Hydrocarbons

What is a hydrocarbon?

A molecule that is formed from carbon and hydrogen atoms only.

What are hydrocarbons which share similar chemical properties grouped into?

Hydrocarbons which share similar chemical properties can be grouped together in homologous series.

How does the molecular formula of neighbouring compounds in a homologous series differ?

The molecular formulae of neighbouring compounds in a homologous series differ by a CH₂ unit.

What properties change as you go down a homologous series? How do they change? How do these properties affect other properties of hydrocarbons?

1. Boiling and melting points: There is a gradual increase in boiling and melting points. This can be attributed to the increase in molecular size and mass, leading to stronger intermolecular forces.

   1. Ease of ignition: Therefore shorter hydrocarbons are easier to ignite because they have lower boiling points, so tend to be gases at room temperature. These gas molecules mix with oxygen in the air to produce a gas mixture which bursts into flames if it comes into contact with a spark.

2. Density: The density of compounds generally increases as you move down the series.

3. Solubility: Generally, as the size of the molecule increases (especially in alcohols), solubility in water decreases. The smaller molecules are more soluble due to their ability to form hydrogen bonds with water.

4. Viscosity: Viscosity measures how easily a substance flows. The stronger the intermolecular forces are between hydrocarbon molecules, the harder it is for the liquid to flow. Fractions containing longer hydrocarbons have a higher viscosity and are very thick (and vice versa for shorter hydrocarbons)

What homologous series of hydrocarbons are most common as fractions of crude oil?

Alkanes

How are physical properties determined in hydrocarbons?

By the size and mass, and therefore the strength of intermolecular forces

What is the word equation for burning hydrocarbons?

Hydrocarbon + oxygen —> carbon dioxide + water

(energy is also released as it’s an exothermic reaction)

What is required for a complete combustion of hydrocarbons?

Sufficient oxygen

What occurs when there is not sufficient oxygen in the air when trying to combust a hydrocarbon?

Incomplete combustion occurs, in which carbon and carbon monoxide may be produced alongside water and carbon dioxide.

Therefore what is the chemical word equation for incomplete combustion of a hydrocarbon?

Hydrocarbon + oxygen —> Carbon dioxide + water + carbon and/or carbon monoxide

What makes carbon monoxide so harmful to the body? What does this result in?

Carbon monoxide can combine with red blood cells and stop your blood from carrying sufficient oxygen around the body. It does this by binding to the haemoglobin in your red blood cells that normally carries oxygen - so less oxygen is able to be transported around your body. A lack of oxygen in the blood supply to the brain can lead to fainting, a coma or even death.

What is soot?

During incomplete combustion, tiny particles of carbon can be released into the atmosphere. When they fall back to the ground, they deposit themselves as the black dust we call soot.

What does soot cause?

Soot makes buildings looks dirty, reduces air quality and can cause or worsen respiratory problems