Hydrocarbons/Fuels Flashcards
● Hydrocarbons are compounds that
contain carbon and hydrogen only
Describe crude oil as:
● A complex mixture of hydrocarbons
● Containing molecules in which carbon atoms are in chains or rings (names,
formulae and structures of specific ring molecules not required)
● An important source of useful substances (fuels and feedstock for the
petrochemical industry)
● A finite resource
Describe and explain the separation of crude oil into simpler, more
useful mixtures by the process of fractional distillation
● Crude oil is a mixture of different hydrocarbons, different hydrocarbons have
different boiling points (longer chain hydrocarbons have higher boiling points)
● The crude oil is heated in the fractionating column and the oil evaporates and
condenses at a number of different temperatures.
● The fractionating column works continuously, heated crude oil is piped in at the
bottom. The vaporised oil rises up the column and the various fractions are
constantly tapped off at the different levels where they condense.
● The fractions can be processed to produce fuels and feedstock for the
petrochemical industry.
Recall the names and uses of the fractions in the fractionating column while distilling crude oil:
● Gases: domestic heating and cooking
● Petrol: fuel for cars
● Kerosene: fuel for aircraft
● Diesel oil: fuel for some cars and trains
● Fuel oil: fuel for large ships and in some power stations
● Bitumen: surface roads and roofs
Explain how hydrocarbons in different fractions differ from each other
in: the number of carbon and hydrogen atoms their molecules contain,
boiling points, ease of ignition, and viscosity and are mostly members of the
alkane homologous series
● Some properties of hydrocarbons depend on the size of their molecules. These
properties influence their use as fuels.
● Shorter the molecules, the less viscous it is. (more runny) and the longer the
molecules, the more viscous it is.
● The shorter the molecules, the lower the temperature at which that fraction
evaporates or condenses – and the lower its boiling point.
● The shorter the molecules – the more flammable it is, so the easier it is to ignite
Explain a homologous series as a series of compounds which:
● Have the same general formula
● Differ by CH 2 in molecular formulae from neighbouring compounds
● Show a gradual variation in physical properties, as exemplified by their boiling
points
● Have similar chemical properties
Explain why the alkanes are saturated hydrocarbons
● They contain no C=C double bonds and are compounds made of hydrogen and
carbon only
What is the first alkene
ethene (NOT METHENE)
Explain why the alkenes are unsaturated hydrocarbons, describing
that their molecules contain the functional group C=C
● Contain one or more C=C double bonds and are compounds made of hydrogen
and carbon only
Recall the addition reaction of ethene with bromine, showing the
structures of reactants and products, and extend this to other alkenes
● Ethene + bromine -> 1,2-dibromoethane
● this reaction works the same for any alkene or any halogen
Explain how bromine water is used to distinguish between alkanes
and alkenes
● Alkenes react with bromine water, turning it from orange to colourless – alkanes
DO NOT react with bromine water
Recall that petrol, kerosene and diesel oil are…
● Non-renewable fossil fuels obtained from crude oil
● And methane is a non-renewable fossil fuel found in natural gas
Explain how cracking involves the breaking down of larger, saturated
hydrocarbon molecules (alkanes) into smaller, more useful ones, some of
which are unsaturated (alkenes)
● Hydrocarbons can be cracked to produce smaller, more useful molecules. This
process involved heating the hydrocarbons to vaporise them.
● The vapours are:
○ Either passed over a hot catalyst (500 degrees)
○ Mixed with steam and heated to a very high temperature so that thermal decomposition reactions can occur. (750 degrees)
● The products of cracking include alkanes and unsaturated hydrocarbons called
alkenes.
○ Alkenes have the general formula C n H 2n
○ The first 2 alkenes are ethene and propene.
○ they are unsaturated because they have a double bond
Explain why cracking is necessary
● Demand for smaller chained alkanes is much greater than that for longer chained
alkanes
● shorter chained hydrocarbons ignite more easily and so are more useful as fuels
