Reversible Reactions, Dynamic Equilibrium & Le Chatelier's Principle
In a reversible reaction, denoted by the symbol ⇌, the products of the reaction can react together to re-form the original reactants; if the forward reaction is exothermic, the reverse reaction is always endothermic by the exact same amount of energy. When a reversible reaction occurs in a closed system, it reaches dynamic equilibrium when the rate of the forward reaction equals the rate of the reverse reaction and the concentrations of reactants and products remain constant. For Higher Tier students, examiners heavily test Le Chatelier's Principle, requiring predictions of how changing temperature, pressure, or concentration shifts the position of equilibrium and alters equilibrium yield.
- Reversible Reactions & Energy Conservation
- Represented by A + B ⇌ C + D. If the forward reaction is exothermic (transfers energy to surroundings), the reverse reaction is endothermic (takes in the exact same amount of energy from the surroundings). Classic examples: heating blue hydrated copper(II) sulfate to form white anhydrous copper(II) sulfate + water, and thermal decomposition of ammonium chloride (NH₄Cl(s) ⇌ NH₃(g) + HCl(g)).
- Dynamic Equilibrium in a Closed System
- When a reversible reaction takes place in a closed apparatus (preventing escape of reactants/products), equilibrium is reached when the rate of the forward reaction equals the rate of the reverse reaction. At equilibrium, the concentrations of reactants and products stay constant (note: they are constant, not necessarily equal).
- Le Chatelier's Principle (Higher Tier)
- If a system is at equilibrium and a change is made to any of the conditions (temperature, pressure, or concentration), then the system responds to counteract (oppose) the change and restore a new equilibrium.
Reversible Reactions, Dynamic Equilibrium & Le Chatelier's Principle: Reaction Profile (ΔH & E_a) + Le Chatelier Yield Simulator
Toggle a catalyst on the enthalpy profile diagram or adjust temperature and pressure to see how dynamic equilibrium shifts.
Modelled on N₂(g) + 3H₂(g) ⇌ 2NH₃(g) (ΔH = −92 kJ/mol) (4 moles gas → 2 moles gas):
Higher pressure shifts equilibrium to the side with fewer gas moles (right, 4 → 2). Lower temperature favours the exothermic forward reaction.
Key Concepts & Exam Definitions (6)
- Reversible Reactions & Energy Conservation
- Represented by A + B ⇌ C + D. If the forward reaction is exothermic (transfers energy to surroundings), the reverse reaction is endothermic (takes in the exact same amount of energy from the surroundings). Classic examples: heating blue hydrated copper(II) sulfate to form white anhydrous copper(II) sulfate + water, and thermal decomposition of ammonium chloride (NH₄Cl(s) ⇌ NH₃(g) + HCl(g)).
- Dynamic Equilibrium in a Closed System
- When a reversible reaction takes place in a closed apparatus (preventing escape of reactants/products), equilibrium is reached when the rate of the forward reaction equals the rate of the reverse reaction. At equilibrium, the concentrations of reactants and products stay constant (note: they are constant, not necessarily equal).
- Le Chatelier's Principle (Higher Tier)
- If a system is at equilibrium and a change is made to any of the conditions (temperature, pressure, or concentration), then the system responds to counteract (oppose) the change and restore a new equilibrium.
- Effect of Temperature on Equilibrium (Higher Tier)
- If temperature is increased, the equilibrium shifts in the endothermic direction, so the yield of the endothermic products increases and exothermic products decreases. If temperature is decreased, the equilibrium shifts in the exothermic direction, increasing the yield of the exothermic products.
- Effect of Pressure on Gaseous Equilibrium (Higher Tier)
- Applies only to reactions involving gases. An increase in pressure shifts the position of equilibrium towards the side with the smaller number of molecules (fewer moles) of gas as shown by the balanced symbol equation. A decrease in pressure shifts equilibrium towards the side with the larger number of molecules (more moles) of gas.
- Effect of Concentration on Equilibrium (Higher Tier)
- If the concentration of a reactant is increased (or a product is removed), the system is no longer at equilibrium; the position of equilibrium shifts to the right (towards the products) to use up the added reactant, until equilibrium is reached again with a higher yield of products.
Quick-Fire Flashcards (6)
Click any card to check the answer, then mark it as Known to track your progress.
What happens when you heat blue hydrated copper(II) sulfate crystals, and what happens when you add water back to the product?
State the two defining features of dynamic equilibrium.
In the reaction 2NO₂(g) (brown) ⇌ N₂O₄(g) (colourless), the forward reaction is exothermic. What happens to the colour of the gas mixture if it is placed in an ice bath?
In the reaction 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), explain how increasing the pressure affects the equilibrium yield of SO₃.
In the reaction H₂(g) + I₂(g) ⇌ 2HI(g), what is the effect of increasing the pressure on the yield of HI and the rate of reaction?
What happens to the position of equilibrium and the rate of reaction when a catalyst is added?
Exam-Style Questions & Mark Schemes (4)
Write or think through your answer first, then click Check Mark Scheme to see the exact points examiners award marks for.
- Q1
Methanol is manufactured by the reversible reaction: CO(g) + 2H₂(g) ⇌ CH₃OH(g). The forward reaction is exothermic (ΔH = -91 kJ/mol). Explain how and why the equilibrium yield of methanol changes if (a) the temperature is increased, and (b) the pressure is increased. (Higher Tier) [6 marks]
Click “Check Mark Scheme” to compare your answer, 1. (a) Temperature: The forward reaction is exothermic, so the reverse reaction is endothermic [1 mark]. 2. By Le Chatelier's Principle, when temperature is increased, the system opposes the change by shifting the position of equilibrium in the endothermic (reverse/left) direction to absorb the extra thermal energy [1 mark]. 3. Therefore, the equilibrium yield of methanol decreases [1 mark]. 4. (b) Pressure: There are 3 moles of gas on the left-hand side (1 mol CO + 2 mol H₂) and only 1 mole of gas on the right-hand side (1 mol CH₃OH) [1 mark]. 5. When pressure is increased, the system opposes the change by shifting the position of equilibrium towards the side with fewer moles of gas (to the right/forward direction) [1 mark]. 6. Therefore, the equilibrium yield of methanol increases [1 mark].
- Q2
When aqueous potassium chromate (yellow) reacts with acid, the following equilibrium is established: 2CrO₄²⁻(aq) (yellow) + 2H⁺(aq) ⇌ Cr₂O₇²⁻(aq) (orange) + H₂O(l). Explain what colour change is observed when concentrated hydrochloric acid is added, and what happens if sodium hydroxide solution is then added. (Higher Tier) [4 marks]
Click “Check Mark Scheme” to compare your answer, 1. Adding concentrated hydrochloric acid increases the concentration of H⁺(aq) ions [1 mark]. 2. By Le Chatelier's Principle, the position of equilibrium shifts to the right (forward direction) to use up the added H⁺ ions, producing more Cr₂O₇²⁻(aq) ions so the solution turns from yellow to orange [1 mark]. 3. Adding sodium hydroxide supplies OH⁻(aq) ions, which react with and remove H⁺(aq) ions (forming water: H⁺ + OH⁻ -> H₂O), decreasing the concentration of H⁺(aq) [1 mark]. 4. The position of equilibrium shifts to the left (reverse direction) to replace the H⁺ ions, forming more CrO₄²⁻(aq) ions so the solution turns back from orange to yellow [1 mark].
- Q3
Hydrated cobalt(II) chloride is a pink solid that decomposes on heating into blue anhydrous cobalt(II) chloride and water vapour: CoCl₂·6H₂O(s) (pink) ⇌ CoCl₂(s) (blue) + 6H₂O(g). Explain how this reaction shows that energy is conserved in reversible reactions, and how anhydrous cobalt(II) chloride can be used as a chemical test for water. [4 marks]
Click “Check Mark Scheme” to compare your answer, 1. The forward reaction requires continuous heating to drive off water, showing that the forward reaction is endothermic [1 mark]. 2. Therefore, the reverse reaction (adding water to blue anhydrous cobalt(II) chloride) must be exothermic and releases the exact same amount of thermal energy that was taken in during the forward reaction [1 mark]. 3. To test for the presence of water, add a few drops of the unknown liquid to blue anhydrous cobalt(II) chloride paper/solid [1 mark]. 4. If water is present, the solid reacts reversibly to form hydrated cobalt(II) chloride, changing colour from blue to pink [1 mark].
- Q4
In an industrial reversible reaction where the forward reaction is exothermic, engineers use a moderately high temperature of 400°C and a catalyst rather than room temperature. Explain why they use 400°C and a catalyst in terms of both rate of reaction and equilibrium yield. (Higher Tier) [4 marks]
Click “Check Mark Scheme” to compare your answer, 1. Because the forward reaction is exothermic, a low temperature would shift equilibrium to the right and give a high equilibrium yield of product [1 mark]. 2. However, at a low temperature, particles have low kinetic energy and few collisions exceed the activation energy, so the rate of reaction would be far too slow to be economically viable [1 mark]. 3. Therefore, 400°C is used as a compromise temperature: it gives a fast enough rate of reaction while still achieving an acceptable equilibrium yield [1 mark]. 4. A catalyst increases the rate of both forward and reverse reactions equally (so equilibrium is reached much faster at lower energy cost) without reducing the equilibrium yield [1 mark].
Common Exam Questions (4)
What does 'dynamic equilibrium' actually mean?
Equilibrium is reached in a closed system (where no reactants or products can escape). 'Dynamic' means both the forward and reverse reactions are still happening simultaneously at the exact same rate. Because they occur at equal rates, there is no overall change in the concentrations of reactants and products.
How does changing temperature affect the position of equilibrium? (Higher Tier)
By Le Chatelier's Principle, the system opposes any change: 1. Increasing the temperature always shifts the equilibrium in the endothermic direction (to absorb the extra heat). 2. Decreasing the temperature always shifts the equilibrium in the exothermic direction (to release heat).
How does changing pressure affect gaseous equilibria? (Higher Tier)
Count the total number of moles of gas on the left-hand side and right-hand side of the balanced symbol equation: 1. Increasing pressure shifts the equilibrium towards the side with fewer moles of gas (to reduce the pressure). 2. Decreasing pressure shifts the equilibrium towards the side with more moles of gas. If both sides have the same number of moles of gas, changing pressure has zero effect on the position of equilibrium.
Does adding a catalyst change the equilibrium yield?
No. Adding a catalyst has zero effect on the position of equilibrium or the final percentage yield because it increases the rate of both the forward reaction and the reverse reaction equally. However, a catalyst is still essential in industry because it allows equilibrium to be reached much faster at lower temperatures.
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