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Chemistry · Practice book · Senior 3 · Unit 2 of 10

Nitrogen and its inorganic compounds

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

Unit 2 Nitrogen and its inorganic compounds

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

Key ideas

  • Nitrogen makes up about 78% of air. It is obtained industrially by fractional distillation of liquid air; in the lab by heating ammonium nitrite (or NH4Cl + NaNO2): NH4NO2 → N2 + 2H2O. It is colourless, odourless, slightly less dense than air and quite unreactive because of the strong triple bond N≡N.
  • Nitrogen reacts at high temperature: with hydrogen to form ammonia, with oxygen (in lightning or car engines) to form NO, and with magnesium to form magnesium nitride (3Mg + N2 → Mg3N2). Uses: making ammonia and fertilisers, food packaging, liquid nitrogen for freezing.
  • Ammonia (NH3): lab preparation by heating an ammonium salt with an alkali (2NH4Cl + Ca(OH)2 → CaCl2 + 2H2O + 2NH3), dried with CaO and collected by upward delivery. It is a colourless, pungent gas, lighter than air, very soluble in water (fountain experiment), forms an alkaline solution and gives white fumes with HCl (NH3 + HCl → NH4Cl).
  • Haber process: N2 + 3H2 ⇌ 2NH3, iron catalyst, about 450 °C and 200 atm. Ammonia is used for fertilisers (ammonium nitrate, ammonium sulfate, urea), nitric acid, cleaning agents and refrigeration.
  • Nitric acid (HNO3) is made by the Ostwald process: ammonia is oxidised to NO over a platinum catalyst, then to NO2, which dissolves in water with oxygen. Dilute nitric acid shows typical acid properties; concentrated nitric acid is a strong oxidising agent.
  • Nitrates: all soluble; on heating, Na/K nitrates give nitrite + oxygen (2NaNO3 → 2NaNO2 + O2); most other metal nitrates give oxide + NO2 + O2 (2Cu(NO3)2 → 2CuO + 4NO2 + O2). Environmental issues: excess nitrate fertilisers cause eutrophication; nitrogen oxides cause acid rain and smog.

Common misconceptions

  • "Nitrogen is reactive because it is the main gas of air." No: Nitrogen is unreactive because of its strong triple bond.
  • "Ammonia can be collected over water." No: Ammonia is very soluble in water; it is collected by upward delivery.
  • "All nitrates decompose in the same way on heating." No: Sodium and potassium nitrates give nitrites and oxygen; most others give oxides, NO2 and O2.

Worked examples

Example 1. Describe the laboratory preparation of dry ammonia gas.

Solution

Mix ammonium chloride with calcium hydroxide and heat gently in a tube sloping downwards.

2NH4Cl + Ca(OH)2 → CaCl2 + 2H2O + 2NH3.

Pass the gas through a tower of calcium oxide to dry it.

Collect by upward delivery (downward displacement of air) since ammonia is lighter than air; test with damp red litmus (turns blue).

Example 2. State the conditions of the Haber process and explain why each is used.

Solution

Equation: N2 + 3H2 ⇌ 2NH3, forward reaction exothermic.

Pressure about 200 atm: high pressure favours fewer gas molecules (more NH3) and speeds up the reaction.

Temperature about 450 °C: a compromise between a good rate and an acceptable yield.

Iron catalyst: speeds up the reaction; unreacted gases are recycled.

Practice questions

  1. Name the industrial method of obtaining nitrogen.

    Show answer

    Fractional distillation of liquid air.

    Common mistake: Saying the Haber process.

  2. Give two uses of nitrogen.

    Show answer

    Making ammonia, food packaging, liquid nitrogen for freezing (any two).

    Common mistake: Saying it supports burning.

  3. Write the equation for the reaction of ammonia with hydrogen chloride.

    Show answer

    NH3 + HCl → NH4Cl.

    Common mistake: Writing NH4Cl2.

  4. Why can ammonia not be dried using concentrated sulfuric acid?

    Show answer

    Ammonia is basic and reacts with the acid.

    Common mistake: Saying sulfuric acid is too weak.

  5. State the catalyst and temperature used in the Haber process.

    Show answer

    Iron; about 450 °C.

    Common mistake: Saying platinum.

  6. Write the equation for heating potassium nitrate.

    Show answer

    2KNO3 → 2KNO2 + O2.

    Common mistake: Writing NO2 as a product.

  7. Give two uses of nitric acid.

    Show answer

    Making fertilisers (ammonium nitrate), explosives, dyes (any two).

    Common mistake: Saying it is used in car batteries.

  8. Explain why nitrogen fertilisers can pollute lakes.

    Show answer

    Nitrates wash into lakes and cause algal blooms (eutrophication) that reduce oxygen.

    Common mistake: Saying they make lakes acidic only.

  9. Calculate the percentage of nitrogen in ammonium nitrate (Mr 80).

    Show answer

    28 ÷ 80 × 100 = 35%.

    Common mistake: Counting only one N atom.

  10. Describe the fountain experiment and what it shows.

    Show answer

    A flask of dry ammonia is inverted with a jet in water containing indicator; water rushes in as a fountain and changes colour, showing ammonia is very soluble and alkaline.

    Common mistake: Saying it shows ammonia is denser than air.

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. Magnesium burns in nitrogen to form:a. sodium nitrite and oxygen
    2. Which drying agent is used for ammonia?b. Calcium oxide
    3. Heating sodium nitrate gives:c. very soluble in water
    4. The fountain experiment shows that ammonia is:d. magnesium nitride, Mg3N2
    Show answer

    1 → d (magnesium nitride, Mg3N2) · 2 → b (Calcium oxide) · 3 → a (sodium nitrite and oxygen) · 4 → c (very soluble in water)

    • 3Mg + N2 → Mg3N2.
    • Acidic drying agents and CaCl2 react with ammonia.
    • 2NaNO3 → 2NaNO2 + O2.
    • Ammonia dissolves so fast that water is sucked up.

  2. Drag and drop

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

    N2 + H2 ⇌ NH3  ·  N2 + 3H2 ⇌ 2NH3  ·  nitrogen monoxide  ·  nitric acid
    1. The balanced equation for the Haber process is .
    2. Ammonium nitrate is made by reacting ammonia with .
    3. Lightning causes nitrogen and oxygen to combine to form .
    Show answer

    1: N2 + 3H2 ⇌ 2NH3 · 2: nitric acid · 3: nitrogen monoxide

    • 1 : 3 : 2 mole ratio.
    • NH3 + HNO3 → NH4NO3.
    • N2 + O2 → 2NO at very high temperature.

  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. Ammonia is collected by upward delivery because it is:a. Urea
    2. The catalyst used in the Haber process is:b. lighter than air and very soluble in water
    3. Nitrogen is unreactive mainly because:c. of the strong triple bond between its atoms
    4. Which fertiliser contains the most nitrogen per kilogram?d. iron
    Show answer

    1 → b (lighter than air and very soluble in water) · 2 → d (iron) · 3 → c (of the strong triple bond between its atoms) · 4 → a (Urea)

    • It rises and cannot be collected over water.
    • Finely divided iron speeds up the reaction.
    • Much energy is needed to break N≡N.
    • Urea is about 46.7% nitrogen.

  4. Drag and drop

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

    Contact process  ·  soluble in water  ·  Haber process  ·  Ostwald process
    1. All nitrates are .
    2. Nitric acid is made industrially by the .
    3. Ammonia is made industrially by the .
    Show answer

    1: soluble in water · 2: Ostwald process · 3: Haber process

    • This is a solubility rule.
    • Ammonia is oxidised over platinum.
    • Nitrogen and hydrogen combine over an iron catalyst.

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