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Chemistry · Practice book · Senior 3 · Unit 9 of 10

Alkanes

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

Unit 9 Alkanes

Key unit competence: Analyse the structure and properties of alkanes in relation to their uses.

Key ideas

  • Alkanes are saturated hydrocarbons (only C–C single bonds) with general formula CnH2n+2: methane CH4, ethane C2H6, propane C3H8, butane C4H10, pentane C5H12, hexane C6H14.
  • IUPAC naming: find the longest carbon chain (meth-, eth-, prop-, but-, pent-, hex-) + '-ane'; name branches (methyl, ethyl) with the number of the carbon they are on, numbering from the end that gives the lowest numbers, e.g. 2-methylpropane.
  • Structural isomers have the same molecular formula but different structures: C4H10 has butane and 2-methylpropane; C5H12 has three isomers.
  • Physical properties: the first four are gases, C5–C17 liquids, larger ones solids (wax). Boiling point increases with chain length and decreases with branching. Insoluble in water, less dense than water.
  • Chemical properties: generally unreactive. They burn: complete combustion gives CO2 and H2O (CH4 + 2O2 → CO2 + 2H2O); incomplete combustion gives CO or soot. With chlorine in UV light they undergo substitution: CH4 + Cl2 → CH3Cl + HCl. Large alkanes can be cracked to smaller alkanes and alkenes.
  • Sources and uses: natural gas and biogas (methane), Lake Kivu methane, crude oil fractions (LPG, petrol, kerosene, diesel, lubricating oil, wax, bitumen); used as fuels, solvents and raw materials for plastics.

Common misconceptions

  • "Branched isomers have higher boiling points than straight chains." No: Branching reduces contact between molecules, so branched isomers have LOWER boiling points.
  • "Alkanes react by addition like alkenes." No: Alkanes are saturated; they react by substitution, not addition.
  • "Incomplete combustion produces only less heat." No: It also produces poisonous carbon monoxide and soot.

Worked examples

Example 1. Draw (describe) and name the structural isomers of C4H10.

Solution

Butane: a straight chain of four carbon atoms, CH3CH2CH2CH3.

2-methylpropane: a chain of three carbon atoms with a methyl group on the middle carbon, CH3CH(CH3)CH3.

Both have formula C4H10 but different structures, so they are structural isomers.

Example 2. Write equations for the complete and incomplete combustion of methane and explain why incomplete combustion is dangerous.

Solution

Complete: CH4 + 2O2 → CO2 + 2H2O.

Incomplete: 2CH4 + 3O2 → 2CO + 4H2O (or carbon/soot).

Carbon monoxide is colourless, odourless and poisonous: it binds to haemoglobin and reduces oxygen transport.

Practice questions

  1. Write the general formula of alkanes.

    Show answer

    CnH2n+2.

    Common mistake: Writing CnH2n.

  2. Name the alkane C6H14.

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    Hexane.

    Common mistake: Writing heptane.

  3. Give two uses of methane.

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    Fuel for cooking and electricity (biogas, Lake Kivu), making hydrogen and ammonia (any two).

    Common mistake: Saying it is used as a medicine.

  4. Why are alkanes called saturated hydrocarbons?

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    They contain only single carbon–carbon bonds and only C and H.

    Common mistake: Saying they are soaked in water.

  5. Write the equation for the complete combustion of propane.

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    C3H8 + 5O2 → 3CO2 + 4H2O.

    Common mistake: Writing 4O2.

  6. What is a structural isomer?

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    A compound with the same molecular formula as another but a different structural formula.

    Common mistake: Saying 'same structure, different formula'.

  7. State the condition for the reaction of methane with chlorine.

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    Ultraviolet light (sunlight).

    Common mistake: Saying a nickel catalyst.

  8. Name CH3C(CH3)2CH3.

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    2,2-dimethylpropane.

    Common mistake: Writing 2-dimethylpropane.

  9. Explain why ethane does not decolourise bromine water.

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    It is saturated; it has no C=C double bond for addition.

    Common mistake: Saying bromine water is too dilute.

  10. Explain why branched alkanes have lower boiling points than straight-chain isomers.

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    Branched molecules are more compact, with less surface contact, so intermolecular forces are weaker.

    Common mistake: Saying they have fewer atoms.

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. Isomers have the same:a. intermolecular forces increase
    2. Which alkane is a liquid at room temperature?b. molecular formula but different structures
    3. The boiling points of alkanes increase with chain length because:c. Hexane
    4. How many structural isomers does butane, C4H10, have?d. 2
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    1 → b (molecular formula but different structures) · 2 → c (Hexane) · 3 → a (intermolecular forces increase) · 4 → d (2)

    • Example: butane and 2-methylpropane.
    • Alkanes from C5 upwards are liquids.
    • Larger molecules attract each other more.
    • Butane and 2-methylpropane.

  2. Drag and drop

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

    propane and butane  ·  methane  ·  carbon dioxide and water  ·  carbon monoxide and water
    1. The main gas in biogas and in Lake Kivu gas is .
    2. Complete combustion of methane gives .
    3. Liquefied petroleum gas (LPG) used for cooking is mainly .
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    1: methane · 2: carbon dioxide and water · 3: propane and butane

    • CH4.
    • CH4 + 2O2 → CO2 + 2H2O.
    • They liquefy easily under pressure.

  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. Alkanes are described as saturated because they:a. 2-methylpropane
    2. Alkanes react with chlorine in sunlight by:b. substitution
    3. Breaking large alkane molecules into smaller ones is called:c. cracking
    4. The IUPAC name of CH3CH(CH3)CH3 is:d. contain only single C–C bonds
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    1 → d (contain only single C–C bonds) · 2 → b (substitution) · 3 → c (cracking) · 4 → a (2-methylpropane)

    • No more hydrogen can be added.
    • H atoms are replaced by Cl atoms.
    • It uses heat and a catalyst.
    • Longest chain 3 C with a methyl on C2.

  4. Drag and drop

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

    carbon monoxide and soot  ·  oxygen  ·  road surfaces  ·  chloromethane and hydrogen chloride
    1. The product of reacting methane with chlorine in UV light (one substitution) is .
    2. Bitumen from crude oil is used for .
    3. Incomplete combustion of alkanes produces .
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    1: chloromethane and hydrogen chloride · 2: road surfaces · 3: carbon monoxide and soot

    • CH4 + Cl2 → CH3Cl + HCl.
    • It is a thick, sticky residue.
    • Too little oxygen leaves CO and C.

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