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H2 Chemistry: physical chemistry decision sheet

Before calculating, label each question as stoichiometry, kinetics or equilibrium. State one assumption beside the equation you choose. This 9476 revision sheet is designed to reveal confusion between a theoretical maximum, a rate and an equilibrium composition.

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Question 1

N₂ + 3H₂ → 2NH₃. There are 0.40 mol N₂ and 0.60 mol H₂. Find the stoichiometric maximum NH₃ amount.

My method: ____________________
My working: ____________________
My check: ____________________

Question 2

For rate = k[A][B], rate = 0.0060 mol/dm³/s when [A] = 0.20 and [B] = 0.30 mol/dm³. Calculate k with units.

My method: ____________________
My working: ____________________
My check: ____________________

Question 3

For H₂ + I₂ ⇌ 2HI, equilibrium concentrations are [H₂] = [I₂] = 0.10 and [HI] = 0.50 mol/dm³. Find Kc.

My method: ____________________
My working: ____________________
My check: ____________________

Question 4

A buffer has pKa = 4.80 and equal concentrations of acid and conjugate base. Find the approximate pH. Explain why a small addition of acid changes the pH only slightly.

My method: ____________________
My working: ____________________
My check: ____________________

Worked answers

Answer 1

  1. Compare 0.40/1 and 0.60/3. Hydrogen limits the extent to 0.20 mol.
  2. Maximum NH₃ = 2 × 0.20 = 0.40 mol, assuming complete consumption of the limiting reagent.

Answer 2

  1. k = 0.0060/(0.20 × 0.30) = 0.10.
  2. For overall second order, the units are dm³ mol⁻¹ s⁻¹.

Answer 3

  1. Kc = [HI]²/([H₂][I₂]).
  2. Kc = 0.50²/(0.10 × 0.10) = 25.

Answer 4

  1. pH ≈ pKa + log₁₀(1) = 4.80.
  2. The conjugate base consumes added H⁺. The concentration ratio changes slightly; pH is resistant to change, not perfectly fixed.

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Scope and review

Official 2026 syllabus. Original AI-assisted practice; selected numerical results and their displayed working are automatically checked. This does not verify every explanation. No teacher review or exam-board endorsement is claimed.

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