Organic (Y1) - Alkanes

Question 1

How do you refine crude oil?

Answer 1

Fractional distillation: industrially.

Question 2

What is petroleum?

Answer 2

Petroleum is a mixture consisting of mainly alkane hydrocarbons.
Petroleum Fractions: Mixture of hydrocarbons with a similar chain length and boiling point range.

Question 3

10 Key Points about fractional distillation (Try put these on a que card and learn them)

Answer 3

1) Oil is pre-heated.
2) Then passed into column.
3) The fractions condense at different heights.
4) The temperature of column decreases upwards.
5) The separation depends on boiling point.
6) Boiling point depends on the size of molecules.
7) The larger the molecule the larger the van der waals forces.
8) Similar molecules (size, bp, mass) condense together.
9) Small molecules condense at the top at the lower temperatures.
10) Big molecules condense at the bottom at higher temperatures.

Question 4

What is fractional distillation in terms of VDWs forces?

Answer 4

Fractional distillation is a physical process involving the splitting of weak van der waals forces between molecules.

Question 5

What is use of vacuum distillation unit?

Answer 5

Heavy residues from the fractionating column are distilled again under a vacuum. Lowering the pressure over a liquid will lower its boiling point.

Question 6

What’s the point of vacuum distillation unit?

Answer 6

Vacuum distillation allows heavier fractions to be further separated without high temperatures which could break them down.

Question 7

Describe the process of fractional distillation in the laboratory

Answer 7

1) Heat the flask with a bunsun burner/electric mantle.
2) Causes vapours of all the components in the mixture to be produced.
3) Vapours pass the fractionating column.
4) The vapour of the substance with the lower boiling point reaches the top of the fractionating column first.
5) The thermometer should be at or below the boiling point of the most volatile substance.
6) The vapours with higher boiling points condense back into the flask.
7) Only the most volatile vapour passes into the condenser.
8) The condenser coos the vapour and condenses to a liquid and is collected.

Question 8

Define cracking

Answer 8

Conversion of large hydrocarbons to smaller hydrocarbon molecules by braking of c-c bonds.

Higher Mr alkanes -> smaller Mr alkanes + alkenes (+ hydrogen).

Question 9

List 3 economic reasons for cracking

Answer 9

1) 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 HCs and to supply demand for shorter ones, longer HCs are cracked.
3) Products of cracking are MORE VALUABLE than starting materials (e.g. ethene used to make poly(ethene), branched alkanes for motor fuels etc.)

Question 10

State the conditions of thermal cracking.

Answer 10

• High pressure (7000kPa)

- High temperature (400C to 900C)

Question 11

What is mostly produced from thermal cracking?

Answer 11

• Produce mostly alkenes e.g. ethene used for making polymers and ethanol.
• Sometimes produces hydrogen used in the Haber process and margarine manufacture.

Question 12

Can you think some example equations - for example what are the products of C8H18 & Products of C12H26

Answer 12

C6H14 + C2H4

C10H22 + C2H4

Bonds can be broken anywhere in the molecule by C-C bond fission and C-H Bond fission

Question 13

State the conditions of catalytic cracking

Answer 13

• Slight/moderate pressure
• High temperature (450C)
• Zeolite catalyst

Question 14

What’s mostly produced from catalytic cracking?

Answer 14

Produces branched and cyclic alkanes and aromatic hydrocarbons.

Question 15

Use of Catalytic cracking?

Answer 15

Used for making motor fuels.

Question 16

Why catalytic cracking?

state 2 reasons, one economic reason and one reason related to the combustion of the products.

Answer 16

• Branched and cyclic hydrocarbons burn more cleanly and are used to give fuels a higher octane number.
• Cheaper than thermal cracking becasue it saves energy as lower temperatures and pressures are used.

Question 17

Fuel - describe it.

Answer 17

Releases heat energy when burnt.

Question 18

When does complete combustion occur?

Answer 18

In excess oxygen alkanes will burn with complete combustion.

Question 19

What are the products of complete combustion of alkanes?

Answer 19

CO2 and H20.

Question 20

Why are alkanes used as fuels

Answer 20

Alkanes readily burn inthe presence of oxygen. This combustion of alkanes is highly exothermic, explaining their use as fuels.

Question 21

When does incomplete combustion occur, what’s produced and which is better in terms of more energy, incomplete/complete combustion?

Answer 21

Incomplete combustion occurs when there’s a lack of oxygen. It produces CO and or C and water - soot produced can cause global dimming.
Incomplete combustion produces less energy per mole than complete combustion.

Question 22

Pollution from combustion - S in coal/ some fuels - state exations of formation.

Answer 22

S + O2 –> SO2.

Ch3SH + 3O2 –> SO2 + CO2 + 2H2O

Question 23

What causes acid rain?

Answer 23

SO2 when it dissolves in atmospheric water and can produce acid rain.

Question 24

How is SO2 removed?

Answer 24

SO2 can be removed from waste gases from furnaces by flue gas desulfurization. The gases pass through a scrubber containing basic Calcium Oxide which react with acidic sulfur dioxide in a neutralisation reaction.

SO2 + CaO –> CaSO3

Question 25

How are nitrogen oxides formed?

Answer 25

N2 + O2 –> 2NO

N2 + 2O2 –> 2NO2

Question 26

Environmental consequence of Nitrogen oxides?

Answer 26

NO is toxic and can form acidic gas NO2. NO2 is toxic and acidic and forms acid rain.

Question 27

Environmental consequence of CO

Answer 27

Toxic

Question 28

Environmental consequence of CO2

Answer 28

Contributes to global warming

Question 29

Environmental consequence of unburnt hydrocarbons

Answer 29

Contributes to formation of smog

Question 30

Environmental consequence of Soot

Answer 30

Global dimming and respiratory problems.

Question 31

Catalytic converters - Metals used and shape of metals

Answer 31

• Platinum, palladium, rhodium
• Converters have a ceramic honeycomb coated with thin layer of catalyst metals
• give a large surface area

Question 32

Global warming and greenhouse gases: What does burning alkanes contribute to Greenhouse gases?

Answer 32

Carbon dioxide is released - it’s a green house gas and is largely responsible for global warming. Earth is thought to be getting warming, and scientists believe it is due to increasing amounts of greenhouse gases in the atmosphere.

Question 33

Synthesis of halogeonalkanes:

What’s used to make a free radical

Answer 33

UV light is used to make the free radical. Br2 + UV –> 2BR*2

Question 34

Define free radical

Answer 34

A free radical is a reactive species which possess an unpaired electron.

Question 35

What’s homolytic fission?

Answer 35

The Cl=Cl bond is broken and possess an unpaired electron.

Question 36

What does all propagation steps contain?

Answer 36

All propagation steps have a free radical in the reactants and in the products.

Free radical may get regenerated.

Question 37

There is a major step and a minor step. How are the minor step written?

Answer 37

Minor step should be written in the structural formula not molecular formula.

Question 38

Why is the termination step named termination step?

Answer 38

Collisions of two free radicals does not generate further free radicals hence chain has terminated.

Question 39

If excess halogen is present what may be formed?

Answer 39

Further substitution of the halogen can occur. e.g.
CH3Cl + CL2 –> CH2Cl2 + HCl
CH2CL2 + CL2 –> CHCL3 + HCl
CHCL3 + CL2 –> CCl4 + HCl