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chemistry a level > alkanes > Flashcards

alkanes Flashcards

1
Q

what are alkanes

A

alkanes are saturated hydrocarbons - they contain only carbon -carbon and carbon - hydrogen singe binds

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2
Q

what are alkanes used for

A

alkanes are the least reactive organic compounds

they are used for:

  1. fuels
  2. lubricants
  3. starting materials for a range of other compounds - this means they are very important to industry
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3
Q

what is the source of alkanes

A

the main source for alkanes is crude oil

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4
Q

what is the general formula for alkanes

A

the general formula for chain alkanes is CnH2n+2

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5
Q

what are the different structure of alkanes

A

hydrocarbons may be unbranched chains , branched chains or rings

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6
Q

what are unbranched alkanes

A

unbranched chains are often called straight chains but the C-C-C angles is 109.5 degrees.
This means that the chains are not actually straight

In an unbranched alkane, each carbon has two hydrocarbons except the end carbons which have one extra

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7
Q

what is the general formula for ring alkanes

A

ring alknes have the general molecular formula of Cn H2n

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8
Q

what is the polarity of alkanes

A

alkanes are almost non - polar because the electronegativity of both carbon and hydrogen are so similar.

As a result, the only intermolecular forces between their molecules are weak Van der Waals forces, and the larger the molecule, the stronger the Van der Waals forces

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9
Q

what is the boiling point of alkanes

A

due to the increasing intermolecular forces, the boiling points of alkanes increase as the chain length increases

the shorter chains are gases at room temperature

Pentane, with five carbons, is a liquid with a low boiling point of 309K (36C)

At a chain length of about 18 carbons, the alkanes become solids at room temperature

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10
Q

what is the trend of melting points in branched chains

A

alkanes with branched chains have lower melting points than straight chain alkanes the the same number of carbon atoms

This is because they cannot pack together as closely as branched chains and so the Van der Waals forces are not effective

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11
Q

what s the solubility of alkanes

A

Alkanes are insoluble in water. This is because water molecules are held together by hydrogen bonds which are much stronger than he Van der Waal’s forces that act between alkane molecules.

However, alkanes do mix with other relatively non -polar liquids

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12
Q

how do alkanes react

A

alkanes are relatively unreactive, they have strong carbon - carbon and carbon - hydrogen bonds

Thy do not react with acids, bases, oxidising agents, and reducing agents

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13
Q

what happens when alkanes are burned in oxygen

A

when they burn in plentiful supply of oxygen, they form carbon dioxide and water

OR

in a restricted supply of oxygen, they form carbon monoxide and carbon )

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14
Q

why are crude oils important

A

crude oil is at the present the world’s main source of organic chemicals

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15
Q

how are fossil fuels formed

A

fossil fuels are formed by the breakdown of plant and animal remains at the high pressures and temperatures deep below the Earth’s surface

because it forms slowly, it effectively non - renewable

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16
Q

what is crude oil

A

crude oils are a mixture mostly of alkanes , both unbranded and branched

crude oils from different sources have different compositions

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17
Q

what is fractional distillation

A

to convert crude oil into useful products you have to separate the mixture

This is done by heating it and collecting the fractions that boil over different ranges of temperatures

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18
Q

what are the different fractions composed of

A

each fraction is composed of:

  1. hydrocarbons of similar chain lengths and therefore similar properties
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19
Q

how do you carry out fractional distillation/ how does it work

A

METHOD:

  1. the crude oil is first heated in a furnace ( at 350C)
  2. a mixture of liquid and vapour passes into a tower that is cooler at the top than at the bottom
  3. the vapours pass up the tower via a series of trays containing bubble caps until they arrive at a tray that is sufficiently cool ( at a lower temperature than their boiling point). Then they condense to liquid
  4. the mixture of liquids that condenses on each tray is piped off
  5. the shorter chain hydrocarbons condense in the trays nearer to the top of the tower, where it is cooler, because they have lower boiling points
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20
Q

what is the thick residue that collects at the base of the fractional tower called

A

the thick residue that collects at the base of the tower is called tar or bitumen. It can be used for road surfacing but, as suppy often exceeds demand, this fraction is often further processed to give more valuable products

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21
Q

what is cracking

A

longer chain fractions are not as useful and therefore of lower value economically

Most crude oil has more longer chain fractions than is wanted and not of the naphtha fraction ( which has a shorter chain and are not in high demand)

The shorter chain products are economically more valuable than the longer chain material

to meet the demand for the shorter chain hydrocarbons, many if the longer chains are broken into shorter lengths (cracked)

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22
Q

what are the products of thermal cracking

A

shorter more useful chains are produced especially petrol

some of the products of the long-chain alkanes are the more reactive alkenes

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23
Q

what are alkenes used for

A

alkenes are used as chemical feed stock (which means they supply industries with the starting materials to make different product) and are converted into a huge range of other compounds e.g. polyethene and a range of everyday materials

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24
Q

what are the different ways in which alkanes are cracked

A

since alkanes are very unreactive and harsh conditions are required to break them down. There are a number of different ways of carrying out cracking:

  1. Thermal cracking
  2. Catalytic cracking
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25
Q

what is thermal cracking

A

thermal cracking involves heating alkanes to a high temperature, 700K -1200K (1000C) under high pressure, up to 7000kPa (70atm)

the C-C bonds break in such a way that one electron from the pair in the covalent bond goes to each carbon atom

These fragments are called free radicals

Free radicals are highly reactive intermediates and react in a number of ways to form a variety of shorter chain molecules

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26
Q

how do the C - C bonds in alkanes break

A

any number of C - C may break and the chain does not necessarily break in the middle
-Hydrogen may also be produced

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27
Q

why is there a high proportion of alkenes when hydrocarbons are cracked

A

as there is not enough hydrogen atoms to produce two alkanes, one of the new chains must have a C = C and therefore be an alkene

28
Q

how do we reduce the decomposition of products formed by thermal cracking

A

thermal cracking tends to produce a high proportion of alkenes

to avoid too much decomposition (ultimately to carbon and hydrogen) the alkanes are kept in these conditions for a very short time (typically one second)

29
Q

what is catalytic cracking

A

Catalytic cracking involves
catalysts to crack hydrocarbons
This form of cracking is used to produce motor fuels

30
Q

at what temperature and pressure does catalytic cracking takes place

A

catalytic cracking takes place at a lower temperature (approximately 500C ) and lower pressure ( but more than atmosphere)

31
Q

what type of catalyst is used in catalytic cracking

A

it uses zeolite catalyst, consisting of silicon dioxide and aluminium oxide (aluminium silicates)

32
Q

what structure is the zeolite structure

A

zeolites have a honeycomb structure with an enormous surface area. They are also acidic.

33
Q

what are the products produced by catalytic cracking

A

the products are mostly branched alkanes, cycloalkanes (rings) and aromatic compounds

34
Q

what are the advantages of catalytic cracking

A

it is cheaper because it does not require high pressure or temperatures

Catalysts speed up reactions so less time and money is spent

35
Q

how can we carry out catalytic cracking in a laboratory

A

in the laboratory, catalytic cracking can be carried out using an aluminium oxide catalyst

the products are mostly gases, showing that they have chain lengths of less than C5 and the mixture decolourises bromine solution. This test is a test for carbon-carbon double bond showing that the product contains alkenes

36
Q

what happens when alkanes are burned in a limited supply of oxygen

A

in a limited supply of oxygen, the poisonous gas carbon monoxide, CO is formed

This is called incomplete combustion

with even less oxygen, carbon (soot) is produced e.g. when a bunsen burner is used with a closed air hole, the flame is yellow and a black sooty deposit appears on the apparatus

water is also produced in each of these reactions

37
Q

when does incomplete combustion usually happen

A

incomplete combustion often happens with longer chain hydrocarbons, which need more oxygen to burn compared with shorter chains

38
Q

what are the pollutants that are released when burning fuels

A

all hydrocarbon-based fuels derived form crude oil may produce polluting when they burn:

They include the following:

  1. nitrogen oxides
  2. sulfur dioxide
  3. carbon particulates
  4. unburnt hydrocarbons
  5. CO2
39
Q

what are nitrogen oxides

A

nitrogen oxides (NOx) is produced when there is enough energy for nitrogen and oxygen in the air to combine

e.g.
N2(g) + O2(g) - 2NO(g)

The nitrogen monoxide can reach further to produce nitrogen dioxide:
2NO(g) + O2 - 2NO2

This happens in a petrol engine at the high temps present, when the sparks ignite the fuel. These oxides react with water vapour and oxygen in the air to form acid. They are therefore contributors to acid rain and photochemical smog

Nitrogen monoxide is produced when the high pressure and temperature in a car engine cause the nitrogen and oxygen atoms in the air to react

40
Q

how are sulfur dioxides formed

A

Some fossil fuels contain sulfur
When they are burnt, the sulfur reactor formsukfur dioxide gas (SO2)

When sulfur dioxide gets in the atmosphere, it dissolves the moisture and is converted to sulfuric acid. This is what causes acid rain- the same process occurs with nitrogen oxides

These oxides combine with water vapour and oxygen in the air to form sulfuric acid

41
Q

why is acid rain damaging

A

acid rain destroys trees and vegetation, as well as corroding buildings and statues and killing fish in lakes

42
Q

what are carbon particles

A

carbon particles, are called particulates which can exacerbate asthma and cause cancer

43
Q

What are unburned hydrocarbons

A

Engined don’t burn all the fuel molecules

Some these come out as unburned

44
Q

what is CO2

A

CO2, is a greenhouse gas. It is always produced hydrocarbons burn

Greenhouse gases in the atmosphere are very good at absorbing infrared energy. Try emit some of the energy back towards the earth keeping it warm- this js called the greenhouse effect

Although carbon is necessary in the atmosphere, its level is rising and this is a cause of the earth’s temperature and consequent climate change

45
Q

what is flue gas desulfurisation

A

large numbers of power stations generate electricity by burning fossil fuels such as coal or natural gas

These compounds contain sulfur compounds and one of the products of their combustion is sulfur dioxide, SO2, a gas that causes acid rain by combining with oxygen and water in the atmosphere to form sulfuric acids

SO2(g)+1/2O2(g) +H2(l) - H2SO4

these gases that are called flue gases so the process of removing the sulfur dioxide is called flue gas desulfuration

46
Q

how can we carry out flu gas desulfurisation

A

one method:

a slurry of calcium oxide (lime =) and water is sprayed onto the flue gas which reacts with the calcium oxide ad water to form calcium sulfite, which can be further oxidised to calcium sulfate

CaO(s) +2H2O(l) +SO2 +1/2O - CaSO4 .2H2O

Alternatively:
you can use calcium carbonate rater than calcium oxide:
CaCO3(s+ 1/2O3(g) +SO(g) - CaSO4+CO2

47
Q

what do catalytic converters do

A

internal combustion engines produce most of the pollutants listed previously

(except sulfur dioxide because sulfur is removed from petrol)

therefore all new cars with petrol engines are now equipped with catalytic converters in their exhaust systems to reduce the output of carbon monoxide, nitrogen oxides, and unburnt hydrocarbons in the exhaust gas mixture

48
Q

what is the shape of catalytic converters/ what are they made out of

A

honeycomb made of ceramic material coated with platinum and rhodium metals are the catalysts

49
Q

why are catalytic converters the shape of a honeycomb

A

honeycombs provide an enormous surface are, so a little of these expensive metals goes a long way

50
Q

how do catalytic converters work

A

as the polluting gases pass over the catalyst, they react with each other to form less harmful products by the following reactions:

2CO + 2NO(g) - N2(g) +2CO2
carbon monoxide+ nitrogen oxide - nitrogen + carbon dioxide

hydrocarbon + nitrogen oxide - nitrogen + carbon dioxide + water

2NO(g) - N2 + O2

51
Q

what is the greenhouse effect

A

greenhouses (CO2) traps infrared radiation so the earth’s atmosphere heats up
This is the greenhouse effect

52
Q

why is the greenhouse effect important

A

the greenhouse effect it is impostant for life because, without other greenhouse gases, the earth would be too cold to sustain human life

53
Q

why has there been an increase in the levels of greenhouse gases in the atmosphere

A

due to the industrial revolution, fossil fuels have been used to fuel industrial plants and the levels of CO2 has been rising.

This causes the eath’s temp has been rising too and scientists believe this is due to the increased level of CO2

54
Q

what happens when you expose an alkane and a halogen to UV light

A

When you put a mixture of an alkane and a halogen into bright sunlight (UV light) to form a halogenalkane

This reaction is called a photochemical reaction as it is started by uv light

55
Q

how is the halogenalkane bromohexane formed

A

if you put a mixture of hexane and a little liquid of bromine into a test tube and leave it in the dark, it will stay a red-brown colour ( the colour of bromine)

However, if we shine ultraviolet light onto it, the mixture becomes colourless and mist fumes of hydrogen bromide appear

This is a substitution reaction :
one or more of the hydrogen atoms in the alkane has been replaced by a bromine atom and hydrogen bromide is given off as a gas

the haloalkane is also produced

56
Q

what is the stages in a chain reaction

A

the reaction from above: an alkane and bromine
is a chain reaction

the three stages are:
initiation, propagation and termination

the reaction between any alkane and halogen goes by the same mechanism

57
Q

what is initiation

A

imagine a reaction between methane and chlorine

first, (initiation) step of the reaction is breaking the CL -CL bond to form two chlorine atoms

the chlorine molecule absorbs the energy of a single quantum of ultraviolet (UV) light
The energy of one quantum of UV light is more than the CL -CL bond energy, so the bond will break

Since both atoms are the same, the CL- CL bond breaks homolytically, that is one electron going to each chlorine atom

This results in two separate chlorine atoms. They are called free radicals. The dot next to the CL is used to show the unpaired electron

free radicals are highly reactive

The C-H bond in the alkane needs more energy to break than is available in a quantum of UV radiation and therefore does not break

58
Q

what happens during propagation

A

we are still considering the chlorine and alkane reaction

propagation takes place in two stages:

  1. the chlorine free radical takes a hydrogen atom from methane to form hydrogen chloride, a stable compound. This leaves a methyl free radical CH3
  2. The methyl free radical is also very reactive and reacts with a chlorine molecule. This produces another chlorine free radical and a molecule of chloromethane - a stable compound

the effect of these two steps is to produce hydrogen chloride and chloromethane, and a new CL free radical.

This is ready to react with more methane and the two steps are repeated. This is the chain part of the chain reaction

These steps may take place thousand of times before the radicals are destroyed in the termination

59
Q

what is termination

A

Termination is when free radicals are removed. This can happen in any of the following three ways ( in regards to the reaction with alkanes and chlorine) :

  1. Two chlorine free radicals react together to give chlorine CL2
  2. A chlorine free radical and a methyl free radical react to give chloromethane
  3. Two methyl free radical react together to give ethane
60
Q

what are the other products that are produced in the chain reaction of chlorine with alkanes

A

the main ones are chloromethane and hydrogen chloride

some ethane

dichloromethane - in the propagation stage if a chlorine radical reacts with some chloromethane that has already formed

with longer chain alkanes there w9ill be many isomers formed because the CL can replace any of the hydrogen atoms

chain reactions are not very useful because they produce such a mixture of products. They will also occur without light at high temperatures

61
Q

why are chain reactions not very useful

A

chain reactions are not very useful because they produce such a mixture of products. They will also occur without light at high temperatures

62
Q

what is ozone

A

ozone is a molecule made from three oxygen atoms O3

The ozone layer is important because it protects the earth from the harmful exposure to too many ultraviolet (UV) rays. Ultraviolet can cause skin cancer in people by damaging DNA

63
Q

how is the ozone layer damaged by the free radical

A

chlorine free radicals are formed from CFCs because the C - CL bonds breaks homolytically in the presence of UV radiation to produce free radicals, CL

Cl•+ O3• - O2(g) + ClO•

ClO• + O3 - 2O2 + Cl•

Because the chlorine free radical is regenerated, it only takes a little of it to destroy loads of ozone molecules

64
Q

Why are these unburned hydrocarbons bad dor the environment

A

These hydrocarbons react with nitrogen oxides in the presence of sunlight to form ground level ozone (O3)

This is a major component of smog
Ground level ozone irritates people’s eyes, aggravates respiratory problems and even causes lung damage

65
Q

What are the three main pollutants from vehicle exhausts

A

Nitrogen oxides

Unburnt hydrocarbons

Carbon monoxide

Catalytic convertes of Cars remove these pollutants from the exhaust

66
Q

What is the equation for the removal for unburnt hydrocarbons

A

It is just the equation for the complete combustion of a hydrocarbons

chemistry a level (24 decks)

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