3.3 Organic Chemistry: .2 Alkanes

Question 1

what are alkanes?

Answer 1

hydrocarbons that only contain only hydrogen and carbon

-saturated compounds (they only have a single covalent bonds)

Question 2

what is the general molecular formula for alkanes?

Answer 2

C_nH_2n+2

Question 3

give the name for these alkane:

CH₄

C₂H₆

C₃H₈

C₄H₁₀

C₅H₁₂

C₆H₁₄

C₇H₁₆

C₈H₁₈

C₉H₂₀

C₁₀H₂₂

Answer 3

CH4 = methane

C2H6 = ethane

C3H8 =propane

C4H10 =butane

C5H12 =pentane

C6H14 =hexane

C7H16 =heptane

C8H18 =octane

C9H20 =nonane

C10H22 =decane

Question 4

how is crude oil (aka petroleum) seperated?

Answer 4

• 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

Question 5

what is a fraction?

Answer 5

a group of hydrocarbons with similar boiling points and similar chain lenghts

Question 6

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)

Answer 6

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

Question 7

what are the three types of trends in fractional distillation?

Answer 7

going down the column the trends are:

• the hydrocarbon chain increases
• the boiling point increases
• the temperature in the column increases

Question 8

what is cracking?

Answer 8

cracking is the break down of long hydrocarbon chains into smaller more useful ones

Question 9

give two economic reasons for cracking long chain alkanes

Answer 9

• 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

Question 10

what are the two types of cracking?

Answer 10

thermal and catalytic

Question 11

in thermal cracking what is the;

• conditions used
• way of fission
• intermediate chemicals
• products produced

Answer 11

• 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

Question 12

in catalytic cracking what is the;

• conditions used
• way of fission
• intermediate chemicals
• products produced

Answer 12

• 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

Question 13

why does using a catalyst cut costs in catalytic cracking?

Answer 13

• the reactions can be done at a low pressure and lower temperature
• the catalyst also speeds up the reaction, saving time

Question 14

why would we not crack octaine industrially?

Answer 14

octane has a short enough chain lenght to be in demand

Question 15

an alkane containing 12 carbon atoms is cracked to produce hexane, butene and ethene.

write an blanced eqautioon for this reaction

Answer 15

C12H6 –> C6H14 + C4H8 + C2H4

Question 16

what is a fuel?

Answer 16

a chemical which releases heat when it is burnt

Question 17

what are the differences between complete and incomplete combustion?

Answer 17

-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

Question 18

what is carbon monoxide and why is it dangerous?

Answer 18

• carbon monoxide is a poisonous gas
• it can combine with haemoglobin and prevent oxygen from passing around the body.
• this can lead to death

Question 19

what problems does soot (Carbon from incomplete combustion) cause?

Answer 19

• breathing problems
• it can also build up in egines, meaning they down work properly

Question 20

which two compounds are produced when an alkane burns completely?

Answer 20

carbon dioxide and water

Question 21

what are the three green house gasses?

Answer 21

• Carbon dioxide
• water vapour
• methane

Question 22

what has been increasing the amount of green house gases in our atmosphere?

Answer 22

• 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

Question 23

describe how the increase in green house gases have lead to global warming

(green house effect)

Answer 23

• 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)

Question 24

how can the green house effect be reduced?

Answer 24

using more renewable resources instead of fossil fuels

Question 25

what are the gases that cause acid rain?

Answer 25

sulphur dioxide (SO2) and nitrogen oxide (NO)

Question 26

how is acid rain caused by sulphur dioxide (prodcue an eqaution to show this)

Answer 26

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

Question 27

how is acid rain caused by nitrogen oxide? produce and eqaution

Answer 27

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

Question 28

what are the effects of acid rain?

Answer 28

- 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

Question 29

how can acid rain be reduced (reducing SO2)

Answer 29

+-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

Question 30

how can acid rain be produced (reducing NO)

Answer 30

- cars are now a days hav catalytic converters - they convert harmful gases into less harmful gases 2NO + 2CO --\> 2CO2 + N2

Question 31

what are the catalysts used in catalytic converters

Answer 31

- Rhodium - Platinium - Palladium

Question 32

what is the free radical susbstituion reaction when halogens react with organic compounds

Answer 32

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

Question 33

how is a free radical formed?

Answer 33

it is formed when a covalent bond breaks by homolytic fission (equal spitting of a bond) to form a particle with an unpaire electron

Question 34

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

Answer 34

- 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•

Question 35

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

Answer 35

- 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)

Question 36

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

Answer 36

- 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•

Question 37

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

Answer 37

- 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

Question 38

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)

Answer 38

(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

Question 39

what is the disadvantage of free radical substitution?

Answer 39

-a mixture of products and the desired product would have to be seperated from the other products