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AS CHEMISTRY > ALKANES > Flashcards

ALKANES Flashcards

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1
Q

What is petroleum

A

A mixture consisting mainly of alkane hydrocarbons

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2
Q

Define petroleum fraction

A

A mixture of hydrocarbons with a similar chain length and boiling point range

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3
Q

Outline fractional distillation

A

Oil is heated and passed into a column

The fractions condense at different height as the temperature of the column decreases upwards

Therefor separation depends on boiling point which depends on molecule size

Larger molecules have larger van der waals forces and condense at the bottom of the column at higher temperatures

Smaller molecules have smaller van der waals forces and condense at the top of the column at lower temperatures

Similar (size/Bp/mass) molecules condense together and are tapped off in a petroleum fraction

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4
Q

What is fractional distillation

A

The physical process involving the splitting of weak van der waals forces between molecules

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5
Q

Outline vacuum distillation

A

Heavy residues from the fractionating column are distilled under a vacuum

Lowering the pressure over a liquid lowers its boiling point

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6
Q

Outline fractional distillation in a lab

A

Heat the flask with a Bunsen burner or electric mantle so vapours of all the components are produced

Vapours pass up the fractionating column

Lower bp molecules reach the top of the fractionating column first

The vapours with higher bp condense into the flask

Only the most volatile vapour passes into the condenser which cools the vapour into a liquid

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7
Q

Define cracking

A

Conversion of large hydrocarbons to smaller hydrocarbon molecules by breaking C-C bonds

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8
Q

Outline the general formula for cracking

A

High Mr alkane -> smaller Mr alkane + alkenes + (hydrogen)

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9
Q

Give an economic reason for cracking

A

Petroleum fractions with shorter C chains are in more demand than larger fractions

Make use of excess larger hydrocarbons

The products of cracking are more valuable than the starting materials

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10
Q

Why does cracking require high temperatures

A

It is a chemical process involving the splitting of strong covalent bonds

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11
Q

What are the two types of cracking

A

Thermal

Catalytic

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12
Q

Outline the conditions for thermal cracking

A

High pressure (7000 kPa)

High temperature (400-900C)

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13
Q

Outline the conditions for catalytic cracking

A

Low pressure

High temperature (450C)

Zeolite catalyst

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14
Q

What are the products of thermal cracking? What can they be used for?

A

Mostly alkenes
- making polymers/ethanol

Sometimes hydrogen
- haber process and margarine manufacturing

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15
Q

What are the products of catalytic cracking?

A

Branched and cyclic alkanes

Aromatic Hydrocarbons

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16
Q

Why is catalytic cracking useful

A

Branched and cyclic hydrocarbons burn more cleanly and give fuels a higher octane number

Cheaper than thermal cracking because of lower temperature and pressures used

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17
Q

Define a fuel

A

Releases heat energy when burnt

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18
Q

Why are alkanes useful as fuels

A

Alkanes readily burn in the presence of oxygen

The reaction is highly exit hermit

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19
Q

What are the products of complete combustion?

A

CO2

H2O

20
Q

What are products of incomplete combustion?

A

CO

C (soot)

H2O

21
Q

What defines complete combustion

A

Excess O2

22
Q

What defines incomplete combustion

A

A limited amount of O2

23
Q

Why is incomplete combustion unbenefitial?

A

Produces less energy per mole

Carbon (s) can cause global dimming

24
Q

What are the two reactions producing SO2

A

S+O2->SO2

CH3SH+3O2 -> SO2 + CO2 + 2H2O

25
Q

Why is SO2 hazardous?

A

It dissolves in atmospheric water which can produce acid rain

26
Q

How can the effects of SO2 be reduced?

A

Remove SO2 from waste gases from fur crave

By flue has desulphurisation

27
Q

Outline flue gas desulphurisation

A

Gases pass through a scrubber
containing basic calcium oxide
Which reacts with acidic sulphur dioxide
In a neutralisation reaction

SO2 + CaO -> CaSO3

28
Q

What can the product of flue gas desulphurisation be used for? What is the product?

A

Calcium sulphite

Can be used to make calcium sulphate

Used in plaster board

29
Q

What reactions form Nitrogen oxides

A

N2 + O2 -> 2NO

N2 + 2O2 -> 2NO2

30
Q

Why do engines form nitrogen oxides

A

The high temperature and spark in the engine

Provides sufficient energy to break strong N2 bonds

31
Q

What is the environmental consequence of nitrogen oxides?

A

NO is toxic and can form NO2

NO2 is toxic and acid and forms acid rain

32
Q

What is the environmental consequence of carbon monoxide?

A

Toxic

33
Q

What is the environmental consequence of carbon dioxide?

A

Contributed to global warming

34
Q

What is the environmental consequence of I burnt hydrocarbons?

A

Contributed to smog formation

35
Q

What is the environmental consequence of soot?

A

Global dimming

Respiratory problems

36
Q

What is a catalytic converter?

A

A ceramic honeycomb coated with thin layers of catalyst metals

Platinum palladium rhodium

Remove CO, NOx and unburnt hydrocarbons from exhaust gases

37
Q

What do catalytic converters convert pollutants into

A

CO2, N2 and H2O

38
Q

What is a greenhouse gas?

A

A gas that traps Earths radiated infra red energy in the atmosphere

39
Q

What are the green house gases?

A

CO2
CH4
H2O

40
Q

Why are alkanes generally unreactive with reagents?

A

The C-C bind and the C-H bond are relatively strong

41
Q

What conditions must synthesis of chloro alkanes occur under?

Why?

A

UV light

Supplies the energy to break the Cl-Cl bond

Broken in preference to others as it’s the weakest bond

42
Q

What are the three steps of free radical substitution

A

Initiation
Propagation
Termination

43
Q

Outline initiation

A

Cl-Cl bond is broken in homolytic fission

Under UV Libby

Forming free radicals

44
Q

Outline propagation

A

Chlorine free radicals react with a hydrogen on the alkane

Forming a free radical and HCl

The new free radical reacts with Cl2

to form the final product and a Cl free radical

45
Q

Outline termination

A

Two free radicals react

To form whole molecules

Left in structural formula

46
Q

What are the potential products of chloroalkane synthesis in excess Cl2

A

CH2Cl2

CHCl3

CCl4

AS CHEMISTRY (17 decks)

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