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Chemistry · Practice book · Senior 4 · Unit 9 of 17

Trends in chemical properties of group 14 elements and their compounds

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

Unit 9 Trends in chemical properties of group 14 elements and their compounds

Key unit competence: Interpret the periodic trends in the properties of group 14 elements and their compounds to explain their chemical behaviour and uses.

Key ideas

  • Group 14: C (non-metal), Si and Ge (metalloids), Sn and Pb (metals); configuration ns2 np2. Metallic character increases down the group; carbon and silicon form giant covalent structures (diamond, graphite, silicon), tin and lead are metallic.
  • Oxidation states +4 and +2. The +4 state is most stable for C and Si; the +2 state becomes more stable down the group (inert pair effect), so Pb2+ compounds are more stable than Pb(IV); PbO2 is a strong oxidising agent, while Sn(II) is a reducing agent.
  • Oxides: CO2 and SiO2 are acidic; CO is neutral; GeO2, SnO, SnO2, PbO and PbO2 are amphoteric. Nature changes from acidic (top) to amphoteric/basic (bottom).
  • Chlorides: CCl4 does not hydrolyse (carbon has no available d orbitals); SiCl4 hydrolyses rapidly: SiCl4 + 2H2O → SiO2 + 4HCl. PbCl2 is more stable and more ionic than PbCl4. Thermal stability of tetrachlorides decreases down the group.
  • Tests: Pb2+ gives a yellow precipitate with KI (PbI2) and a white precipitate with dilute HCl (PbCl2, soluble in hot water); Sn2+ reduces HgCl2 and gives a white precipitate with NaOH soluble in excess. CO32− gives CO2 with acid; HCO3− gives no precipitate with MgSO4 in the cold (carbonate does).
  • Uses: carbon (fuels, electrodes, diamonds), silicon (chips, solar cells, glass, cement), germanium (transistors, optical fibres), tin (coating steel cans, solder, bronze; cassiterite mined in Rwanda), lead (car batteries, radiation shielding). Diagonal relationship links Be–Al and B–Si.

Common misconceptions

  • "All group 14 elements prefer the +4 oxidation state." No: Down the group the +2 state becomes more stable; lead mainly forms Pb(II).
  • "CCl4 hydrolyses like SiCl4." No: CCl4 does not hydrolyse because carbon has no low-energy d orbitals to accept water's lone pair.
  • "Tin is a non-metal because it is in the same group as carbon." No: Metallic character increases down the group; tin and lead are metals.

Worked examples

Example 1. Explain the change in metallic character down group 14 with evidence.

Solution

Carbon (non-metal): giant covalent, does not conduct (except graphite), acidic oxide.

Silicon and germanium: metalloids, semiconductors.

Tin and lead: metals, conduct electricity, form positive ions and amphoteric oxides.

Down the group atoms get larger, ionisation energies fall, so electrons are lost more easily: metallic character increases.

Example 2. Compare the reactions of CCl4 and SiCl4 with water.

Solution

CCl4: no reaction; carbon has no available d orbitals and is shielded by four Cl atoms, so water cannot attack it.

SiCl4: reacts vigorously, giving white fumes of HCl: SiCl4 + 2H2O → SiO2 + 4HCl.

Silicon can accept a lone pair from water into its empty 3d orbitals.

Practice questions

  1. Write the configuration of silicon (Z = 14).

    Show answer

    1s2 2s2 2p6 3s2 3p2.

    Common mistake: Writing 3p4.

  2. Give two uses of tin.

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    Coating cans, solder, bronze, pewter (any two).

    Common mistake: Saying fertiliser.

  3. Write the equation for the hydrolysis of SiCl4.

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    SiCl4 + 2H2O → SiO2 + 4HCl.

    Common mistake: Writing SiCl4 + H2O → SiO + 2HCl2.

  4. State the inert pair effect for group 14.

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    The ns2 electrons become reluctant to bond down the group, so +2 becomes more stable.

    Common mistake: Saying the elements become noble gases.

  5. Why is lead(IV) oxide an oxidising agent?

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    Pb(IV) readily gains electrons to form the more stable Pb(II).

    Common mistake: Saying it loses electrons.

  6. Give the test for Pb2+ ions.

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    Yellow precipitate with KI (or white precipitate with HCl, soluble in hot water).

    Common mistake: Saying green flame.

  7. Classify the oxides CO, CO2, SnO2.

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    Neutral, acidic, amphoteric.

    Common mistake: Calling CO acidic.

  8. Give one reason why lead is no longer used in water pipes and paints.

    Show answer

    Lead compounds are toxic and damage the nervous system, especially in children.

    Common mistake: Saying lead is too expensive.

  9. Name the element in group 14 used in optical fibres and transistors.

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    Germanium.

    Common mistake: Saying carbon.

  10. Calculate the percentage of tin in cassiterite, SnO2.

    Show answer

    119 ÷ 151 × 100 = 78.8%.

    Common mistake: Using Sn = 50 (atomic number).

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. Lead is used in:a. Yellow
    2. Which element shows a diagonal relationship with boron?b. Silicon
    3. Which group 14 element is a metal?c. car batteries and radiation shielding
    4. Pb2+ ions give what colour precipitate with potassium iodide?d. Lead
    Show answer

    1 → c (car batteries and radiation shielding) · 2 → b (Silicon) · 3 → d (Lead) · 4 → a (Yellow)

    • Dense and used in lead–acid cells.
    • Similar properties across the diagonal.
    • Metallic character increases down the group.
    • Lead(II) iodide is bright yellow.

  2. Drag and drop

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

    computer chips and solar cells  ·  oxidising agent  ·  silicon dioxide  ·  car batteries
    1. Lead(IV) oxide is a strong .
    2. Silicon is used in .
    3. Sand is mainly .
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    1: oxidising agent · 2: computer chips and solar cells · 3: silicon dioxide

    • Pb(IV) tends to gain electrons to become Pb(II).
    • It is a semiconductor.
    • SiO2.

  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. A white precipitate with dilute HCl that dissolves in hot water indicates:a. Pb2+
    2. Carbon dioxide is:b. silicon dioxide and hydrogen chloride
    3. Silicon tetrachloride reacts with water to give:c. acidic
    4. Carbon monoxide is:d. a neutral oxide and reducing agent
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    1 → a (Pb2+) · 2 → c (acidic) · 3 → b (silicon dioxide and hydrogen chloride) · 4 → d (a neutral oxide and reducing agent)

    • PbCl2 is soluble in hot water.
    • It forms carbonic acid.
    • SiCl4 + 2H2O → SiO2 + 4HCl.
    • It reduces metal oxides.

  4. Drag and drop

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

    drying agent  ·  ns2 np2  ·  amphoteric  ·  reducing agent
    1. Group 14 elements have outer configuration .
    2. Lead(II) oxide is .
    3. Tin(II) chloride is a good .
    Show answer

    1: ns2 np2 · 2: amphoteric · 3: reducing agent

    • Four outer electrons.
    • It reacts with acids and alkalis.
    • Sn(II) is oxidised to Sn(IV).

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