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Organic - Alkanes Flashcards

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1
Q

What are alkanes?

A

Alkanes are a homologous series of saturated hydrocarbons with the general formula CnH2n+2.

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2
Q

Are alkanes reactive?

A

Alkanes are very unreactive because they’re non-polar, although they do burn and react with halogens. Harsh conditions are required to break them down.

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3
Q

How does the carbon chain affect the boiling point of alkanes?

A

The longer the carbon chain, the higher the boiling point due to stronger van der Waals’ forces between molecules (because there are more electrons in the molecules).

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4
Q

How do the isomers of an alkane affect the boiling point?

A

For alkanes that are isomers, the more branched the carbon chain, the lower the boiling point due to weaker van der Waals’ forces between molecules (due to molecules not being able to pack as close together).

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5
Q

How are deposits of crude oil and natural gas formed?

A

Deposits of crude oil and natural gas usually occur together and they are formed by the slow decay of marine animals and plants, over millions of years, under heat and pressure in the absence of air.

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6
Q

What is crude oil composed of?

A

Although the exact composition of crude oils vary around the world, all are a complex mixture consisting mainly of alkanes (including cycloalkanes, some aromatics and other compounds containing some sulphur and oxygen).

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7
Q

How is crude oil used?

A

Crude oil has no use in its raw form, so to provide useful products its components must be partly separated (and if necessary modified) - the separation uses the differences in the physical properties of alkanes.

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8
Q

How is crude oil separated?

A

The compounds in crude oil have different boiling points and this is used to separate them by fractional distillation at an oil refinery.

C-H bonds are virtually non-polar, so there are only van der Waals’ forces between molecules. As the alkane chain gets longer, the melting and boiling points increase due to greater van der Waals’ forces.

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9
Q

What is the basic idea of the separation process?

A
  1. The crude oil is first heated in a furnace.
  2. A mixture of liquid and vapour is passed into a tower which is hot at the bottom and cold at the top.
  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. Molecules will condense at different heights as they have different boiling points.
  5. The mixture of liquids that condenses on each tray is piped off.
  6. The larger the molecule (with higher boiling points), the lower down the column it condenses.
  7. This produces a fraction.
  8. The thick residue that collects at the base of the tower is called tar or bitumen. It can be used for road surfaces and roofing but, as supply often exceeds demand, this fraction is often further processes to give more valuable products.
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10
Q

What is a fraction?

A

A fraction is a mixture of compounds (hydrocarbons) with similar boiling points.

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11
Q

What happens to the hydrocarbons as the carbon chain gets longer?

A

As the carbon chain gets longer, the hydrocarbons:

  • become more viscous
  • become harder to ignite
  • become less volatile
  • have higher boiling points
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12
Q

Give the order of the fractions from the lowest boiling point to the highest. What are their uses?

A
  • fuel gases (Calor gas, LPG)
  • gasoline (petrol for cars)
  • naphtha (petrochemicals)
  • kerosene (jet fuel)
  • diesel/gas oil (fuel for diesel engines)
  • residue: distilled further at lower pressures to give fuel oil, lubricating oil, waxes, and bitumen
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13
Q

Why is the residue from the primary distillation separated further?

A

The residue from the primary distillation (first distillation) contains useful substances, such as fuel oil, lubricating oil, waxes and bitumen, that boil above 350 degrees Celsius at atmospheric pressure. However, they would decompose at such high temperatures, so they are separated further by distillation at a lower pressure (vacuum distillation).

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14
Q

Which fractions are most in-demand?

A

The petroleum fractions (lower boiling points) with shorter carbon chains (e.g. petrol and naphtha) are in more demand than larger fractions. However, they are the least abundant.

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15
Q

How do you supply demand for shorter carbon chains?

A

To make use of excess larger hydrocarbons and supply demand for shorter ones, longer hydrocarbons are cracked.

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16
Q

What is cracking?

A

Cracking is the thermal decomposition of alkanes. Carbon-carbon bonds are broken in cracking. 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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17
Q

What are the conditions for thermal cracking?

A 
temperature - 900 degrees Celcius
pressure - 70 atm
mechanism - homolytic fission
catalyst - none
products - alkenes
uses of products - making polymers
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18
Q

What are the conditions for catalytic cracking?

A 
temperature - 450 degrees Celcius
pressure - 1-2 atm (slight pressure)
mechanism - heterolytic fission
catalyst - zeolites
products - motor fuels, aromatic hydrocarbons, cycloalkanes, branched alkanes)
uses of products - fuel
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19
Q

What is reforming?

A

Reforming is a process where straight chain hydrocarbons are converted into branched chain alkanes and cyclic alkanes. Both these products burn more efficiently and are used in petrol for cars.

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20
Q

How do alkanes undergo combustion?

A

Alkanes readily burn in the presence of oxygen and release vast amounts of energy, this combustion of alkanes being highly exothermic, explaining their use as fuels.

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21
Q

What are the products of complete combustion?

A

Alkanes burn in a plentiful supply of oxygen to produce carbon dioxide and water only.

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22
Q

What are the products of incomplete combustion?

A

In a limited supply of air, alkanes will burn to form water and carbon monoxide. If the supply of oxygen is further limited, solid carbon particles (soot) are formed.

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23
Q

Why isn’t hydrogen produced during combustion?

A

Any amount of oxygen present binds to hydrogen first to produce water.

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24
Q

What is the polarity of alkanes?

A

Alkanes are almost no-polar because the electronegativities of 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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25
Q

What are the boiling points of alkanes?

A

The increasing intermolecular force is why 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. At a chain length of about 18 carbons, the alkanes become solids at room temperature. The solids have a waxy feel.

Alkanes with branched chains have lower melting points than straight chain alkanes with the same number of carbon atoms. This is because they cannot pack together closely as unbranched chains and so the van der Waals forces are not so effective.

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26
Q

What is 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 the van der Waal’s forces that act between alkane molecules. However, alkanes do mix with other relatively non-polar liquids.

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27
Q

How do alkanes react?

A

Alkanes are relatively unreactive. They have strong carbon-carbon and carbon-hydrogen bonds. They do not react with acids, bases, oxidising agents, and reducing agents. However, they do burn and they will react with halogens under suitable conditions. They burn in a plentiful supply of oxygen to form carbon dioxide and water (or, in a restricted supply of oxygen, to form carbon monoxide or carbon).

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28
Q

How is sulphur dioxide produced?

A

Crude oil contains small amounts of other compounds dissolved in it. These come from other elements in the original plants and animals the oil was formed from, for example, some contain sulphur. These produce sulphur dioxide when they are burnt. This is one of the causes of acid rain - sulphur dioxide reacts with oxygen high in the atmosphere to form sulphur trioxide. This reacts with water in the atmosphere to form sulphuric acid.

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29
Q

What is broken during fractional distillation?

A

Fractional distillation is a physical process so no covalent bonds within the molecules are broken. It is the van der Waals forces between the molecules that are broken during vaporisation and reform on condensing.

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30
Q

What is fracking?

A

Many areas of the UK have resources of natural gas trapped within shale rock. This gas can be extracted by drilling into the shale and forcing pressurised water mixed with sand into the shale. This causes the rather soft shale rock to break up or fracture (giving the term fracking, short for hydraulic fracturing) releasing the trapped gas which flows to the surface. A number of chemicals are added to the water such as hydrochloric acid to help break up the shale and methanol to prevent corrosion in the system.

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31
Q

Why are people opposed to fracking?

A
  • They do not like the infrastructure of wells and the associated traffic in their ‘background’.
  • There is concern about the amount of water used.
  • They worry about the chemical additives polluting water supplies.
  • Occasionally fracking appears to have caused small earthquakes.
  • Burning natural gas produces carbon dioxide (a cause of global warming).

Set against these objectives is the appeal of gas supplies for many years which are not subject to control by other countries.

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32
Q

What are seven different pollutants?

A
  • CO2
  • CO
  • C (soot)
  • H2O
  • SO2
  • NOx
  • unburned hydrocarbons
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33
Q

What is CO2? (how is it formed? what are the problems it causes? what are the ways to reduce the problem?)

A

How formed - complete combustion of fuels containing C
Problem caused - greenhouse gas
Ways to reduce the problem - burn less fossil fuels

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34
Q

What is CO? (how is it formed? what are the problems it causes? what are the ways to reduce the problem?)

A

How formed - incomplete combustion of fuels containing C
Problem caused - toxic
Ways to reduce the problem - ensure a good supply of oxygen when burning fuels

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35
Q

What is C (soot)? (how is it formed? what are the problems it causes? what are the ways to reduce the problem?)

A

How formed - incomplete combustion of fuels containing C
Problem caused - blackens buildings, can cause respiratory problems, global dimming
Ways to reduce the problem - ensure a good supply of oxygen when burning fuels

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36
Q

What is H2O? (how is it formed? what are the problems it causes? what are the ways to reduce the problem?)

A

How formed - combustion of fuels containing H
Problem caused - greenhouse gas
Ways to reduce the problem -

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37
Q

What is SO2? (how is it formed? what are the problems it causes? what are the ways to reduce the problem?)

A

How formed - combustion of S containing compounds in fuel
Problem caused - acid rain
Ways to reduce the problem - remove S from fuel before burning, flue gas desulfurisation

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38
Q

What is NOx? (how is it formed? what are the problems it causes? what are the ways to reduce the problem?)

A

How formed - reaction of N2 in the air with O2 in the air at very high temperatures (in engines and furnaces) when the sparks ignite the fuel
Problem caused - acid rain and photochemical smog
Ways to reduce the problem - use catalytic converters in cars

39
Q

What are unburned hydrocarbons? (how is it formed? what are the problems it causes? what are the ways to reduce the problem?)

A

How formed - some of the fuel may not actually burn
Problem caused - wastes fuel, can react with NOx to form ground level ozone which causes respiratory problems, photochemical smog
Ways to reduce the problem - ensure engines are well-tuned and there is a good supply of oxygen

40
Q

How do you decrease the amount of sulphur dioxide released into the atmosphere?

A

To decrease the amount of sulphur dioxide released into the atmosphere, most of the sulphur containing compounds are removed from petrol and diesel before use. However, removal of sulphur before combustion is not economically viable for fuels used in power stations. The sulphur dioxide is removed from the combustion emissions instead in a process known as flue gas desulfurisation.

41
Q

What is flue gas desulphurisation?

A

SO2 can be removed from the waste gases from furnaces (e.g. coal fired power stations) by flue gas desulfurization. The gases are passed through a wet semi-solid mixture, a slurry, containing calcium oxide or calcium carbonate. Calcium oxide and calcium carbonate are bases. They neutralise the acidic sulphur dioxide to form calcium sulphite (CaSO3). This product has little commercial value but rather than dump it in waste pits, it is oxidised to calcium sulphate (CaSO4) which is commercially useful as a construction material.

42
Q

What kind of reaction is flue gas desulphurisation?

A

This is an acid-base reaction as CaCO3 and CaO are bases and SO2 is an acidic oxide (non-metal oxide).

43
Q

What does flue gas desulphurisation form?

A

The reaction forms gypsum (calcium sulphate IV) which is a saleable product as it is used to make builders’ plaster and plasterboard.

44
Q

What is used in the internal combustion engine?

A

Petrol (gasoline fraction, which consists of liquid alkanes) is used in the internal combustion engine where the alkanes are vaporised and combusted with air.

45
Q

What do catalytic converters do?

A

A catalytic convertor reduces the amount of carbon monoxide, nitrogen oxides and unburnt hydrocarbons released into the air from an internal combustion engine, by converting them into less toxic gases. It is fitted to the exhaust system of the engine.

46
Q

What is the appearance of converters? Why?

A

At its centre is a honeycomb of ceramic material covered in a thin layer of platinum and rhodium. The platinum and rhodium are the catalysts. The thin layer coating the honeycomb structure provides a large surface area for the reaction, increasing the rate of conversion and also ensuring that as little as possible of the expensive catalytic metals are required.

47
Q

What are greenhouse gases?

A

Greenhouse gases are gases that absorb the IR radiation given off by the Earth, but do not absorb the higher frequency UV/visible radiation given off by the sun.

48
Q

Which molecules absorb IR radiation?

A

Molecules that contain polar bonds absorb IR radiation to make bonds vibrate. Therefore, molecules such as CO2 (the C=O bonds are polar), H2O (the O-H bonds are polar) and CH4 (the C-H bonds are slightly polar). Molecules such as O2 and N2 do not absorb IR radiation and are not greenhouse gases as they do not have polar bonds.

49
Q

What are the increasing levels of greenhouse gases in the atmosphere resulting in?

A

The Earth is getting warmer and many scientists believe it is due to increasing amounts of greenhouse gases in the atmosphere. The burning of fossil fuels (including alkanes) releases carbon dioxide into the atmosphere. Carbon dioxide levels have risen significantly in recent years due to increasing burning of fossil fuels. Carbon dioxide is a particularly effective greenhouse gas and its increase is thought to be largely responsible for global warming. Methane levels have also increased significantly largely due to agriculture (e.g. rice production) and farming (e.g. cows).

50
Q

What are the conditions needed for thermal cracking?

A

During thermal cracking, the long chain alkanes are heated to a very high temperature, typically between 1000K and 1200K, at extremely high pressures, up to 70 atm, for a very short space of time, 1 second.

51
Q

Why are these conditions needed for thermal cracking?

A

These conditions produce cracking products which contain shorter chain alkanes and are rich in alkenes. The high temperature and pressures employed in thermal cracking could decompose the molecule completely and produce carbon and hydrogen. To prevent this, the time during which these conditions are applied is very short.

52
Q

What are the conditions needed for catalytic cracking?

A

During catalytic cracking, the long chain alkane is heated under pressure in the presence of a zeolite catalyst. The temperature used is approximately 800-1000K and the pressure used between 1-2 atm.

53
Q

What is a zeolite? What is its structure?

A

A zeolite is an acidic mineral which has a honeycomb structure made from aluminium oxide and silicon dioxide. The honeycomb structure gives it a large surface area which increases the rate of reaction.

54
Q

What products do modern catalytic cracking units produce?

A

Modern catalytic cracking units are operated continuously for up to three years at a time producing a product that consists mainly of branched alkanes, cycloalkanes and aromatic compounds. This method of cracking is used to produce fuels for road vehicles.

55
Q

Why are high temperatures needed for catalytic cracking? How does it compare to thermal cracking?

A

While the temperatures employed in the catalytic cracking process are lower than those in the process of thermal cracking, they are still high. High temperatures are needed as the carbon-carbon bonds in the alkane molecules, which are cracked, are extremely strong and therefore difficult to break.

56
Q

What is methane?

A

Methane is a major component of natural gas, the gas found with oil and coal deposits.

57
Q

What is butane?

A

Butane is a major component of liquid petroleum gas produced by the fractional distillation of crude oil.

58
Q

What is carbon monoxide?

A

Carbon monoxide is a toxic, colourless and odourless gas which is often referred to as a ‘silent killer’. Carbon monoxide reacts with the haemoglobin in red blood cells preventing them from carrying the oxygen to all of the cells in your body. It is absorbed 200 times faster than oxygen and is so difficult to eliminate, it is classified as a cumulative poison.

Carbon monoxide can be removed using a catalytic converter.

59
Q

Why must all gas fuelled appliances be regularly maintained?

A

All gas fuelled appliances must be regularly maintained to prevent the formation of carbon monoxide due to a lack of oxygen present for combustion. Carbon monoxide detectors can warn of dangerous levels of the gas.

60
Q

What are the symptoms of carbon monoxide poisoning from mild exposure?

A

Slight headache, nausea, vomiting, unusual tiredness.

61
Q

What are the symptoms of carbon monoxide poisoning from medium exposure?

A

Severe throbbing, headache, drowsiness, disorientation, confusion, fast heart rate.

62
Q

What are the symptoms of carbon monoxide poisoning from extreme exposure?

A

Unconsciousness, convulsions, cardio-respiratory failure, death.

63
Q

What is the primary reaction facilitating the change from chemical energy to kinetic energy? What are the unwanted side reactions?

A

The primary reaction facilitating the change from chemical energy to kinetic energy is the combustion of the alkane fuel in oxygen. However, the pressure and extreme temperature of the combustion chamber cause unwanted side reactions.

The normally unreactive nitrogen, approximately 78% of the air intake, combines with oxygen producing a series of nitrogen oxides, mainly NO and NO2 (NOx). Sulphur dioxide can also be present in the exhaust mixture. The sulphur originates from impurities in crude oil which end up in the fuel. Consequently, a chemical cocktail of combustion products are released into the environment by our use of alkane-based fuels in the internal combustion engine. This mixture contains a number of pollutants including NOx, CO, carbon particles, SO2 and unburnt hydrocarbons.

64
Q

What is the greenhouse effect?

A

The Earth is surrounded by an atmosphere of several layers of gas. Infrared radiation from the sun passes through these layers to reach the Earth and warms it up. Infrared radiation from the Earth travels back through the atmosphere where some of it is prevented from escaping into space by atmospheric gases, such as carbon dioxide, water and methane. The effect of trapping the energy from the sun is known as The Greenhouse Effect and the gases which cause the phenomenon are known as greenhouse gases. This is an important natural process as without it the average temperature of the atmosphere at the surface of the Earth would be 60 degrees Celsius lower and the Earth would not be able to sustain life,

65
Q

What is global warming?

A

Global warming is the term given to the increasing average temperature of the atmosphere at the surface of the Earth. It is caused by changing the balance of the concentration of greenhouse gases. Recent human activity has rapidly increased the concentration of carbon dioxide in the atmosphere, due to increased burning of fossil fuels. As a result, more heat from the sun is trapped and so the Earth’s temperature increases.

66
Q

Where is water vapour found?

A

occurs naturally in the atmosphere

67
Q

Where is carbon dioxide found?

A

produced during respiration and as a product of combustion

68
Q

Where is methane found?

A

produced as a product of digestion by cows and sheep, and by other natural processes

69
Q

What is acid rain?

A

Rain water is a weak acid, pH 5.5, due to the naturally occurring carbon dioxide present in the atmosphere. Acid rain is rain water which is more acidic than this, with a pH lower than 5.5. The nitrogen oxides produced by high temperature combustion of fossil fuels contribute to acid rain but the main contributor is the sulphur dioxide gas produced when the impurities in fossil fuels are burned. The sulphur dioxide reacts with water in the air to produce sulphurous acid (H2SO3), which is oxidised in the air to form sulphuric acid (H2SO4).

70
Q

What is the impact of acid rain?

A

Acid rain destroys trees and vegetation, corrodes buildings and kills fish in lakes. Acid rain may fall far from the source of the polluting gases and as a consequence, spreads the effects of acid rain to many other areas. This makes acid rain a global environmental issue.

71
Q

What is photochemical smog?

A

Photochemical smog is caused by pungent, toxic gases and minute solid particles suspended in the air close to the surface of the Earth. Smog is formed when nitrogen oxides, sulphur dioxide and unburnt hydrocarbon fuels react with sunlight. It also includes carbon atoms produced when hydrocarbon fuels are burned in a very limited supply of air.

72
Q

What is the impact of photochemical smog?

A

Smog can form in almost any climate in industrialised cities and causes a variety of health problems from relatively minor red irritated eyes and nasal congestion to severe lung diseases such as emphysema.

73
Q

What are the benefits of cracking?

A
  • Shorter, more useful chains are produced, especially petrol.
  • Some of the products are alkenes, which are more reactive than alkanes.
74
Q

What are free radicals?

A

Thermal cracking involves heating alkanes to a high temperature, and under a high pressure. Carbon-carbon bonds break in such a way that one electron from the pair in the covalent bond goes to each carbon atom. So initially two shorter chains are produced, each ending in a carbon atom with an unpaired electron. 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.

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

75
Q

What are the products obtained from cracking separated by?

A

fractional distillation

76
Q

What do combustion reactions do?

A

Combustion reactions give out heat and have large negative enthalpies of combustion. The more carbons present, the greater the heat output. For this reason they are important as fuels. Fuels are substances that release heat energy when they undergo combustion. They also store a large amount of energy for a small amount of weight.

77
Q

Describe the concentration of water vapour in the atmosphere.

A

The concentration of water vapour, the most abundant greenhouse gas, in the atmosphere tends to stay roughly the same (except locally - by waterfalls, for example) because of the equilibrium that exists between water vapour and liquid water. However, if the temperature of the atmosphere rises, there will be more water vapour in the air and therefore more greenhouse warming. This may be offset by greater cloud formation and clouds reflect solar radiation. The role of water is therefore recognised as very important but as yet not fully understood.

78
Q

What are carbon neutral activities?

A

Activities that produce no carbon dioxide emissions OVERALL are referred to as carbon neutral. (give out the same amount they take in)

79
Q

What is a free radical substitution reaction?

A

In free radical substitution reactions, hydrogen atoms on alkanes or halogenalkanes are replaced by halogen atoms (e.g. F, Cl, Br, I). For every one hydrogen atom that is replaced, one X2 molecule is used and one HX molecule is formed.

80
Q

What are the conditions needed for a free radical substitution reaction?

A
  • ultraviolet light to give enough energy to break the halogen bond and start the reaction
  • excess methane to reduce further substitution
81
Q

What happens during the chlorination of methane?

A

homolytic breaking of covalent bonds

82
Q

What is the overall equation for a free radical substitution reaction?

A

For every hydrogen atom replaced by F/Cl/Br/I, one molecule of F2/Cl2/Br2/I2 is used and one molecule of HF/HCl/HBr/HI is released.

In order to replace many hydrogen atoms with F/Cl/Br/I atoms, an excess of F2/Cl2/Br2/I2 is used. In order to only replace one hydrogen atom, an excess of the alkane/halogenalkane is used.

83
Q

What is initiation?

A

Photochemical reactions. When exposed to ultraviolet light, a molecule of a halogen F2/Cl2/Br2/I2 absorbs the energy of a single quantum of the UV light. The energy of one quantum of UV light is greater than the covalent bond energy, so it breaks apart homolytically (one electron going to each atom) into two halogen atom reactive intermediates called free radicals F/Cl/Br/I (this is called photodissociation). The ultraviolet light provides the energy to break the covalent bond between the two halogen atoms. Some bonds aren’t broken because they need more energy to break than is available in a quantum of ultraviolet radiation.

(Free radicals are species with an odd number of electrons - they are very reactive - the atom with the odd number of electrons is shown with a *).

84
Q

What is propagation?

A

(molecule + radical -> different molecule + different radical)

For every hydrogen that is replaced, there is one pair of propagation reactions.

Step One: The alkane/halogenalkane reacts with the F/Cl/Br/I - this removed a hydrogen atom from the alkane/halogenalkane - this produces HF/HCl/HBr/HI and a carbon based free radical. (The hydrogen is replaced and the halogen radical reformed as a catalyst).

Step Two: The carbon based radical from step one reacts with F2/Cl2/Br2/I2 to put an atom of F/Cl/Br/I onto the carbon based radical - this also produces another halogen free radical (F/Cl/Br/I) to continue the chain reaction in another step one.

This is the chain part of the chain reaction. These steps may take place thousands of times to result in multiple substitutions before the radicals are destroyed in the termination step. Condition of the reaction can be altered to favour the termination step and limit the number of substitutions.

85
Q

What is termination?

A

(2 radicals -> molecule)

Termination is the step in which the free radicals are removed. If two free radicals collide, they will form a molecule and stop the chain reaction. Any two free radicals involved in the mechanism could collide in this way.

Notice that in every case, two free radicals react to form a stable compound with no unpaired electrons.

86
Q

What is a chain reaction?

A

Free radical substitution reactions start off a chain reaction which takes place in three stages - initiation, propagation, and termination.

87
Q

Why are chain reactions not 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.

88
Q

Why are chain reactions important?

A

It is believed that chlorofluorocarbons (CFCs) in the stratosphere are destroying the ozone layer.

89
Q

What is ozone?

A

Ozone is a molecule made from three oxygen atoms, 03. It decomposes to oxygen. Too much ozone at ground level causes lung irritation and degradation of paints and plastics, but high in the atmosphere it has a vital role.

90
Q

Why is the ozone layer important?

A

The ozone layer is important because it protects the Earth from the harmful exposure to too many ultraviolet (UV) rays. Without this protective layer, life on Earth would be very different. For example, plankton in the sea, which are at the very bottom of the food chain of the oceans, need protection from too much UV radiation. Also, too much UV radiation causes skin cancer in people by damaging DNA.

91
Q

How are chlorine free radicals formed?

A

Chlorine free radicals are formed from CFCs because the C-Cl bond breaks homolytically in the presence of UV radiation to produce chlorine free radicals, Cl. Ozone molecules are then attacked by these. The resulting free radicals also attack ozone and regenerate Cl. Adding the two equations, the chlorine free radical is not destroyed in this process. It acts as a catalyst in the breakdown of ozone to oxygen.

92
Q

What are alternatives to CFCs?

A

In 1987, 24 countries signed up the Montreal protocol to ban the uses of CFCs. However, some countries still use CFCs and there are many fridges in use or to be disposed of. HFCs (hydrofluorocarbons) can be used as a replacement. They have no Cl so there is no ozone depletion potential.

93
Q

Why does the C-Cl bond break but not the C-F?

A

C-F has a higher bond enthalpy so it is not broken by UV light. C-Cl is a weaker bond so it can be broken by UV light.

94
Q

What is the purpose of zeolites?

A

Using a zeolite catalyst lowers the temperature and pressure needed for cracking to occur. This lowers costs and speeds up the process.

Chemistry (38 decks)

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