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Chemistry · Practice book · Senior 3 · Unit 3 of 10

Sulfur and its inorganic compounds

Senior 3 Chemistry · 14 questions with answers. Try each question on paper, then tap it to see the answer.

Unit 3 Sulfur and its inorganic compounds

Key unit competence: Relate the physical and chemical properties of sulfur and its inorganic compounds to their preparation, uses and effects on the environment.

Key ideas

  • Sulfur occurs free in volcanic areas and underground deposits (extracted by the Frasch process), and combined in ores such as iron pyrite (FeS2), zinc blende (ZnS), galena (PbS) and gypsum (CaSO4·2H2O).
  • Allotropes of sulfur: rhombic (alpha, stable below 96 °C) and monoclinic (beta, stable above 96 °C); plastic sulfur forms when molten sulfur is poured into cold water. Sulfur is a yellow, brittle solid, insoluble in water, a non-conductor.
  • Sulfur burns with a blue flame to form sulfur dioxide (S + O2 → SO2), combines with metals when heated (Fe + S → FeS) and is used to make sulfuric acid, to vulcanise rubber, in fungicides and matches.
  • Sulfur dioxide: prepared by Na2SO3 + 2HCl → 2NaCl + H2O + SO2 or Cu + hot conc. H2SO4. Colourless, choking smell, denser than air, very soluble (forms sulfurous acid). It is a reducing agent: turns acidified potassium dichromate(VI) from orange to green and decolourises acidified potassium manganate(VII); it bleaches (by reduction). Uses: making H2SO4, bleaching paper and wool, food preservative.
  • Contact process: 2SO2 + O2 ⇌ 2SO3 over vanadium(V) oxide at about 450 °C and 1–2 atm; SO3 is absorbed in concentrated H2SO4 to form oleum (H2S2O7), which is diluted to give H2SO4. Concentrated sulfuric acid is a dehydrating agent (chars sugar), an oxidising agent and a drying agent; dilute sulfuric acid shows typical acid properties.
  • Tests: sulfate ions give a white precipitate with acidified barium chloride; sulfite ions give a white precipitate with barium chloride that dissolves in dilute HCl, releasing SO2. Sulfur oxides cause acid rain, which damages buildings, plants, soils and aquatic life.

Common misconceptions

  • "Sulfur dioxide is collected over water." No: SO2 is very soluble; it is collected by downward delivery (upward displacement of air).
  • "Water should be added to concentrated sulfuric acid to dilute it." No: Always add the acid slowly to water, because much heat is released.
  • "In the Contact process, SO3 is dissolved directly in water." No: That forms a dangerous acid mist; SO3 is absorbed in concentrated H2SO4 to form oleum.

Worked examples

Example 1. Describe the stages of the Contact process with equations and conditions.

Solution

Burn sulfur in air: S + O2 → SO2.

Oxidise SO2 with air over vanadium(V) oxide at about 450 °C and 1–2 atm: 2SO2 + O2 ⇌ 2SO3.

Absorb SO3 in concentrated sulfuric acid to form oleum: SO3 + H2SO4 → H2S2O7.

Dilute oleum with water: H2S2O7 + H2O → 2H2SO4.

Example 2. Explain why sulfur dioxide bleaches petals but the colour may return later.

Solution

SO2 is a reducing agent: in water it removes oxygen from the coloured dye, forming a colourless compound.

Air slowly re-oxidises the reduced dye.

So the colour can return (bleaching by SO2 is temporary, unlike bleaching by chlorine).

Practice questions

  1. Name two ores of sulfur.

    Show answer

    Iron pyrite (FeS2), zinc blende (ZnS), galena (PbS), gypsum (any two).

    Common mistake: Naming bauxite.

  2. Write the equation for sulfur burning in oxygen.

    Show answer

    S + O2 → SO2.

    Common mistake: Writing SO3.

  3. State two uses of sulfur.

    Show answer

    Making sulfuric acid, vulcanising rubber, fungicides, matches (any two).

    Common mistake: Saying it is used as fuel in cars.

  4. Describe the test for sulfur dioxide.

    Show answer

    Turns acidified potassium dichromate(VI) paper from orange to green.

    Common mistake: Using limewater only (CO2 also turns it milky).

  5. Name the catalyst and temperature of the Contact process.

    Show answer

    Vanadium(V) oxide; about 450 °C.

    Common mistake: Saying iron.

  6. What is observed when concentrated sulfuric acid is added to sugar?

    Show answer

    The sugar turns black (carbon), swells and gives off steam.

    Common mistake: Saying it dissolves into a clear solution.

  7. Why is SO3 not dissolved directly in water in the Contact process?

    Show answer

    The reaction is very exothermic and forms a fine acid mist that is hard to collect.

    Common mistake: Saying SO3 is insoluble in water.

  8. Write the ionic equation for the sulfate test.

    Show answer

    Ba2+(aq) + SO42−(aq) → BaSO4(s).

    Common mistake: Including chloride ions.

  9. Give two effects of acid rain.

    Show answer

    Damages buildings and statues, kills trees, acidifies lakes and kills fish, leaches soil nutrients (any two).

    Common mistake: Saying it causes ozone depletion.

  10. Calculate the volume of SO2 at STP produced by burning 16 g of sulfur.

    Show answer

    0.5 mol × 22.4 = 11.2 dm3.

    Common mistake: Using 32 g/mol for SO2.

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. Concentrated sulfuric acid turns sugar black because it is a:a. iron(II) sulfide
    2. Sulfur is used to harden rubber in a process called:b. White precipitate with acidified barium chloride
    3. Iron and sulfur heated together form:c. dehydrating agent
    4. Which test identifies sulfate ions?d. vulcanisation
    Show answer

    1 → c (dehydrating agent) · 2 → d (vulcanisation) · 3 → a (iron(II) sulfide) · 4 → b (White precipitate with acidified barium chloride)

    • It removes water from sugar, leaving carbon.
    • Sulfur forms cross-links between rubber chains.
    • Fe + S → FeS, a new compound.
    • BaSO4 is insoluble.

  2. Drag and drop

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

    sulfuric acid  ·  orange to green  ·  pouring boiling sulfur into cold water  ·  sulfurous acid
    1. Sulfur dioxide dissolves in water to form .
    2. Plastic sulfur is formed by .
    3. Sulfur dioxide turns acidified potassium dichromate(VI) from .
    Show answer

    1: sulfurous acid · 2: pouring boiling sulfur into cold water · 3: orange to green

    • SO2 + H2O → H2SO3.
    • Long chains of sulfur atoms are 'frozen' quickly.
    • SO2 reduces dichromate(VI) to chromium(III).

  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 ore contains sulfur?a. It absorbs water strongly
    2. Sulfur burns in air with a blue flame to form:b. reduction
    3. Which property of concentrated sulfuric acid lets it dry gases?c. sulfur dioxide
    4. Sulfur dioxide bleaches coloured flowers by:d. Iron pyrite, FeS2
    Show answer

    1 → d (Iron pyrite, FeS2) · 2 → c (sulfur dioxide) · 3 → a (It absorbs water strongly) · 4 → b (reduction)

    • Pyrite is iron sulfide.
    • S + O2 → SO2.
    • It is hygroscopic.
    • It removes oxygen from the dye; the colour may return in air.

  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  ·  vanadium(V) oxide  ·  reducing agent  ·  concentrated sulfuric acid
    1. Sulfur dioxide decolourises acidified potassium manganate(VII) because it is a .
    2. The catalyst in the Contact process is .
    3. In the Contact process, sulfur trioxide is absorbed in .
    Show answer

    1: reducing agent · 2: vanadium(V) oxide · 3: concentrated sulfuric acid

    • It reduces purple MnO4− to colourless Mn2+.
    • V2O5 speeds up SO2 oxidation.
    • This avoids a dangerous acid mist.

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