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Chemistry · Practice book · Senior 4 · Unit 8 of 17

Trends in chemical properties of group 13 elements and their compounds

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

Unit 8 Trends in chemical properties of group 13 elements and their compounds

Key unit competence: Interpret the variations in the properties of group 13 elements and their compounds in relation to their electronic configurations and uses.

Key ideas

  • Group 13: B, Al, Ga, In, Tl; configuration ns2 np1. Boron is a metalloid; the rest are metals. Aluminium is the most abundant metal in the Earth's crust (bauxite, Al2O3·2H2O).
  • Common oxidation state +3; the +1 state becomes more stable down the group (thallium(I) is stable) due to the inert pair effect.
  • Aluminium reacts with oxygen to form a protective oxide layer (4Al + 3O2 → 2Al2O3); with dilute HCl (2Al + 6HCl → 2AlCl3 + 3H2); with hot NaOH (2Al + 2NaOH + 6H2O → 2Na[Al(OH)4] + 3H2); with chlorine to form AlCl3, which exists as the dimer Al2Cl6 in the vapour (dative bonds).
  • Aluminium oxide and hydroxide (and gallium's) are amphoteric: Al2O3 + 6HCl → 2AlCl3 + 3H2O and Al2O3 + 2NaOH + 3H2O → 2Na[Al(OH)4].
  • Boron is anomalous: very small, high ionisation energy; forms covalent compounds; B2O3 is acidic; boric acid H3BO3 is a weak acid; BF3 is electron-deficient (Lewis acid). Diagonal relationship with silicon.
  • Test for Al3+: white precipitate with NaOH that dissolves in excess; white precipitate with ammonia solution that does NOT dissolve in excess. Uses: Al in cans, foil, cables, aircraft and window frames; alum in water treatment; Al2O3 in abrasives; boron in borosilicate glass and as borax.

Common misconceptions

  • "Aluminium is unreactive because it does not corrode." No: Aluminium is quite reactive but is protected by a thin, tough oxide layer.
  • "Al(OH)3 is only basic." No: It is amphoteric: it dissolves in both acids and excess alkali.
  • "All group 13 elements are metals." No: Boron is a metalloid with mainly covalent chemistry.

Worked examples

Example 1. Show, with equations, that aluminium oxide is amphoteric.

Solution

With hydrochloric acid (acting as a base): Al2O3 + 6HCl → 2AlCl3 + 3H2O.

With sodium hydroxide (acting as an acid): Al2O3 + 2NaOH + 3H2O → 2Na[Al(OH)4].

Reacting with both an acid and a base means it is amphoteric.

Example 2. Describe how to test for Al3+ ions and distinguish them from Zn2+.

Solution

Add NaOH solution dropwise: both give a white precipitate that dissolves in excess (amphoteric hydroxides).

Add aqueous ammonia to a fresh sample: both give a white precipitate.

In excess ammonia, the zinc precipitate dissolves (forms [Zn(NH3)4]2+) but the aluminium precipitate does not.

Practice questions

  1. Write the configuration of aluminium (Z = 13).

    Show answer

    1s2 2s2 2p6 3s2 3p1.

    Common mistake: Writing 3s1 3p2.

  2. Give two uses of aluminium and the property behind each.

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    Cans/foil (light, non-toxic, corrosion resistant); cables (conductor, light); aircraft (light, strong alloy) (any two).

    Common mistake: Saying it is magnetic.

  3. Write the equation for aluminium burning in oxygen.

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    4Al + 3O2 → 2Al2O3.

    Common mistake: Writing AlO.

  4. Define amphoteric.

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    Able to react with both acids and bases.

    Common mistake: Saying 'neither acidic nor basic'.

  5. State what is meant by the inert pair effect.

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    Reluctance of the outer ns2 electrons to take part in bonding in heavy p-block elements.

    Common mistake: Saying the elements are inert gases.

  6. Name the dimer formed by aluminium chloride.

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    Al2Cl6.

    Common mistake: Writing AlCl6.

  7. Why is BF3 described as a Lewis acid?

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    It can accept a pair of electrons (boron has an incomplete octet).

    Common mistake: Saying it releases H+.

  8. Write the ionic equation for the precipitation of aluminium hydroxide.

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    Al3+(aq) + 3OH−(aq) → Al(OH)3(s).

    Common mistake: Writing AlOH.

  9. Give one anomalous property of boron.

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    Metalloid / covalent compounds / acidic oxide / high melting point (any one).

    Common mistake: Saying it reacts violently with water.

  10. Calculate the mass of aluminium in 51 kg of Al2O3.

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    51 × 54/102 = 27 kg.

    Common mistake: Using 27/102.

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. Aluminium reacts with hot NaOH solution to give hydrogen and:a. weak acid
    2. Boron shows a diagonal relationship with:b. silicon
    3. Aluminium oxide is:c. amphoteric
    4. Boric acid, H3BO3, is a:d. sodium aluminate
    Show answer

    1 → d (sodium aluminate) · 2 → b (silicon) · 3 → c (amphoteric) · 4 → a (weak acid)

    • Shows amphoteric character of Al.
    • Similar charge density and properties.
    • It reacts with acids and alkalis.
    • Boron oxide is acidic.

  2. Drag and drop

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

    turns brown  ·  coagulate fine particles  ·  dissolves in excess NaOH  ·  Al2Cl6 dimers
    1. Alum is used in water treatment to .
    2. Aluminium chloride vapour exists as .
    3. With NaOH solution, Al3+ gives a white precipitate that .
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    1: coagulate fine particles · 2: Al2Cl6 dimers · 3: dissolves in excess NaOH

    • Particles clump and settle.
    • Dative bonds link two AlCl3 units.
    • Forms soluble aluminate.

  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. With ammonia solution, Al3+ gives a white precipitate that:a. does not dissolve in excess
    2. Borosilicate glass (Pyrex) is used for laboratory glassware because it:b. Aircraft bodies
    3. Which is a use of aluminium?c. resists sudden temperature changes
    4. Aluminium is extracted from its oxide by:d. electrolysis of molten Al2O3 in cryolite
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    1 → a (does not dissolve in excess) · 2 → c (resists sudden temperature changes) · 3 → b (Aircraft bodies) · 4 → d (electrolysis of molten Al2O3 in cryolite)

    • Distinguishes Al from Zn.
    • Low thermal expansion.
    • It is light, strong as an alloy and resists corrosion.
    • Aluminium is too reactive for carbon reduction.

  4. Drag and drop

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

    aluminium chloride and hydrogen  ·  aluminium oxide and chlorine  ·  +3  ·  ns2 np1
    1. Group 13 elements have the outer configuration .
    2. Aluminium reacts with dilute hydrochloric acid to give .
    3. The most common oxidation state of aluminium is .
    Show answer

    1: ns2 np1 · 2: aluminium chloride and hydrogen · 3: +3

    • Three outer electrons.
    • 2Al + 6HCl → 2AlCl3 + 3H2.
    • It loses all three outer electrons.

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