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Chemistry · Practice book · Senior 2 · Unit 6 of 9

Preparation of oxygen and its properties

Senior 2 Chemistry · 14 questions with answers. Try each question on paper, then tap it to see the answer.

Unit 6 Preparation of oxygen and its properties

Key unit competence: Analyse the preparation, properties and roles of oxygen and its allotropes in supporting life and maintaining environmental balance.

Key ideas

  • Laboratory preparation: decomposition of hydrogen peroxide with manganese(IV) oxide as a catalyst, 2H2O2 → 2H2O + O2; or heating potassium manganate(VII) or potassium chlorate (with MnO2): 2KClO3 → 2KCl + 3O2.
  • Industrial preparation: fractional distillation of liquid air (oxygen boils at −183 °C, nitrogen at −196 °C) and electrolysis of water.
  • Oxygen is collected over water (it is only slightly soluble) or by upward displacement of air (it is slightly denser than air). Other methods: downward delivery (upward displacement) for denser gases and gas syringe for measured volumes.
  • Physical properties: colourless, odourless, tasteless gas, slightly soluble in water, slightly denser than air. Test: it relights a glowing splint.
  • Chemical properties: supports combustion. Metals burn to give basic oxides (2Mg + O2 → 2MgO); non-metals give acidic oxides (S + O2 → SO2, C + O2 → CO2). It is needed for respiration and causes rusting.
  • Allotropes of oxygen: dioxygen (O2) and ozone (O3). The ozone layer absorbs harmful ultraviolet radiation. Uses of oxygen: hospitals (breathing support), welding and cutting (oxy-acetylene flame), steel making, rocket fuel, diving and mountain climbing.

Common misconceptions

  • "Manganese(IV) oxide is a reactant that produces oxygen." No: It is a catalyst: it speeds up the decomposition of H2O2 and is not used up.
  • "Oxygen itself burns." No: Oxygen does not burn; it supports the burning of other substances.
  • "A glowing splint test shows hydrogen." No: A glowing splint relights in oxygen; hydrogen gives a squeaky pop with a lighted splint.

Worked examples

Example 1. Describe the laboratory preparation of oxygen from hydrogen peroxide, including how it is collected and tested.

Solution

Put manganese(IV) oxide in a flask and add hydrogen peroxide from a dropping funnel.

2H2O2 → 2H2O + O2; MnO2 is a catalyst.

Collect the gas over water in a gas jar (discard the first jar of air).

Test: a glowing splint relights.

Example 2. Calculate the mass of hydrogen peroxide needed to produce 1.12 dm3 of oxygen at STP. (H = 1, O = 16)

Solution

n(O2) = 1.12 ÷ 22.4 = 0.05 mol.

From 2H2O2 → 2H2O + O2, n(H2O2) = 2 × 0.05 = 0.1 mol.

Mr H2O2 = 34; mass = 0.1 × 34 = 3.4 g.

Practice questions

  1. Write the equation for the decomposition of hydrogen peroxide.

    Show answer

    2H2O2 → 2H2O + O2.

    Common mistake: Writing H2O2 → H2 + O2.

  2. State the test for oxygen.

    Show answer

    It relights a glowing splint.

    Common mistake: Saying it puts out a flame.

  3. Name two methods of collecting oxygen.

    Show answer

    Over water; upward displacement of air (downward delivery); gas syringe.

    Common mistake: Saying downward displacement of air (used for lighter gases).

  4. Give two uses of oxygen.

    Show answer

    Hospitals, welding, steel making, rockets, diving (any two).

    Common mistake: Saying 'to put out fires'.

  5. What is ozone and where is the ozone layer?

    Show answer

    An allotrope of oxygen, O3; in the upper atmosphere (stratosphere).

    Common mistake: Saying it is in the soil.

  6. Write equations for burning carbon and calcium in oxygen.

    Show answer

    C + O2 → CO2; 2Ca + O2 → 2CaO.

    Common mistake: Writing CaO2.

  7. Why is oxygen not collected by downward displacement of air?

    Show answer

    It is slightly denser than air, so it would not stay in an upside-down jar.

    Common mistake: Saying because it is soluble.

  8. Explain the role of MnO2 in the preparation of oxygen.

    Show answer

    Catalyst: speeds up decomposition of H2O2 and is unchanged at the end.

    Common mistake: Calling it a reactant.

  9. Calculate the volume of oxygen at STP needed to burn 0.2 mol of magnesium.

    Show answer

    2Mg + O2 → 2MgO: 0.1 mol O2; 0.1 × 22.4 = 2.24 dm3.

    Common mistake: Using a 1 : 1 ratio.

  10. Explain why oxygen is described as the 'active' part of air.

    Show answer

    It reacts in burning, respiration and rusting; nitrogen does not.

    Common mistake: Saying oxygen is the largest part.

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. Oxygen boils at −183 °C and nitrogen at −196 °C. During fractional distillation of liquid air, which boils off first?a. Nitrogen
    2. Oxygen makes up about what fraction of air?b. Colourless and odourless
    3. Oxygen is used with acetylene for welding because:c. One fifth
    4. Which is a physical property of oxygen?d. the mixture burns with a very hot flame
    Show answer

    1 → a (Nitrogen) · 2 → c (One fifth) · 3 → d (the mixture burns with a very hot flame) · 4 → b (Colourless and odourless)

    • Nitrogen has the lower boiling point.
    • About 21%.
    • The oxy-acetylene flame reaches about 3000 °C.
    • Oxygen has no colour or smell.

  2. Drag and drop

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

    measure the volume of gas produced  ·  carbon dioxide  ·  oxygen and water  ·  potassium chloride
    1. Rusting of iron needs .
    2. Heating potassium chlorate with MnO2 gives oxygen and .
    3. A gas syringe is used to collect gases when you want to .
    Show answer

    1: oxygen and water · 2: potassium chloride · 3: measure the volume of gas produced

    • Both are required for rust to form.
    • 2KClO3 → 2KCl + 3O2.
    • It is graduated for volume.

  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 test identifies oxygen gas?a. fractional distillation of liquid air
    2. The balanced equation for decomposition of hydrogen peroxide is:b. Helping patients with breathing difficulties
    3. Which use of oxygen is found in hospitals?c. It relights a glowing splint
    4. Industrially, oxygen is obtained from:d. 2H2O2 → 2H2O + O2
    Show answer

    1 → c (It relights a glowing splint) · 2 → d (2H2O2 → 2H2O + O2) · 3 → b (Helping patients with breathing difficulties) · 4 → a (fractional distillation of liquid air)

    • Oxygen supports combustion strongly.
    • Left: 4 H and 4 O; right: 4 H and 4 O.
    • Pure oxygen supports patients.
    • Liquid air is separated by boiling points.

  4. Drag and drop

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

    an acidic solution  ·  acidic  ·  ultraviolet radiation  ·  basic
    1. The ozone layer protects living things from .
    2. Carbon dioxide formed by burning carbon in oxygen dissolves in water to give .
    3. The oxide formed when magnesium burns in oxygen is .
    Show answer

    1: ultraviolet radiation · 2: an acidic solution · 3: basic

    • Ozone absorbs harmful UV rays.
    • CO2 + H2O → H2CO3, a weak acid.
    • Metal oxides are basic.

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