C7 - organic chemistry Flashcards
what is a hydrocarbon?
any compound that is formed from carbon and hydrogen atoms only
what is the general formula for alkanes?
CnH2n+2,
what are alkanes?
a homologous series of saturated hydrocarbons
what is a homologous series?
a group of organic compounds that react in a similar way
what are the first four alkanes?
methane, ethane, propane, butane
what is a displayed formula?
a drawing showing all the atoms and bonds in a molecule
what is the formula for methane?
CH4
what is the formula for Ethane?
C2H6
what is the formula for propane?
C3H8
what is the formula for Butane?
C4H10
what are the properties of shorter hydrocarbons?
- less viscous
- more volatile (lower boiling points)
- more flammable
what are the properties of longer hydrocarbons?
- more viscous
- less volatile (higher boiling points)
- less flammable
what are short-chain hydrocarbons used for? why?
they are used as ‘bottled gases’ - stored under pressure as liquids in bottles - because of their low boiling points
what is the equation for the complete combustion of a hydrocarbon?
hydrocarbon + oxygen -> carbon dioxide + water
during the combustion of a hydrocarbon, what happens to the carbon and hydrogen?
they are both oxidised
why are hydrocarbons used as fuels?
they release a lot of energy when they combust completely
write a balanced symbol equation for the complete combustion of methane
CH4 + 2O2 -> CO2 + 2H20
write a balanced symbol equation for the complete combustion of propane
C3H8 + 5O2 -> 3CO2 + 4H2O
how is crude oil made?
from the remains of an ancient biomass, made up mainly of plankton, that’s been buried in mud for millions of years
what type of fuel is crude oil?
a fossil fuel
is crude oil renewable?
no
what does crude oil consist of?
different lengths of hydrocarbons and a bit of sulfur
what are the 4 steps to separating hydrocarbons by fractional distillation?
- the oil is heated until most of it has turned into gas. The gasses enter a fractionating column, and everything that remains liquid is drained off
- in the fractionating column there’s a temperature gradient (it’s hot at the bottom and cooler as you go up)
- the longer hydrocarbons have high boiling points. They condense back into liquids early on, when they’re near the bottom. The shorter hydrocarbons have lower boiling points. They condense and drain out much later on, near to the top of the column where it’s cooler.
- you end up with the crude oil mixture separated out into different fractions. Each fraction contains a mixture of hydrocarbons that all contain a similar number of carbon atoms, so have similar boiling points
which fuels are obtained from the fractional distillation of crude oil, in order from the highest boiling point (collected at the bottom) to the lowest boiling point (collected at the top)
bitumen, heavy fuel oil, diesel oil, kerosene, petrol, LPG (liquefied petroleum gas)
which two hydrocarbons does LPG mostly contain?
propane and butane
what are the three oils that count as heavy fuel oil?
heating oil, fuel oil, lubricating oil
what are the two main uses of crude oil?
- oil provides the fuel for most modern transport. Diesel oil, kerosene, heavy fuel oil and LPG (liquid petroleum gas) all come from crude oil
- the petrochemical industry uses some of the hydrocarbons from crude oil as a feedstock to make new compounds for use in things like polymers, solvents, lubricants and detergents
what are organic compounds?
compounds containing carbon atoms
what is cracking?
splitting up long-chain hydrocarbons
why do we use cracking?
short-chain hydrocarbons are flammable so make good fuels and are in high demand. However, long-chain hydrocarbons form thick gloopy liquids like tar which aren’t very useful, so we use cracking to turn a lot of the longer alkane molecules produced from fractional distillation into smaller, more useful ones.
what are some of the products of cracking useful as?
fuels, e.g. petrol for cars and paraffin for jet fuel
what does cracking produce other than alkanes?
alkenes
which are more reactive: alkanes or alkenes?
alkenes
what are alkenes used for?
they’re used as a starting material when making lots of other compounds and can be used to make polymers
what is the test for alkenes?
bromine water - when in the presence of an alkene it turns from bright orange to colourless
what type of reaction is cracking?
a thermal decomposition reaction - it breaks molecules down by heating them
what are the two methods of cracking?
- catalytic cracking
2. steam cracking
what are the 3 steps to catalytic cracking?
- you heat and vaporise long-chain hydrocarbons
- then the vapour is passed over a hot powdered aluminium oxide catalyst
- the long-chain molecules split apart on the surface of the specks of catalyst
how do you perform steam cracking?
vaporise long-chain hydrocarbons, mix them with steam and then heat them to a very high temperature
what is the word equation for cracking?
long-chain hydrocarbon molecule -> shorter alkane molecule + alkene
how do you balance a chemical equation for cracking?
make sure that there is the same number of carbon molecules on both sides
what is the general formula for alkenes?
CnH2n
