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Chemistry · Practice book · Senior 3 · Unit 8 of 10

Introduction to organic chemistry

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

Unit 8 Introduction to organic chemistry

Key unit competence: Analyse the nature, properties, compositions and applications of organic compounds.

Key ideas

  • Organic chemistry is the chemistry of carbon compounds (except CO, CO2, carbonates and cyanides, which are inorganic). Organic compounds occur in living things, crude oil, natural gas, coal and biomass.
  • Carbon forms so many compounds because it is tetravalent (forms 4 covalent bonds), can bond to other carbon atoms in long chains, branches and rings (catenation) and forms single, double and triple bonds.
  • Differences: organic compounds are mainly covalent, have low melting and boiling points, are often flammable, usually insoluble in water but soluble in organic solvents, and react slowly; inorganic compounds are often ionic, have high melting points and conduct when dissolved or molten.
  • A homologous series is a family of organic compounds with the same functional group and general formula; successive members differ by CH2; they show a gradual change in physical properties and similar chemical properties. Examples: alkanes CnH2n+2, alkenes CnH2n, alkynes CnH2n-2, alcohols CnH2n+1OH, carboxylic acids.
  • A functional group is the atom or group of atoms that gives the series its characteristic reactions: C=C (alkenes), C≡C (alkynes), –OH (alcohols), –COOH (carboxylic acids).
  • Applications: fuels (methane, petrol, kerosene), plastics, medicines, soaps and detergents, foods (carbohydrates, fats, proteins), alcoholic drinks, fibres, dyes and pesticides.

Common misconceptions

  • "All compounds containing carbon are organic." No: CO, CO2, carbonates and cyanides are treated as inorganic.
  • "Organic compounds can only be made by living things." No: Wöhler made urea in the laboratory in 1828; most organic compounds can be synthesised.
  • "Members of a homologous series have identical physical properties." No: They have similar CHEMICAL properties; physical properties change gradually.

Worked examples

Example 1. State three reasons why carbon forms a very large number of compounds.

Solution

Carbon is tetravalent: each atom forms four covalent bonds.

Carbon atoms join to each other in long chains, branched chains and rings (catenation).

Carbon forms single, double and triple bonds, and bonds with other elements such as H, O, N and halogens.

Example 2. Give three characteristics of a homologous series, using alkanes as an example.

Solution

Same general formula: CnH2n+2 (methane CH4, ethane C2H6, propane C3H8).

Successive members differ by CH2 and show a gradual change in physical properties such as boiling point.

Similar chemical properties (all burn and undergo substitution with halogens).

Practice questions

  1. Define organic chemistry.

    Show answer

    The study of carbon compounds, except simple ones like oxides, carbonates and cyanides.

    Common mistake: Saying 'chemistry of living things only'.

  2. Give two natural sources of organic compounds.

    Show answer

    Crude oil, natural gas, coal, plants and animals (any two).

    Common mistake: Naming limestone.

  3. What is catenation?

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    The ability of atoms of an element (carbon) to bond to each other in chains or rings.

    Common mistake: Confusing it with allotropy.

  4. State two differences between organic and inorganic compounds.

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    Organic: covalent, low m.p., flammable, insoluble in water. Inorganic: often ionic, high m.p., soluble, conduct (any two pairs).

    Common mistake: Saying organic compounds conduct.

  5. Write the general formula of alkynes.

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    CnH2n-2.

    Common mistake: Writing CnH2n.

  6. What is a functional group?

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    The atom or group of atoms that gives a homologous series its characteristic reactions.

    Common mistake: Saying 'the carbon chain'.

  7. Write the formula of the fourth alkane.

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    C4H10 (butane).

    Common mistake: Writing C4H8.

  8. Name the functional group in ethanol.

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    Hydroxyl, –OH.

    Common mistake: Writing hydroxide ion OH−.

  9. Give three everyday uses of organic compounds.

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    Fuels, plastics, medicines, soaps, food, fabrics (any three).

    Common mistake: Listing metals.

  10. Explain why organic compounds usually have low melting points.

    Show answer

    They are made of small molecules held by weak forces.

    Common mistake: Saying covalent bonds are weak.

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. The molecular formula of the alkane with 5 carbon atoms is:a. carbon compounds
    2. Organic chemistry is the study of:b. C5H12
    3. The functional group of alkenes is:c. CH2
    4. Successive members of a homologous series differ by:d. C=C double bond
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    1 → b (C5H12) · 2 → a (carbon compounds) · 3 → d (C=C double bond) · 4 → c (CH2)

    • 2 × 5 + 2 = 12.
    • It excludes simple carbon oxides and carbonates.
    • The double bond makes alkenes reactive.
    • Each member adds one CH2 unit.

  2. Drag and drop

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

    ethanol  ·  CnH2n+2  ·  methane  ·  functional group and general formula
    1. Natural gas is mainly .
    2. The general formula of the alkanes is .
    3. A homologous series is a family with the same .
    Show answer

    1: methane · 2: CnH2n+2 · 3: functional group and general formula

    • Methane is the main component.
    • For example C2H6.
    • Members differ by CH2.

  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 formula belongs to the same homologous series as methane?a. Crude oil
    2. Which source provides most organic fuels?b. CnH2n
    3. Which is the general formula of the alkenes?c. C3H8
    4. Who first made an organic compound, urea, from an inorganic one?d. Friedrich Wöhler
    Show answer

    1 → c (C3H8) · 2 → a (Crude oil) · 3 → b (CnH2n) · 4 → d (Friedrich Wöhler)

    • C3H8 fits CnH2n+2.
    • Crude oil is a mixture of hydrocarbons.
    • For example ethene C2H4.
    • He heated ammonium cyanate in 1828.

  4. Drag and drop

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

    hydrolysis  ·  –OH  ·  catenation  ·  covalent and have low melting points
    1. The ability of carbon atoms to bond to each other in chains is called .
    2. Most organic compounds are .
    3. The functional group of alcohols is .
    Show answer

    1: catenation · 2: covalent and have low melting points · 3: –OH

    • This allows millions of compounds.
    • They are made of molecules.
    • Hydroxyl group.

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