# Alkanes and Alkenes

> Chemistry · CIE IGCSE 0620
> Source: https://www.owlsprep.com/study/cie-0620-u11-alkanes-and-alkenes/

This guide covers the structure, general formulae, key properties and characteristic reactions of alkanes and alkenes for CIE IGCSE Chemistry 0620, including the bromine water test for unsaturation, aligned with 2026-2028 Core and Extended syllabus requirements.

**Prerequisites:** [Introduction to organic chemistry, homologous series and functional groups](https://www.owlsprep.com/study/cie-0620-u11-introduction-to-organic-chemistry/)

## Learning objectives

- Describe the general formula and homologous series characteristics of alkanes and alkenes
- Draw structural and displayed formulae for unbranched alkanes/alkenes up to 4 carbon atoms
- Explain the difference between saturated and unsaturated hydrocarbons
- Describe the core reactions of alkanes and alkenes, including the bromine water unsaturation test
- Recall Extended-only alkene addition reactions (hydrogenation, hydration) and the cracking process
- Distinguish between alkanes and alkenes using chemical tests and structural properties

## Core: Structure and Classification of Alkanes and Alkenes

Alkanes and alkenes are the two simplest homologous series of hydrocarbons, which are compounds containing only carbon and hydrogen atoms.

**Saturated Hydrocarbon** — A hydrocarbon with only single covalent bonds between all carbon atoms, so no additional atoms can be added to its structure.

*Example:* Methane ($CH_4$), ethane ($C_2H_6$)

Alkanes are saturated hydrocarbons with the general formula $C_nH_{2n+2}$, where $n$ is the number of carbon atoms ($n \geq 1$). Alkenes are unsaturated hydrocarbons containing one C=C double functional group, with the general formula $C_nH_{2n}$, where $n \geq 2$ (you need at least 2 carbons to form a double bond).

**Worked example:** Give the molecular formula of propane (alkane with 3 carbon atoms) and propene (alkene with 3 carbon atoms).

1. For propane (alkane): Use the general formula $C_nH_{2n+2}$ with $n=3$. Substitute: $2(3)+2=8$, so molecular formula is $C_3H_8$.
2. For propene (alkene): Use the general formula $C_nH_{2n}$ with $n=3$. Substitute: $2(3)=6$, so molecular formula is $C_3H_6$.

> **Exam tip:** Remember alkenes have two fewer hydrogen atoms than the corresponding alkane, because the C=C double bond replaces one C-C single bond and two C-H single bonds.

## Core: Key Reactions of Alkanes

Alkanes are relatively unreactive except for two core reactions you need to memorise: complete combustion, and substitution with halogens in ultraviolet (UV) light.

- **Complete combustion**: Alkanes burn in excess oxygen to produce carbon dioxide and water, releasing large amounts of energy. This makes them widely used as household and transport fuels.
- **Substitution with halogens**: In UV light (e.g. direct sunlight), alkanes react with halogens like chlorine or bromine. One hydrogen atom is replaced by a halogen atom, forming a halogenoalkane and a hydrogen halide.

**Worked example:** Write the word equation for the complete combustion of methane, and the word equation for the reaction between methane and chlorine in UV light.

1. Complete combustion of methane: $\text{methane + oxygen} \rightarrow \text{carbon dioxide + water}$
2. Substitution reaction of methane with chlorine: $\text{methane + chlorine} \rightarrow \text{chloromethane + hydrogen chloride}$

> **Exam tip:** If oxygen is limited, incomplete combustion produces carbon monoxide or soot instead of carbon dioxide, but you only need to recall complete combustion equations for Core tier questions.

## Core: Key Reactions of Alkenes and Test for Unsaturation

Alkenes are more reactive than alkanes due to the presence of the C=C double bond, which can break to form single bonds and add new atoms in a process called an addition reaction.

**Addition Reaction** — A reaction where a small molecule adds across a double (or triple) bond, breaking one bond of the double bond to form two new single bonds, resulting in a single saturated product.

- **Combustion**: Alkenes burn in excess oxygen to produce carbon dioxide and water, but they burn with a smokier, yellower flame than alkanes due to their higher carbon-to-hydrogen ratio.
- **Bromine water test for unsaturation**: Orange bromine water adds across the C=C double bond of alkenes to form a colourless dibromoalkane, so the orange colour disappears when mixed with an alkene. No reaction occurs with alkanes, so the orange colour remains.

**Worked example:** A student adds bromine water to two test tubes, one containing hexane (alkane) and one containing hexene (alkene). Describe the observations for each test tube and explain the reaction occurring.

1. Hexane test tube: Orange bromine water remains orange. No reaction occurs, as alkanes are saturated and cannot undergo addition reactions.
2. Hexene test tube: Orange bromine water turns colourless. An addition reaction occurs, where bromine adds across the C=C double bond in hexene to form colourless dibromohexane.

> **Exam tip:** You must explicitly state that bromine water *changes from orange to colourless* for a positive alkene test. Stating it 'goes clear' is not enough for full marks, as clear solutions can still be coloured.

## Extended Only: Additional Alkene Reactions and Cracking

> **Extended Only Content**
>
> This section covers Supplement syllabus content, required only for students taking Paper 2 and Paper 4 (Extended tier). Core students can skip this section.

For Extended tier, you need to memorise two additional addition reactions of alkenes, plus the industrial process of cracking used to produce alkenes from large alkane molecules.

- **Hydrogenation (addition of hydrogen)**: Alkenes react with hydrogen gas at 150°C with a nickel catalyst to form the corresponding alkane. This reaction is used to turn liquid unsaturated vegetable oils into solid margarine.
- **Hydration (addition of steam)**: Alkenes react with steam at 300°C, 60-70 atm pressure, with a phosphoric acid catalyst to form an alcohol. This is the industrial method to produce ethanol from ethene.
- **Cracking**: Large, less useful alkane molecules from crude oil are broken down at 600-700°C with an aluminium oxide catalyst (or high temperature alone) into smaller alkanes and alkenes. Alkenes from cracking are used to make plastics and other industrial chemicals.

**Worked example:** Write the word equation for the hydrogenation of ethene, and the hydration of ethene. Include the conditions required for each reaction.

1. Hydrogenation of ethene: $\text{ethene + hydrogen} \rightarrow \text{ethane}$. Conditions: 150°C, nickel catalyst.
2. Hydration of ethene: $\text{ethene + steam} \rightarrow \text{ethanol}$. Conditions: 300°C, 60-70 atm pressure, phosphoric acid catalyst.

> **Exam tip:** Always include full reaction conditions for industrial Extended reactions, as they are usually awarded 1-2 separate marks in exam questions.

## Common pitfalls

- **Wrong:** Using the general formula $C_nH_{2n}$ for alkanes instead of alkenes
  - Why it fails: Confusing the general formulae of the two homologous series
  - Correct: Remember alkanes are saturated (maximum H atoms) so use $C_nH_{2n+2}$; alkenes have one C=C double bond so use $C_nH_{2n}$
- **Wrong:** Stating bromine water 'goes clear' instead of 'turns from orange to colourless' for the unsaturation test
  - Why it fails: Examiners require explicit mention of the colour change; 'clear' does not confirm a colour change has occurred
  - Correct: Always write the full colour change: *orange to colourless* for a positive alkene test
- **Wrong:** Writing carbon monoxide or soot as products of complete combustion
  - Why it fails: Confusing complete and incomplete combustion reactions
  - Correct: Complete combustion uses excess oxygen, so only carbon dioxide and water are produced; carbon monoxide/soot are products of incomplete combustion with limited oxygen
- **Wrong:** Writing substitution reactions for alkenes
  - Why it fails: Alkenes have a reactive C=C double bond and undergo addition reactions, not substitution
  - Correct: Alkanes undergo substitution reactions with halogens in UV light; alkenes undergo addition reactions with halogens with no UV light required
- **Wrong:** Forgetting catalysts/conditions for Extended hydrogenation/hydration reactions
  - Why it fails: Exam questions award separate marks for correct reaction conditions
  - Correct: Memorise the exact temperature, pressure and catalyst for each industrial Extended reaction

## Cheatsheet

| Property | Alkanes | Alkenes |
| --- | --- | --- |
| General formula | $C_nH_{2n+2}$ ($n \geq 1$) | $C_nH_{2n}$ ($n \geq 2$) |
| Bond type | Only C-C single bonds (saturated) | One C=C double bond (unsaturated) |
| Reactivity | Relatively unreactive | More reactive |
| Combustion flame | Clean blue flame | Smoky yellow flame |
| Bromine water test | No change (stays orange) | Orange → colourless |
| Core reactions | Combustion, substitution (UV light) | Combustion, addition (bromine) |
| Extended reactions | Feedstock for cracking | Hydrogenation, hydration, polymerisation |

## What's next

Now that you have mastered the structure and reactions of alkanes and alkenes, you are ready to move on to the next homologous series in organic chemistry: alcohols and carboxylic acids, which build on the functional group reaction principles you have learned here. For Extended tier students, you will also apply your knowledge of alkenes to polymerisation reactions, where small alkene monomers join together to form long polymer chains used in plastics and industrial materials. Be sure to practice drawing displayed formulae for unbranched alkanes and alkenes up to 4 carbon atoms, and writing word equations for all core and extended reactions, as these are frequently tested in both Paper 3 (Core) and Paper 4 (Extended) exams. Revisit the bromine water test regularly, as it is a common 2-3 mark question across both tiers.

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