Alkanes and Halogenoalkanes Flashcards
Alkanes are ________ hydrocarbons
Saturated
Every carbon atom in an alkane has four single bonds with other atoms. It is impossible for carbon to make any more bonds so alkanes are saturated.
What is the general formula of an alkane?
CnH2n+2
You can get ____________ too. They have a ring of carbon atoms with two hydrogens attached to each carbon.
What is different about the general formula?
cycloalkanes
CnH2n (1 ring) - still saturated
Crude oil is mainly alkanes but it isn’t very useful - describe fractional distillation
1) The crude oil is vaporised at about 350 degrees Celsius
2) The vaporised crude oil goes into a fractionating column and rises up through the trays. The largest hydrocarbons don’t vaporise at all, because their boiling point is too high - they just run to the bottom to form a gooey residue (used for road surfacing etc)
3) As the crude oil vapour goes up the column, it gets cooler. Because the alkane molecules have different chain lengths, they have different boiling points, so each fraction condenses at a different temperature. The fractions are drawn off at different levels in the column.
4) The hydrocarbons with the lowest bp don’t condense. They are drawn off as gases at the top of the column.
Heavy fraction can be ________ to make smaller molecules
cracked
Give an example of how decane could be cracked
C10H22 –> C2H4 + C8H18
involves breaking the c-c bonds
Describe the properties of thermal cracking
high temp (up to 1000 degrees Celsius)
high pressure (up to 70 atm)
produces a lot of alkenes
these alkenes are used to make a lot of valuable products, like polymers(plastics). a good example is poly(ethene) which is made from ethene.
Describe the properties of catalytic cracking
uses a zeolite catalyst
slight pressure and high temp (around 450 degrees celsius)
produces mostly aromatic hydrocarbons and motor fuels
using a catalyst cuts costs because the reaction can be done at low temp and pressure and catalyst speeds up reaction saving time and money
Alkanes are useful _____
fuels
burning a small amount releases a lot of energy
If you burn(oxidise) alkanes with plenty of oxygen, you get carbon dioxide and water - its a complete __________ reaction
combustion
they are burned in power stations, central heating systems and power car engines.
however they do produce pollutants
Incomplete combustion happens when there’s not enough _______
oxygen
What happens when incomplete combustion takes place?
You get carbon monoxide gas instead of carbon dioxide - it is poisonous and binds to same sites of haemoglobin molecules as the oxygen so oxygen cannot be carried around the body.
Carbon monoxide can be removed from exhaust gases by catalytic convertors on cars
Carbon particulates can be formed also
Soot causes breathing problems and also can build up in engines causing them to not work correctly
Unburnt ___________ and oxides of _______ can cause smog
hydrocarbons
nitrogen
How are oxides of nitrogen (NOx) produced?
the high pressure and temperature in a car engine cause the nitrogen and oxygen atoms from the air react together
When do hydrocarbons and nitrogen oxides react? And what can they cause?
react in the presence of sunlight to form ground level ozone which is a major component of smog
ground level ozone irritates people eyes, aggravates respiratory problems and can cause lung damage.
catalytic converters on cars remove unburnt hydrocarbons ad oxides of nitrogen from the exhaust.
