DONATSciences and Maths
Chemistry · Practice book · Senior 4 · Unit 12 of 17

Trends in chemical properties of group 17 elements and their compounds

Senior 4 Chemistry · 14 questions with answers. Try each question on paper, then tap it to see the answer.

Unit 12 Trends in chemical properties of group 17 elements and their compounds

Key unit competence: Interpret the periodic trends in the properties of group 17 elements and their compounds to explain their reactivity, uses and hazards.

Key ideas

  • Group 17 (halogens): F2 (pale yellow gas), Cl2 (greenish-yellow gas), Br2 (red-brown liquid), I2 (grey-black solid, purple vapour); configuration ns2 np5; diatomic molecules. Melting and boiling points increase down the group (stronger London forces).
  • Preparation: Cl2 by oxidising conc. HCl with MnO2 or KMnO4; industrially by electrolysis of brine. Fluorine only by electrolysis (most powerful oxidising agent).
  • Reactivity and oxidising power decrease down the group (atoms larger, attract electrons less): Cl2 displaces Br2 and I2 from their salts; Br2 displaces I2. With hydrogen: F2 explodes in the dark, Cl2 reacts in sunlight, I2 only partially on heating.
  • Hydrogen halides: HF, HCl, HBr, HI; acid strength increases (HF weak due to strong H–F bond and hydrogen bonding), thermal stability decreases, and reducing power of halide ions increases down the group. HF has the highest boiling point (hydrogen bonding) and etches glass.
  • With alkali: cold dilute NaOH gives chlorate(I): Cl2 + 2NaOH → NaCl + NaOCl + H2O (bleach); hot concentrated NaOH gives chlorate(V): 3Cl2 + 6NaOH → 5NaCl + NaClO3 + 3H2O (disproportionation). Chlorates and iodates are oxidising agents.
  • Tests: halide ions with acidified AgNO3: AgCl white (soluble in dilute NH3), AgBr cream (soluble in conc. NH3), AgI yellow (insoluble in NH3). Uses: chlorine (water treatment, PVC, bleach), fluoride (toothpaste), iodine (antiseptic, iodised salt to prevent goitre), bromine (flame retardants). Hazards: toxic gases, CFCs and ozone depletion.

Common misconceptions

  • "HF is the strongest hydrohalic acid because F is most electronegative." No: HF is a weak acid; the H–F bond is very strong and HF forms hydrogen bonds.
  • "Iodine is more reactive than chlorine because it is larger." No: Reactivity of halogens decreases down the group.
  • "All silver halides dissolve in ammonia." No: AgCl dissolves in dilute ammonia, AgBr only in concentrated, AgI not at all.

Worked examples

Example 1. Describe and explain the displacement reactions between halogens and halide ions, using chlorine, bromine and iodine.

Solution

Chlorine + bromide: Cl2 + 2Br− → 2Cl− + Br2 (orange solution).

Chlorine + iodide: Cl2 + 2I− → 2Cl− + I2 (brown solution).

Bromine + iodide: Br2 + 2I− → 2Br− + I2.

Iodine does not displace chlorine or bromine.

Oxidising power decreases down the group (Cl2 > Br2 > I2) because larger atoms attract electrons less.

Example 2. Explain how silver nitrate and ammonia are used to identify chloride, bromide and iodide ions.

Solution

Acidify the solution with dilute nitric acid, then add silver nitrate.

Chloride: white precipitate (AgCl), dissolves in dilute ammonia.

Bromide: cream precipitate (AgBr), dissolves only in concentrated ammonia.

Iodide: yellow precipitate (AgI), insoluble even in concentrated ammonia.

Practice questions

  1. Write the configuration of chlorine (Z = 17).

    Show answer

    1s2 2s2 2p6 3s2 3p5.

    Common mistake: Writing 3p6.

  2. Give the physical state and colour of each halogen at room temperature.

    Show answer

    F2 pale yellow gas, Cl2 greenish-yellow gas, Br2 red-brown liquid, I2 grey-black solid.

    Common mistake: Calling iodine a gas.

  3. Explain why boiling points increase down group 17.

    Show answer

    Molecules get larger with more electrons, so London forces increase.

    Common mistake: Saying covalent bonds get stronger.

  4. Write the equation for chlorine with cold dilute NaOH.

    Show answer

    Cl2 + 2NaOH → NaCl + NaOCl + H2O.

    Common mistake: Writing NaClO3.

  5. What is disproportionation?

    Show answer

    A reaction in which the same element is both oxidised and reduced.

    Common mistake: Saying 'two elements react'.

  6. Why is HF a weak acid?

    Show answer

    The H–F bond is very strong and HF forms hydrogen bonds, so it ionises only slightly.

    Common mistake: Saying fluorine is not electronegative.

  7. Give one use each of fluorine compounds and iodine.

    Show answer

    Fluoride: toothpaste/PTFE; iodine: antiseptic/iodised salt.

    Common mistake: Saying iodine is used in toothpaste.

  8. Write the ionic equation for the formation of silver bromide.

    Show answer

    Ag+(aq) + Br−(aq) → AgBr(s).

    Common mistake: Writing AgBr2.

  9. State the trend in reducing power of halide ions and give a reason.

    Show answer

    Increases down the group; larger ions lose electrons more easily.

    Common mistake: Saying it decreases.

  10. Explain why chlorine is a better bleach in water than dry chlorine.

    Show answer

    In water it forms HOCl, which bleaches by oxidation.

    Common mistake: Saying dry chlorine is more reactive.

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. Which halogen is used as an antiseptic in solution?a. HF
    2. Halogens have outer configuration:b. decreases
    3. Thermal stability of hydrogen halides down the group:c. ns2 np5
    4. Which hydrogen halide has the highest boiling point?d. Iodine
    Show answer

    1 → d (Iodine) · 2 → c (ns2 np5) · 3 → b (decreases) · 4 → a (HF)

    • Tincture of iodine.
    • Seven outer electrons.
    • H–X bonds become weaker.
    • Hydrogen bonding in HF.

  2. Drag and drop

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

    purple  ·  strengthen tooth enamel  ·  sodium chloride and sodium chlorate(I)  ·  colourless
    1. Fluoride is added to toothpaste to .
    2. Chlorine reacts with cold dilute NaOH to form .
    3. Iodine vapour is .
    Show answer

    1: strengthen tooth enamel · 2: sodium chloride and sodium chlorate(I) · 3: purple

    • Fluorapatite resists acid attack.
    • Household bleach.
    • Iodine sublimes to a violet vapour.

  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. HF is used to:a. chloride
    2. The reducing power of halide ions down the group:b. increases
    3. A white precipitate with acidified silver nitrate that dissolves in dilute ammonia indicates:c. etch glass
    4. Down group 17, boiling points:d. increase
    Show answer

    1 → c (etch glass) · 2 → b (increases) · 3 → a (chloride) · 4 → d (increase)

    • It reacts with silicon dioxide.
    • Larger ions lose electrons more easily.
    • AgCl dissolves forming [Ag(NH3)2]+.
    • Larger molecules, stronger London forces.

  4. Drag and drop

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

    bromide  ·  chlorine atoms  ·  grey solid  ·  red-brown liquid
    1. A precipitate with acidified AgNO3 is cream and dissolves only in concentrated ammonia. The halide is .
    2. CFCs damage the ozone layer by releasing .
    3. At room temperature, bromine is a .
    Show answer

    1: bromide · 2: chlorine atoms · 3: red-brown liquid

    • Step 1. Cream colour.
      Step 2. Solubility only in conc. NH3: AgBr.
    • Cl atoms catalyse ozone breakdown.
    • Bromine is one of two liquid elements.

Take the Unit 12 quiz → · Back to contents