Organic Chemistry

Organic chemistry

Organic chemistry is the chemistry of carbon compounds, and it is everywhere: in fuels, plastics, medicines and food. Most of it starts with crude oil, a thick mixture of molecules made only of carbon and hydrogen, which are called hydrocarbons. By heating the oil and separating it, we get useful fuels such as petrol and diesel. This module looks at the main families of organic molecules: the alkanes, which are fairly unreactive and burn as fuels; the alkenes, which react more easily and can be joined into plastics; and a little on alcohols. It also covers how fuels burn, how long-chain molecules are cracked into smaller ones, and how small units are linked into polymers. Throughout, substances and reactions are described in words, the way a written answer should read.

Words for organic chemistry

Learn these terms before the organic substances are matched up. They name the ideas used in this topic.

Match each organic substance to what it does

Alkanes against alkenes

The two main hydrocarbon families look similar but behave differently, and telling them apart is a common exam task.

Which hydrocarbon is this?

A hydrocarbon reacts easily and can be joined together to make a plastic. Which family does it belong to?

• The alkenes, because they have a reactive double bond
• The alkanes
• A metal
• A salt

How fuels burn

Burning a fuel is one of the most useful reactions in chemistry. When a fuel burns completely in plenty of air, it reacts with the oxygen to release energy, and it produces carbon dioxide and water. This is called complete combustion, and it is what we want in a heater or an engine. If there is too little air, the burning is incomplete: it releases less energy and can produce carbon monoxide, a colourless, poisonous gas, along with soot. That is why fuel-burning appliances need a good air supply and must be checked. When you describe combustion in an answer, name the substances in words and say what is made, rather than reaching for a formula.

Pick the true facts about hydrocarbons

Select every statement about hydrocarbons that is true.

• Hydrocarbons contain only carbon and hydrogen
• Alkanes have single bonds only
• Alkenes have a double bond and are more reactive
• Hydrocarbons are metals
• Hydrocarbons are made only of water

Order how crude oil becomes fuels

Put the stages of separating crude oil into useful fuels into a sensible order.

• The crude oil is heated until it vaporises
• The vapours rise up a tall column
• They cool as they rise
• Each part condenses at its own level
• The separate fuels are collected

Complete the organic facts

A molecule made of only carbon and hydrogen is called a _____. The family with single bonds only, such as methane, are the _____. The family with a double bond, such as ethene, are the _____. When many small alkene units join into one long chain, they form a _____.

Tap the two alkenes

Read these four substances. Tap the TWO that are alkenes.

• ethene
• propene
• methane
• ethanol

A paper chain of links

Making a plastic is like building a paper chain. On their own, the little paper strips are small and separate, easy to handle one at a time. But loop one into the next, again and again, and you build a single chain far longer than any strip. Plastics are made in the same way: many small, identical molecules are joined end to end into one enormous chain. The small units are simple; the long chain has completely new properties, strong and flexible, that no single unit had. That is why a handful of small molecules can become a bag, a bottle or a pipe, just by linking them up.

Work out the hydrogen atoms

An alkane has four carbon atoms. Using the rule that the number of hydrogen atoms is twice the number of carbons plus two, how many hydrogen atoms does it have? Give the number only.

Reason about the organic reaction

Read each situation and choose the best answer.

• You want to turn long-chain molecules from oil into smaller, more useful ones. What is this process called?
• A hydrocarbon burns completely in plenty of air. What is produced?
• Many small alkene units join end to end. What have you made?

Build an organic chemistry point

Choose the word for each gap to complete one point about organic chemistry.

Explain organic chemistry

A friend asks what organic chemistry is about. Using what you have learned, explain the main ideas, describing substances in words.

• Explain what a hydrocarbon is
• Explain the difference between alkanes and alkenes
• Explain what happens when a fuel burns completely
• Explain what cracking is for
• Explain how a polymer is made