Chlorine disproportionation and bleach quiz

1. Which equation shows the disproportionation of chlorine in water?

a) Cl₂ + 2H₂O → 4HCl + O₂
b) Cl₂ + H₂O ⇌ HCl + HClO
c) Cl₂ + 2NaOH → NaCl + NaClO + H₂O
d) 2HCl + Na₂CO₃ → 2NaCl + H₂O + CO₂

2. In the reaction Cl₂ + H₂O ⇌ HCl + HClO, what are the oxidation states of chlorine in HCl and HClO respectively?

a) 0 and +1
b) −1 and +1
c) −1 and 0
d) +1 and −1

3. In chlorine water, the bleaching ion ClO⁻ is called the __________________ ion. (Give the name in words.)

4. Which statement best describes a disproportionation reaction?

a) A reaction in which a substance is only oxidised.
b) A reaction in which a substance is only reduced.
c) A reaction in which the same element is simultaneously oxidised and reduced.
d) Any reaction involving a change in state.

5. What colour changes are seen when universal indicator is added to chlorine water?

a) It stays green.
b) It turns blue then yellow.
c) It turns red then quickly becomes colourless as the solution is bleached.
d) It turns purple and remains purple.

6. Why are swimming pools usually kept at pH 7.2–7.4 when treated with chlorine?

a) To convert most chlorine into Cl₂(g), which is the strongest disinfectant.
b) To maximise the concentration of ClO⁻, the most powerful oxidising agent.
c) To have a high proportion of HClO present while keeping the water comfortable and not too corrosive.
d) Because pH has no effect on the relative amounts of HClO and ClO⁻.

7. In chlorinated pool water, the main disinfectant species is ____________________. (Give its name in words.)

8. What happens if the pH of a chlorinated swimming pool drifts too high?

a) More HClO forms and disinfection becomes more effective.
b) More ClO⁻ forms, which is a weaker disinfectant, so the pool is less effectively disinfected.
c) Chlorine is converted completely into chloride ions, so disinfection improves.
d) The pH has no effect on disinfecting power.

9. Which statement about bromine water and disproportionation is correct?

a) Bromine water disproportionates extensively and is a strong bleach like chlorine water.
b) Bromine water does not react with water at all.
c) The equilibrium Br₂ + H₂O ⇌ HBr + HBrO lies far to the right, so the solution is colourless.
d) The equilibrium Br₂ + H₂O ⇌ HBr + HBrO lies far to the left, so only small amounts of HBrO/BrO⁻ form and the solution stays brown.

10. Which equation represents the disproportionation of chlorine in cold dilute sodium hydroxide, used in bleach manufacture?

a) Cl₂ + H₂O ⇌ HCl + HClO
b) Cl₂ + 2NaOH → NaCl + NaClO + H₂O
c) 2Cl₂ + 2H₂O → 4HCl + O₂
d) 3Cl₂ + 6NaOH → 5NaCl + NaClO₃ + 3H₂O

11. Household bleach solution is essentially a mixture of sodium chloride and sodium _____________________, NaClO.

12. In sodium chlorate(I), NaClO, and sodium chlorate(V), NaClO₃, what are the oxidation states of chlorine respectively?

a) 0 and +1
b) −1 and +1
c) +1 and +5
d) −1 and +5

13. Which equation represents the further disproportionation of sodium chlorate(I) on heating?

a) 3NaOCl → 2NaCl + NaClO₃
b) Cl₂ + 2NaOH → NaCl + NaClO + H₂O
c) 2Cl₂ + 2H₂O → 4HCl + O₂
d) NaClO₃ → NaCl + 3/2 O₂

14. In a disproportionation reaction, the same element is both oxidised and _____________.

15. Why must chlorine water used in the lab be freshly prepared?

a) Because chlorine slowly freezes out of solution.
b) Because chlorine water quickly evaporates even in a closed bottle.
c) Because in sunlight chlorine water decomposes, releasing oxygen gas and leaving only chloride ions.
d) Because HClO rapidly turns into HCl and Cl₂ in the dark.

16. What is the main public health benefit of adding chlorine to drinking water?

a) It improves the taste and smell of the water.
b) It removes all dissolved salts from the water.
c) It kills or inactivates harmful microorganisms that cause waterborne diseases.
d) It increases the hardness of the water.

17. What is one important risk associated with chlorinating drinking water?

a) Chlorine always makes the water strongly alkaline.
b) Chlorine reacts with organic matter to form potentially harmful organochloro by-products.
c) Chlorine converts all dissolved oxygen into carbon dioxide.
d) Chlorine removes all natural minerals from the water.

18. When chlorine reacts with organic matter in water it can form harmful _________________ compounds that may be carcinogenic.

19. Which statement about iodine and water is correct?

a) Iodine readily disproportionates in water to give iodate(I) and iodide ions.
b) Iodine is practically insoluble in water and does not undergo the same disproportionation reactions as chlorine and bromine.
c) Iodine dissolves well in water to give a colourless solution.
d) Iodine dissolved in water is a much stronger bleach than chlorine water.

20. Iodine is often used medically in a “tincture”. What does this typically consist of?

a) Iodine dissolved in pure water.
b) Iodine dissolved in liquid chlorine.
c) Iodine dissolved in ethanol with potassium or sodium iodide and water.
d) Iodine dissolved in sodium hydroxide solution.