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Chemistry · Practice book · Senior 4 · Unit 10 of 17

Trends in chemical properties of group 15 elements and their compounds

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

Unit 10 Trends in chemical properties of group 15 elements and their compounds

Key unit competence: Interpret the periodic trends in the properties of group 15 elements and their compounds to explain their chemical behaviour and uses.

Key ideas

  • Group 15: N, P (non-metals), As, Sb (metalloids), Bi (metal); configuration ns2 np3. Nitrogen is relatively inert because of its very strong N≡N triple bond (945 kJ/mol); phosphorus is much more reactive (white P ignites in air, stored under water).
  • Oxidation states range from −3 to +5; the +3 state becomes more stable down the group (inert pair effect). Hydrides: NH3, PH3, AsH3; basic strength and thermal stability decrease down the group; NH3 has the highest boiling point because of hydrogen bonding.
  • Ammonia: made by the Haber process; basic (NH3 + H2O ⇌ NH4+ + OH−); forms complexes ([Cu(NH3)4]2+, deep blue); reducing agent (2NH3 + 3CuO → 3Cu + N2 + 3H2O).
  • Nitric acid: made by the Ostwald process; strong acid and strong oxidising agent. Dilute HNO3 with copper gives NO (3Cu + 8HNO3 → 3Cu(NO3)2 + 2NO + 4H2O); concentrated gives NO2.
  • Phosphorus: allotropes white (P4, reactive, toxic) and red (polymeric, stable, used in matches). P4O10 is a powerful dehydrating agent and dissolves to form phosphoric acid, H3PO4 (tribasic, weak), used in fertilisers, soft drinks and rust removal. PCl3 and PCl5 hydrolyse in water.
  • Tests: NO3−: warm with NaOH and aluminium foil (Devarda's alloy), ammonia is released (or brown-ring test); PO43−: yellow precipitate with ammonium molybdate in nitric acid. Uses: fertilisers (ammonium nitrate, phosphates), explosives, matches, food additives.

Common misconceptions

  • "Phosphorus is unreactive like nitrogen." No: Phosphorus has weak P–P single bonds in P4 and is very reactive; white phosphorus ignites in air.
  • "Nitric acid reacts with copper to give hydrogen like other acids." No: Nitric acid is an oxidising agent; it gives nitrogen oxides (NO or NO2), not hydrogen.
  • "Phosphoric acid is a strong acid like nitric acid." No: H3PO4 is a weak, tribasic acid.

Worked examples

Example 1. Explain why nitrogen is unreactive while white phosphorus is very reactive.

Solution

Nitrogen exists as N2 with a triple bond (bond energy 945 kJ/mol); much energy is needed to break it.

White phosphorus is P4: four P atoms in a tetrahedron joined by single bonds with 60° angles.

These bonds are weak and strained (about 200 kJ/mol), so phosphorus reacts readily, even igniting in air.

Example 2. Write equations for the reactions of copper with dilute and with concentrated nitric acid and name the gases.

Solution

Dilute: 3Cu + 8HNO3 → 3Cu(NO3)2 + 2NO + 4H2O; gas: nitrogen monoxide (colourless, turns brown in air).

Concentrated: Cu + 4HNO3 → Cu(NO3)2 + 2NO2 + 2H2O; gas: nitrogen dioxide (brown).

No hydrogen forms because nitric acid acts as an oxidising agent.

Practice questions

  1. Write the configuration of phosphorus (Z = 15).

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    1s2 2s2 2p6 3s2 3p3.

    Common mistake: Writing 3p5.

  2. Name the two main allotropes of phosphorus.

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    White and red phosphorus.

    Common mistake: Naming diamond and graphite.

  3. Give one use of red phosphorus.

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    Safety matches (striking surface).

    Common mistake: Saying fertiliser directly.

  4. Write the equation for ammonia dissolving in water.

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    NH3 + H2O ⇌ NH4+ + OH−.

    Common mistake: Writing a one-way arrow and calling it strong.

  5. What is seen when excess ammonia solution is added to copper(II) sulfate solution?

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    A pale blue precipitate forms, then dissolves to a deep blue solution.

    Common mistake: Saying a white precipitate.

  6. State the test for phosphate ions.

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    Add ammonium molybdate in nitric acid and warm: yellow precipitate.

    Common mistake: Saying white precipitate with silver nitrate only.

  7. Why is P4O10 used to dry gases?

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    It reacts strongly with water.

    Common mistake: Saying it is a base.

  8. Give two uses of nitric acid.

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    Fertilisers, explosives, dyes, purifying metals (any two).

    Common mistake: Saying car batteries.

  9. Explain why bismuth is a metal although nitrogen is a non-metal.

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    Metallic character increases down the group as atoms get larger and ionisation energies fall.

    Common mistake: Saying bismuth has fewer protons.

  10. Write the equation for the hydrolysis of PCl5.

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    PCl5 + 4H2O → H3PO4 + 5HCl.

    Common mistake: Writing H3PO3.

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. Ammonia reduces hot copper(II) oxide to:a. copper(II) nitrate, nitrogen monoxide and water
    2. Dilute nitric acid reacts with copper to give:b. Ostwald process
    3. Nitric acid is manufactured by the:c. Nitrogen dioxide
    4. Concentrated nitric acid with copper gives which brown gas?d. copper
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    1 → d (copper) · 2 → a (copper(II) nitrate, nitrogen monoxide and water) · 3 → b (Ostwald process) · 4 → c (Nitrogen dioxide)

    • 2NH3 + 3CuO → 3Cu + N2 + 3H2O.
    • Nitric acid is an oxidising agent.
    • Catalytic oxidation of ammonia.
    • Cu + 4HNO3 → Cu(NO3)2 + 2NO2 + 2H2O.

  2. Drag and drop

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

    bleach  ·  fertilisers and soft drinks  ·  phosphoric acid and hydrogen chloride  ·  catches fire in air
    1. Phosphorus pentachloride reacts with water to give .
    2. Phosphoric acid is used in .
    3. White phosphorus is stored under water because it .
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    1: phosphoric acid and hydrogen chloride · 2: fertilisers and soft drinks · 3: catches fire in air

    • PCl5 + 4H2O → H3PO4 + 5HCl.
    • It gives a tangy taste and makes phosphate fertilisers.
    • It ignites spontaneously at about 30 °C.

  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. Phosphate ions give a yellow precipitate with:a. a deep blue solution
    2. The test for nitrate ions uses NaOH and:b. aluminium foil, releasing ammonia
    3. Ammonia solution with copper(II) ions in excess forms:c. ns2 np3
    4. Group 15 elements have outer configuration:d. ammonium molybdate in nitric acid
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    1 → d (ammonium molybdate in nitric acid) · 2 → b (aluminium foil, releasing ammonia) · 3 → a (a deep blue solution) · 4 → c (ns2 np3)

    • Characteristic test for PO43−.
    • Nitrate is reduced to ammonia.
    • [Cu(NH3)4]2+ complex.
    • Five outer electrons.

  4. Drag and drop

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

    ammonia  ·  decreases  ·  strong triple bond  ·  stibine
    1. Basic strength of group 15 hydrides down the group .
    2. Nitrogen is unreactive mainly because of its .
    3. The hydride with the highest boiling point in group 15 is .
    Show answer

    1: decreases · 2: strong triple bond · 3: ammonia

    • NH3 is the strongest base.
    • N≡N bond energy is very high.
    • Hydrogen bonding in NH3.

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