ORGANIC CHEMISTRY - alkanes

Question 1

what are alkanes?

Answer 1

They are saturated hydrocarbons with single C-C bonds.
general formula: CnH2n+2
every carbon atom has 4 single bonds with other atoms

Question 2

Are there bonds polar? why? why not?

Answer 2

they are non-polar because carbon and hydrogen have similar electronegativities

Question 3

which intermolecular forces do they have? why?

Answer 3

They have induced dipole-dipole/ van der waals’ forces as bonds are non-polar

Question 4

Are alkanes soluble in water?

Answer 4

They are insoluble in water because the hydrogen bonds in water are stronger than the alkanes’ van der waals’ forces of attraction.

Question 5

How reactive are alkanes?

Answer 5

very unreactive

Question 6

which reactions will alkanes undergo?

Answer 6

combustion and reactions with halogens

Question 7

What is crude oil? How is it formed? Is it renewable? Why?

Answer 7

crude oil a mixture of fractions formed at high temperatures and pressures deep below the earths surface over millions of years. Therefore it is not renewable.

Question 8

Name the fractions from high to low boiling point?

Answer 8

Gases - fuel on site
Gasoline/Petrol/Naphtha - cars
Kerosine/paraffin - jet fuel
diesel - lorries/taxis
lubricating oil/waxes - candles, engine oil
fuel oil - ships/power stations
tar/bitumen - roads/roofing

Question 9

What is fractional distillation? How does it work?

Answer 9

The crude oil is vaporised at about 350°. The vaporise crude oil goes into a fractionating column and rises up.The largest hydrocarbons don’t vaporise at all, because their boiling points are too high. As the crude oil vapour goes up the fractionating column, it gets cooler. Because the hydrocarbon molecules have different chain length, they have different boiling points, so each fraction condenses at different temperature. The fraction are drawn off at different levels in the column. The hydrocarbons with the lowest boiling point do not condense. They are drawn off as gas is at the top of the column.

Question 10

why are alkanes cracked?

Answer 10

To turn a long chain alkane, (which is not economically valuable) into a shorter chain alkane )which is more economically valuable as it can be used as fuel) and an alkene.

Question 11

what are the conditions for thermal cracking?

Answer 11

700-1200K temperature
7000kPa pressure

Question 12

What is the intermediate for cracking reaction?

Answer 12

free radicals - Free radicals are highly reactive atoms that have one unpaired electron. They are unstable as there is a tendency for unpaired electrons to pair up and so the free radicals react very quickly.

Question 13

what are the main products of thermal cracking?

Answer 13

alkenes

Question 14

what are the conditions for catalytic cracking?

Answer 14

Lower temp (720K)
Lower pressure (but above atmospheric)
Zeolite catalyst with a honey comb structure to give a large surface area.

Question 15

what are the main products of catalytic cracking?

Answer 15

cycloalkanes, aromatic hydrocarbons, branched alkanes

Question 16

what is a fuel?

Answer 16

something which releases heat energy when combusted

Question 17

what are the five main fuels, comprising of alkanes?

Answer 17

methane, butane , propane, petrol, paraffin

Question 18

what is incomplete combustion?

Answer 18

combustion in a limited supply of oxygen
CO is produced which is a poisonous gas
C particulates produced which causes global dimming.

Question 19

which type of hydrocabons are more likely to undergo incomplete combustion?

Answer 19

longer chain hydrocarbons.

Question 20

what is the environmental impact of CO?

Answer 20

it is toxic/poisonous

Question 21

what is the environmental impact of nitrogen oxides?

Answer 21

forms nitric acid which leads to acid rain and photochemical smog

Question 22

what is the environmental impact of sulfur impurities/sulfur dioxide?

Answer 22

form sulphuric acid which leads to acid rain

Question 23

what is the environmental impact of Carbon (soot)?

Answer 23

asthma ,global dimming, cancer

Question 24

what is the environmental impact of unburnt hydrocarbons?

Answer 24

photochemical smog

Question 25

what is the environmental impact of co2?

Answer 25

greenhouse gas, leading to global warming, increasing global temperature ,speeds up climate change.

Question 26

what is the environmental impact of water vapour?

Answer 26

green house gas –> global warming, increases global temperatures, speeds up climate change

Question 27

what are flue gases?

Answer 27

gases emitted by power stations

Question 28

what are catalytic converters made up of?

Answer 28

ceramaic honey comb coated with platinum, palladium rhodium metals

Question 29

what do catalytic converters catalyse?

Answer 29

2CO (g) + 2NO(g) –>N2 + 2CO2

hydrocarbons +NO —> N2 + Co2 + H2O

Question 30

what are green house gases?

Answer 30

gases which trap infrared radiaiton, making the earth act like a greenhouse.

Question 31

what is the green house effect and how does it contribute to global warming?

Answer 31

green house gases trap infrared radiation in the atmosphere, atmosphere heats –> global warming

Question 32

define carbon neutral activities?

Answer 32

activities that produce no carbon dioxide emissions?

Question 33

how are halogenalkanes formed from alkanes?

Answer 33

By free radical substitution reactions

Question 34

What are the three stages of the free radical substitution reaction?

Answer 34

Initiation
Propagation
Termination

Question 35

Explain the first stage of free radical substitution?

Answer 35

INITIATION
- one or more molecules react to form radicals
- Bond between halogen breaks due to homolytic fission so the reaction is endothermic
- Happens in the presence of UV light which provides energy to break the bonds

Question 36

Explain the second stage of free radical substitution?

Answer 36

PROPAGATION
- a molecule and free radical react to form a new molecules and free radical pair
- a bond breaks and a new bond forms
- reaction occurs in pairs - one propagation step produces a radical which then reacts in a second step

Question 37

Explain the third stage of free radical substitution?

Answer 37

TERMINATION
- two free radicals react to form one molecule
- a bond forms so this step is exothermic.

Question 38

What are the conditions needed for the formation a free radical chlorine (halogen) atom?

Answer 38

Presence of UV light

Question 39

what is the function of the ozone layer?

Answer 39

To protect the earth from harmful exposure of too many UV rays

Question 40

How do CFC’s break down the ozone layer?

Answer 40

Free radical substitution

Question 41

write an equation for the overall decomposition of ozone?

Answer 41

2O3 —> 3O2

Question 42

write the free radical substitution equations to show how Cl free radicals catalyse the breakdown of O3?

Answer 42

Cl2 –> Cl* + Cl*
O3 + Cl* —> ClO* +O2
ClO* + O3 —> 2O2 + Cl*

overall: 2O3 —> 3O2

Question 43

what are CFC’s?

Answer 43

any of a class of compounds of carbon, hydrogen, chlorine, and fluorine, typically gases used in refrigerants and aerosol propellants. They are harmful to the ozone layer in the earth’s atmosphere owing to the release of chlorine atoms on exposure to ultraviolet radiation.

Question 44

how is ozone formed?

Answer 44

1. oxygen absorbs UV and photodissociates

O2 —> O* + O*

2. Oxygen radicals react with oxygen and form ozone ( O3)
   O2 + O* —-> O3