Chemistry - C7 Organic Chemistry

What is a hydrocarbon?

What is a hydrocarbon?

a compound made of only hydrogen and carbon atoms

what is an alkane and what is the general formula for them?

saturated compounds

Cn + H(2n+2)

C-C bonds (single bonds)

what are the first four alkanes?

Methane CH4 Ethane C2H6 Propane C3H8 Butane C4H10

What is a homologous series?

A homologous series is a bunch of organic compounds that have the same general formula and similar chemical properties.

what are the properties of a hydrocarbon as the length changes?

1. The shorter the carbon chain, the more runny a hydrocarbon is - the less viscous it is

2. hydrocarbons with shorter carbon chains are more volatile and more flammable

what are the physical properties of alkanes?

1. First few in series of gases, then changes to liquid, then to solids

2. boiling points and viscosity increase as molecules get bigger

3. poor reactivity

4. volatility and flammability decrease as the molecule gets bigger

what happens during the combustion of hydrocarbons?

exothermic reaction

complete combustion:

1. carbon and hydrogen atoms are completely oxidised

2. produces carbon dioxide, water (waste products), and energy

hydrocarbon + oxygen —> carbon dioxide + water (+energy)

incomplete combustion: 3) occurs when the supply of air or oxygen is poor 4) produces carbon monoxide, water and carbon

why are hydrocarbons used as fuels

Due to the amount of energy released when they combust completely

what is crude oil?

finite resource - cannot be replaced as it is used up

formed from the remains of plants and animals, mainly plankton, that died millions of years ago and were buried in mud

over millions of years, with high temperatures and pressure, the remains turn to crude oil which can be drilled up from rocks where it is found

fossil fuel

aqa exam mark scheme -> plankton, buried in mud, over millions of years

explain fractional distillation of crude oil - aqa mark scheme specific

crude oil is heated hydrocarbons vaporise temperature gradient in the column vapours condense fractions collect at different levels depending on the boiling point

explain fractional distillation

oil is heated until most of it vaporises

the gases enter the fraction into column -> in the column there is a temperature gradient: hot at the bottomcold at the top

the longer hydrocarbons have high boiling points, they condense back into liquid and drain out of the column early on when they are near the bottom

shorter hydrocarbons have lower boiling points, they condense and drain out much later on near to the top of the column where it is cooler

which fraction has the lowest boiling point? In a fractional distillation column, where would you find a long chain hydrocarbons? which fraction is the least volatile? which fraction is the most flammable? which fraction is the most viscous? why do large hydrocarbons have a higher boiling points?

LPG towards the bottom bitumen LPG bitumen more energy is needed to break the intermolecular bonds

what is cracking and why is it useful?

where large hydrocarbon molecules are broken down into smaller hydrocarbons called alkanes and alkenes - thermal decomposition

short-chain hydrocarbons are flammable so make good fuels and are high in demand long-chain hydrocarbons form thick gloopy liquids like tar which isn't useful

so cracking is used to create more useful short-chain hydrocarbons

what is steam cracking?

1. heat the long chain hydrocarbons to vaporise them

2. mix the vapour with steam

3. heat it to a very high temperature

what is catalytic cracking?

1. heat the long chain hydrocarbons to vaporise them

2. the vapour is passed over a hot powdered aluminium oxide catalyst

3. the long-chain molecules split apart on the surface of the specks of the catalyst

what is the bromine water test?

test for alkene/alkanes

alkane: when orange bromine water is added to an alkane -> no reaction will happen and it will stay bright orange

alkene: the bromine water reacts with the alkene to make a colourless compound -> the bromine water is decolourised

what are the uses of crude oil?

oil provided the fuel for most modern transport - cars, planes, trains etc.

petrochemical industry uses some hydrocarbons from crude oil to make new compounds for things like polymers, solvents, lubricants and detergents

describe the fractions in fractional distillation

• at the top - LPG (liquified petroleum gas) which contains mostly propane and butane

• petrol -> fuel for vehicles

• kerosene -> airplane/jet fuel

• diesel oil -> fuel for vehicles heavy fuel oil

• at the bottom - bitumen -> tarmac

what is an alkene?

unsaturated hydrocarbon

Cn + H2n

C=C double bonds

what is flammability?

how easy it is to ignite a substance shorter hydrocarbons are more flammable

what is viscosity?

how easy a gas or liquid flows longer hydrocarbons are more viscous

What is volatility?

how easy a substance will evaporate shorter hydrocarbons are more volatile

Why do longer hydrocarbons have higher boiling points?

they have stronger intermolecular forces of attraction these stronger IM forces require more energy to overcome, meaning the longer hydrocarbons have a higher melting and boiling point