Carbon Compounds: Unit Review

Introduction

This unit began with a single question: why is carbon special? The answer — four covalent bonds and an outstanding ability to bond to itself — led us through chains and rings, families of hydrocarbons, crude oil and its processing, and finally combustion with all its benefits and hazards. This review draws those threads together so that you can see how the structure of carbon compounds explains their properties, their uses and their impact on the world.

Core explanation

Carbon's bonding. Carbon has the electron arrangement 2,4, so it forms four covalent bonds to complete its outer shell. When it forms four single bonds they point to the corners of a tetrahedron at about 109.5°. Carbon–carbon bonds are strong, so carbon undergoes catenation : it forms long chains, branched chains and rings. Carbon can also form double (C=C) and triple (C≡C) bonds. This variety is why organic chemistry, the chemistry of carbon compounds, is such a vast subject.

Hydrocarbons and homologous series. Hydrocarbons contain only hydrogen and carbon. They are organised into homologous series , families with the same general formula whose members differ by CH₂ and show a gradual trend in physical properties.

Series General formula Key bond Example --- --- --- --- Alkanes CₙH₂ₙ₊₂ C–C only (saturated) propane, C₃H₈ Alkenes CₙH₂ₙ one C=C (unsaturated) propene, C₃H₆ Alkynes CₙH₂ₙ₋₂ one C≡C (unsaturated) ethyne, C₂H₂

Names are built from a stem showing the number of carbons (meth-, eth-, prop-, but-, pent-) and an ending showing the series (-ane, -ene, -yne). Isomers share a molecular formula but differ in structure, for example butane and methylpropane.

Properties and reactions. Alkanes are fairly unreactive apart from burning. As chains get longer, boiling point and viscosity rise and flammability falls, because intermolecular forces grow stronger. Alkenes undergo addition reactions across the C=C bond; the decolourisation of orange bromine water is the standard test for unsaturation.

From crude oil to useful products. Crude oil is a mixture of hydrocarbons. Fractional distillation separates it into fractions with similar boiling points. Cracking breaks long, less useful alkanes into shorter alkanes and alkenes, meeting demand for petrol and providing alkenes for making polymers.

Combustion. Complete combustion of a hydrocarbon in plenty of oxygen gives carbon dioxide and water and is strongly exothermic, because forming strong C=O and O–H bonds releases more energy than breaking C–H, C–C and O=O bonds requires. With limited oxygen, incomplete combustion gives toxic carbon monoxide and soot. Burning fuels also releases SO₂, NOₓ and particulates, and adds CO₂ to the atmosphere, strengthening the greenhouse effect. Cleaner fuels, efficient technologies and good fire safety help us use combustion responsibly.

Step-by-step reasoning

To answer a question about an unfamiliar hydrocarbon:

1. Count carbons and hydrogens and compare with the general formulae to find its series. 2. Decide whether it is saturated or unsaturated. 3. Predict physical properties from chain length. 4. Predict reactions: addition if it has C=C; combustion for all hydrocarbons. 5. Write and balance its combustion equation.

Visual explanation

Imagine a concept map with "carbon: 4 bonds + catenation" at the centre. Branches lead to "chains, branches, rings", then to "alkanes / alkenes / alkynes", then to "crude oil → distillation → cracking", and finally to "combustion", which splits into "energy", "CO and soot", "pollutants" and "climate".

Real-world analogy

The unit is like following a river from its source to the sea. The spring is carbon's bonding; streams join to become the families of hydrocarbons; the river is shaped by refineries into fuels and materials; and at the delta, combustion spreads its effects into energy, air quality and climate.

Real-world example

A litre of petrol links every idea in the unit: its molecules are alkane chains built by catenation, separated from crude oil by fractional distillation and partly made by cracking. Burning it in an engine releases energy, carbon dioxide, water and small amounts of CO and NOₓ that a catalytic converter reduces.

Why?

Why does one element support so much chemistry? Carbon sits in the middle of its period, so it shares electrons rather than gaining or losing them, and its small atoms form strong bonds to one another and to hydrogen, oxygen and nitrogen. Few other elements combine all these features.

Common misconception

"All hydrocarbons are alkanes." Alkanes are only one series. Alkenes, alkynes, cyclic hydrocarbons and aromatic compounds such as benzene are also hydrocarbons, with different bonding and reactivity.

Worked example

Question: Butane, C₄H₁₀, is used in lighters. Identify its series, state whether it is saturated and write a balanced equation for its complete combustion.

Reasoning: C₄H₁₀ fits CₙH₂ₙ₊₂ with n = 4, so it is an alkane with only single bonds. Products: 4CO₂ and 5H₂O need 8 + 5 = 13 oxygen atoms, which is 6½ O₂; double everything.

Answer: Alkane; saturated; 2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O.

Quick check

1. What is the general formula of the alkenes? Answer: CₙH₂ₙ.

Exam focus

Expect questions that link topics: naming a hydrocarbon, predicting its properties, choosing a test for unsaturation and writing its combustion equation, then evaluating environmental effects. Always balance equations and use key terms precisely.

Advanced insight

Carbon's closest relative, silicon, also forms four bonds, but Si–Si bonds are weaker and Si–O bonds much stronger, so silicon chemistry is dominated by rock-forming silicates rather than long chains. This contrast highlights how finely balanced bond strengths shape the chemistry of an element.

Summary

Carbon forms four covalent bonds and catenates into chains, branches and rings. Hydrocarbons form homologous series: saturated alkanes, unsaturated alkenes and alkynes. Crude oil is separated by fractional distillation and upgraded by cracking. Combustion releases energy but, when incomplete or impure, releases CO, soot and pollutants, and its CO₂ drives climate change.

Practice questions

1. Explain what is meant by catenation and why carbon shows it so well. Answer: Catenation is atoms of one element bonding to each other in chains or rings; carbon does it well because C–C bonds are strong and each carbon forms four bonds. 2. Describe a test to distinguish an alkane from an alkene. Answer: Add bromine water: the alkene decolourises it from orange to colourless, while the alkane leaves it orange. 3. Why is cracking carried out at oil refineries? Answer: To break long-chain alkanes, which are in low demand, into shorter, more useful alkanes such as petrol and into alkenes for making polymers. 4. Write a balanced equation for the complete combustion of propane, C₃H₈. Answer: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O. 5. State two environmental problems caused by burning fossil fuels. Answer: For example, carbon dioxide emissions increase the greenhouse effect, and sulfur dioxide causes acid rain.