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Chemistry · Practice book · Senior 3 · Unit 4 of 10

Chlorine and its inorganic compounds

Senior 3 Chemistry · 14 questions with answers. Try each question on paper, then tap it to see the answer.

Unit 4 Chlorine and its inorganic compounds

Key unit competence: Relate the properties of chlorine and its compounds to their uses, describe how they are prepared and their effects on health and the environment.

Key ideas

  • Laboratory preparation of chlorine: oxidise concentrated hydrochloric acid with manganese(IV) oxide (heat) or potassium manganate(VII): MnO2 + 4HCl → MnCl2 + 2H2O + Cl2. Pass through water (removes HCl) and concentrated H2SO4 (dries); collect by downward delivery.
  • Chlorine is a greenish-yellow, poisonous gas with a choking smell, denser than air, fairly soluble in water. Test: it turns damp blue litmus red and then bleaches it. Industrially it is made by electrolysis of brine (concentrated NaCl solution).
  • Chlorine is a strong oxidising agent: it combines with metals (2Fe + 3Cl2 → 2FeCl3) and hydrogen (H2 + Cl2 → 2HCl); it displaces bromine and iodine from their salts (Cl2 + 2KBr → 2KCl + Br2); with water it forms HCl and HOCl (chloric(I) acid, which bleaches and kills germs); with cold dilute NaOH it forms bleach (NaOCl).
  • Hydrogen chloride: prepared by NaCl + H2SO4 → NaHSO4 + HCl (conc. acid); colourless, choking, very soluble in water to give hydrochloric acid; forms white fumes with ammonia. In methylbenzene it does not show acidic properties (no ions).
  • Hydrochloric acid reacts with metals (salt + H2), bases, carbonates, and is oxidised by MnO2 to chlorine. Test for chloride ions: white precipitate with acidified silver nitrate (Ag+ + Cl− → AgCl), which darkens in light; bromide gives a cream ppt and iodide a yellow ppt.
  • Uses: water treatment and swimming pools, bleaches, PVC plastics, disinfectants, HCl for cleaning metals and making salts; NaCl for food, preservation and making chlorine and NaOH. Hazards: chlorine is toxic and irritates lungs; CFCs damage the ozone layer; some chlorinated pesticides persist in the environment.

Common misconceptions

  • "Chlorine bleaches dry coloured cloth." No: Chlorine bleaches only in the presence of water, through chloric(I) acid (HOCl).
  • "Hydrogen chloride is always acidic." No: Dry HCl gas or HCl in methylbenzene contains no ions and is not acidic; it is acidic only in water.
  • "Iodine can displace chlorine from sodium chloride." No: A more reactive halogen displaces a less reactive one; chlorine displaces iodine, not the reverse.

Worked examples

Example 1. Describe the laboratory preparation of dry chlorine gas.

Solution

Heat manganese(IV) oxide with concentrated hydrochloric acid in a flask: MnO2 + 4HCl → MnCl2 + 2H2O + Cl2.

Pass the gas through water to remove hydrogen chloride.

Pass it through concentrated sulfuric acid to dry it.

Collect by downward delivery in a fume cupboard (chlorine is denser than air and poisonous).

Example 2. Explain how chlorine bleaches and why it kills germs in water.

Solution

Chlorine reacts with water: Cl2 + H2O → HCl + HOCl.

HOCl (chloric(I) acid) releases oxygen that oxidises dyes to colourless compounds: bleaching.

The same oxidising action destroys bacteria, so chlorinated water is safe to drink.

Practice questions

  1. Write the equation for the preparation of chlorine from MnO2.

    Show answer

    MnO2 + 4HCl → MnCl2 + 2H2O + Cl2.

    Common mistake: Writing MnCl4.

  2. Give two physical properties of chlorine.

    Show answer

    Greenish-yellow, choking smell, denser than air, fairly soluble in water, poisonous (any two).

    Common mistake: Saying colourless.

  3. What happens when damp blue litmus is placed in chlorine?

    Show answer

    It turns red and then is bleached white.

    Common mistake: Saying it turns blue.

  4. Give two uses of chlorine.

    Show answer

    Water treatment, bleaches, PVC, disinfectants, pesticides (any two).

    Common mistake: Saying it is used in food.

  5. Write the equation for chlorine reacting with hydrogen.

    Show answer

    H2 + Cl2 → 2HCl.

    Common mistake: Writing H2Cl2.

  6. How is hydrogen chloride prepared in the laboratory?

    Show answer

    By warming sodium chloride with concentrated sulfuric acid.

    Common mistake: Using dilute sulfuric acid.

  7. Describe the test for chloride ions.

    Show answer

    Add dilute nitric acid then silver nitrate: white precipitate (darkens in light).

    Common mistake: Using barium chloride.

  8. Explain why hydrogen chloride in water conducts electricity.

    Show answer

    It ionises into H+ and Cl− ions.

    Common mistake: Saying water conducts by itself.

  9. Write the equation for chlorine displacing iodine from KI.

    Show answer

    Cl2 + 2KI → 2KCl + I2.

    Common mistake: Writing KCl2.

  10. Give one health hazard and one precaution when using chlorine.

    Show answer

    It irritates and damages lungs; work in a fume cupboard / well-ventilated area.

    Common mistake: Saying it is safe to smell directly.

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. When chlorine dissolves in water it forms:a. hydrochloric acid and chloric(I) acid
    2. Which halogen is the most powerful oxidising agent of chlorine, bromine and iodine?b. acidified silver nitrate
    3. The test for chloride ions uses:c. Chlorine
    4. Hydrogen chloride gas is prepared by reacting sodium chloride with:d. concentrated sulfuric acid
    Show answer

    1 → a (hydrochloric acid and chloric(I) acid) · 2 → c (Chlorine) · 3 → b (acidified silver nitrate) · 4 → d (concentrated sulfuric acid)

    • Cl2 + H2O → HCl + HOCl.
    • Oxidising power decreases down group 17.
    • A white precipitate of AgCl forms.
    • NaCl + H2SO4 → NaHSO4 + HCl.

  2. Drag and drop

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

    make it hard  ·  kill germs  ·  manganese(IV) oxide  ·  electrolysis of brine
    1. Industrially, chlorine is made by .
    2. Chlorine is added to drinking water to .
    3. Chlorine is prepared in the lab by oxidising concentrated hydrochloric acid with .
    Show answer

    1: electrolysis of brine · 2: kill germs · 3: manganese(IV) oxide

    • Brine is concentrated sodium chloride solution.
    • It disinfects water.
    • MnO2 oxidises HCl to chlorine.

  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. Which plastic is made using chlorine?a. white (bleached)
    2. Which substance is a common household bleach?b. PVC (polyvinyl chloride)
    3. Chlorine is collected by downward delivery because it is:c. Sodium chlorate(I) (NaOCl)
    4. Damp blue litmus paper in chlorine first turns red and then:d. denser than air
    Show answer

    1 → b (PVC (polyvinyl chloride)) · 2 → c (Sodium chlorate(I) (NaOCl)) · 3 → d (denser than air) · 4 → a (white (bleached))

    • PVC contains chlorine atoms.
    • It releases HOCl, which bleaches.
    • It sinks and fills the jar from the bottom.
    • The acid turns it red; HOCl then bleaches it.

  4. Drag and drop

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

    greenish-yellow  ·  bromide  ·  sodium chloride, water and carbon dioxide  ·  sodium and chlorine
    1. Hydrochloric acid reacts with sodium hydrogencarbonate to give .
    2. The colour of chlorine gas is .
    3. A solution gives a cream precipitate with acidified silver nitrate. The anion is .
    Show answer

    1: sodium chloride, water and carbon dioxide · 2: greenish-yellow · 3: bromide

    • Typical acid + hydrogencarbonate reaction.
    • Chlorine is a pale greenish-yellow gas.
    • Step 1. AgCl white, AgBr cream, AgI yellow.
      Step 2. Cream means Br−.

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