3.3.2 Alkanes

Question 1

What does it mean by alkanes being saturated hydrocarbons?

Answer 1

each carbon is bonded 4 times with the maximum number of hydrogens possible.
no c=c bonds

Question 2

How do cycloalkane general formula differ from normal alkanes?

Answer 2

alkanes = CnH2n+2
cycloalkane = CnH2n

Question 3

Where are alkanes found?

Answer 3

in crude oil

Question 4

What is Crude oil

Answer 4

mixture of different length hydrocarbons

Question 5

What is petroleum?

Answer 5

mixture of mainly alkane hydrocarbons that can be separated by fractional distillation

Question 6

At what temperature is crude oil heated to in order to vaporise?

Answer 6

350 degrees Celsius

Question 7

Describe the process of fractional distillation of crude oil

Answer 7

• vaporised oil enters the column and rises through trays. LONGEST HYDROCARBONS dont vaporise and run down to the bottom
• the column has a TEMPERATURE GRADIENT (cooler at the top)
• as vapour rises parts of the mixture condense at different temperatures (due to different chain lengths having different boiling points)
• the fractions are drawn off at different levels
• the shortest hydrocarbons dont condense at 20 degrees Celsius so come off as a gas at the top of the column

Question 8

What is petrol, kerosine, diesel oil, fuel oil, bitumen and gas used for?

Answer 8

p = used in petrol cars
k= jet fuel and heating
f = used in ships and power stations
b = roofing and tarmac
g = LPG and stove gas

Question 9

define cracking

Answer 9

breaking a longer chain alkane to shorter hydrocarbons

Question 10

Why do we crack heaver fractions/long hydrocarbons?

Answer 10

heavier fractions e.g fuel oil are in lower demand than lighter fractions such as petrol which are much more valuable

Question 11

What are the 2 types of cracking?

Answer 11

thermal and catalytic

Question 12

What are the conditions and products of thermal cracking?

Answer 12

• high temperature of 1000 degrees
• pressure of 70atm
• produces mainly alkenes

Question 13

What are the uses of products from thermal cracking?

Answer 13

alkenes can be used to make polymers such as plastics. e.g poly(propene)

Question 14

what are conditions and products of catalytic cracking?

Answer 14

• high temperature of 450 degrees
• slight pressure
• zeolite catalyst
• products are mainly AROMATIC HYDROCARBONS (contain benzene rings) and Motor fuel

Question 15

What are the products of catalytic cracking useful for?

Answer 15

aromatic hydrocarbons are useful in fuels for vehicles

Question 16

why is a zeolite needed in catalytic cracking?

Answer 16

using a zeolite catalyst lowers the temp and pressure needed for cracking to happen.
this lowers COST and SPEEDS up the process

Question 17

Describe the complete combustion of alkanes (reactants and products)

Answer 17

alkanes burn completely in oxygen to form carbon dioxide and water

Question 18

Why are alkanes good fuels?

Answer 18

most burn readily to produce large amounts of energy

Question 19

What occurs during incomplete combustion of alkanes?

Answer 19

alkanes burn with limited oxygen supply producing carbon monoxide and or carbon (soot)

Question 20

Why is carbon monoxide poisonous

Answer 20

it bonds to haemoglobin in the blood and prevents oxygen bonding.

Question 21

how can carbon monoxide be remove from the back on an engine?

Answer 21

with a catalytic converter

Question 22

what damage can soot cause?

Answer 22

breathing problems
make buildings dirty
clog engines

Question 23

What is the greenhouse effect

Answer 23

carbon dioxide absorbs infra red radiation from the sun but emit some back into earth

Question 24

How does burning fossil fuels lead to global warming?

Answer 24

burning fossil fuels makes carbon dioxide and this is a greenhouse gas.

this increases the greenhouse effect as more infra red radiation (heat) is being emmitted back towards earth causing global temperatures to increase and hence global warming.

Question 25

What 3 things make up the ozone layer?

Answer 25

sunlight

hydrocarbons

nitrogen dioxide

Question 26

where do most of the hydrocarbons and nitrogen dioxide in the ozone layer come from?

Answer 26

cars (nitrogen and oxygen from air combine under high pressures and temperatures in engines)

and factories

Question 27

What is photochemical smog

Answer 27

when solid carbon particulates and ozone mix

Question 28

how is photochemical smog bad

Answer 28

harms the respiratory system in animals and damages plants

Question 29

What can release of sulphur dioxide cause and how?

Answer 29

• sulfur dioxide is an acidic gas which reacts with water in atomosphere to form sulphuric acid and falls as ACID RAIN

-acid rain damages plants, kills fish and causes erosion of buildings

(nitrogen oxide does the same)

Question 30

How is sulphur dioxide formed

Answer 30

some fossil fuels contain sulfur based impurities and when burned the sulfur reacts with oxygen to form sulphur dioxide

Question 31

How do we remove sulfur dioxide from flue gases?

Answer 31

• via wet scrubbing (dissolving calcium carbonate or oxide in water)
• alkali is used to neutralise sulphur dioxide in flue gases
• the dissolved mixture is sprayed on acidic sulphur dioxide gas

Question 32

What are the 3 steps of free radical chain reactions

Answer 32

initiation
propagation
termination

Question 33

Describe initiation step

Answer 33

• photochemical reaction
• radicals are made (bonds broken) with UV light

Question 34

describe propagations step

Answer 34

• radical reacts with non-radical molecule
• new radical formed and go on to react with other non-radicals
  -(chain reaction)

Question 35

describe termination step

Answer 35

• 2 radicals form a non-radical molecule

Question 36

what is photodissociation

Answer 36

dissociation of a chemical compound by the action of light

Question 37

draw out the free-radical substitution of methane with chlorine

Answer 37

initiation : cl2 —> 2 Cl.

Propagation : cl. + ch4 —> ch3. + HCL
CH3. + Cl2 —> CH3CL + CL.

Termination : cl. + ch3. —> CH3CL + HCL