C7 (organic chemistry) Flashcards
how is crude oil formed?
- found in rocks - it’s a finite resource (will run out some day).
- 300-400 million years ago, organisms (such as plankton) died, sank to seabed. sediment fell on top of them. sediment and the water on top of them created high pressures and temperature, preventing them decomposing. formed crude oil, then extracted from the rocks.
what is a hydrocarbon?
a compound of carbon and hydrogen only. (e.g. crude oil is made up of a mix of different hydrocarbons. the temperature and pressure the solution is exposed to determines the types of hydrocarbons in the mixture).
what are the four types of alkanes and their formulas?
- methane (CH4)
- ethane (C2H6)
- propane (C3H8)
- butane (C4H10)
what is the formula for alkanes?
Cn (H2 x n +2)
However many carbon atoms there are, double it and add two to find the number of hydrogen atoms.
what is an alkane?
scientists say that alkanes are saturated molecules. this is because the carbon atoms are fully covalently bonded to the hydrogen atoms.
why is the process of fractional distillation important?
- crude oil is made up of many hydrocarbons with different boiling points.
- we must separate them in order for the hydrocarbons in the oil to be useful, and we do this through fractional distillation.
- in fractional distillation, crude oil is separated into fractions - these contain hydrocarbons with a similar number of carbon atoms.
describe the process of fractional distillation:
- takes place in large columns in oil refineries.
- the crude oil is heated to a very high temperature, causing it to boil.
- all the hydrocarbons evaporate and turn into a gas, which is then fed into the bottom of a fractional distillation column (above).
- the column is hotter at the bottom and cooler at the top.
- the hydrocarbon vapours now rise up the column.
- the different hydrocarbons condense (turn into liquid) at different points up the fractionating column, when they reach their boiling point. the liquid fractions are then removed.
- very long chain hydrocarbons have very high boiling points, and so are all removed at the bottom of the fractionating column.
- very short chain hydrocarbons have very low boiling points, and do do not condense. these are removed at the top of the fractionating column as gases.
describe fractions and their uses:
fractions contain hydrocarbons with a similar number of carbon atoms.
- petrol and diesel fuels cars.
- kerosene is used as jet fuel.
- heavy fuel oil is used to power ships.
- liquified petroleum gas is used in camping stoves.
- some fractions are used as feedstock (chemical used to make other chemicals) for the petrochemical industry.
- solvents
- lubricants
- detergents
- polymers
how does hydrocarbon chain length affect viscosity?
viscosity = the thickness of a fluid.
- fluids with a high viscosity flow slowly, e.g. honey.
- as the hydrocarbon molecule size increases, the molecules get more viscous. very long chain hydrocarbons flow very slowly.
- this is because it takes more energy for longer chain lengths to flow over itself.
how does hydrocarbon chain length affect flammability?
flammability = how easy hydrocarbons combust.
- short chain hydrocarbons are very flammable - methane is the gas used in bunsen burners.
- as the size of hydrocarbon molecules increases, the molecules become less flammable.
- this is because it’s less able to evaporate and convert into gas form.
how does hydrocarbon chain length affect the boiling point?
boiling point = temperature at which a liquid turns into a gas.
- short chain hydrocarbons have low boiling points.
- as chain length increases, the number of intermolecular forces increases - more energy is needed to separate longer molecules, meaning they have higher boiling and melting points.
what are the two main features of alkanes?
- general formula of Cn (H2n+2)
- only have single covalent bonds between carbon atoms.
what is the disadvantage of using long-chain hydrocarbons as fuels?
- not very flammable, don’t make good fuels.
- short chain hydrocarbons are flammable, and are under high demand - finite resource.
- must convert long-chain hydrocarbons into shorter-chain hydrocarbons.
- this process is called cracking.
what is cracking?
- thermal decomposition reaction.
long chain hydrocarbon broken down to produce smaller, more useful molecules. - this makes one shorter chain alkane (used for fuels in cars).
- also makes an alkene molecule.
what are the required conditions for cracking?
- catalytic cracking: high temperature and catalyst (speeds up reaction).
- heat long chain hydrocarbons, vaporise them.
- use hot powdered aluminium oxide as the catalyst, and pass the vaporised hydrocarbon over it.
- as the long hydrocarbons come into contact with the catalyst, they split apart.
- steam cracking: high temperature and steam.
- heat long chain hydrocarbons, vaporise them.
- mix the vaporised hydrocarbons with steam, and heat them to a very high temperature.
- this causes the long hydrocarbons to split apart.
describe alkenes:
- (e.g. ethene) has double covalent bond between two carbon atoms. this makes them unsaturated - alkanes only have single bonds, making them saturated.
- very useful. used by adding them together to make polymers. the double bond can break to form two or more bonds to connect the adjacent atoms.
- used as starting material for other useful chemicals.
- more reactive than alkanes - has a double bond which can easily split open.
what are the four alkenes and their formulas?
- methene (CH2)
- ethene (C2H4)
- propene (C3H6)
- butene (C4H8)
how do you test for alkenes?
- uses bromine water, which is orange.
- if you shake an alkene solution with bromine water, the bromine water will turn from orange to colourless.
- the bromine molecules replace the double bond in the alkene molecule. there is now no bromine molecule to keep the orange colour of the solution - this won’t happen with alkanes, as there is no double bond to break.
why do you get one alkane and one alkene when you split long chain hydrocarbons?
- take decane, which splits to make heptane and propene.
- as decane is ten carbons long, it splits to make heptane (7 carbons) and propene (3 carbons).
- if you did the same for hydrogens, there aren’t enough hydrogens left for both products to be saturated and have all single bonds.
- therefore, one of the products has to have a double bond, making it an alkene, rather than an alkane.
what is a homologous series?
a group of similar compounds with similar properties.
- alkanes and alkenes are both homologous series.