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Chemistry · Practice book · Senior 4 · Unit 7 of 17

Trends in chemical properties of group 2 elements and their compounds

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

Unit 7 Trends in chemical properties of group 2 elements and their compounds

Key unit competence: Interpret the periodic trends in the properties of group 2 elements and their compounds to explain their chemical reactivity and uses.

Key ideas

  • Group 2 (alkaline earth metals): Be, Mg, Ca, Sr, Ba, Ra; configuration ns2; form M2+ ions. Occurrence: limestone and marble (CaCO3), dolomite (CaCO3·MgCO3), gypsum (CaSO4·2H2O), magnesite (MgCO3).
  • Reactivity increases down the group (first + second ionisation energies decrease). With water: Mg reacts very slowly with cold water but burns in steam (Mg + H2O → MgO + H2); Ca, Sr, Ba react with cold water (Ca + 2H2O → Ca(OH)2 + H2) more vigorously down the group. All burn in oxygen to form MO.
  • Hydroxides: solubility increases down the group (Mg(OH)2 sparingly soluble, Ba(OH)2 quite soluble), so basic strength increases. Sulfates: solubility decreases down the group (MgSO4 soluble, BaSO4 insoluble, used in the sulfate test and in barium meals).
  • Thermal stability of carbonates and nitrates increases down the group because larger cations polarise the anion less: MgCO3 decomposes easily, BaCO3 needs very high temperatures. 2Mg(NO3)2 → 2MgO + 4NO2 + O2.
  • Beryllium is anomalous: very small, high charge density, so its compounds are covalent (e.g. BeCl2), its oxide and hydroxide are amphoteric, and it does not react with water. It shows a diagonal relationship with aluminium.
  • Test for Ba2+: white precipitate with dilute sulfuric acid or sulfate ions; apple-green flame. Flame colours: Ca brick-red, Sr crimson, Ba apple-green. Uses: Mg in alloys and flares, MgO in furnace linings, Mg(OH)2 antacid, CaO/Ca(OH)2 in agriculture and building, CaCO3 in cement and glass, CaSO4 plaster of Paris, BaSO4 for X-ray meals.

Common misconceptions

  • "Solubility of all group 2 compounds increases down the group." No: Hydroxides become more soluble but sulfates become LESS soluble down the group.
  • "Barium meals are dangerous because barium is toxic." No: Soluble barium compounds are toxic, but BaSO4 is so insoluble that it passes through the body safely.
  • "Magnesium reacts vigorously with cold water like calcium." No: Magnesium reacts only very slowly with cold water but burns in steam.

Worked examples

Example 1. Describe the reactions of magnesium and calcium with water and explain the difference.

Solution

Magnesium reacts very slowly with cold water but burns with a bright white light in steam: Mg + H2O → MgO + H2.

Calcium reacts steadily with cold water, fizzing and forming a cloudy solution of calcium hydroxide: Ca + 2H2O → Ca(OH)2 + H2.

Calcium is more reactive because its outer electrons are further from the nucleus and more shielded, so the ionisation energies are lower.

Example 2. Explain the trends in solubility of group 2 hydroxides and sulfates and give one use of each trend.

Solution

Hydroxides: solubility increases down the group; Mg(OH)2 is sparingly soluble (safe antacid), Ca(OH)2 slightly soluble (limewater, lime for soils).

Sulfates: solubility decreases down the group; MgSO4 is soluble (Epsom salts), BaSO4 is insoluble.

The insolubility of BaSO4 is used in the test for sulfate ions and in barium meals for X-rays.

Practice questions

  1. Write the equation for calcium burning in oxygen.

    Show answer

    2Ca + O2 → 2CaO.

    Common mistake: Writing CaO2.

  2. Give the trend in reactivity of group 2 metals and a reason.

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    Increases down the group; outer electrons further from nucleus and more shielded.

    Common mistake: Saying it decreases like halogens.

  3. Name two group 2 compounds used in medicine.

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    Mg(OH)2 (antacid), BaSO4 (barium meal), MgSO4 (Epsom salts) (any two).

    Common mistake: Naming barium chloride.

  4. Write the equation for the thermal decomposition of calcium carbonate.

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    CaCO3 → CaO + CO2.

    Common mistake: Writing Ca + CO2.

  5. State two anomalous properties of beryllium.

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    Covalent compounds; amphoteric oxide/hydroxide; no reaction with water (any two).

    Common mistake: Saying it is the most reactive.

  6. Why is limewater used to test for CO2?

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    It forms a white precipitate of CaCO3.

    Common mistake: Saying it changes colour.

  7. Give the flame colour of strontium.

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    Crimson (red).

    Common mistake: Saying lilac.

  8. Explain why BaSO4 can be swallowed for X-rays although barium ions are poisonous.

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    It is so insoluble that almost no Ba2+ ions are released into the body.

    Common mistake: Saying barium is not toxic.

  9. Write the equation for magnesium nitrate decomposition.

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    2Mg(NO3)2 → 2MgO + 4NO2 + O2.

    Common mistake: Writing magnesium nitrite.

  10. Explain why calcium oxide is used to treat acidic soils.

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    It is a basic oxide (forms Ca(OH)2) that neutralises soil acids.

    Common mistake: Saying it adds nitrogen.

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 group 2 metal is used in flares and fireworks for bright white light?a. Magnesium
    2. Limestone is mainly:b. 2+
    3. Calcium reacts with cold water to form:c. calcium hydroxide and hydrogen
    4. Group 2 metals form ions with charge:d. calcium carbonate
    Show answer

    1 → a (Magnesium) · 2 → d (calcium carbonate) · 3 → c (calcium hydroxide and hydrogen) · 4 → b (2+)

    • Magnesium burns with a brilliant white flame.
    • CaCO3.
    • Ca + 2H2O → Ca(OH)2 + H2.
    • They lose two electrons.

  2. Drag and drop

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

    apple-green  ·  covalent  ·  ns2  ·  crimson
    1. Beryllium chloride is mainly .
    2. Group 2 elements have outer configuration .
    3. The flame colour of barium is .
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    1: covalent · 2: ns2 · 3: apple-green

    • Small Be2+ polarises chloride ions strongly.
    • Two electrons in the outer s orbital.
    • Ba gives apple-green.

  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. Which compound is used as an antacid?a. Magnesium carbonate
    2. Heating calcium nitrate produces:b. calcium oxide, nitrogen dioxide and oxygen
    3. The solubility of group 2 hydroxides down the group:c. Magnesium hydroxide
    4. Which carbonate decomposes most easily on heating?d. increases
    Show answer

    1 → c (Magnesium hydroxide) · 2 → b (calcium oxide, nitrogen dioxide and oxygen) · 3 → d (increases) · 4 → a (Magnesium carbonate)

    • Milk of magnesia neutralises stomach acid.
    • Group 2 nitrates give oxide, NO2 and O2.
    • Ba(OH)2 is the most soluble.
    • Small Mg2+ polarises carbonate most.

  4. Drag and drop

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

    brick-red  ·  increases  ·  white precipitate with dilute sulfuric acid  ·  decreases
    1. The flame colour of calcium is .
    2. The test for Ba2+ ions is .
    3. Reactivity of group 2 metals down the group .
    Show answer

    1: brick-red · 2: white precipitate with dilute sulfuric acid · 3: increases

    • Ca gives brick-red.
    • Ba2+ + SO42− → BaSO4.
    • Electrons are lost more easily.

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