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Chemistry · Practice book · Senior 6 · Unit 7 of 14

Quantitative chemical equilibrium

Senior 6 Chemistry · 14 questions with answers. Try each question on paper, then tap it to see the answer.

Unit 7 Quantitative chemical equilibrium

Key unit competence: Write expressions for Kc and Kp, calculate equilibrium constants and interpret them in relation to the yield of industrial processes.

Key ideas

  • A reversible reaction can go both ways (⇌). In a closed system it reaches dynamic equilibrium: forward and backward rates are equal, concentrations stay constant, both reactions continue. Equilibrium can be reached from either side.
  • Le Châtelier's principle: if a condition of a system at equilibrium is changed, the position of equilibrium shifts to oppose the change. Concentration: adding reactant shifts right. Pressure: increase shifts to the side with fewer gas moles. Temperature: increase shifts in the endothermic direction. A catalyst speeds up both directions equally and does NOT change the position.
  • For aA + bB ⇌ cC + dD: Kc = [C]c[D]d ÷ ([A]a[B]b) (law of mass action). Pure solids and liquids are left out. A large K (>103) means mainly products; a small K means mainly reactants. Only temperature changes K.
  • For gases, Kp uses partial pressures: pA = xA × Ptotal. Kp = Kc(RT)Δn, where Δn = moles of gaseous products − moles of gaseous reactants.
  • The reaction quotient Qc has the same form as Kc but uses any concentrations: Qc < Kc: reaction goes forward; Qc > Kc: goes backward; Qc = Kc: at equilibrium.
  • Industry: Haber process N2 + 3H2 ⇌ 2NH3 (ΔH = −92 kJ mol−1): about 450 °C, 200 atm, iron catalyst, ammonia removed. Contact process 2SO2 + O2 ⇌ 2SO3 (ΔH = −197 kJ mol−1): about 450 °C, 1–2 atm, V2O5 catalyst. The temperature is a compromise between yield and rate.

Common misconceptions

  • "At equilibrium the reactions stop." No: Equilibrium is dynamic: both reactions continue at equal rates.
  • "At equilibrium the concentrations of reactants and products are equal." No: They are constant, not necessarily equal.
  • "A catalyst increases the yield at equilibrium." No: A catalyst only makes equilibrium reached faster; the position and K are unchanged.

Worked examples

Example 1. Explain the choice of conditions for the Haber process.

Solution

Equation: N2 + 3H2 ⇌ 2NH3, ΔH = −92 kJ mol−1.

Pressure about 200 atm: 4 mol → 2 mol gas, so high pressure increases yield and rate; higher still would cost too much and be dangerous.

Temperature about 450 °C: low T gives higher yield (exothermic) but slow rate; 450 °C is a compromise.

Iron catalyst speeds up the reaction; ammonia is condensed out and unreacted gases recycled: overall conversion about 98 %.

Example 2. 2.0 mol PCl5 is heated in a 1.0 dm3 flask: PCl5 ⇌ PCl3 + Cl2. At equilibrium 0.50 mol Cl2 is present. Find Kc.

Solution

Change: PCl5 −0.50, PCl3 +0.50, Cl2 +0.50.

Equilibrium: [PCl5] = 1.5, [PCl3] = 0.50, [Cl2] = 0.50 mol dm−3.

Kc = (0.50 × 0.50) ÷ 1.5 = 0.167 mol dm−3.

Units: (mol dm−3)2 ÷ (mol dm−3) = mol dm−3.

Practice questions

  1. State two characteristics of dynamic equilibrium.

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    Closed system; forward rate = backward rate; constant concentrations; reachable from either side (any two).

    Common mistake: Saying the reactions stop.

  2. State Le Châtelier's principle.

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    If a system at equilibrium is disturbed, the position of equilibrium shifts to oppose the disturbance.

    Common mistake: Saying the system always moves to the right.

  3. Write Kc for N2 + 3H2 ⇌ 2NH3.

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    [NH3]2 ÷ ([N2][H2]3)

    Common mistake: Forgetting the powers.

  4. Write Kp for 2SO2 + O2 ⇌ 2SO3.

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    p(SO3)2 ÷ (p(SO2)2 × p(O2))

    Common mistake: Using concentrations instead of partial pressures.

  5. Explain the effect of increasing temperature on K for an endothermic reaction.

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    K increases; the equilibrium shifts towards products.

    Common mistake: Saying K stays the same.

  6. Why is ammonia removed from the Haber reactor?

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    Removing product shifts the equilibrium right and it is the useful product; unreacted gases are recycled.

    Common mistake: Saying ammonia poisons the catalyst.

  7. Define partial pressure.

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    The pressure a gas in a mixture would exert if it alone occupied the whole volume; pA = xAP.

    Common mistake: Saying it is half the total pressure.

  8. What is meant by the position of equilibrium?

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    The relative amounts of reactants and products at equilibrium.

    Common mistake: Confusing it with the rate.

  9. Why does a catalyst not change K?

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    It lowers the activation energy of both the forward and backward reactions equally.

    Common mistake: Saying it speeds only the forward reaction.

  10. Give the conditions of the Contact process.

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    About 450 °C, 1–2 atm, V2O5 catalyst, excess air.

    Common mistake: Giving iron as the catalyst.

Matching and drag-and-drop

  1. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. Increasing temperature on an exothermic forward reaction:a. iron
    2. The catalyst in the Contact process is:b. [CO2]
    3. The catalyst in the Haber process is:c. vanadium(V) oxide
    4. For CaCO3(s) ⇌ CaO(s) + CO2(g), Kc equals:d. shifts equilibrium to the left
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    1 → d (shifts equilibrium to the left) · 2 → c (vanadium(V) oxide) · 3 → a (iron) · 4 → b ([CO2])

    • Favours the endothermic backward reaction.
    • V2O5.
    • Finely divided Fe.
    • Solids are left out.

  2. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    [HI]2/([H2][I2])  ·  proceeds forward  ·  [HI]/([H2][I2])  ·  increases the amount of SO3
    1. For H2 + I2 ⇌ 2HI, Kc is .
    2. If Qc < Kc, the reaction .
    3. Adding more O2 to 2SO2 + O2 ⇌ 2SO3 at equilibrium .
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    1: [HI]2/([H2][I2]) · 2: proceeds forward · 3: increases the amount of SO3

    • Products over reactants with powers.
    • More products must form.
    • Shifts right.

  3. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. Equilibrium can only be established in:a. forward and backward rates are equal
    2. The relationship between Kp and Kc is:b. there are equal moles of gas on both sides
    3. Pressure has no effect on the position of H2 + I2 ⇌ 2HI because:c. a closed system
    4. Dynamic equilibrium means:d. Kp = Kc(RT)Δn
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    1 → c (a closed system) · 2 → d (Kp = Kc(RT)Δn) · 3 → b (there are equal moles of gas on both sides) · 4 → a (forward and backward rates are equal)

    • Nothing can escape.
    • Δn = gas moles of products − reactants.
    • 2 mol ⇌ 2 mol.
    • Both reactions continue.

  4. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    partial pressures  ·  to the right  ·  mainly reactants  ·  mainly products at equilibrium
    1. Increasing the pressure on N2 + 3H2 ⇌ 2NH3 shifts the equilibrium .
    2. Kp is expressed in terms of .
    3. A very large Kc means .
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    1: to the right · 2: partial pressures · 3: mainly products at equilibrium

    • 4 mol gas → 2 mol gas.
    • For gaseous equilibria.
    • Position lies far to the right.

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