6. Organic Molecules (REACTIONS OF ALKANES)

Question 1

What is the general formula for alkanes

Answer 1

Cn H2n+2

Question 2

Are alkanes and cycloalkanes saturated or unsaturated

Answer 2

Saturated - all carbon bonds are single

Question 3

What are the 2 types of bond fission that break a covalent bond

Answer 3

Heterolytic fission
Homolytic fission

Question 4

What is heterolytic fission

Answer 4

When the covalent bond breaks unevenly so electrons are distributed unevenly to form a cation and ion. The cation receives both of the electrons in the bonding pair

Question 5

What is homolytic fission

Answer 5

When the covalent bond breaks evenly so electrons are distributed equally to form 2 radicals

Question 6

Define what is meant by the term radicals

Answer 6

Radicals are particles that have an unpaired electron
(represented by a dot)
–> due to unpaired electron, radicals are highly reactive

Question 7

What is free radical substitution and what does it require

Answer 7

When a hydrogen atom gets substituted by a halogen (chlorine/bromine) in a photochemical reaction ( a reaction that requires UV light)

Question 8

What are the three stages of free radical substitution

Answer 8

• Initiation - Creation of radicals
• Propagation (2 stages) - reactions that produce new radicals
• Termination - Removal of radicals
  (This is a chain reaction)

Question 9

Describe the first stage 1 of free radical substitution (making chloromethane from reaction of chlorine and methane)

Answer 9

Initiation - sunlight / uv light provides enough energy to break the Cl-Cl bond. This is called photodissociation. The bond equally breaks forming 2 highly reactive radicals through process of HOMOLYTIC fission

Question 10

Describe stage 2 of free radical substitution

Answer 10

Propagation -
1) Chlorine radical react with methane. Chlorine radical removes a hydrogen to produce a methyl radical and the second product - HCL
2) This methyl radical can then react with a chlorine molecule (not a chlorine a radical) to produce the desired product of chloromethane and a chlorine radical
3) New chlorine radical can react with methane and so on…… causes a chain reaction

Question 11

Describe stage 3 of free radical substitution

Answer 11

Termination - 2 radicals react together to form a stable non - radical

Question 12

What are the problems with free radical substitution? How to combat this when making chloromethane

Answer 12

You create a mixture of products

–> Excess Cl2 promotes further substitution as its a chain reaction, creating more products and therefore issues for separating the mixture as di,tri and tetrachloromethane can be produced)

–> Add excess methane to combat as there is more chance of a chlorine radical colliding with a methyl molecule and not a chloromethane molecule

Question 13

Define what is meant by an aliphatic alkane

Answer 13

Straight chain organic compounds e.g. C2H5OH

Question 14

Define what is aromatic alkane

Answer 14

Organic compounds containing the benzene ring

Question 15

What are alkanes used for

Answer 15

Fuels and lubricants

Question 16

Describe the process of crude oil fractional distillation

Answer 16

1) heated in furnaces and passed as a vapour into fractionating column from the bottom
2) temperature gradient: hotter near bottom
3) as the vapour rises, different fractions condense at different heights depending on boiling temperatures

Question 17

Describe where the crude oil forms factions

Answer 17

• larger molecules with longer chains and higher boiling temperatures condense at the bottom
• smaller molecules with more bracnhes and shorter chains and lower boiling temperatures condense near the top (pack less closley reducing stength of london forces)
• some of the hydrocarbons are dissolved gases so rise to the top of column without it condensing

Question 18

Define cracking

Answer 18

splitting of large hydrocarbons to smaller alkenes and alkanes by breakage of C-C bonds

Question 19

Give the reasons why cracking is done

Answer 19

1) The petroleum fractions with shorter C chains (e.g. petrol and naphtha) are in more demand than larger fractions.
2) To make use of excess larger hydrocarbons and to supply demand for shorter ones, longer hydrocarbons are cracked.
3) The products of cracking are more useful and valuable than the starting materials (e.g. ethene used to make poly(ethene) and ethane-1,2-diol, and ethanol) The smaller alkanes are used for motor fuels which burn more efficiently.

Question 20

Describe the process of cracking

Answer 20

Passing heavier fractions through a heated catalyst (usually ZEOLITE)

Question 21

Why do we reform hydrocarbons

Answer 21

branched and cyclic hydrocarbons burn more effeciently and easily ( branches get in the way so less london forces as they pack togethor less) than straight chain hydrocarbons and are used to give fuels a higher octane number - used for making motor fuels
Reform hudrocarbons to also reduce reduce knocking

Question 22

Describe what is meant by a fuel

Answer 22

A material that releases energy when it burns

Question 23

When do alkanes undergo complete combustion

Answer 23

when burning in excess oxygen

Question 24

When do alkanes undergo incomplete combustion

Answer 24

When there is a limited supply of oxygen

Question 25

Give the products of complete combustion

Answer 25

Carbon dioxide and water
(in an exam you would be expected to balance this reaction)

Question 26

Give the products of incomplete combustion

Answer 26

Carbon monoxide and/or carbon and water producing a sooty flame

Question 27

Describe the formation and environmental consequence of nitrogen oxides

Answer 27

form from the reaction between N2 found in air or nitrogen impurities in fuel and O2 found in air inside the car engine, the high temperature and spark in the engine has enough energy to break N-N bond
- NO toxic and can form smog
- NO2 is toxic and an impurity and can form acid rain (similar to SO2)

Question 28

Describe the formation and environmental consequence of carbon monoxide

Answer 28

formed from incomplete combustion eg in faulty heating appliances
- odourless, colourless toxic gas by forming a strong bond with haemoglobin in red blood cells, preventing the binding of oxygen

Question 29

Describe the formation and environmental consequence of carbon dioxide

Answer 29

formed during complete combustion of alkanes
-contributes to global warming

Question 30

Describe the formation and environmental consequence of soot/ carbon particulates

Answer 30

formed during incomplete combustion of alkanes
- contribute to global dimming

Question 31

What do catalytic converters do

Answer 31

Remove CO, NO and unburned hydrocarbons from exhaust gases turning them into less harmful CO2, N2 and H2O

Question 32

How are catalytic converters used

Answer 32

Harmful gases are adsorbed onto the surface of catalysts, where they react to form harmless gases.
these catalysts (platinum/palladium/rhodium) are presented as a thin layer over ceramic honeycomb for large SA

Question 33

Give 2 examples of renewable plant based fuels (biofuels) and how they form

Answer 33

Alcohols eg: ethanol - from the fermentation of sugar
Biodiesel - formed from reacting vegetable oils with a mixture of alkali and methanol

Question 34

Give advantages of biofuels

Answer 34

1) reduction of fossil fuels
2) more carbon neutral
3) allows fossil fuels to be used as feedstock for organic compounds
4) no risk for large scale pollution eg exploitation of fossil fuels

Question 35

Give disadvantages of biofuels

Answer 35

1) less food crops may be grown
2) land not used for food crops
3) rainforests cut down for farmland
4) shortage of fertile soils