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A Level Chem Paper 1 > Alkanes > Flashcards

Alkanes Flashcards

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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

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
involving the splitting of
strong covalent bonds so
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 alkenes e.g. ethene used
for making polymers and ethanol
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

19
Q

What is a negative for the release of Carbon

A

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

20
Q

What is Petroleum

A

A mixture consisting mainly of alkane hydrocarbons

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

flue gas desulfurisation.

A

SHYT

24
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
25
Q

What environmental consequence does the pollutant Nitrogen oxide(s) have

A

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

26
Q

What environmental consequence does the pollutant Carbon monoxide (CO) have

A

toxic

27
Q

What environmental consequence does the pollutant carbon dioxide have

A

Contributes towards global warming

28
Q

What environmental consequence does the pollutant unburnt hydrocarbons have

A

Contributes towards formation of smog

29
Q

What environmental consequence does the pollutant soot have

A

Global dimming and respiratory problems

30
Q

What are the 3 main greenhouse gasses

A

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

31
Q

Out of the three greenhouse gases name them in abundant order

A

Water CO2 CH4

32
Q

Mechanism of 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.
33
Q

Why have carbon dioxide levels risen

A

Burning of fossil fuels

34
Q

What do 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

35
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.

36
Q

Why do alkanes not react with many reagents?

A

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

37
Q

What are the three steps in free radical subsituition

A

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

38
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*

39
Q

What is a free radical

A

A reactive species which possesses an unpaired electron.

40
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)

41
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.

42
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.

43
Q
A

A Level Chem Paper 1 (11 decks)

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