5 high-yield, syllabus-aligned questions on Organic chemistry, covering 17 individually creditable mark points. Each one shows the answer that scores full marks under our guide, which phrase earns which mark, and a common incomplete answer — so you can see the difference rather than guess at it.
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Every question and solution is reviewed by examiners versed in WAEC and JAMB, each with more than thirty years of experience.
Mark allocations are TopMarks’ own, written against the published syllabus and Chief Examiners’ reports. They are a guide to how these answers are usually credited, not an official marking scheme.
Define4 marks
Define a homologous series and state three of its characteristics.
TopMarks model answer
A homologous series is a family of organic compounds having the same general formula, in which successive members differ by a CH₂ group. Members of the series have similar chemical properties because they contain the same functional group. Their physical properties, such as boiling point, show a gradual change as the molecular mass increases. All members can be prepared by the same general method.
Suggested marking guide
1Same general formula, differing by CH₂
2Same functional group, similar chemical properties
3Gradual change in physical properties
4Same general method of preparation
Why it scores
Four marks, and one of them is the definition while three are characteristics. Reading the question that way tells you how much to write. Note the pairing that carries the second mark: SAME functional group gives SIMILAR chemical properties, while GRADED molecular mass gives GRADED physical properties. Confusing which properties are similar and which are graded is the classic error here.
A common incomplete answer
“Compounds with the same general formula whose members differ by CH₂.”
What it costs: Three marks of four. The definition is credited, but the question asked for three characteristics as well and none was given.
Key ideas to include: general formula, CH₂, functional group, gradation, boiling point.
Describe4 marks
State the difference between a saturated and an unsaturated hydrocarbon, and describe a test that would distinguish between them.
TopMarks model answer
A saturated hydrocarbon contains only single carbon-to-carbon bonds, whereas an unsaturated hydrocarbon contains at least one carbon-to-carbon double or triple bond. To distinguish them, add a few drops of bromine water to each. The unsaturated hydrocarbon rapidly decolourises the reddish-brown bromine water by an addition reaction, while the saturated hydrocarbon does not decolourise it in the dark.
Suggested marking guide
1Saturated has only single bonds
2Unsaturated has a double or triple bond
3Bromine water named as the reagent
4The observation: it is decolourised
Why it scores
A test question is never complete without the OBSERVATION. “Add bromine water” describes an action; “the reddish-brown colour disappears” is the evidence, and evidence is what the mark is for. Every test question in Chemistry follows this pattern: reagent, then observation, then conclusion.
A common incomplete answer
“Unsaturated hydrocarbons contain double bonds.”
What it costs: Three marks of four. One half of the difference is credited. The saturated side is missing, and no test at all was described.
Key ideas to include: single bond, double bond, bromine water, decolourise, addition reaction.
Explain3 marks
Explain what is meant by isomerism and name the two isomers of C₄H₁₀.
TopMarks model answer
Isomerism is the existence of two or more compounds that have the same molecular formula but different structural formulae, and therefore different physical and sometimes different chemical properties. The two isomers of C₄H₁₀ are butane, an unbranched chain of four carbon atoms, and 2-methylpropane, a three-carbon chain carrying a methyl branch on the second carbon atom.
Suggested marking guide
1Same molecular formula but different structures
2Butane named
32-methylpropane named
Why it scores
The definition mark hinges on holding two things in the same sentence: SAME molecular formula, DIFFERENT structural formula. Saying only “different structures” leaves out what makes them isomers rather than simply two different compounds.
A common incomplete answer
“Isomers are compounds with the same molecular formula but different structures.”
What it costs: Two marks of three. The definition is exactly right and fully credited — but the question also asked for the two isomers by name, and neither was given.
State three uses of ethanol and one method by which it is prepared industrially.
TopMarks model answer
Ethanol is used as a solvent in the manufacture of paints, perfumes and drugs. It is used as a fuel, either alone or blended with petrol. It is used as an antiseptic to sterilise the skin and surgical instruments, and it is the active component of alcoholic drinks. Industrially it is prepared by the fermentation of glucose from starch or molasses, using the enzyme zymase in yeast, or by the catalytic hydration of ethene with steam.
Suggested marking guide
1Use as a solvent
2Use as a fuel
3Use as an antiseptic or in beverages
4A correct industrial preparation
Why it scores
Three uses plus one preparation equals four marks. That arithmetic is the whole instruction. A candidate who writes a paragraph on fermentation and gives one use has answered a question that was not asked and collected two marks out of four.
A common incomplete answer
“Ethanol is used as a fuel and in alcoholic drinks.”
What it costs: Two marks of four. Both uses are credited, but a third use is missing and the industrial preparation — a whole mark on its own — was not attempted.
Explain the term functional group and identify the functional group present in ethanoic acid.
TopMarks model answer
A functional group is the atom or group of atoms in an organic molecule that is responsible for the characteristic chemical properties of that molecule. The functional group in ethanoic acid is the carboxyl group, -COOH.
Suggested marking guide
1Atom or group of atoms responsible for the chemical properties
2The carboxyl group identified
Why it scores
Naming the group is only half the mark scheme; you must also say what a functional group DOES, which is to determine the chemical behaviour of the whole molecule. That is why all carboxylic acids react alike however long their carbon chain.
A common incomplete answer
“It is the reactive part of a molecule. In ethanoic acid it is the OH group.”
What it costs: Both marks. “Reactive part” is too loose to be credited, and -OH is the hydroxyl group of an alcohol; the acid’s functional group is the whole carboxyl group, -COOH.
Key ideas to include: functional group, carboxyl, -COOH, characteristic properties.
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