alkanes Flashcards
(29 cards)
boiling points of alkanes.
The longer the carbon chain, the higher the bp of alkanes due to stronger VDWs forces between molecules (as there are more electrons in the molecules).
For alkanes that are isomers, the more branched the carbon chain, the lower the bp due to weaker VDWs forces between molecules as the molecules are not able to pack as closely together.
How is crude oil formed?
Deposits of crude oil and natural gas usually occur together and they are formed by the slow decay of marine animals and plants, over millions of years, under heat and pressure in the absence of air.
What does crude oil consist of?
Although the exact composition of crude oils vary around the world, all are a complex mixture consisting mainly of alkanes (including cycloalkanes, some aromatics and other compounds some S and O).
The are the uses of crude oil?
Crude oil has no use in its raw form, so to provide useful products its components must be partly separated and if necessary modified.
How is crude oil separated by fractional distillation?
The crude oil is vaporised.
The vapour is passed into a tower which is hot at the bottom and cold at the top.
As the vapour rises it cools.
Molecules will condense at different heights as they have different bps.
The larger the molecule (with higher bps), the lower down the column it condenses.
This produces fractions - a fraction is a mixture of hydrocarbons with similar bps.
The residue from the primary distillation contains useful substances such as fuel oil, lubricating oil, waves and bitumen that boil above 350°C at atmospheric pressure.
However, they would decompose at temperatures above 350°C , so they are separated further by distillation at lower pressure (where their bps are lower).
What happens to hydrocarbons as the C chain gets longer?
Become more viscous.
Harder to ignite.
Less volatile.
Have higher bps.
Why are alkanes cracked?
The petroleum fractions with shorter C-chains are better fuels than ones with longer C-chains and so the shorter C-chain fractions are in greater demand.
This leads to greater demand than supply for these fractions, but a greater supply than demand for the fractions with longer C-chains.
Cracking is done to make use of excess fractions with longer C-chains and meet supply demand for shorter alkanes.
what is cracking?
Cracking is the thermal decomposition of alkanes to produce higher value shorter C-chain products.
C-C bonds are broken in cracking.
There are 2 types of cracking: thermal cracking and catalytic cracking.
Thermal cracking.
temperature - 900°C
pressure - 70atm
catalyst - none
products - alkenes
Catalytic cracking.
temperature - 450°C
pressure - 1-2atm
catalyst - zeolites (a type of aluminosilicate catalyst)
products - motor fuels (cyclic alkanes, branched alkanes, aromatics)
Give information about CO2 as a pollutant.
how is it formed?
complete combustion of fuels containing C
problems it causes?
greenhouse gas
ways to reduce the problem?
burn less fossil fuels
Give information about CO as a pollutant?
how is it formed?
incomplete combustion of fuels containing C
problems it causes?
toxic
way to reduce the problem?
ensure a good supply of oxygen when burning fuels
Give information about C as a pollutant?
how is it formed?
incomplete combustion of fuels containing C
problem it causes?
blackens buildings, can cause respiratory problems and global dimming
ways to reduce the problem?
ensure a good supply of oxygen when burning fuels
Give information about H2O as a pollutant?
how is it formed?
combustion of fuels containing H
problem it causes?
not a problem
Give information about SO2 as a pollutant?
how is it formed?
combustion of S containing compounds in fuel
problem it causes?
acid rain
ways to reduce the problem?
remove S from fuel before burning, flue gas desulfurisation
Give information about NO/NO2 as a pollutant?
how is it formed?
reaction of N2 in the air with O2 at very high temperatures (in engines and furnaces)
problem it causes?
acid rain
ways to reduce the problem?
use catalytic converters in cars
Give information about unburned hydrocarbons as pollutants?
how is it formed?
some of the fuel may not actually burns
problem it causes?
wastes fuel
ways to reduce the problem?
ensure engines are well tuned and there is a good supply of oxygen
Explain flue gas desulfurisation.
The gases pass through a filter containing calcium oxide or calcium carbonate which reacts with the sulphur dioxide.
CaO + SO2 —> CaSO3
or
CaSO3 + SO2 —> CaSO3 + CO2
This is an acid-base reaction as CaSO3 and CaO are bases and SO2 is an acidic oxide.
The reaction forms calcium sulphate (IV) which is used to make plasterboard.
The internal combustion engine
Petrol is used in the internal combustion engine where the alkanes are vaporised and combusted with air.
Catalytic converters.
Catalytic converters remove CO, NO/NO2 and unburned hydro carbons from the exhaust gases, turning them into CO2, N2 and H2O.
2CO + 2NO —> 2CO2 + N2
C8H18 + 25NO —> 8CO2 + 12.5N2 + 9H2O
Converters have a ceramic honeycomb coated with a thin layer of catalyst metals (Pt, Pd, Rh) - to give a large surface area.
What are greenhouse gases?
Greenhouse gases are gases that absorbs the IR radiation given off by the earth, but do not absorb the higher frequency UV/visible radiation given off by the sun.
They are molecules that contain polar bonds which absorb IR radiation to make bonds vibrate.
Global warming.
The earth is currently getting warmer.
Most scientists believe it is due to increasing amounts of greenhouse gases in the atmosphere.
However, the earth does have variations in temperature over time, and some scientists think the current warming may be a natural variation.
However, many scientists think the recent increase has been more rapid than would be expected in any natural variation.
CO2 in atmosphere.
The burning of fossil fuels (including alkanes) releases carbon dioxide into the atmosphere.
Carbon dioxide levels have risen significantly in recent years largely due to large scale burning of fossil fuels and deforestation.
CH4 in the atmosphere.
Methane levels have also increased significantly largely due to the increasing world population leading to increased
- cattle farming
- rice production
- landfill
