organic chemistry

aqa chemistry gcse

What are fossil fuels?

Dead organic matter such as plankton buried in mud and compressed and exposed to high pressures and temperatures over millions of years

Give one example of a fossil fuel

Crude oil

What is a hydrocarbon?

Molecules made up of only hydrogen and carbon atoms

What is a functional group?

An atom or group of atoms that give characterisation properties to a series of molecules

What are alkanes?

Saturated hydrocarbons containing only C-C and C-H single bonds that have the formula C_nH_2n+2 where n is the number of carbon atoms

What is crude oil?

A mixture of hydrocarbons such as alkanes that can be separated using a process called fractional distillation

What is methane’s molecular formula?

CH₄

What is ethane’s molecular formula?

C₂H₆

What is propane’s molecular formula?

C₃H₈

What is butane’s molecular formula?

C₄H₁₀

What is pentane’s molecular formula?

C₅H₁₂

How does fractional distillation work and what is it used for?

It’s used to separate crude oil into simpler and more useful mixtures (eg. petrol, diesel) Different hydrocarbons have different boiling points, so heated crude oil enters the fractioning column, which is heated (around 350^oC) at the bottom and cool (around 25^oC) at the top. Vapors from the evaporating oil rises through the column, with each hydrocarbon condensing at their boiling point and leaving the column at different heights. Small hydrocarbon molecules with very low boiling points leave the column as gases, and large hydrocarbon molecules with very high boiling points leave the column as liquid at the bottom.

Properties of small hydrocarbon molecules (higher in the column)

Low boiling points, very volatile, flow and ignite easily

Properties of large hydrocarbon molcules (lower in the column)

High boiling points, not very volatile, do not flow or ignite easily

What are alkenes?

Unsaturated hydrocarbons that contain one C=C double bond where not all carbons have the maximum number of hydrogens attached with the general formula C_nH_2n

What is ethene’s molecular formula?

C₂H₄

What is propene’s molecular formula?

C₃H₆

What is butene’s molecular formula?

C₄H₈

What is pentene’s molecular formula?

C₅H₁₀

What is hexene’s molecular formula?

C₆H₁₂

How are alkenes made?

Made from alkanes via cracking

What does cracking do?

It takes large chain alkanes and converts them to smaller chain alkanes and an alkene (e.g. C₁₀H₂₂ → C₅H₁₂ + C₅H₁₀)

Why are small chain alkanes more useful?

They are easily combustible and are better fuels

How does cracking work?

Fractions containing large hydrocarbons are heated to 600-700^oC to vaporise them, then passed over a catalyst of silica or Al₂O₃ - this breaks the covalent bonds in the molecules, causing thermal decomposition reactions

What are addition reactions?

When one of the double bonds breaks; each carbon can have species added on to make up the fourth bond

What is hydrogenation and what does it require?

The addition of H₂, requires nickle catalyst

What bonds does hydrogen form/break into when added to alkenes?

-H and -OH

What does addition polymerisation turn alkenes into?

Polyalkenes (plastics)

What is a monomer?

A small molecule which can bond together with other monomers to form a polymer, is a single part of a repeating unit

What is a polymer?

A large molecule made up of multiple repeating monomer units

What is combustion?

Burning with oxygen

What happens in complete combustion?

Burns with excess oxygen, water and carbon dioxide gas formed

What happens in incomplete combustion?

Burns with limited oxgyen, can produce carbon monoxide (poisonous) or carbon (soot, particulate, contributes to global dimming) along with water instead of oxygen

What happens when alkenes burn with limited oxygen?

Incomplete combustion occurs and a smoky yellow flame is produced and less energy released

What are the 4 types of polymers?

Low density, high density, thermosoftening and thermosetting

What are the properties of low density polymers?

Flexible, unreactive, weaker than high density polymers, becomes softer at lower temps, are formed by branched polymer chains (molecules randomly arranged)

What are the properties of high density polymers?

Strong, flexible, resists shattering and chemical attacks, made up of molecules that are lined up closely/structured, less branching of polymer chains than in low density polymers

What are the properties of thermosoftening plastics?

Melt when heated, can be recycled, don’t have covalent bonds between neighbouring polymer molecules (no cross links), so molecules can move over each other when heated (plastic melts)

What are the properties of thermosetting plastics?

Don’t melt when heated, instead char and burn, resistant to high temperatures, covalent bonds are strong and prevent polymer melting when heated