Practice questions: Carbon Compounds

Attempt each question before reading the answer. These are original questions written in SPM style to test the same content standards; they are not past-year papers. For the multiple-choice items, write your choice, then check the letter and the one-line reason. For the structured questions, write a full answer first, then compare with the model.

Paper 1-style multiple-choice questions

1. What is the general formula of the alkene homologous series?

• A. CₙH₂ₙ₊₂
• B. CₙH₂ₙ
• C. CₙH₂ₙ₋₂
• D. CₙH₂ₙ₊₁OH

Answer: B. Alkenes have one carbon–carbon double bond and follow CₙH₂ₙ; CₙH₂ₙ₊₂ is the alkane formula.

2. Which compound is a member of the same homologous series as ethanol?

• A. Ethene
• B. Ethanoic acid
• C. Propan-1-ol
• D. Ethyl ethanoate

Answer: C. Propan-1-ol is also an alcohol (–OH group); the others belong to different series.

3. What is the correct name for CH₃CH₂CH₂CH₃?

• A. Propane
• B. Butane
• C. But-1-ene
• D. Butan-1-ol

Answer: B. Four carbons in a saturated chain with only single bonds is butane.

4. Ethene decolourises bromine water at room temperature. This reaction is an example of

• A. substitution.
• B. addition.
• C. neutralisation.
• D. oxidation.

Answer: B. The double bond opens and bromine adds across it, an addition reaction.

5. Which reagent and condition converts an alkane to a halogenoalkane?

• A. bromine water at room temperature
• B. chlorine in ultraviolet light
• C. steam and a phosphoric acid catalyst
• D. hydrogen and a nickel catalyst

Answer: B. Alkanes undergo substitution with a halogen only in ultraviolet light.

6. How many structural isomers have the molecular formula C₅H₁₂?

• A. 1
• B. 2
• C. 3
• D. 4

Answer: C. Pentane, 2-methylbutane and 2,2-dimethylpropane are the three isomers.

7. Ethanol is oxidised by acidified potassium dichromate(VI). What is the colour change?

• A. purple to colourless
• B. orange to green
• C. green to orange
• D. colourless to brown

Answer: B. Dichromate(VI) changes from orange to green as ethanol is oxidised to ethanoic acid.

8. Which product is formed when ethanol undergoes dehydration?

• A. ethane
• B. ethene
• C. ethanoic acid
• D. ethyl ethanoate

Answer: B. Dehydration removes water from ethanol to give the alkene, ethene.

9. Ethyl ethanoate is formed from ethanol and which other reactant?

• A. ethene
• B. ethane
• C. ethanoic acid
• D. sodium ethanoate

Answer: C. An ester forms from an alcohol and a carboxylic acid; here ethanol + ethanoic acid.

10. Which statement about fats and oils is correct?

• A. Both are saturated esters.
• B. Fats are unsaturated; oils are saturated.
• C. Fats are saturated and solid; oils are unsaturated and liquid.
• D. Oils have no ester linkage.

Answer: C. Fats are saturated and solid at room temperature; oils are unsaturated and liquid.

Structured questions

Question 1 (homologous series and naming).

(a) State three characteristics of a homologous series. (b) Draw the structural formula of but-1-ene and name one isomer of it.

Model answer. (a) Members share the same general formula and functional group, differ from the next member by a CH₂ unit, and show a gradual change in physical properties (with similar chemical properties). (b) But-1-ene is CH₂=CH–CH₂–CH₃. One isomer is but-2-ene (CH₃–CH=CH–CH₃); 2-methylprop-1-ene is also acceptable.

Question 2 (reactions of an alkene).

Ethene, C₂H₄, reacts in several ways. (a) Write the equation for the complete combustion of ethene. (b) Describe a chemical test, with the observation, that shows ethene is unsaturated. (c) Name the polymer formed when many ethene molecules join, and the type of reaction.

Model answer. (a) C₂H₄ + 3O₂ → 2CO₂ + 2H₂O. (b) Add bromine water and shake; the brown bromine water is decolourised at room temperature, showing the presence of a carbon–carbon double bond. (c) Poly(ethene), formed by addition polymerisation.

Question 3 (alcohols and esters).

Ethanol can be made by fermentation and reacts to form an ester. (a) Write the equation for the fermentation of glucose to ethanol. (b) Ethanol is warmed with ethanoic acid and a little concentrated sulfuric acid. Name the ester formed and write the equation. (c) State the role of the concentrated sulfuric acid and one use of esters.

Model answer. (a) C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ (enzymes in yeast, absence of air). (b) Ethyl ethanoate: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O. (c) The concentrated sulfuric acid acts as a catalyst; esters are used as flavourings or in perfumes.

Question 4 (carboxylic acids and distinguishing tests).

A student has two colourless liquids, ethanol and ethanoic acid, in unlabelled test tubes. (a) Describe how the student could use a carbonate to identify the ethanoic acid, stating the observation. (b) Write the equation for the reaction between ethanoic acid and sodium carbonate. (c) State one similarity and one difference in how ethanol and ethanoic acid react with a reactive metal such as magnesium.

Model answer. (a) Add a little sodium carbonate to each liquid. The one that produces effervescence (a gas that turns limewater milky, showing carbon dioxide) is the ethanoic acid; the ethanol shows no reaction. (b) 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂. (c) Similarity: both can react with magnesium to release hydrogen gas. Difference: ethanoic acid, being an acid, reacts readily at room temperature, while ethanol reacts far less readily under the same conditions.

How to use these questions

Mark yourself strictly: award a point only where the formula is correct, the condition or catalyst is stated, the equation is balanced, and any test gives both the observation and the conclusion. Where you dropped a mark, return to the revision notes for that standard and try a similar question again. A one-to-one teacher can set you targeted questions on the standards you find hardest, which is the fastest way to lift your SPM Chemistry score.