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Practice · Chemistry · 2025 syllabus · R3.3

IB Chemistry R3.3 Electron sharing reactions: practice questions

5 exam-style questions on R3.3 Electron sharing reactions, SL and HL: 3 multiple choice and 2 structured. Each one comes with the mark scheme, a model answer and the reasoning behind every mark. Free with your email.

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R3.3-01 · Paper 1A style · multiple choice · 1 mark

Spotting a propagation step

Ethane reacts with chlorine in ultraviolet light. Which equation is a propagation step in the accepted mechanism of this reaction? [1]

  1. Cl2 → 2Cl•
  2. •C2H5 + Cl• → C2H5Cl
  3. C2H6 + Cl• → •C2H5 + HCl
  4. C2H6 + Cl• → C2H5Cl + H•
Show the answer and the reasoning

Answer: C. C is correct. A propagation step uses up one radical and makes another, so the chain continues. A is initiation: radicals are made from a molecule. B is termination: two radicals join. D has one radical on each side, but it is not part of the accepted mechanism: the chlorine radical removes a hydrogen atom to form HCl, so free hydrogen atoms are not formed. In May 2025, 18 % chose a distractor of this type.

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The 5 questions in R3.3

  1. R3.3-01 Spotting a propagation stepPaper 1A style · multiple choice · 1 mark
  2. R3.3-02 Which bond breaks firstPaper 1A style · multiple choice · 1 mark
  3. R3.3-03 How a halogen forms radicalsPaper 1A style · multiple choice · 1 mark
  4. R3.3-04 Brominating ethanePaper 2 style · structured question · 6 marks
  5. R3.3-05 Two ways a bond can breakPaper 2 style · structured question · 4 marks

More of R3 What are the mechanisms of chemical change?

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Every Biology and Chemistry question here is original, written for practice in the style of the papers set since the 2025 syllabus, with invented data. Developed independently from the International Baccalaureate Organization, which has not endorsed it. Found a mistake? Email vytis@limeadmissions.com with the question id.