Reversible reactions and equilibrium

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Notes de leçon

Big idea: reversible reactions and equilibrium

  • Key concept: Relationships. At dynamic equilibrium the forward and reverse reaction rates are equal. Concentrations remain constant but the reactions continue.
  • Related concepts: Models and evidence. Use a scientific explanation to make predictions, then test it against observations.
  • Global context: Scientific and technical innovation. Industrial processes balance equilibrium yield, rate and energy costs.

Reversible Reactions

  • In reversible reactions, products can react to reform reactants; the reaction occurs in both directions.
  • The symbol ⇌ is used to indicate a reversible reaction.
  • If the forward reaction is exothermic, the reverse reaction is endothermic (same amount of heat transferred).
  • Example: N₂ + 3H₂ ⇌ 2NH₃ (forward exothermic, reverse endothermic).
  • Hydrated salts contain water of crystallisation; heating removes water to form anhydrous salts.
  • Copper(II) sulfate: CuSO₄·5H₂O (blue) ⇌ CuSO₄ (white) + 5H₂O; forward endothermic, reverse exothermic.
  • Cobalt(II) chloride: CoCl₂·6H₂O (pink) ⇌ CoCl₂ (blue) + 6H₂O; forward endothermic, reverse exothermic.
  • These colour changes are used as tests for water (blue to pink for CoCl₂, white to blue for CuSO₄).

Hydrated copper(II) sulfate (blue) heated to anhydrous copper(II) sulfate (white); adding water reverses the change

Hydrated copper(II) sulfate (blue) heated to anhydrous copper(II) sulfate (white); adding water reverses the change

Equilibrium

  • A reversible reaction reaches equilibrium in a closed system (no reactants or products can escape).
  • At equilibrium: rate of forward reaction = rate of reverse reaction; concentrations of reactants and products remain constant.
  • Equilibrium is dynamic – molecules continuously interconvert at equal rates.
  • Example: In the Haber process, initially forward rate is high; as reactants decrease and products increase, rates become equal.

Le Chatelier's Principle

  • Le Chatelier's Principle: If a change is made to a system at equilibrium, the system moves to oppose the change.
  • Temperature: Increase temperature favours the endothermic direction; decrease favours the exothermic direction.
  • Pressure: Increase pressure shifts equilibrium to the side with fewer gas molecules; decrease shifts to side with more gas molecules.
  • Concentration: Increase reactant concentration shifts equilibrium to the right (more products); increase product concentration shifts to the left.
  • Catalyst does not affect the position of equilibrium – it speeds up both forward and reverse reactions equally, reaching equilibrium faster.

The Haber Process

  • Manufactures ammonia via N₂(g) + 3H₂(g) ⇌ 2NH₃(g) (forward exothermic).
  • Raw materials: Nitrogen from fractional distillation of liquid air; Hydrogen from natural gas (methane).
  • Conditions: 450 °C, 200 atm, iron catalyst.
  • Compromise conditions: Lower temperature favours yield but slows rate; higher pressure favours yield but is expensive and dangerous.
  • Catalyst (iron) allows equilibrium to be reached faster, enabling a lower operating temperature.
  • Process stages: Compress gases → react over catalyst → cool to liquefy ammonia → recycle unreacted N₂ and H₂.
  • Yield increases with increasing pressure and decreasing temperature (see data tables).

The production of ammonia by the Haber process

The production of ammonia by the Haber process

The Contact Process

  • Manufactures sulfuric acid via main stage: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) (forward exothermic).
  • Catalyst: Vanadium(V) oxide (V₂O₅).
  • Conditions: 450 °C, 2 atm (200 kPa).
  • Compromise: Low temperature favours yield but slows rate; pressure is kept low (≈1 atm) because equilibrium already gives ~96% yield and high pressure would liquefy SO₂.
  • SO₃ is absorbed into 98% sulfuric acid to form oleum (H₂S₂O₇), then diluted to give concentrated H₂SO₄.
  • SO₃ is not absorbed directly into water to avoid forming a dangerous mist of sulfuric acid.

Think like a scientist

  • Use a teacher-approved model to compare equilibrium composition at different temperatures in a closed reacting system.
  • Comparison: the temperature of the closed system. Outcome: the equilibrium proportion of a chosen product.
  • Control: keep the starting composition and total amount fixed. Explain why this makes the comparison fairer.
  • Evidence: Use a consistent method, repeated observations where appropriate and a table with labelled quantities and units. Keep unexpected results and investigate their cause.
  • Safety: Practical activities need teacher supervision and an appropriate risk assessment. Use the provided data or simulation where the investigation specifies it.
  • Inquiry task: State a testable question, predict the outcome using the science, then explain how your observations would support or challenge the prediction.

Evaluate the science

  • Industrial processes balance equilibrium yield, rate and energy costs.
  • A catalyst speeds approach to equilibrium but does not change the equilibrium composition at a fixed temperature.
  • Evaluation task: Link your conclusion to evidence, identify a limitation and suggest a specific improvement. Distinguish a measured result from an explanation of its cause.

Diapos

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  1. 1.What does the symbol ⇌ mean in a chemical equation?

    Easy
    • AThe reaction is reversible
    • BThe reaction goes to completion
    • CThe reaction is exothermic
    • DThe reaction is at equilibrium

  2. 2.When anhydrous cobalt(II) chloride is added to water, what colour change is observed?

    Easy
    • ABlue to pink
    • BPink to blue
    • CWhite to blue
    • DBlue to white

  3. 3.Which of the following is a condition needed for a reversible reaction to reach equilibrium?

    Easy
    • AOpen system
    • BClosed system
    • CHigh temperature
    • DPresence of a catalyst

  4. 4.In the Haber process, what is the source of nitrogen?

    Easy
    • ANatural gas
    • BAir
    • CWater
    • DAmmonia

  5. 5.For the reversible reaction N2(g) + 3H2(g) ⇌ 2NH3(g), the forward reaction is exothermic. Which change would increase the yield of ammonia at equilibrium?

    Medium
    • AIncreasing the temperature
    • BDecreasing the pressure
    • CAdding a catalyst
    • DDecreasing the temperature

  6. 6.In the Contact process, the main stage reaction is 2SO2(g) + O2(g) ⇌ 2SO3(g). The forward reaction is exothermic. What is the approximate temperature used for this stage?

    Easy
    • A200 °C
    • B450 °C
    • C800 °C
    • D1000 °C

  7. 7.Which statement is correct about a catalyst in a reversible reaction at equilibrium?

    Medium
    • AIt increases the yield of products
    • BIt speeds up the forward reaction only
    • CIt increases the rate of both forward and reverse reactions equally
    • DIt shifts the equilibrium to the right

  8. 8.Hydrated copper(II) sulfate is heated. Which statement describes this reaction?

    Easy
    • AEndothermic, blue to white
    • BExothermic, white to blue
    • CEndothermic, white to blue
    • DExothermic, blue to white

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