C8.8–9 Dynamic Equilibrium and Altering Conditions

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: what happens when a reversible reaction reaches equilibrium in a closed system, and how changing conditions moves the position of equilibrium. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Explain what is meant by dynamic equilibrium in a closed system
• State Le Chatelier’s principle (Higher tier)
• Predict and explain the effect of changing concentration, temperature and pressure on the position of equilibrium (Higher tier)
• Explain why a catalyst does not change the position of equilibrium

Key words

• Closed system: A system where no substances can get in or out (but energy can be transferred).
• Dynamic equilibrium: The state where the forward and reverse reactions happen at exactly the same rate, so the amounts of reactants and products stay constant.
• Position of equilibrium: The relative amounts of reactants and products at equilibrium. “Shifts right” means more products.
• Le Chatelier’s principle: If a system at equilibrium is subjected to a change in conditions, the system responds to counteract the change.

Dynamic equilibrium

When a reversible reaction happens in a closed system, the forward reaction starts fast and slows down as reactants are used up. The reverse reaction starts at zero and speeds up as products build up. Eventually the two rates are equal. From then on, the amounts of reactants and products stay constant: the reaction has reached equilibrium.

Key fact: Equilibrium is dynamic: both reactions are still happening, at the same rate. Nothing looks as if it is changing, because each forward reaction is balanced by a reverse one.

Le Chatelier’s principle (Higher tier)

If we change the conditions of a system at equilibrium, the position of equilibrium shifts to counteract (oppose) the change. The system then settles at a new equilibrium. We use this to predict what happens to the amount of product.

• Add more reactant (raise its concentration); Equilibrium shifts… to the right: more products form; Why uses up some of the added reactant
• Remove product; Equilibrium shifts… to the right: more products form; Why replaces some of the product removed
• Raise the temperature; Equilibrium shifts… in the endothermic direction; Why takes in energy, opposing the rise
• Lower the temperature; Equilibrium shifts… in the exothermic direction; Why gives out energy, opposing the fall
• Raise the pressure (gases); Equilibrium shifts… to the side with fewer molecules of gas; Why fewer molecules lower the pressure
• Lower the pressure (gases); Equilibrium shifts… to the side with more molecules of gas; Why more molecules raise the pressure
• Add a catalyst; Equilibrium shifts… no shift; Why speeds up forward and reverse reactions equally; equilibrium is reached sooner

Changing concentration (Higher tier)

Yellow chromate ions and orange dichromate ions are in equilibrium in solution: 2CrO₄²−(aq) + 2H⁺(aq) ⇌ Cr₂O₇²−(aq) + H₂O(l) (yellow ⇌ orange) Adding acid raises the concentration of H⁺, so the equilibrium shifts right and the solution turns more orange. Adding alkali removes H⁺, so it shifts left and turns yellow.

Key fact: If the concentration of a reactant is increased, more products form until equilibrium is reached again. If the concentration of a product is decreased, more reactants react.

Why does this happen? Adding reactant makes the forward reaction faster for a while (more frequent collisions). As more product builds up, the reverse reaction speeds up too, until the two rates are equal again at a new equilibrium with more product.

Changing temperature and pressure (Higher tier)

Temperature. Colourless dinitrogen tetroxide is in equilibrium with brown nitrogen dioxide: N₂O₄(g) ⇌ 2NO₂(g). The forward reaction is endothermic. Warming a sealed tube shifts the equilibrium right (endothermic direction), so it turns darker brown. Cooling it in ice makes it paler.

Pressure. Count the molecules of gas on each side of the balanced equation. In N₂(g) + 3H₂(g) ⇌ 2NH₃(g) there are 4 molecules on the left and 2 on the right. Raising the pressure shifts the equilibrium to the right, making more ammonia. In H₂(g) + I₂(g) ⇌ 2HI(g) both sides have 2 molecules, so pressure has no effect on the position. Higher pressure still makes both the forward and reverse reactions faster, by the same amount, so equilibrium is reached sooner.

Worked example: predicting the yield

N₂(g) + 3H₂(g) ⇌ 2NH₃(g). The forward reaction is exothermic.
Higher pressure → shifts right (4 → 2 molecules) → more ammonia.
Higher temperature → shifts in the endothermic direction (left) → less ammonia, but the rate is faster.

Answer: high pressure and low temperature give the highest equilibrium yield

Exam tip: In a 3-mark “predict and explain” answer: (1) say which way the equilibrium shifts, (2) give the reason (endothermic direction / fewer gas molecules), (3) state what happens to the yield.

Common mistakes

• Saying the reactions stop at equilibrium. Both carry on, at equal rates.
• Saying concentrations are equal at equilibrium. They are constant.
• Saying a catalyst increases the equilibrium yield. It only gets to equilibrium faster.
• Counting solids, liquids or ignoring the big numbers when judging pressure. Count moles of gas in the balanced equation.
• Saying a temperature rise favours the exothermic direction. It favours the endothermic direction.

Revision questions

Try these, then check your answers below each one.

Question 1. Which statement describes a reaction at dynamic equilibrium? (Choose from: The forward and reverse reactions happen at the same rate, Both reactions have stopped, The amounts of reactants and products are always equal, Only the forward reaction is happening.)

Answer: The forward and reverse reactions happen at the same rate

Working: At dynamic equilibrium both reactions continue at equal rates, so the amounts of reactants and products stay constant (but not necessarily equal).

Question 2. The graph shows the concentrations of a reactant and a product during a reversible reaction in a closed flask. At which time, P, Q or R, is equilibrium first reached? (Choose from: P, Q, R.)

Answer: Q

Working: At Q both concentrations stop changing, so the forward and reverse rates have become equal. At P they are still changing; at R the mixture is still at equilibrium but it was reached earlier.

Question 3. What effect does adding a catalyst have on a reaction at equilibrium? (Choose from: It does not change the position of equilibrium, but equilibrium is reached faster, It shifts the equilibrium to the right, It shifts the equilibrium to the left, It stops the reverse reaction.)

Answer: It does not change the position of equilibrium, but equilibrium is reached faster

Working: A catalyst speeds up the forward and reverse reactions equally, so the yield is unchanged; equilibrium is just reached sooner.

Related lessons

• Previous lesson: C8.6–7 Reversible Reactions and Energy and Reversible Reactions
• Next lesson: C8 RP Required Practical: Rates of Reaction
• In this chapter: C8.1 Rate of Reaction
• In this chapter: C8.2 Collision Theory and Surface Area
• In this chapter: C8.3 The Effect of Temperature

Continue in the full course

This lesson is section C8.8–9 (AGCSC2506) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.