# Hydrocarbons

> Alkanes, alkenes and alkynes, their shapes, intermolecular forces and boiling points, solubility, safe handling, and the environmental, economic and social effects of using fuels from the Earth.

From HSC Chemistry on WhizzNotes, free study notes for NSW students: https://whizznotes.com/chemistry/hydrocarbons/. Updated 3 November 2026.
Year 11 and Year 12 boxes go deeper than the Year 10 text around them.

### Story time: The black gold under the desert
In 1908, after years of drilling and running out of money, a British team in Persia was about to give up. Then, at 4 am one morning, oil gushed 15 metres into the air. That well started the Middle East oil industry.

Crude oil is a thick black mixture of thousands of **hydrocarbons**, molecules made only of hydrogen and carbon. From it come petrol, diesel, jet fuel, gas for cooking, and the starting materials for most plastics and medicines. It changed the world, but burning it has also changed the climate.

**In short:** Hydrocarbons are compounds of only carbon and hydrogen. Alkanes have only single bonds (saturated), while alkenes have a C=C double bond and alkynes a triple bond (unsaturated), and longer chains have higher boiling points, which is how crude oil is separated.

## What is a hydrocarbon?

A **hydrocarbon** contains **only carbon and hydrogen**.

| Series | Bonds | General formula | First member | Saturated? |
|---|---|---|---|---|
| Alkanes | C−C single only | $\mathrm{C_nH_{2n+2}}$ | methane $\mathrm{CH_4}$ | Saturated |
| Alkenes | one C=C double | $\mathrm{C_nH_{2n}}$ | ethene $\mathrm{C_2H_4}$ | Unsaturated |
| Alkynes | one C≡C triple | $\mathrm{C_nH_{2n-2}}$ | ethyne $\mathrm{C_2H_2}$ | Unsaturated |

**Saturated** means every carbon has the most hydrogen atoms possible (no double or triple bonds). **Unsaturated** compounds have double or triple bonds and can **add** more atoms.

**Cycloalkanes** like cyclohexane, $\mathrm{C_6H_{12}}$, are rings of carbons with single bonds. They have the same formula as the alkenes but are saturated.

## Shapes

The shape around each carbon depends on its bonds (VSEPR):

| Bonds on the carbon | Shape | Angle | Example |
|---|---|---|---|
| Four single bonds | Tetrahedral | 109.5° | every C in ethane |
| One double, two single | Trigonal planar | 120° | each C in ethene (flat molecule) |
| One triple, one single | Linear | 180° | each C in ethyne |

## Intermolecular forces and boiling points

Hydrocarbon molecules are **non-polar**. The only forces between them are **dispersion forces** (weak temporary attractions caused by moving electrons).

- **Longer chains have higher boiling points**: more electrons and more surface contact, so stronger dispersion forces.
- **Branched chains have lower boiling points** than straight chains of the same formula: they are more compact, with less surface contact.

| Alkane | Carbons | Boiling point (°C) | State at 25 °C |
|---|---|---|---|
| Methane | 1 | −162 | Gas |
| Butane | 4 | −1 | Gas |
| Pentane | 5 | 36 | Liquid |
| Octane | 8 | 126 | Liquid |
| 2,2-dimethylpropane | 5 | 10 | Gas |

Alkenes and alkynes have similar boiling points to alkanes of the same size.

## Solubility

Hydrocarbons are **insoluble in water**. Water molecules hold each other tightly with hydrogen bonds, and non-polar hydrocarbons cannot form hydrogen bonds to break into that network. Hydrocarbons **do** dissolve in other non-polar solvents ("like dissolves like"). Being less dense than water, they float on top, which is why oil spills spread across the sea.

## Fractional distillation

Crude oil is separated by **fractional distillation**. It is heated so most of it boils, and the vapour rises up a tall column that is hot at the bottom and cool at the top. Each fraction condenses at a different height:

- Small molecules (low boiling point) rise to the top: refinery gas, petrol.
- Large molecules (high boiling point) condense near the bottom: diesel, lubricating oil, bitumen.

## Safe handling

Hydrocarbons are **flammable**, and their vapours can catch fire well away from the liquid. Small ones are also **volatile** (evaporate easily). In the lab:

- Keep them away from flames. Heat with a water bath or heating mantle, not a Bunsen burner.
- Use a fume hood, because vapours can be toxic.
- Store in sealed, labelled containers. Read the **Safety Data Sheet (SDS)**.

## Effects of using hydrocarbons

| Area | Effects |
|---|---|
| **Environmental** | $\mathrm{CO_2}$ from burning adds to the enhanced greenhouse effect and **global warming**. Extra $\mathrm{CO_2}$ causes **ocean acidification**. Sulfur and nitrogen oxides cause **acid rain**. Incomplete combustion gives toxic CO and soot. Oil spills harm wildlife. |
| **Economic** | Fuels power transport and industry; oil sales earn money for countries; prices can rise and fall sharply. Cleaning up pollution costs money. |
| **Social** | Cheap energy improves living standards. Air pollution harms health. Supplies are **non-renewable** and will run out, so future generations will need other sources. |

Note: the hole in the **ozone layer** was caused by CFCs (chlorofluorocarbons), not by burning hydrocarbons.

**Mind map**

```text
Hydrocarbons
  Types
    Alkanes: CnH2n+2, saturated
    Alkenes: CnH2n, C=C
    Alkynes: CnH2n−2, C≡C
  Shapes
    Tetrahedral 109.5°
    Trigonal planar 120°
    Linear 180°
  Forces
    Non-polar
    Dispersion only
    Longer chain, higher bp
    Branched, lower bp
  Properties
    Insoluble in water
    Flammable, volatile
  Crude oil
    Fractional distillation
  Effects
    Global warming
    Acid rain
    Non-renewable
```

### Year 11: Intermolecular forces compared
The strength of intermolecular forces, from weakest to strongest:

**dispersion forces < dipole-dipole forces < hydrogen bonds**

- **Dispersion** forces act between **all** molecules. They are the only forces in non-polar molecules like hydrocarbons.
- **Dipole-dipole** forces act between **polar** molecules, such as chloromethane or propanone.
- **Hydrogen bonds** form when H is bonded to **N, O or F** and is attracted to an N, O or F on another molecule.

So for molecules of similar size, boiling point increases: **alkane < haloalkane or ketone < alcohol**. For example, propane (−42 °C) < propanone (56 °C) < propan-1-ol (97 °C).

**Carboxylic acids boil highest.** The −COOH group has both a C=O and an O−H, so acid molecules hydrogen bond more strongly than alcohols (pairs of acid molecules even hold together in twos). For similar molar masses: pentane < butan-1-ol < propanoic acid.

### Year 12: Why carbon makes so many compounds
Carbon has four valence electrons, so it forms four strong covalent bonds. C−C bonds are strong enough to make long stable chains (this is called **catenation**), and carbon can form single, double and triple bonds and rings. Silicon, below carbon in group 14, makes much weaker Si−Si bonds, so it cannot form long stable chains in the same way.

**Combustion energy and efficiency.** All hydrocarbons burn in excess oxygen to make carbon dioxide and water:

$$\mathrm{C_8H_{18}(l)} + \tfrac{25}{2}\mathrm{O_2(g)} \rightarrow \mathrm{8CO_2(g)} + \mathrm{9H_2O(l)}$$

In limited oxygen, **incomplete combustion** produces carbon monoxide or soot (C) instead of $\mathrm{CO_2}$, and releases less energy.

Compare the equations: $\mathrm{2C_8H_{18}} + \mathrm{17O_2} \rightarrow \mathrm{16CO} + \mathrm{18H_2O}$ needs 17 mol of oxygen, while complete combustion of the same octane needs 25. Less oxygen is used, less energy is released, and the CO is a poisonous gas.

**Telling isomers apart.** Cyclohexane and hex-1-ene are both $\mathrm{C_6H_{12}}$, but cyclohexane has only single bonds. Hex-1-ene turns bromine water colourless at once; cyclohexane does not.

## Quick summary

- Hydrocarbons contain only C and H. Alkanes are saturated; alkenes and alkynes are unsaturated.
- Shapes: tetrahedral (single bonds), trigonal planar (C=C), linear (C≡C).
- Only dispersion forces: longer chains have higher boiling points, branching lowers them.
- Hydrocarbons are insoluble in water, flammable and volatile.
- Fractional distillation separates crude oil by boiling point.
- Burning hydrocarbons causes global warming, ocean acidification and acid rain, and the supply is non-renewable.

## Quiz

Try these questions from the chapter quiz.

1. A hydrocarbon contains:
   - A. Only carbon and hydrogen
   - B. Carbon, hydrogen and oxygen
   - C. Only carbon
   - D. Carbon and any other element
   - Answer: A. Only carbon and hydrogen. Hydro (hydrogen) + carbon.

2. Alkanes are described as saturated because they have:
   - A. Only single C−C bonds
   - B. A double bond
   - C. A triple bond
   - D. Oxygen atoms
   - Answer: A. Only single C−C bonds. Every carbon holds as many hydrogens as possible.

3. The general formula of the alkanes is:
   - A. $\mathrm{C_nH_{2n}}$
   - B. $\mathrm{C_nH_{2n+2}}$
   - C. $\mathrm{C_nH_{2n-2}}$
   - D. $\mathrm{C_nH_n}$
   - Answer: B. $\mathrm{C_nH_{2n+2}}$. For example, propane is $\mathrm{C_3H_8}$.

The whole chapter quiz is on the page, for signed-in students: https://whizznotes.com/chemistry/hydrocarbons/#quiz