DONATSciences and Maths
Chemistry · Practice book · Senior 4 · Unit 11 of 17

Trends in chemical properties of group 16 elements and their compounds

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

Unit 11 Trends in chemical properties of group 16 elements and their compounds

Key unit competence: Interpret the periodic trends in the properties of group 16 elements and their compounds to explain their chemical behaviour and uses.

Key ideas

  • Group 16 (chalcogens): O, S (non-metals), Se, Te (metalloids), Po (metal); configuration ns2 np4. They gain 2 electrons to form X2− or share 2 electrons. Oxygen is diatomic; sulfur forms S8 rings.
  • Oxidation states: oxygen −2 (except peroxides −1 and OF2 +2); sulfur −2, +4 and +6. Electronegativity decreases down the group.
  • Hydrides: H2O, H2S, H2Se, H2Te. Boiling point of H2O is anomalously high (hydrogen bonding); acidity of the hydrides increases down the group (H–X bond weaker); thermal stability decreases down the group.
  • Sulfuric acid is made by the Contact process (2SO2 + O2 ⇌ 2SO3, V2O5, 450 °C, 1–2 atm). It is a strong diprotic acid; concentrated it is a dehydrating, oxidising and drying agent.
  • Oxoanions of sulfur: sulfite SO32− (reducing agent), sulfate SO42−, thiosulfate S2O32− (used in iodometric titrations: 2S2O32− + I2 → S4O62− + 2I−). Thiosulfate reacts with acid to give a sulfur precipitate.
  • Tests: SO42− white ppt with acidified BaCl2; SO32− white ppt with BaCl2 that dissolves in dilute HCl giving SO2, which turns acidified dichromate green; S2−/H2S turns lead(II) ethanoate paper black. Uses: H2SO4 (fertilisers, batteries), sulfur (vulcanising), selenium (photocopiers, solar cells), oxygen (respiration, welding).

Common misconceptions

  • "H2S boils higher than H2O because it is heavier." No: Water boils much higher because of hydrogen bonding.
  • "Acidity of group 16 hydrides decreases down the group." No: Acidity increases down the group because the H–X bond becomes weaker.
  • "Thiosulfate and sulfate are the same ion." No: Thiosulfate S2O32− has one O replaced by S; it behaves as a reducing agent.

Worked examples

Example 1. Explain the trend in boiling points of the hydrides of group 16.

Solution

H2S (−60 °C), H2Se (−41 °C), H2Te (−2 °C): boiling point rises with molecular size as London forces increase.

H2O (100 °C) is far higher than expected.

This is because water forms strong hydrogen bonds (O is small and highly electronegative), which need much more energy to overcome.

Example 2. Describe tests to distinguish sulfate, sulfite and sulfide ions.

Solution

Sulfate: add dilute HCl then BaCl2: white precipitate that does not dissolve.

Sulfite: BaCl2 gives a white precipitate that dissolves in dilute HCl, releasing SO2 (turns acidified potassium dichromate orange to green).

Sulfide: add dilute acid; H2S (rotten-egg smell) turns lead(II) ethanoate paper black.

Practice questions

  1. Write the configuration of sulfur (Z = 16).

    Show answer

    1s2 2s2 2p6 3s2 3p4.

    Common mistake: Writing 3p6.

  2. Give the oxidation states of S in H2S, SO2 and H2SO4.

    Show answer

    −2, +4, +6.

    Common mistake: Giving +2 for H2S.

  3. State two properties of concentrated sulfuric acid.

    Show answer

    Dehydrating agent, oxidising agent, drying agent, strong acid when diluted (any two).

    Common mistake: Saying it is a base.

  4. Write the equation for the reaction of thiosulfate with iodine.

    Show answer

    2S2O32− + I2 → S4O62− + 2I−.

    Common mistake: Writing S2O32− + I2 → SO42−.

  5. Why is H2Te a stronger acid than H2S?

    Show answer

    The H–Te bond is weaker, so H+ is released more easily.

    Common mistake: Saying Te is more electronegative.

  6. What is seen when dilute HCl is added to sodium thiosulfate solution?

    Show answer

    A pale yellow precipitate of sulfur forms; the solution becomes cloudy; SO2 smell.

    Common mistake: Saying a white precipitate of barium sulfate.

  7. Give one use each of sulfur and selenium.

    Show answer

    Sulfur: vulcanising rubber/sulfuric acid; selenium: photocopiers/solar cells.

    Common mistake: Swapping the uses.

  8. Explain why H2O is a liquid but H2S is a gas at room temperature.

    Show answer

    Water molecules form hydrogen bonds; H2S has only weak dipole and London forces.

    Common mistake: Saying water is heavier.

  9. State the conditions of the Contact process.

    Show answer

    V2O5 catalyst, about 450 °C, 1–2 atm.

    Common mistake: Saying iron catalyst and 200 atm.

  10. Describe the test for H2S.

    Show answer

    Turns moist lead(II) ethanoate paper black.

    Common mistake: Saying it relights a glowing splint.

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. Group 16 elements have outer configuration:a. tetrathionate and iodide ions
    2. Water has an unusually high boiling point because of:b. ns2 np4
    3. Thiosulfate ions react with iodine to form:c. vanadium(V) oxide
    4. The catalyst in the Contact process is:d. hydrogen bonding
    Show answer

    1 → b (ns2 np4) · 2 → d (hydrogen bonding) · 3 → a (tetrathionate and iodide ions) · 4 → c (vanadium(V) oxide)

    • Six outer electrons.
    • O–H...O hydrogen bonds.
    • 2S2O32− + I2 → S4O62− + 2I−.
    • V2O5.

  2. Drag and drop

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

    doubles  ·  reducing agents  ·  decreases  ·  +4
    1. Sulfite ions decolourise acidified potassium manganate(VII) because they are .
    2. Thermal stability of group 16 hydrides down the group .
    3. The oxidation state of sulfur in SO2 is .
    Show answer

    1: reducing agents · 2: decreases · 3: +4

    • SO32− is oxidised to SO42−.
    • H–X bonds weaken.
    • S + 2(−2) = 0.

  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. Oxygen has an oxidation state of −1 in:a. chlorine and sulfur dioxide
    2. The most electronegative element in group 16 is:b. oxygen
    3. Concentrated sulfuric acid is used as a drying agent for:c. hydrogen peroxide
    4. Sulfur commonly exists as:d. S8 rings
    Show answer

    1 → c (hydrogen peroxide) · 2 → b (oxygen) · 3 → a (chlorine and sulfur dioxide) · 4 → d (S8 rings)

    • Peroxide ion O22−.
    • Electronegativity decreases down.
    • It reacts with basic or reducing gases.
    • Crown-shaped S8.

  4. Drag and drop

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

    pale yellow precipitate of sulfur  ·  +2  ·  chalcogens  ·  +6
    1. Sodium thiosulfate solution with dilute acid gives a .
    2. The oxidation state of sulfur in H2SO4 is .
    3. Group 16 elements are also called .
    Show answer

    1: pale yellow precipitate of sulfur · 2: +6 · 3: chalcogens

    • Used in rate experiments.
    • 2(+1) + S + 4(−2) = 0.
    • Ore-forming elements.

Take the Unit 11 quiz → · Back to contents