Class 10 Chemistry: Hydrocarbons and Functional Groups Made Clear

Organic chemistry is easier to organise when you look at a structure before trying to remember its name. Count the carbon atoms, inspect the bonds and identify any functional group. Those three checks tell you much more than the molecular formula alone.

This introduction compares simple hydrocarbons, alcohols and carboxylic acids. It uses condensed structural formulas so that the position of the important atoms remains visible. Follow the naming scope assigned in your textbook for longer or branched examples.

Independent topic practice for Class 10 students. Check your textbook, examination scheme and assigned learning outcomes against the official FBISE assessment frameworks and model papers. These are original exercises, not an official answer key or a predicted examination paper.

What you will practise

• Identify hydrocarbons and carbon bonding
• Compare alkanes, alkenes and alkynes
• Understand a homologous series
• Find alcohol and carboxylic acid groups
• Connect structure with reactions

Identify hydrocarbons and carbon bonding

A hydrocarbon contains only carbon and hydrogen. Ethane, C₂H₆, is a hydrocarbon; ethanol, C₂H₅OH, is not, because it also contains oxygen. This is a composition definition, not a judgement about whether a substance can burn.

Carbon commonly forms four covalent bonds in the neutral organic structures studied here. In a displayed formula, count the total bond order around each carbon. A double bond counts as two and a triple bond as three. Missing hydrogens can often be found by completing four bonds at each carbon.

Compare alkanes, alkenes and alkynes

Acyclic alkanes have carbon–carbon single bonds only and follow CₙH₂ₙ₊₂. Acyclic alkenes with one carbon–carbon double bond follow CₙH₂ₙ. Acyclic alkynes with one carbon–carbon triple bond follow CₙH₂ₙ₋₂. These formulas require the stated structural assumptions.

The formula CₙH₂ₙ alone does not prove that a substance is an alkene: a cycloalkane can have the same general formula. When a question supplies a structure, use it. When it supplies only a formula, state what can and cannot be concluded.

Understand a homologous series

Members of a homologous series share a functional group or structural pattern and a general formula within the series. Successive members differ by CH₂. They show similar chemical properties and gradual changes in many physical properties as chain length increases.

Methane, ethane and propane are successive alkanes. Their molecular formulas are CH₄, C₂H₆ and C₃H₈. The difference in relative molecular mass between successive members is approximately 14 using C = 12 and H = 1. Similar chemistry does not mean identical boiling points or identical physical states.

Find alcohol and carboxylic acid groups

Alcohols contain an –OH group bonded to a suitable saturated carbon atom in the simple examples used here. Ethanol is CH₃CH₂OH. Carboxylic acids contain –COOH, as in ethanoic acid, CH₃COOH. The carboxyl group includes both a carbonyl and a hydroxyl as one characteristic group.

Do not classify a compound merely by spotting the letters OH somewhere in its formula. The arrangement matters. Ethanoic acid contains OH within COOH but is classified as a carboxylic acid. Condensed structures preserve this distinction better than a molecular formula such as C₂H₄O₂ alone.

Connect structure with reactions

Alkenes can undergo addition across their carbon–carbon double bond. Ethene adds hydrogen to form ethane under suitable catalytic conditions. Alkanes are saturated and do not have that double bond available for the same addition pattern.

Complete combustion of a hydrocarbon in sufficient oxygen forms carbon dioxide and water. Incomplete combustion can produce carbon monoxide or carbon as well. Balance carbon first, hydrogen next and oxygen last. Do not use a flame or chemical test outside supervised laboratory conditions.

Worked examples and model answers

1. Classify a structure

Classify CH₃CH₂CH₃.

It contains only carbon and hydrogen and only single carbon–carbon bonds. It is propane, an alkane.

2. Complete a formula

An acyclic alkene with one double bond contains four carbons.

CₙH₂ₙ gives C₄H₈. The molecular formula alone would not identify a unique structural isomer.

3. Balance combustion

Balance the complete combustion of methane.

CH₄ + 2O₂ → CO₂ + 2H₂O. There is one carbon, four hydrogens and four oxygens on each side.

Practice MCQs

Attempt all six before opening the answers. Write a reason beside each choice so that a correct guess is not mistaken for understanding.

1. A hydrocarbon contains:

1. C and H only
2. C and O only
3. H and O only
4. any carbon compound

Answer and explanation

A. C and H only Other elements exclude it from the hydrocarbon category.

2. Ethene contains a carbon–carbon:

1. single bond only
2. double bond
3. triple bond
4. ionic bond

Answer and explanation

B. double bond Ethene is an alkene.

3. Successive homologous members differ by:

1. H₂
2. CH₂
3. CO₂
4. OH

Answer and explanation

B. CH₂ The extra CH₂ adds approximately 14 to relative molecular mass.

4. The carboxylic acid group is:

1. –OH
2. –COOH
3. –Cl
4. –NH₂

Answer and explanation

B. –COOH The full carboxyl group identifies the series.

5. Complete methane combustion forms:

1. CO₂ and H₂O
2. CO only
3. H₂ only
4. C and H₂

Answer and explanation

A. CO₂ and H₂O Sufficient oxygen supports complete combustion.

6. For an acyclic alkane with n carbons, the formula is:

1. CₙHₙ
2. CₙH₂ₙ
3. CₙH₂ₙ₊₂
4. CₙH₂ₙ₋₂

Answer and explanation

C. CₙH₂ₙ₊₂ This assumes a saturated open-chain hydrocarbon.

Written self-check

1. Write the molecular formula of pentane.
2. Identify the functional group in CH₃CH₂OH.
3. Balance C₂H₄ + O₂ → CO₂ + H₂O.
4. Explain why C₃H₆ is not enough information to prove an alkene structure.

Suggested answers

1. C₅H₁₂.
2. Alcohol hydroxyl group, –OH.
3. C₂H₄ + 3O₂ → 2CO₂ + 2H₂O.
4. The same formula can represent a cycloalkane, so structural information is needed.

Use your mistakes to choose the next task

Mark the MCQs only after completing the written self-check. If you chose the correct answer but cannot explain it, return to the relevant heading and write a two-sentence explanation in your own words. If a calculation went wrong, identify whether the problem came from the starting relationship, the substitution, the arithmetic or the unit. That gives you a specific correction to practise.

On your next attempt, change the order of the questions and keep the answers covered. Explain one idea aloud, then answer one question on paper without consulting the notes. A successful immediate retry is useful, but a second correct attempt on a different day gives better evidence that you can recall and apply the idea independently.

Further study

Use the official FBISE framework and model-paper page to select the subject, level and scheme that apply to you. Match the learning outcomes with your assigned textbook before deciding how much detail to revise.

More practice on this blog: Pakistan Studies Chapter 1 · History of Pakistan MCQs.