Studied by 3 people
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?
The shorter the carbon chain, the more runny a hydrocarbon is - the less viscous it is
hydrocarbons with shorter carbon chains are more volatile and more flammable
what are the physical properties of alkanes?
First few in series of gases, then changes to liquid, then to solids
boiling points and viscosity increase as molecules get bigger
poor reactivity
volatility and flammability decrease as the molecule gets bigger
what happens during the combustion of hydrocarbons?
exothermic reaction
complete combustion:
carbon and hydrogen atoms are completely oxidised
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?
heat the long chain hydrocarbons to vaporise them
mix the vapour with steam
heat it to a very high temperature
what is catalytic cracking?
heat the long chain hydrocarbons to vaporise them
the vapour is passed over a hot powdered aluminium oxide catalyst
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
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