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AS Level Chemistry > Alkanes > Flashcards

Alkanes Flashcards

1
Q

What is cracking

A

conversion of large hydrocarbons to smaller hydrocarbon molecules by breakage of C-C bonds

High Mr alkanes —> smaller Mr alkanes+ alkenes + (hydrogen)

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2
Q

What are the Economic reasons for cracking(3)

A
  • The alkanes with shorter C chains are in more demand than larger fractions.
  • To make use of excess larger hydrocarbons and to supply demand
    for shorter ones, longer hydrocarbons are cracked.
  • The products of cracking are more valuable than the starting
    materials (e.g. ethene used to make poly(ethene), branched alkanes
    for motor fuels, etc.)
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3
Q

Why does cracking require large amounts of energy

A

It is a chemical process which involves the splitting of strong covalent bonds so it requires high temperatures

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4
Q

What are the two types of cracking

A

Thermal and catalytic. They need different conditions and are
used to produce different products

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5
Q

What are the conditions required for Thermal cracking

A

High pressure (7000 kPa)
High temperature (400°C to 900°C)

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6
Q

What are the conditions needed for catalytic cracking

A

Slight or moderate pressure
High temperature (450°C)
Zeolite catalyst

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7
Q

What does thermal cracking produce

A
  • Mostly small alkenes e.g. ethene used for making polymers and ethanol
  • Slightly larger alkanes
  • Sometimes produces hydrogen which is used in the Haber Process and in margarine manufacture.
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8
Q

An example equation of thermal cracking

A

.
C8H18 –> C6H14 + C2H4
C12H26 –> C10H22 + C2H4
Bonds can be broken anywhere in the molecule by C-C bond fission and C-H bond fission.

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9
Q

What does catalytic cracking produce

A

branched and cyclic alkanes and aromatic hydrocarbons

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10
Q

What is the use of the products of catalytic cracking

A

Making motor fuel

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11
Q

Why is catalytic cracking cheaper than thermal cracking

A

g because it saves
energy as lower temperatures and pressures are used

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12
Q

Define what a Fuel is

A

something that releases heat energy when burnt

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13
Q

What are the conditions required for complete combustion

A

An excess oxygen

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14
Q

What are the products of complete combustion

A

CO2 and H2O

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15
Q

What are the conditions required for incomplete combustion

A

a limited amount of oxygen

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16
Q

What are the products of incomplete combustion

A

CO (which is very toxic) and/or C (producing a sooty
flame)

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17
Q

Why are alkanes used as fuels

A

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

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18
Q

What is the differnevce between CC and IC (comple and incomplete combustion)

A

Incomplete combustion produces
less energy per mole than
complete combustion

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19
Q

What is a negative for the release of Carbon

A

Carbon (soot) can cause global
dimming- reflection of the sun’s
light

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20
Q

What is Petroleum

A

A mixture consisting mainly of alkane hydrocarbons

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21
Q

What are the steps for fractional distillation? (6)

A
  • Crude oil is heated until it evaporates.
  • Crude oil vapour is put into a fractionating column at the bottom and rises upwards.
  • The temperature is highest at the bottom of the column.
  • Long-chain hydrocarbons condense at the bottom
  • Short-chain hydrocarbons have lower boiling points.
  • They condense at lower temperatures nearer the top.
22
Q

What causes acid rain

A

SO2 will dissolve in atmospheric water which produces acid rain

23
Q

How are nitrogen oxides formed

A
  • Nitrogen oxides form from the reaction between N2 and O2 inside the car engine.
  • The high temperature and spark in the engine provides sufficient energy to break strong N2 bond
24
Q

What environmental consequence does the pollutant NO and NO2 have

A

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

25
Q

What consequence does the pollutant Carbon monoxide (CO) have

A

CO is a toxic and odourless gas which can cause dizziness, loss of consciousness and eventually death
The CO binds well to haemoglobin which therefore cannot bind oxygen and carbon dioxide
Oxygen is transported to organs
Carbon dioxide is removed as waste material from organs

26
Q

What environmental consequence does the pollutant carbon dioxide have

A

Contributes towards global warming

27
Q

What environmental consequence does the pollutant unburnt hydrocarbons have

A

Contributes towards formation of smog

28
Q

What environmental consequence does the pollutant soot have

A

Global dimming and respiratory problems

29
Q

What are the 3 main greenhouse gasses

A

Carbon dioxide (CO2)
Methane (CH4)
Water vapour (H2O)

30
Q

Out of the three greenhouse gases name them in abundant order

A

Water CO2 CH4

31
Q

Explain the greenhouse effect (4)

A
  • UV wavelength radiation passes through the atmosphere to the Earth’s surface and heats up Earth’s surface.
  • The Earth radiates out infrared long wavelength radiation.
  • The C=O Bonds in CO2 absorb infrared radiation so the IR radiation does not escape from the atmosphere.
  • This energy is transferred to other molecules in the atmosphere by collisions so the atmosphere is warmed.
32
Q

Why have carbon dioxide levels risen

A

Burning of fossil fuels

33
Q

What do Catalytic converts contain?

A

Converters have a ceramic honeycomb coated with a thin layer of catalyst metals platinum, palladium, rhodium– to give a large surface area

34
Q

What do catalytic converters do?

A

Removes CO, NOx and unburned hydrocarbons (e.g. octane, C8H18)
from the exhaust gases, turning them into ‘harmless’ CO2 , N2 and H2O.

35
Q

Why do alkanes not react with many reagents?

A

Because the C-C bond and the C-H bond are relatively strong

36
Q

What are the three steps in free radical subsituition

A

Step one: Initiation
Step two: Propagation
Step three: Termination

37
Q

What happens in the first step of FRS

A

The UV light supplies the energy to break the Cl-Cl bond. It is broken in preference to the others because it is the weakest.
Cl2 –> 2Cl*

38
Q

What is a free radical

A

A reactive species which possesses an unpaired electron.

39
Q

How are free radical represented

A

Free radicals do not have a charge and are represented by a *( not actually an asterisk just a black circle)

40
Q

What happens at Step two: Propagation

A

The chlorine free radicals are very reactive and remove an H from the methane leaving a methyl free radical
CH4 + Cl. —> HCl + .CH3
.CH3 + Cl2 –> CH3Cl + Cl.
The methyl free radical reacts with a Cl2 molecule to produce the main product and another Cl free radical

All propagation steps have a FreeRadicalInTheReactantsAndInTheProducts.

41
Q

What happens as the Cl free radical is regenerated?

Regenerated= Cl gets a free radical again

A

It can react with several more alkane molecules in a chain reaction.

42
Q

How is crude oil formed

A

From the remains of plants and animals from millions of years ago that have undergone high pressures and temperatures

43
Q

What are alkanes

A

Saturated hydrocarbons with the general formula CnH2n+2

44
Q

How do you test for alkenes

A

Addbromine water and if alkenes are present it goes from orange to colourless

45
Q

What is a use of alkenes

A

Prpoduction of plastics

46
Q

What is flue gas desulphurisation

A

Neutralising Sulphur dioxide by reacting it with calcium oxide (SO2+CaO–>CaSO3)

add oxygen to caso3–> cas04 is useful as it can be used in plastering

47
Q

How do nitrogen oxides form

A

The engine of a car reacts with nitrogen and oxygen in the atmosphere with heat and pressure forming 2NO

48
Q

What are catalytic converters

A

Platinium o rhodium (the catalysts used)
The presence of the catalysts removes the toxic gases

49
Q

what is the role of the mesh or honeycomb structure of the catalyst converters

A

Large surface area

50
Q

Write down two ways catalytic converters can remove nitrogen oxides`

A

2NO–> N2+O2
2NO+2CO–>N2+CO2

51
Q

What do unburnt hydrocarbons lead to

A

Respiratory problems

AS Level Chemistry (14 decks)

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