alkanes

Question 1

what are alkanes

Answer 1

alkanes are saturated hydrocarbons with the general formula of CnH2n+2

Question 2

what does saturated mean

Answer 2

no double bonds - maximum number of elements attached

Question 3

what are cycloalkanes

Answer 3

general formula of CnH2n in a circle form

Question 4

where are alkanes found

Answer 4

in crude oil which is a mixture of different length hydrocarbons

Question 5

how does the fractional distillation of crude oil work

Answer 5

crude oil is vaporised using a furnace of around 350
the column has a temperature gradient. it is cooler at the top
as the vapour rises parts of the mixture condense at different temperatures
this is because there are different chain lengths and so different boiling points
the fractions are drawn off at different levels

Question 6

uses of different fractions of alkanes

Answer 6

gas- used in lpg and stove gas
bitumen- used in roofing and tarmac
kerosine- used as jet fuel and heating
diesel oil- used as diesel fuel
fuel oil- used in ships and power stations
petrol- used in cars

Question 7

longest hydrocarbons don’t … and come out at the …

Answer 7

vaporise
bottom

Question 8

shortest hydrocarbons don’t … and come out at the …

Answer 8

condense
top

Question 9

what is cracking

Answer 9

breaking a longer chain alkane into shorter hydrocarbons

Question 10

why is cracking used

Answer 10

heavier fractions like fuel oil are in lower demand than lighter fractions like petrol which are higher demand

Question 11

what are the two types of cracking and explain both

Answer 11

thermal cracking- 1000°C and 70 atm, products are mostly alkenes which are used to make polymers such as plastics
catalytic cracking- high temp and slight pressure 450°C- zeolite catalyst used to lower temp and pressure lowering costs and time, produces morosely aromatic hydrocarbons useful in fuels for vehicles

Question 12

complete combustion of alkanes

Answer 12

alkane + oxygen => carbon dioxide + water

Question 13

what is wrong with incomplete combustion of alkanes

Answer 13

CO is poisonous as it bonds to haemoglobin in the blood and prevent oxygen bonding (catalytic converter can remove CO)
soot can cause breathing problems, make buildings dirty and clog up engines

Question 14

what is ozone

Answer 14

sunlight, hydrocarbons and nitrogen dioxide form ozone
oxides of nitrogen are made when orogen and oxygen are found in the air to combine under high pressure and temps, car engines provide these conditions

Question 15

what can prevent nitrogen oxides

Answer 15

catalytic converters help to reduce the amount of unburnt hydrocarbons and nitrogen oxides in atmosphere

Question 16

what is photochemical smog

Answer 16

solid carbon particulates and ozone mix we create photochemical smog

Question 17

why is photochemical smog bad

Answer 17

harms the respiratory system in animals and damages plants, ozone is also toxic to humans

Question 18

how does acid rain form

Answer 18

come fossil fuels contain sulfur based impurities
when burned the sulfur reacts w oxygen producing sulfur dioxide
this is an acidic gas that can react with water to form sulfuric acid that falls as acid rain

Question 19

what is wet scrubbing

Answer 19

an alkali is used to naturalise sulfur dioxide in flue gases
dissolve calcium carbonate or oxide and spray on acidic sulfur dioxide gas

Question 20

process of free radical substitution eg: making haloalkanes

Answer 20

initiation
sunlight break bond in a process called photodissociation which is a photochemical reaction
bond breaks equally producing 2 highly reactive radicals

propagation
when a radical reacts w a non radical molecule. new radicals are created which then go to react w other non radicals. this is a chain reaction

termination
when 2 radicals react they form non radical molecule ending the chain reaction