3.3 Organic Chemistry: .2 Alkanes Flashcards
what are alkanes?
hydrocarbons that only contain only hydrogen and carbon
-saturated compounds (they only have a single covalent bonds)
what is the general molecular formula for alkanes?
CnH2n+2
give the name for these alkane:
CH4
C2H6
C3H8
C4H10
C5H12
C6H14
C7H16
C8H18
C9H20
C10H22
CH4 = methane
C2H6 = ethane
C3H8 =propane
C4H10 =butane
C5H12 =pentane
C6H14 =hexane
C7H16 =heptane
C8H18 =octane
C9H20 =nonane
C10H22 =decane
how is crude oil (aka petroleum) seperated?
- by using fractional distillation
- this method works because the hydrocarbons have different boiling points
- first of all the crude oil is heated so that it turns into a vapour at about 350 degrees celcius
- the vapour enters the fractionating column which is little bit cooler
- this means that long chained hydrocarbons with very high boiling condense and they collect at the bottom
- the rest of the vapour rises up the column where it is a bit cooler. the next longest chains condense and collect there. this continues all the way up the column
- at the top the chemicals are still gases because they have low boiling points and it is never cool enough to condense these
what is a fraction?
a group of hydrocarbons with similar boiling points and similar chain lenghts
for each fraction state what the aproximate number of carbons in the alkane chain would be expected and what the uses are:
refinery gases (really)
gasoline (petrol) (good)
Naptha (nice)
kerosene (parrafin) (kangaroos)
Diesel (gas oil) (dont)
fuel oil (fight)
bitumen (back)
refinery gases: usually have 1 - 4 carbons
used for bottled gas (camping gas), LPG
gasoline petrol): usally have 5- 12 carbons
used for fuel for cars
naptha: usually have 7-14 carbons
processed to make petrochemicals
kerosene (parrafin): usually have 11- 15 carbons
used for jet engines, central heating fuel
diesel (gas oil): usually have 15-19 carbons
used for fuel for diesel engines in cars, trucks etc
fuel oil: usually has 30- 40
used for fuel for big ships
bitumen: usally has 50+ carbons
used for road surfacing and asphalt for roofs
what are the three types of trends in fractional distillation?
going down the column the trends are:
- the hydrocarbon chain increases
- the boiling point increases
- the temperature in the column increases
what is cracking?
cracking is the break down of long hydrocarbon chains into smaller more useful ones
give two economic reasons for cracking long chain alkanes
- some fractions have large supply but little demand e.g bitumen
- where as some shorter chains have a greater deman but little supply e.g petrol
- so cracking is used to meet this demand by cracking the heavier fractions into the more valuble and useful fractions
- alkenes are also more useful than alkanes as starting materials fo further chemical reactions
what are the two types of cracking?
thermal and catalytic
in thermal cracking what is the;
- conditions used
- way of fission
- intermediate chemicals
- products produced
- conditions used= high temp (up to 1000 degrees celcius) and high pressure (up to 70 atm)
- way of fission= homolytic fission (eqaul splitting of a covalent bond)
- intermediate chemicals= the homolytic fission produces free radicals
- products produced= smaller alkanes and smaller alkenes e.g polyethene
in catalytic cracking what is the;
- conditions used
- way of fission
- intermediate chemicals
- products produced
- conditions used= zeolite catalyst (hydrated aluminosillicate) with high pressure and temperature
- way of fission= hetrolytic (uneqaul splitting of covalent bond)
- intermediate chemicals= from the hetrolytic fission a carbocation is produced
- products produced= motor fuels and hydrocarbons that are Aromatic (compounds that contain benzene rings -6 carbon atoms with three double bonds), _B_ranched and _C_yclic
why does using a catalyst cut costs in catalytic cracking?
- the reactions can be done at a low pressure and lower temperature
- the catalyst also speeds up the reaction, saving time
why would we not crack octaine industrially?
octane has a short enough chain lenght to be in demand
an alkane containing 12 carbon atoms is cracked to produce hexane, butene and ethene.
write an blanced eqautioon for this reaction
C12H6 –> C6H14 + C4H8 + C2H4
what is a fuel?
a chemical which releases heat when it is burnt
what are the differences between complete and incomplete combustion?
-complete;
burning in a plentiful supply of oxygen
CO2 is produced
-incomplete;
burning in limited supply of oxygen
CO (carbon monoxide) or C (carbon/soot) is produced
what is carbon monoxide and why is it dangerous?
- carbon monoxide is a poisonous gas
- it can combine with haemoglobin and prevent oxygen from passing around the body.
- this can lead to death
what problems does soot (Carbon from incomplete combustion) cause?
- breathing problems
- it can also build up in egines, meaning they down work properly
which two compounds are produced when an alkane burns completely?
carbon dioxide and water
what are the three green house gasses?
- Carbon dioxide
- water vapour
- methane
what has been increasing the amount of green house gases in our atmosphere?
- an increasing population has meant that there is an icreased use in fossil fuels so more CO2 and H2O are produced
- deforestation means that less CO2 and H2O is taken in for photosynthesis
describe how the increase in green house gases have lead to global warming
(green house effect)
- ultra violet radiation is emitted towards the earth from the sun
- this is then reflected back by the earth but at a smaller frequency (infrared radiation)
- the green houses gases in out atmosphere are really good at absorbing the infrared energy. the emit sone of the enrgy they absorb back towards the earth keeping it warm
- therfore the more green house gases in the atomosphere the more infrared is emited back towards the earth and the warmer the earth gets
- this leads to global warming
- the effects of global warming are; ice caps melting, sea levels rising resulting in changing weather conditions (climate change)
how can the green house effect be reduced?
using more renewable resources instead of fossil fuels
what are the gases that cause acid rain?
sulphur dioxide (SO2) and nitrogen oxide (NO)
how is acid rain caused by sulphur dioxide (prodcue an eqaution to show this)
when fossil fuels burn the sulphur in them combines with the oxygen in the air to make sulphur dioxide
S + O2 –> SO2
this gass then combines with water to make acid rain
how is acid rain caused by nitrogen oxide? produce and eqaution
in cars the spark of the ignition causes the nitrogen and oxygen in the air to combine and form nitrogenoxide
N2 + O2 –> 2NO
this gas combines with rain water to make acid rain
what are the effects of acid rain?
- acid rain corrodes metlas
- it can erode rocks
- destroy plantation if the soil is too acidic
- affect aqautiic animals (fish) if the lake are too acidic
how can acid rain be reduced (reducing SO2)
+-in indursty chimneys are lined with oxide or calcium carbonate
-this neutralises the SO2 to reduce the amoind of acid rain producded
CaO + 2H2O + SO2 –> CaSO3 + 2H2O
CaCO3 + 2H2O + SO2 –> CaSO3 + 2H2O + CO2
how can acid rain be produced (reducing NO)
- cars are now a days hav catalytic converters
- they convert harmful gases into less harmful gases
2NO + 2CO –> 2CO2 + N2
what are the catalysts used in catalytic converters
- Rhodium
- Platinium
- Palladium
what is the free radical susbstituion reaction when halogens react with organic compounds
this is where halogen react with alkanes in photochemical reaction- reactions that are started by ultraviolet light
-a hydrogen atom is substituted by chlorine or bromine
how is a free radical formed?
it is formed when a covalent bond breaks by homolytic fission (equal spitting of a bond) to form a particle with an unpaire electron
In the reaction of chlorine reacting with methane to form:
CH4 + Cl2 -UV-> CH3Cl + HCl
describe the initiation reaction and give a eqation to represent this
- sunlight provides enough energy to break the Cl-Cl bond (this is called photodissociation)
- the bond splits eqaully and each atom gets to keep one electron, the atom becomes a highly reactive free radical because of its unpaired electron
Cl2 –UV–> 2Cl•
In the reaction of chlorine reacting with methane to form:
CH4 + Cl2 -UV-> CH3Cl + HCl
describe the 1st propogation reaction and give a eqation to represent this
- in the first propogation step the HALOGEN RADICAL with the ALKANE
- the halogen radical ALWAYS TAKES A HYDROGEN
- a HYDROGEN HALIDE and ORGANIC COMPOUND RADICAL is made
- Cl• + CH4 –> HCl + CH3• (better if the radical is on top of Carbon)
In the reaction of chlorine reacting with methane to form:
CH4 + Cl2 -UV-> CH3Cl + HCl
describe the 2nd propogation reaction and give a eqation to represent this
- in the second propogation step, the new ORGANIC COMPOUND RADICAL always reacts with the HALOGEN
- the halogen-halogen (e.g Cl-Cl) bond breaks by homolytic fission
- a HALOGENALKANE and a Halogen radical is made
CH3• + Cl2 –> CH3Cl + Cl•
In the reaction of chlorine reacting with methane to form:
CH4 + Cl2 -UV-> CH3Cl + HCl
describe the termination reaction and give a eqation to represent this
- in the termination step the free radicals combine
- there will ALWAYS be only THREE reactions possible:
HALOGEN radical and HALOGEN radical reacting together
Cl• + Cl• –> Cl2
ORGANIC COMPOUND radical and ORGANIC COMPOUND radical
CH3• + CH3 •–> CH3CH3
ORGANIC COMPOUND radical and HALOGEN radical
CH3• + Cl• –> CH3Cl
if the chlorine’s is in excess in the free radical substitution of methane and chlorine produce equations for the further reactions.
state which substituions they would occur at (e.g 2nd, 3rd, 4th)
(this is you first reaction) 1st sub: CH4 + Cl2 –> CH3Cl + HCl
2rd sub: CH3Cl + Cl2 –> CH2Cl2 + HCl
3th: CH2Cl2 + Cl2 –> CHCl3 + HCl
4th: CHCl3 + Cl2 –> CCl4 + HCl
what is the disadvantage of free radical substitution?
-a mixture of products and the desired product would have to be seperated from the other products
