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Chem Mock > Alkanes > Flashcards

Alkanes Flashcards

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1
Q

What are aliphatic

A

chemical compound that belonging to the organic class in which the atoms are connected by single, double, or triple bonds such as Alkanes, Alkenes and alkynes

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2
Q

What are aromatic

A

cyclic compounds containing double bonds such as arenes

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3
Q

What is crude oil

A

mixture of hydrocarbons containing alkanes, cycloalkanes and arenes

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4
Q

fractions from fractional distillation

A

refinery gas, gasoline, kerosene, diesel, fuel oil, bitumen

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5
Q

Cracking

A

excess heavier fractions are broken down into smaller, more useful compounds
These more useful compounds include alkanes and alkenes of lower relative formula

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6
Q

Product of cracking

A

When a large hydrocarbon is cracked, a smaller alkane and alkene molecules are formed

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7
Q

Types of cracking

A

Thermal cracking requires high temperatures and high pressure and produces alkanes and a lot of alkenes

Catalytic cracking uses a lower temperature and slight pressure in the presence of a catalyst such as a zeolite or aluminium oxide to produce mainly aromatic hydrocarbons

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8
Q

Reforming alkanes

A

Many of the alkanes in these fuels are straight chain alkanes
These straight chain alkanes are more likely to explode, rather than combust, inside the engine
This is known as knocking and makes the combustion less efficient
To reduce this straight chain alkanes are reformed into
Branched alkanes

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9
Q

Catalyst for reforming

A

platinum

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10
Q

What is complete combustion

A

When alkanes are burnt in excess (plenty of) oxygen, complete combustion will take place and all carbon and hydrogen will be oxidised to carbon dioxide and water respectively

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11
Q

What is incomplete combustion

A

When alkanes are burnt in only a limited supply of oxygen, incomplete combustion will take place and not all the carbon is fully oxidised
Some carbon is only partially oxidised to form carbon monoxide.
carbon will be produced in the form of soot

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12
Q

what do Car exhaust fumes include

A

include toxic gases such as carbon monoxide (CO), oxides of nitrogen (NO/NO2) and volatile organic compounds (VOCs)

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13
Q

What is carbon monoxide

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

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14
Q

how are oxides of nitrogen caused

A

nitrogen is too unreactive to react with oxygen in air
However, in a car engine, high temperatures and pressures are reached causing the oxidation of nitrogen to take place

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15
Q

how is photochemical smog made

A

Car exhaust fumes also contain unburnt hydrocarbons from fuels and their oxides (VOCs)
In air, the nitrogen oxides can react with these VOCs to form peroxyacetyl nitrate (PAN) which is the main pollutant found in photochemical smog

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16
Q

how is acid rain formed

A

Nitrogen oxides can also dissolve and react in water with oxygen to form nitric acid which can cause corrosion of buildings, endangers plant and aquatic life

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17
Q

how is soot made

A

Solid carbon particles (or particulates) released from incomplete combustion clump together to form soot which gradually falls back to the ground

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18
Q

what problems can particulates cause

A

If they are inhaled they can damage the lungs and cause respiratory problems

They can cover buildings and statues, making them look unclean and accelerating corrosion

They can reflect sunlight back into space reducing the amount of light reaching the earth, this is called global dimming

19
Q

how can pollution be reduced

A

catalytic converters which have precious metals (such as platinum) are coated on a honeycomb to provide a large surface area

20
Q

what reactions take place in catalytic converter

A

Oxidation of unburnt hydrocarbons
Oxidation of CO to CO2
Reduction of NO to N2
as more stable

21
Q

Catalytic removal of NO and CO

A

The nitrogen oxides are reduced on the surface of the hot catalyst to form the unreactive and harmless nitrogen gas which is then released from the vehicle’s exhaust pipe into the atmosphere

2CO (g) + 2NO (g) → 2CO2 (g) + N2 (g)

22
Q

Alternative fuels

A

Biofuels are renewable fuels
Renewable meaning that they can be replaced over a short period of time
The bio part of biofuel meaning that it comes from living matter

23
Q

Types of biofuel

A

Biodiesel - made by refining renewable fats and oils

Bioethanol - made by fermentation

Biogas - made / released when organic waste breaks down

24
Q

Benefit and disadvantages of biofuels

A

Biofuels are often considered as carbon neutral
This is because as the plants grow they absorb carbon dioxide which is equivalent to the amount of carbon dioxide that will be released when they are burnt

Biodiesel and biogas can reduce the amount of waste going to landfill as the waste can be used to produce them

Biofuel production could provide money for less developed countries as they have the space to grow the crops required

The cost of converting engines and machinery to run on biofuels instead of petrol / diesel

countries don’t have the space to be able to produce enough plants to make the biofuels because the land is needed for food production

25
Q

What is free radical substitution

A

a hydrogen atom gets substituted by a halogen

26
Q

why is ultraviolet needed for this substitution

A

alkanes are very unreactive

27
Q

What are the three steps of free-radical substitution

A

In the initiation step, the halogen bond (Cl-Cl or Br-Br) is broken by UV energy to form two radicals

These radicals create further radicals in a chain reaction called the propagation step

The reaction is terminated when two radicals collide with each other in a termination step

28
Q

explain initiation step

A

Cl-Cl or Br-Br is broken by energy from the UV light

This produces two radicals in a homolytic fission reaction each atom takes one electron from covalent bond

29
Q

what is heterolytic and homolytic

A

homolytic fission is splitting a bond to produce two particles which are free radicals.

Heterolytic fission produces two particles which are different because one is a positive ion and the other a negative ion.

30
Q

explain propagation

A

Free radicals are very reactive and will attack the unreactive alkanes
A C-H bond breaks homolytically (each atom gets an electron from the covalent bond). An alkyl free radical is produced
This can attack another chlorine/bromine molecule to form the halogenoalkane and regenerate the chlorine/bromine free radical
This free radical can then repeat the cycle

progression (growing) of the substitution reaction in a chain reaction

31
Q

Will all hydrogen be eventually substituted

A

yes if there is enough halogen present.

32
Q

explain termination step

A

when the chain reaction terminates (stops) due to two free radicals reacting together and forming a single unreactive molecule
different possible equations with the different alkyles

33
Q

if we have propane how will substitution occur

A

put it next to the carbon with radical in brackets

34
Q

characteristics homologous series

A

they have the same general formula

neighbouring compounds differ from each other by a -CH2 - group

they have the same functional group / display similar chemical properties

they show a gradual change / trend in physical properties

35
Q

uses of cracking hydrocarbons

A

shorter chain alkanes and alkenes formed

Alkenes are useful starting materials in organic synthesis / used for making polymers / plastics

Shorter chain alkanes are more in demand / higher value / can be used as fuel

36
Q

Why do isomers of alkanes have lower boiling temperatures

A

due to branching which decreases the surface area as the molecule will be less tightly packed therefore the intermolecular forces / van der vaals forces are decreased.

37
Q

why are sigma bonds stronger

A

more effective overlap

38
Q

one reason why cracking reactions are carried out in industry and suggest why high temperatures are used in this process other than to speed up the reaction.

A

(Greater) demand for smaller alkanes more useful and to break c-c bond or break down the hydrocarbon chain

39
Q

what is a fuel

A

substance that produces heat or energy

40
Q

reason why companies reform alkanes

A

has more efficient combustion
allows smoother burning
increases octane number
reduces knocking / less likely to produce pre-ignition

41
Q

why can different alkanes in crude oil be seperated by fractional distillation

A

as they contain a different boiling temperature due to difference in chain length

42
Q

why do isomers of the unbranched hexane have a lower boiling temperature

A

because isomers have branched chain that have lower surface areas so don’t pack closely hence london forces are weaker

43
Q

what is the one product that is always produced during forming

A

Hydrogen

Chem Mock (34 decks)

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