Organic Chemistry

Introduction and Alkanes – Revision Notes

Key Definitions and Terminology

**Organic chemistry**: The study of carbon-containing compounds (excluding carbon dioxide, carbon monoxide, and carbonates, which are classified as inorganic).
**Hydrocarbon**: A compound containing **only** hydrogen and carbon atoms.
**Homologous series**: A family of organic compounds with the same general formula, similar chemical properties, and a gradual trend in physical properties. Each successive member differs by –CH₂–.
**Molecular formula**: Shows the actual number of each type of atom in one molecule (e.g. C₃H₈).
**Displayed (structural) formula**: Shows all the atoms and all the bonds between them drawn out in full.
**Saturated**: Describes a molecule that contains only **single** covalent bonds between carbon atoms (no C=C double bonds).

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Main Concepts

1. Why Carbon Forms So Many Compounds

Carbon has four electrons in its outer shell, so it can form **four covalent bonds**.
Carbon atoms can bond to each other in chains, branched chains, and rings, creating an enormous variety of structures.

2. The Alkanes – An Introduction

Alkanes are the simplest homologous series of hydrocarbons.
They are **saturated** hydrocarbons – every carbon-carbon bond is a single bond.
**General formula: CₙH₂ₙ₊₂** (where *n* = number of carbon atoms).

3. The First Four Alkanes

|---------|-------------------|-----------------|---------------------|

| Methane | CH₄ | 1 | Gas |

| Ethane | C₂H₆ | 2 | Gas |

| Propane | C₃H₈ | 3 | Gas |

| Butane | C₄H₁₀ | 4 | Gas |

4. Trend in Physical Properties

As the chain length (number of carbon atoms) **increases**:
**Boiling point increases** – longer molecules have stronger intermolecular forces (more surface contact between molecules), so more energy is needed to separate them.
**Viscosity increases** – longer molecules flow less easily.
**Flammability decreases** – longer molecules are harder to ignite.
These gradual trends are a characteristic feature of any homologous series.

5. Combustion of Alkanes

Alkanes are widely used as **fuels** because they release large amounts of energy when burned.
**Complete combustion** (excess oxygen):
Products: **carbon dioxide + water**
General word equation: alkane + oxygen → carbon dioxide + water
**Incomplete combustion** (limited oxygen):
Products can include **carbon monoxide** (toxic) and/or **carbon/soot (particulates)**, along with water.
This is dangerous and wasteful of fuel.

6. Displayed Formulae of Alkanes

Every bond must be drawn as a straight line.
Each carbon makes **4 bonds**; each hydrogen makes **1 bond**.
Example – displayed formula of methane:

```

H —— C —— H

```

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Worked Examples and Real-World Applications

Worked Example 1 – Using the General Formula

Q: Predict the molecular formula of the alkane with 5 carbon atoms.

A: General formula = CₙH₂ₙ₊₂.

With n = 5: H atoms = 2(5) + 2 = 12.

Molecular formula = C₅H₁₂ (pentane).

Worked Example 2 – Writing a Balanced Equation for Complete Combustion

Q: Write a balanced symbol equation for the complete combustion of propane (C₃H₈).

$$C_3H_8 + 5O_2 \rightarrow 3CO_2 + 4H_2O$$

*Step-by-step:*

Balance C: 3 carbons → 3CO₂
Balance H: 8 hydrogens → 4H₂O
Count O on the right: (3 × 2) + (4 × 1) = 10 oxygen atoms → 5O₂

Real-World Application

**Methane** is the main component of natural gas, used for heating and cooking.
**Propane** and **butane** are sold as LPG (liquefied petroleum gas) for portable stoves and gas barbecues.
Incomplete combustion in poorly ventilated gas appliances produces carbon monoxide – a colourless, odourless, toxic gas. This is why carbon monoxide detectors are fitted in homes.

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Exam Technique Tips (Edexcel-Specific)

**When drawing displayed formulae, show every single bond as a line.** Edexcel mark schemes will **not** award the mark if you write "CH₃–CH₃" instead of drawing out all C–H bonds individually. Always draw all atoms and all bonds explicitly.
**When asked to explain the trend in boiling points of alkanes, link chain length → intermolecular forces → energy.** A full-mark answer must include: (i) longer molecules have **stronger intermolecular forces** (or "more forces between molecules"), AND (ii) therefore **more energy is needed to overcome/break these forces**. Simply stating "bigger molecules have higher boiling points" without an explanation will not gain full marks.

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*Focus your revision on being able to recall the first four alkanes, use the general formula, draw displayed formulae accurately, and write balanced equations for complete and incomplete combustion.*

Subtopics

Revision Method

Introduction and Alkanes