explain the effect of raising the temperature on the yield of ammonia. [3]

The forward reaction is exothermic, so the reverse reaction is endothermic. Raising the temperature shifts the equilibrium in the endothermic direction, because that direction absorbs the added heat and so opposes the increase in temperature. The endothermic direction here is the reverse reaction, so the equilibrium shifts to the left and the yield of ammonia decreases.

Position of equilibrium against rate

A. POSITION is how much product there is when equilibrium is reached. Changed by temperature and by pressure; NOT by a catalyst.

B. RATE is how quickly equilibrium is reached. Changed by temperature, by pressure AND by a catalyst. The two are independent.

Not this: At equilibrium the reaction has stopped, and the amounts of reactants and products are equal.

This: Both reactions are STILL HAPPENING, at equal rates — that is what dynamic means. And the concentrations are CONSTANT, not equal: a mixture can be 95% product and be perfectly at equilibrium.

Mark-losing trap. DYNAMIC means both reactions continue. Nothing has stopped.

Mark-losing trap. Constant does NOT mean equal. The amounts stop changing; they do not become the same.

Mark-losing trap. A catalyst changes WHEN equilibrium is reached, never WHERE it lies.

Mark-losing trap. Pressure has NO effect if both sides have the same number of gas molecules.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [1 mark] — At dynamic equilibrium, how do the rates of the forward and reverse reactions compare?
2. Grade 7 · Explain [3 marks] — Explain why equilibrium can only be reached in a closed system.
3. Grade 8 · Predict [3 marks] — For N₂ + 3H₂ ⇌ 2NH₃, predict the effect of increasing the pressure on the yield of ammonia, and explain why.
4. Grade 9 · Explain [6 marks] — Select every statement that belongs in a full-mark explanation of what dynamic equilibrium is and why adding a catalyst does not change its position.
5. 9+ · Analyse [6 marks] — For N₂ + 3H₂ ⇌ 2NH₃ the forward reaction is exothermic. A student argues that the yield of ammonia would be maximised by using the lowest possible temperature, so industrial plants should be run cold. Select every statement that belongs in a full-mark analysis.

The people behind this science

Two ways into the same idea — the one whose hypothesis makes counting gas molecules mean something, and the one who established why heating pushes a reaction one particular way. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Amedeo Avogadro — the one whose hypothesis makes counting gas molecules mean something

The pressure rule on this page works by counting gas molecules on each side of an equation, and that only predicts anything because equal numbers of gas molecules occupy equal volumes — Avogadro's hypothesis. Four molecules on the left and two on the right means the left side takes up twice the volume, which is why compressing the mixture favours the right. He is the right figure to ask why a count of molecules translates directly into a volume, and why that was not obvious for fifty years.

• “Why does counting gas molecules tell you about volume?”
• “What does it mean that equal volumes hold equal numbers?”
• “Would compressing a gas mixture change what it contains?”
• “Why did chemists take so long to accept your hypothesis?”
• “How would you have tested it if you could?”

James Prescott Joule — the one who established why heating pushes a reaction one particular way

The temperature rule on this page says an equilibrium shifts in the endothermic direction when heated, which only makes sense if heat is a quantity of energy that a reaction can absorb. Joule is the person who established heat as energy rather than as a fluid, and who showed it could be converted into and out of other forms at a fixed rate. He is the right figure to ask what it means to say a direction of reaction 'absorbs' the heat you have added — and why absorbing it opposes the change.

• “What does adding heat to a system actually do?”
• “What does it mean for a reaction to absorb heat?”
• “Why should absorbing heat oppose a rise in temperature?”
• “How did you establish that heat is a form of energy?”
• “Can energy be moved around without being lost?”

Then defend it

On Incandio a lesson is not finished when the questions come out right. You teach the idea back to Ember, an AI apprentice who asks the awkward question, and then you argue it against Marcellin Berthelot in a structured debate marked against descriptors you can read before you enter. Learn it, teach it, then defend it — all three happen on this page once the app loads.

Carry on through the course

• Previous lesson: Reversible Reactions
• Next lesson: What Organic Chemistry Is
• Exothermic and Endothermic Reactions — Reactions that warm their surroundings and reactions that chill them — and why the thermometer tells you which.
• Calorimetry Calculations — Turning a temperature rise into an energy in joules, and then into a value per mole.
• Energy Diagrams and Bond Energies — Why breaking bonds costs energy, why making them repays it, and why the balance decides everything.
• Rates of Reaction — Four things that make a reaction faster, and two ways of measuring whether they did.
• Collision Theory — One explanation that accounts for every factor on the previous page.
• All of Chemistry · Incandio Science