Alkanes Flashcards
how to name alkanes ?
find the longest chain
find the methyl group and its position on the c chain
then state the position of the methyl group and include the name of the carbon chain.
what is meant by an alkane ?
alkanes are saturated hydrocarbons with the general formula CnH2n+2
alkanes are hydrocarbons #. what is meant by a hydrocarbon ?
a compound that contains only hydrogen and carbon atoms
what is meant by alkanes being saturated ?
it means that each carbon is bonded 4 times with the maximum number of hydrogens possible
what are cycloalkanes described as in terms of being saturated ?
cycloalkanes are still saturated
what kind of isomer of alkanes are cycloalkanes ?
functional group isomers
what is the general formula of cycloalkanes ?
CnH2n
where are alkanes found in ?
crude oil
what is another word for crude oil ?
petroleum
what is crude oil / petroleum ?
a mixture of different length hydrocarbons
how can we separate crude oil ?
using a fractionating column that uses fractional distillation
why does crude oil need to be separated ?
so that it can be separated into smaller more useful alkanes called fractions
1st stage of fractional distillation
crude oil is vaporised in a blast furnace to about 350C so that it turns into a gas
2nd stage
vaporised crude oil goes into a fractionating column and rises up through the trays. the longest hydrocarbons don’t vaporise and run down to the bottom to form a gooey residue
why do the longest hydrocarbons not vaporise and run to the bottom ?
because their boiling points are too high.
what does the column have in terms of temperature ?
the column has a temperature gradient. it is cooler at the top and hotter at the bottom
why is it that as the vapour rises, parts of the mixture condense at different temperatures ?
- because there are different chain lengths and hence different boiling points
what happens to the shortest hydrocarbons ? and why ?
they won’t condense so will come off as gases at the top of the column
- because they have the lowest boiling points
fraction = gases
number of carbons + uses ?
1-4 carbons
-liquified petroleum gas (LPG)
-camping gas
fraction = petrol (gasoline)
number of carbons + uses ?
temp ?
5-12 carbons
- petrol
40 C
fraction = naphtha
number of carbons and uses ? temp ?
7-14 carbons
-processed to make petrochemicals
110 C
fraction = kerosene (paraffin)
number of carbons and uses ? temp ?
11-15 carbons
- jet fuel
- petrochemicals
- central heating fuel
180 C
fraction = gas oil (diesel)
number of carbons + uses ?
temp ?
15-19 carbons
- diesel fuel
- central heating fuel
250 C
fraction = mineral oil (lubricating )
number of carbon + uses ?
temp ?
20-30 carbons
- lubricating oil
340 C
what are the fractions called in the residue at the bottom of the fractionating column ?
- fuel oil
- waxes
- grease
- bitumen
fraction = fuel oil
number of carbons + uses ?
30-40 carbons
- fuel for ships and power stations
fraction = wax , grease
number of carbons + uses ?
40-50
-candles
- lubrication
fraction = bitumen
number of carbons + uses ?
50 + carbons
- roofing
- road surfacing
what kind of fractions are more demanded ?
the lighter fractions of crude oil such as petrol and naphtha
which fractions are not as highly demanded ?
the heavier fractions of crude oil such as bitumen
what is meant by cracking ?
breaking long chain alkanes (the heavier fractions ) into shorter more useful hydrocarbons including alkenes
how can decane crack
C10H22 ?
C10H22 —> C2H4 + C8H18
decane —> ethene + octane
what are the 2 products of cracking ?
alkane + alkene
what are the 2 types of cracking ?
- thermal cracking
- catalytic cracking
conditions for thermal cracking ?
high temperature ( up to 1000C)
high pressure ( 70 atm)
what are the products of thermal cracking ?
mainly alkenes
what are the products of thermal cracking used for ?
the alkenes produced are used to make polymers ( plastics )
e,g propene is used to make poly(propene)
what are he conditions for catalytic cracking ?
-high temperature ( 450C)
- slight pressure
- uses a zeolite catalyst (hydrated aluminosilicate )
what are the products of catalytic cracking ?
- aromatic hydrocarbons and these are useful in motor fuels
- branched alkanes
- cycloalkanes
what do aromatic compounds contain ?
they contain benzene rings
benzene rings have 6 carbon atoms with 3 double bonds
they are also pretty stable because the electrons are delocalised around the carbon ring
what does the zeolite catalyst do to the reaction ?
using a zeolite catalyst lowers the temperature and pressure needed for cracking to occur
- this lowers the cost and speeds up the reaction by saving time and therefore saving money
what is formed during the complete combustion of alkanes ?
carbon dioxide and water
why are alkanes good fuels ?
alkanes are good fuels as burning just a small amount releases large amounts of energy
what are the uses for alkanes ?
they are burnt in power stations, central heating systems and used to power vehicles
show the complete combustion of butane ?
C4H10 + 6.5O2 —-> 4CO2 + 5H2O
what is formed during incomplete combustion ?
-carbon monoxide and water
or
- carbon soot
and water
why is carbon monoxide bad for human health ?
carbon monoxide is a colourless, odourless, toxic gas
- it binds to haemoglobin in the blood and prevents oxygen bonding so oxygen cannot be carried around the body
how can carbon monoxide be removed ?
using a catalytic converter
what health issues does carbon soot cause ?
breathing problems
makes buildings dirty
clogs up engines
global dimming
what does burning fossil fuels lead to ?
global warming
what does burning fossil fuels produce ?
carbon dioxide CO2
- which is a greenhouse gas
what is meant by the greenhouse effect ?
when greenhouses gases such as CO” in the atmosphere absorb infrared radiation from the sun and emit some of it back into the earth which in turn keeps it warm
what do most scientists agree on ?
that by increasing the amount of carbon dioxide in the atmosphere we are increasing the global temperatures which leads to global warming
where does ozone occur ?
at the lowest level of the atmosphere
what does the ozone layer contain ?
- sunlight
- hydrocarbons
- nitrogen dioxide
from where does a great deal of hydrocarbons and nitrogen dioxides come from ?
cars and factories
how is photochemical smog produced ?
when solid carbon particulates and ozone mix
what are the health problems caused by smog ?
it harms the respiratory system in animals and damages plants
what are the problems caused by ground-level ozone ?
- irritates peoples eyes
- aggravates respiratory problems
- causes lung damage
how are oxides of nitrogen formed ?
they are made when nitrogen and oxygen found in the air combine under high pressure and temperature. car engines provide these conditions
what machinery helps to reduce the amount of unburnt hydrocarbons and oxides of nitrogen going into the atmosphere ?
catalytic converters in the exhaust
what is produced when we burn fossil fuels that contain sulfur ?
sulfur dioxide
why is sulfur dioxide gas so bad ?
it gets into the atmosphere and dissolves the moisture and is then converted into sulfuric acid which causes acid rain
what does acid rain cause ?
acid rain :
- destroys trees and vegetation
- corrodes buildings and statues
- kills fish in lakes
how do you remove sulfur dioxide in glue gasses ?
through a process called wet scrubbing
- where an alkali is used to neutralise the sulfur dioxide in flue gas
what does wet scrubbing involve ?
involves dissolving calcium carbonate or oxide in water and spraying it on acidic sulphur dioxide gas
- this therefore forms a harmless salt called calcium sulfite
Features of shorter alkanes ?
- lower boiling points
- more volatile
- more flammable
- less viscous
- less coloured
How is the zeolite catalyst arranged ?
It is arranged in a honeycomb structure
That allows us to use a small amount of catalyst for a large surface area
Reduces cost
What kind of flame do shorter alkanes have ?
A clean flame
What kind of flame do larger alkanes produce when burnt ?
A sootier flame
