Knowledge Organiser – Organic Chemistry Basics

Ever wondered what makes up the fuel in your family's car? Crude oil is a fascinating mixture that formed from ancient sea creatures millions of years ago – essentially prehistoric plankton that got buried and cooked under pressure!

Hydrocarbons are the star players here – they're compounds made only of hydrogen and carbon atoms. The most important family is called alkanes, which are saturated molecules (meaning they only have single bonds between atoms). Think of them like a chain where each link is perfectly connected.

You'll need to remember that alkanes follow a pattern called a homologous series with the general formula CₙH₂ₙ₊₂. This means methane is CH₄, ethane is C₂H₆, propane is C₃H₈, and so on. Each one just adds another carbon and two more hydrogens!

Quick Tip: The displayed formula shows every single atom and bond drawn out – it's like seeing the molecule's blueprint!

From Crude Oil to Useful Products

Here's where chemistry gets really clever – we can't use crude oil as it comes out of the ground, so we need to separate it into useful bits. Fractional distillation is like the ultimate sorting machine that uses heat to split crude oil into different fractions.

The process works brilliantly: heat the crude oil until it evaporates, then let different compounds condense at different temperatures as they rise up a tall tower. Lighter molecules like petrol condense higher up (where it's cooler), whilst heavier ones like diesel settle lower down.

Sometimes we get molecules that are too big to be useful, so we use cracking to break them into smaller, more valuable pieces. This process creates both alkanes and alkenes – the difference being that alkenes have double bonds and are unsaturated.

Complete combustion happens when hydrocarbons burn with plenty of oxygen, producing just carbon dioxide and water. This releases loads of energy, which is why fossil fuels are such powerful energy sources.

Remember: Alkenes turn bromine water colourless – this is a dead giveaway test you might see in your exams!