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Chemistry > The Ozone Story > Flashcards

The Ozone Story Flashcards

1
Q

What is electronegativity ?

A

The ability of an atom to attract the bonding electrons in a covalent bond.

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2
Q

How is electronegativity measured ?

A

The Pauling scale

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3
Q

What happens to electronegativity as you go across the periods ?

A

They increase

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4
Q

What happens to electronegativity as you go down the groups ?

A

They decrease

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5
Q

What is the most electronegative ion ?

A

Flourine

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6
Q

How can covalent bonds be polarised ?

A

By differences in electronegativity

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7
Q

How is a covalent bond non-polar ?

A

The bonding electrons sit in orbital between two nuclei. If the atoms have similar or identical electronegativities, the electrons sit roughly midway between the two nuclei and the bond will be non-polar.

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8
Q

Why are covalent bonds in diatomic gases non-polar ?

A

They have equal electronegativities, o the bond is non-polar.

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9
Q

How is a covalent bond polar ?

A

Atoms with different electronegativities. The bonding electrons will be pulled towards the more electronegative atom. This causes the electrons to be spread unevenly, and so their will be a charge across the bond (each atom has a partial charge - one atom is slightly negative and the other slightly positive). The bond is said to be polar.

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10
Q

What is caused by a difference in electronegativities in a polar bond ?

A

A dipole

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11
Q

What is a dipole ?

A

A difference in charge between the two atoms caused by a shift in electron density.

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12
Q

What causes a dipole ?

A

In a polar bond, the difference in electronegativity between the two atoms

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13
Q

Predict whether a C-Cl bond will be polar, given that the Pauling electronegativity values of carbon and chlorine are:
C = 2.6
Cl = 3.2

A

The difference between the electronegativities values of carbon and chlorine is 3.2 - 2.6 = 0.6

So the bond will be polar. The chlorine atom will be negative, and the carbon atom positive.

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14
Q

What does the polarity of a bond
depend on ?

A

The shape of the molecule
The polarity of the bonds

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15
Q

If the polar bonds are arranged so they point in the opposite direction, what does this mean ?

A

They cancel each other out, so it is non-polar overall

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16
Q

What happens if the bonds all point in roughly the same direction ?

A

The molecule will be polar

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17
Q

What is the scale electronegativity is measured in ?

A

The Pauling scale

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18
Q

What is the most electronegative element ?

A

Fluorine

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19
Q

What is a dipole ?

A

A difference in charge between the two atoms caused by a shift in electron density.

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20
Q

Why isn’t CO2 a polar molecule even though it has polar bonds ?

A

The bonds are arranged in a linear shape, so the bonds point in opposite directions so they cancel each other out.

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21
Q

MAny covalent molecules have a permanent dipole, due to differences in electronegativities.

a) Define the term electronegativity ?

b) What are the trends in electronegativity as you go across a period and down a group in the periodic table ?

c) Which of the following molecules are polar:
- H2O
- Br2
- CCl4
- SF6

A

a) The ability of an atom to attract the bonding electrons in a covalent bond

b) Electronegativity increases as you go across a period, and decreases as you go down a group.

c) H2O

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22
Q

What are intermolecular bonds ?

A

Forces between molecules

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23
Q

What are three types of intermolecular bonds ?

A
  • Instantaneous dipole - induced dipole
  • Permanent dipole - permanent dipole
  • Hydrogen bonding
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24
Q

What type of intermolecular bonds are present in all molecules ?

A

Instantaneous dipole - induced dipole

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25
Q

How are instantaneous dipole - induced dipole made ?

A
  • Electrons in charge clouds are always moving really quickly. At any particular moment, the electrons in an atom are likely to be more attracted to one side than the other. At this moment, the atom would have an instantaneous dipole.
  • This dipole can induce another instantaneous dipole in the opposite direction on a neighbouring atom. The two dipoles are then attracted to each other.
  • The second dipole can induce yet another dipole in a third atom.
  • Because the electrons are constantly moving, the dipoles are being created and destroyed all the time. Even though the dipoles keep changing, the overall effect is for the atoms to be attracted to each other.
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26
Q

What affects the strength of the intermolecular bonds ?

A

The shape of the molecule

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27
Q

Explain how the shape of the alkanes affect the strength of intermolecular bonds ?

A
  • Alkanes have covalent bonds inside the molecules. Between the molecules there are instantaneous dipole - induced dipole bonds which holds them all together.
  • The longer the carbon chain, th stronger the instantaneous dipole - induced dipole bonds > because there’s moe molecular surface contact and more electrons to interact.
  • So as the molecules get longer, it gets harder to separate them because it takes more energy to overcome the instantaneous dipole - induced dipole bonds.
  • Branched-chain alkanes can’t pack closely together and their molecular surface contact is small compared to straight chain alkanes of similar molecular mass. So fewer instantaneous dipole - induced dipole bonds can form/
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28
Q

The heavier the molecule or atom, the stronger or weaker the bond ?

A

The stronger the bond

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29
Q

What factors make Instantaneous dipole - induced dipole bonds stronger ?

A
  • Larger molecules have larger electron clouds, so stringer instantaneous dipole - induced dipole bonds.
  • Molecules that have a larger surface area have stronger instantaneous dipole - induced dipole bonds because they have a bigger exposed electron cloud
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30
Q

What is needed for a liquid to boil ?

A

The intermolecular bonds need to be overcome. More energy is needed to overcome the stronger intermolecular bonds, so liquids with stronger id - id bonds will have higher boiling points.

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31
Q

What is the trend as you go down group 7 ? and Why ?

A

As you go down the group, the id - id bonds and boiling point increases. This is because the Mr increases, the no. of shells of electrons increase, and so the molecular size increases.

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32
Q

What are permanent dipole - permanent dipole bonds ?

A

The delta + and delta - on polar molecules that cause weak electrostatic forces of attraction between molecules.

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33
Q

Experimentally, how do you show polar molecules ?

A

If you put an electrostatically charged rod next to a jet of polar liquid, like water, the liquid will move towards the rod. Polar molecules contain permanent dipole - permanent dipole bonds. The molecules go to the oppositely charged end and therefore attracted to the rod.

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34
Q

What is the strongest type intermolecular bond ?

A

Hydrogen bonding (when there are loads together )

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35
Q

Explain what instantaneous dipole - induced dipole bonds are ?

A

When electrons in an atom are likely to move to one side of the atom than the other.

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36
Q

Explain what gives rise to permanent dipole - permanent dipole intermolecular bonds ?

A

The delta + and delta - on the molecule

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37
Q

What intermolecular bonds are present in methane ?

A Instantaneous dipole - induced dipole
B Permanent dipole - permanent dipole
C Hydrogen bonding

A

A Instantaneous dipole - induced dipole

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38
Q

The molecules in the table below all have the molecular formula C5H12.

Pentane >36.1oC
2 - methylbutane > 27.7oC
2,2 - dimethylbutane > 9.50oC

Explain the differences in the boiling point of these molecules ?

A
  • The boiling point of a substance depends on the energy needed to overcome the intermolecular forces between the molecules.
  • Pentane is the most linear molecule so it has a greatest surface area contact, and so has the strongest instantaneous dipole - induced dipole bonds. This gives it the highest boiling point.
  • The surface contact of 2-methylbutane is less then that of pentane and that of
    2,2-dimethylbutane is smaller still, meaning that these substances have weaker instantaneous dipole - induced dipole bonds and therefore lower boiling points.
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39
Q

When does hydrogen bonding occur ? and Why ?

A

When hydrogen is covalently bonded to flourine, nitrogen or oxygen. Fluorine, nitrogen and oxygen is very electronegative, so they draw the bonding electrons away from the hydrogen atom. The bond is polarised and so polarised, and hydrogen has such a high charge density because it is so small, that the hydrogen atoms form weak bonds with lone pairs of electrons on the fluorine, nitrogen and oxygen atoms f other molecules.

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40
Q

Give examples of hydrogen bonding ?

A

Ammonia
Water

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41
Q

Why is ice less dense then water ?

A
  • In ice, the water molecules are arranged so that there is the maximum number of hydrogen bonds - the lattice structure formed in this way ‘wastes’ space.
  • As the ice melts, some of the hydrogen bonds are broken and the lattice structure breaks down - allowing molecules to fill the space.
  • This effect means ice if less dense than water which is why ice floats.
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42
Q

Why does hydrogen bonds affect how a substance behaves ?

A
  • Hydrogen bonds are the strongest type of intermolecular bonds and have a huge affect on the properties of substances.
  • Substances that form hydrogen bonds have high melting and boiling points because lots of energy is required to overcome the intermolecular bonds.
  • Hydrides of nitrates, oxygen and fluoride generally have the highest boiling points if you compare them with other hydrides in their group, because of the extra energy needed to break the hydrogen bonds.
  • Substances that form in hydrogen bonds are also soluble in water. This is because they can form hydrogen bonds with the molecules, allowing them to mix and dissolve.
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43
Q

Why can compounds that have hydrogen bonding, have other types of intermolecular forces that can affect there properties ?

A
  • Compounds that have hydrogen bonding can have other intermolecular bonds that affect their properties.
  • E.g. butan-1-ol and butan-2-ol have the same molecular formula and both form one hydrogen bond per molecule. Butan-1-ol has a boiling point of 117oC, but the boiling point of butan-2-ol if only 99oC. This difference id due to the strength of the instantaneous dipole - induced dipole bonds between the molecules.
  • Butan-1-ol is less branched then butan-2-ol, so the surface contact is greater. The molecules can pack closer together, allowing it to for stronger instantaneous dipole - induced dipole bonds. So butan-1-ol has a higher boiling point.
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44
Q

What happens to the surrounding temperature when a liquid evaporates ?

A

They take in heat (exothermic process) and so the temperature around them decreases. The more easily a substance evaporates, the faster its rate of reaction will be, and so the surrounding temperature will also decrease at a faster rate.

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45
Q

What atoms need to be present for hydrogen bonding to occur ?

A

Fluorine
Nitrogen
Oxygen

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46
Q

Name 2 substances that undergo hydrogen bonding ?

A

Ammonia
Water

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47
Q

Why is ice less dense than water ?

A

In ice water molecules are arranged so that there is a maximum number of hydrogen bonds - the lattice structure formed in this way ‘wastes’ lots of space.
As the ice melts, some of the hydrogen bonds are broken an the lattice breaks down - allowing molecules to ‘fill’ the space.
This effect means ice is much less dense than water - which is why ice floats.

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48
Q

Which of the Group 7 hydrides has the highest boiling point ?

A

Fluorine

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49
Q

For each of the following pairs of compounds, state which will have the higher boiling point.
i) Ammonia and methane
ii) Water and hydrogen sulfide
iii) Butane and propan-1-ol

Explain your choices ?

A

i) Ammonia will have the higher boiling point
ii) Water will have a higher boiling point
iii) Propan-1-ol will have the higher boiling point

The molecules of these substances can form hydrogen bonds. These are stronger and take more energy to overcome, in comparison to the intermolecular bonds in the other molecules.

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50
Q

An organic compound used as antifreeze is ethane-1,2-diol. The boiling point of
ethane-1,2-diol is 197oC, whereas the boiling point of ethanol is 78oC. Suggest a reason for this ?

A

Ethane-1,2-diol has stronger intermolecular bonds because there are two alcohol groups, twice as many as in ethanol. Therefore ethane-1,2-diol can form twice as many hydrogen bonds as ethanol.

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51
Q

What is the collision theory ?

A
  • Particles must collide in the right direction, They need to be facing each other the right way.
  • Particles must collide with at least a certain minimum amount of kinetic energy.
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52
Q

What is the activation enthalpy/energy ?

A

The minimum amount of energy needed to react, the particles need this much energy to break the bonds to start the reaction.

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53
Q

How do you increase the reaction, if the substance have a high activation energy ?

A

Heat

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54
Q

What happens if you increase the temperature ?

A

If you increase the temperature, the particles will on average have more kinetic energy and more faster. So, a greater proportion of the molecules will have the activation energy and be able to react. This changes the shape of the Boltzmann Distribution Curve - it pushes it to the right. A higher number of molecules with the activation energy means that the frequency of collisions that result in a reaction will also increase - there will be more successful collisions per unit time. So a small temperature increase can lead to a large increase in reaction rate.

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55
Q

Explain the term ‘activation enthalpy’ ?

A

The minimum amount of energy needed to react, the particles need this much energy to break the bonds to start the reaction.

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56
Q

Nitrogen monoxide and ozone sometimes react to produce nitrogen dioxide and oxygen. A collision between the two molecules does not always lead to a reaction. Explain why ?

A

The molecules don’t always have enough energy. Collisions don’t always happen in the right orientation.

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57
Q

Explain why an increase in temperature results in an increase in the rate of reaction ?

A

At higher temperatures, more particles will have the activation energy and will be able to react. So collisions that result in a reaction/ have the activation energy will happen more frequently.

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58
Q

What happens when you increase the concentration of a reaction ?

A

Increasing the concentration of the reactants in a solution means the particles are closer together on average. If they’re closer, they’ll collide more often. More collisions mean more chances to react.

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59
Q

What happens when you increase the pressure of a reaction ?

A

Increasing the pressure, means that the particles get closer together on average, they collide more often, and so have more chances to react.

60
Q

How can catalysts speed up the rate of reaction ?

A

Catalysts lower the activation enthalpy by providing an alternative pathway for the bonds to be broken and remade. If the activation enthalpy is lower, more particles will have enough energy to react. The catalyst remains unchanged at the end of the reaction.

61
Q

How can increasing the temperature speed up the rate of reaction ?

A

Increasing the temperature gives the particles more kinetic energy, so they they’re more likely to react when they collide. They are moving faster, they collide more frequently.

62
Q

How do you measure the rate of reaction ?

A

Monitor he loss of a reactant or the formation of a product at regular intervals throughout a reaction.

63
Q

What methods can you use to measure the rate of reaction ?

A

Measure the volume of gas produced
Measure the loss of mass as a gas is produced
Measure the change in pH during a reaction
Measure the temperature change
Taking samples at regular intervals and analysing them by titrations.

64
Q

What are homogeneous catalysts ?

A

When the catalyst is in the same state as the reactants.

65
Q

How do homogeneous catalysts work ?

A

A homogeneous catalyst works by forming one or more intermediate compounds with the reactants. The products are then formed by the intermediate compounds.
The activation energy needed to form the intermediates is lower than that needed to make the products directly from the reactants.
If a reaction is sped up by a homogeneous catalyst, its enthalpy profiles will have two humps in it.
The catalyst is reformed again and carries on catalysing the reaction.
The Boltzmann distribution of the catalysed reaction is the same as for the uncatalyzed reaction, but by lowering the activation enthalpy, more particles in the catalysed reaction are able to react.

66
Q

Name 4 factors that affect the rate of reaction ?

A

Concentration
pressure
Catalysts
Temperature

67
Q

Why does decreasing the concentration decrease the rate of reaction ?

A

Decreasing the concentration of reactants in a solution means the particles are further apart on average. If they are further apart, they’ll collide less. Less collisions means less chances to react.

68
Q

Name two methods for measuring reaction rates ?

A

Measuring the volume of gas
Measuring the loss of mass as a gas is produced

69
Q

Enzymes are homogeneous catalysts. Explain what a homogeneous catalyst is ?

A

A catalyst that is in the same state as the reactants

70
Q

Why are there two humps in he enthalpy profile of a homogeneously catalysed reaction ?

A

An intermediate is formed.

71
Q

Explain how homogeneous catalysts speed up chemical ?

A

The homogeneous catalysts form intermediate compounds. The activation enthalpy needed to form the intermediates is lower than that needed to make the products directly from the reactants.

72
Q

Do the boiling points of haloalkanes increase or decrease down the group ?

A

Increase down the group[

73
Q

Why do the boiling points increase down the group ?

A
  • The boiling points of the haloalkanes depend on the strength of the intermolecular bonds - the stronger the bond between the molecules, the higher the boiling point.
  • As you go down group 7 from fluorine to iodine, the atomic radium of the halogen atoms, and the number of the electron shells that they have, increases.
  • This leads to stronger instantaneous dipole - induced dipole forces between molecules - so that you put in more energy to overcome them.
  • So the boiling point of the haloalkanes increases down the group.
74
Q

Is the carbon-halogen bond polar or non-polar ?

A

Polar

75
Q

Why is the carbon-halogen bond ?

A
  • Fluorine, chlorine and bromine are much more electronegative then carbon. So, these carbon-halogen bonds are polar
  • The electronegative halogen pulls electron density away from the carbon, so the carbon is electron deficient. This means it can be attacked by a nucleophile.
  • A nucleophile an electron-pair donor. It donates an electron pair to somewhere without enough electrons.
  • OH-, NH3 and H2O are all nucleophiles that can react with haloalkanes.
76
Q

What is a substitution reaction ?

A

A functional group in a compound is replaced by another functional group.

77
Q

What is a nucleophilic substitution ?

A

When a nucleophile attacks an delta positive carbon and replaces the delta negative atom or group.

78
Q

How do haloalkanes react with hydroxide ions ?

A
  • A nucleophilic reaction
  • Use warm aqueous sodium hydroxide and do the reaction under reflux.
  • General formula
    R-X + NaOH –> ROH + NaX
79
Q

Can water act as a nucleophile ?

A

Yes

80
Q

Haloalkanes react with ammonia to form … ?

A

Amines

81
Q

how do haloalkanes react with ammonia to form amines ?

A
  • Amines are organic compounds. They’re based on ammonia, but one or more of the hydrogen atoms are replaced by alkyl groups
  • If you warm a haloalkane with excess ethanolic ammonia, the ammonia swaps places with the halogen - it is a nucleophilic substitution.
82
Q

Which is the most reactive haloalkane ?

A

Iodoalkane

83
Q

Why is C - I the most reactive ?

A

Most bonds between the carbon atoms and halogen atoms are polarised. The C - F bond is the most polar whilst the C - I bond isn’t polar at all. You might expect that the more polar the bond is, the more likely it is to break, but this is not the case. Experimental evidence shows that iodoalkanes are the most reactive of the first four haloalkanes - so reactivity can’t be due to bond polarisation.

84
Q

What is the experiment to show haloalkane reactions ?

A
  • Put chloroalkane, a bromoalkane and a iodoalkane in 3 different test tubes.
  • To each of these add silver nitrate and soe ethanol.
  • The silver halide compound is insoluble, so forma a precipitate
  • The precipitate forms faster with the iodoalkane - so that must be the most reactive. Bromoalkanes react slower that iodoalkanes, and chloroalkanes the slowest.
85
Q

What decides the reactivity ?

A

The C-F bond is the strongest - it has the highest bond enthalpy. For a reaction to occur the carbon-halogen bond needs to break. The stronger the bond is, the slower the reaction will be.

86
Q

What is a haloalkane ?

A

A alkane with at least one halogen atom

87
Q

State one factor that influences the boiling points of haloalkanes ?

A

The strength of intermolecular forces

88
Q

Why are most carbon-halogen bonds polar ?

A

Fluorine, chlorine and bromine are much more electronegative than carbon. So, the carbon-halogen bonds are polar.

89
Q

Give two examples of nucleophiles that can react with haloalkanes to form alcohols ?

A

Water
Hydroxide ions

90
Q

What types of organic compound would you produce if you reacted ammonia with a haloalkane in ethanol ?

A

Amine

91
Q

2-bromopropane reacts with water in the following reactions:
CH3CHBrCH3 + H2O –> CH3CH(OH)CH3 + HBr

a) i) Name this type of reaction
ii) Give the systematic name for the organic product

b) Under the same conditions, 2-iodopropane was used in place of 2-bromopropane in the reaction above. What difference would you expect in the rate of reaction ? Explain your answer.

A

a) i) nucleophilic substitution
ii) propan-2-ol

b) The reaction would be faster with iodopropane. This is because the C-I bond is weaker than the C-Br bond.

92
Q

What are the two types of bond fission ?

A

Homolytic
Heterolytic

93
Q

What is homolytic bond fission ?

A

When two electrically uncharged ‘radicals’ are formed. Radicals are particles that have an unpaired electron

94
Q

What is heterolytic bond fission ?

A

When two different substances are formed - a positively charged cation (X+) and a negatively charged anion (Y-).

95
Q

What are the three stages in chain reactions ?

A

Initiation
Propagation
Termination

96
Q

What is an initiation stage ?

A

Free radicals are produced

97
Q

What is a propagation reaction ?

A

Free radicals react with molecules and produce new radicals. These go on to react with more molecules, producing even more radicals.

98
Q

What is a termination reaction ?

A

Two radicals react together to form a stable molecule.

99
Q

What is the reaction called when a halogen is reacted with an alkane ?

A

A photochemical reaction

100
Q

What are photochemical reactions started by ?

A

UV light (ultraviolet)

101
Q

How do oxygen radicals form ?

A

With O2 and ozone

102
Q

Where is the ozone layer ?

A

In the stratosphere

103
Q

How is ozone formed ?

A

When UV radiation from the sun hits oxygen molecules

104
Q

Is the ozone layer being replaced ?

A

Yes

105
Q

What happens when the right amount of UV radiation is absorbed by an oxygen molecule ?

A

The oxygen molecule splits into separate atoms or free radicals. The free radicals then combine with the other oxygen molecules to form ozone molecules > O3.

O2 + hv –> O + O
O2 + O –> O3

106
Q

Can UV radiation also reverse the formation of ozone ?

A

Yes
O3 + hv –> O2 + O

107
Q

How is the ozone layer replaced ?

A

The ozone layer is continuously being replaced as UV radiation hits the molecules. As an equilibrium is set up, so the concentrations stay fairly constant.

O2 + O <–> O3

108
Q

What does the ozone protect us from ?

A

UV radiation

109
Q

What is the impact of UV radiation ?

A
  • The Ozone layer is important - it protects us from the most harmful effects of the suns UV radiation.
  • When ozone breaks down, it absorbs high energy UV radiation, so the ozone layer removes all the high energy UVC radiation and about 90% of the UVB.
  • These types of radiation is harmful to humans and most other life on earth.
  • UVB can damage the DNA in cells and cause skin cancer, and is the main cause of sunburn. UVA an also lead to skin cancer. Both types ca cause the skin to age faster.
  • BUT .. UV radiation is not all bad. It is essential for humans. it produced vitamin D. When the is exposed to UV, it tans. This helps protect deeper tissues from the effect of radiation.
110
Q

What are the 3 different frequencies of UV radiation ? (in increasing frequency and radiation)

A

UVA
UVB
UVC

111
Q

Where does ozone also occur ?

A

In the troposphere

112
Q

Why is ozone also in the troposphere ?

A

The effect of sunlight on mixtures of nitrogen dioxide and hydrocarbons. These occur naturally from a variety of sources but vehicle engines and power stations contribute to large amounts.

113
Q

How is photochemical smog produced ?

What does it cause ?

A

In industrialised areas and cities with lots of cars, the ozone mixes with solid particles of carbon and many other substance to create an air pollutant > Mexico City is particularly bad.

Photochemical smog can cause respiratory problems. It can be dangerous for animals and cause damage to plants and materials too. Ozone can be toxic to humans.

114
Q

What are CFC’s ?

A

Chlorofluorocarbons are haloalkanes that have all their hydrogen atoms replaced by chlorine and fluorine atoms.

115
Q

How are Cl radicals formed in the stratosphere, from CFC’s ?

A

C-Cl bonds can be broken down, they are broken down by high energy UV radiation in the stratosphere to form chlorine radicals - these act as catalysts in the breakdown of ozone.

116
Q

Why do less CFC’s get broken down when you go further down the troposphere ?

A

Further down the troposphere only a few CFC’s get broken down by UV radiation because most of the high frequency UV has been absorbed by the ozone layer. The ozone can still be broken down in the troposphere,, it’s much less likely to happen because there are less suitable catalysts.

117
Q

What is formed when UV radiation breaks the carbon and halogen bonds ?

A

The carbon-halogen bond splits homolytically and produces two free radicals.

118
Q

What is happening with the breakdown of ozone by CFC’s by homogeneous catalysts ?

A
  • Chlorine radicals, Cl., are formed when CFC’s are broken down by high energy UV radiation in the stratosphere.
  • These free radicals are catalysts. They reacts with ozone to form an intermediate (ClO.) and an oxygen molecule.

CCl3F + hv –> .CCl2F + Cl. Initiation
Cl. + O3 –> O2 + ClO. Propagation
ClO. + O3 –> 2O2 + Cl. Propagation
Cl. + Cl. –> Cl2 Termanation

119
Q

What are the radicals that can destroy ozone ?

A

NO from nitrogen oxides
Other halogen free radicals from haloalkanes

120
Q

What is bond fission ? Describe two types of bond fission.

A

Bond fission is breaking a covalent bond

The two types of bond fission is heterolytic and homolytic

Heterolytic bond fission is when two different substances are formed, a positively charged cation and a negatively charged anion.
X-Y –> X+ + Y-

Homolytic bond fission is when two radicals are formed.
X-Y –> X. + Y.

121
Q

Write an equation for the intuition step of the photochemical reaction between chlorine and methane ?

A

Cl-Cl –> Cl. + Cl.

122
Q

What are the benefits to humans of the ozone layer ?

A

It protects humans from the most harmful effects of the sun’s ultraviolet radiation.

123
Q

How is ozone formed in the troposphere ?

A

Sunlight on mixtures of nitrogen dioxide and hydrocarbons.

124
Q

When irradiated with UV light, methane gas will react with bromine to form a mixture of several organic compounds.

a) Write an overall equation to show the formation of bromomethane from methane and bromine

b) Write down the two equations in the propagation step for the formation of CH3Br.

c) i. Explain why a tiny amount of ethane is found in the product mixture.
ii. Write the equation for the formation of ethane in this reaction.

A

a) CH4 + Br2 –> CH3Br + HBr

b) Br. + CH4 –> HBr + .CH3
.CH3 + Br2 –> CH3Br + Br.

c) i. Two methyl radicals join together to produce an ethane molecule
ii. .CH3 + .CH3 –> CH3CH3

125
Q

The ‘ozone layer’ lies mostly between 15 and 30 km above the Earths surface.

a) Explain how ozone forms in this part of the atmosphere

b) How does the ozone layer absorb harmful radiation without being permanently destroyed ?

c) Write equations to show how the CFC CCl4 could catalyse the destruction of ozone in the stratosphere ?

A

a) Ozone is formed by the effect of UV radiation from the sun on oxygen molecules. The oxygen molecules split to form oxygen free radicals, which react with more oxygen molecules to form ozone.

b) The ozone molecules absorb harmful UV radiation breaking down to form an oxygen molecule and a free oxygen radical. The radical produced forms more ozone with an O2 molecule so ozone is continually destroyed and remade.

c) CCl4 + hv –> .CCl3 + Cl.
Cl. + O3 –> ClO. + O2
ClO. + O3 –>Cl. + O2
2O3 –> 3O2

126
Q

a) Explain why large amounts of ground-level ozone can be a problem

b) Why are these problems most frequently encountered in heavily industrialised areas ?

A

a) It can cause photochemical smog, which can cause respiratory problems

b) Ground-level ozone forms as a result of sunlight ating on mixtures of nitrogen dioxide and hydrocarbons. which are emitted by power stations.

127
Q

How can gases be measured ?

A

By their percentage concentration

128
Q

How are small gases measured ?

A

Parts per million

129
Q

The percentage concentration of xenon in the atmosphere is 0.000,009%. What is its concentration in parts per million ?

A

0.000,009% means there are 0.000,009 parts of xenon per 100 parts of air.
So in one part of air there will be 0.000,009 / 100 = 0.000,000,09 parts of air.
In a million parts of air there are 0.000,000,09 x 1,000,000 = 0.09 parts of air
So, xenon has a concentration of 0.09 ppm.

130
Q

Does the earths surface absorb radiation ?

A

Yes

131
Q

Why does the sun give out electromagnetic radiation ?

A

Due to the nuclear processes going on in its core

132
Q

What electromagnetic radiation does the sun give out ?

A

Visible radiation
Infrared radiation
Ultraviolet radiation

133
Q

Give the order of the electromagnetic radiation, in increasing frequency and energy ?

A

Radio Waves microwaves
Infrared radiation
Visible light
Ultraviolet radiation
X-trays radiation
Gamma rays

134
Q

What radiation does the earths atmosphere mostly absorb ?

A

Infrared radiation
Ultraviolet radiation

135
Q

How is the earths surface warmed ?

A

The earths surface absorbs radiation from the sun and is warmed. It then re-emits radiation, mostly infrared. The earth emits much lower radiation than the sun, because it is much cooler.

136
Q

How does UV and visible light radiation give electrons more energy ?

A

The electrons in molecules have fixed energy levels that they can jump between. These are called quantised energy levels.
When ultraviolet radiation or visible light hit a molecule of gas the electrons can absorb the energy and jump up to their next energy level. Because the energy needed for these changes is quantised too, only specific frequencies are absorbed. If enough energy is absorbed bonds breakak, forming free radicals.

137
Q

What is the equation for energy ?

A

Energy = Planks constant x Frequency
( J ) ( 6.63 x 10^-34 ) ( Hz )
E = hv

138
Q

What is the energy supplied to a molecule by ultraviolet radiation of frequency 1.2 x 10^15 Hz ?

A

E = hv
E = (6.63 x 10^-34) x (1.2 x 10^15)
E = 7.96 x 10^-19 J

139
Q

What is the equation for frequency ?

A

Frequency = Speed of light / Wavelength
( Hz ) ( 3.00 x 10^8 ) ( m )

140
Q

What is the equation, when you combine the energy equation and the frequency equation ?

A

Energy = (Planks constant x Speed of light) / wavelength
( J ) ( 6.63 x 10^-34 ) ( 3.00 x 10^8 ) ( m )

141
Q

What is the energy supplied to a molecule by ultraviolet radiation with wavelength 1.95 x 10^-7 ?

A

Energy = (Planks constant x Speed of light) / wavelength
Energy = ((6.63 x 10^-34) x (3.00 x 10^8)) / 1.95 x 10^-7
Energy = 1.02 x 10^-18 J

142
Q

When irradiated with UV light of wavelength 242 nm, oxygen gas decomposes. Calculate the enthalpy of the O=O bonds broken at this wavelength.

A

E = (planks constant x speed of light) / wavelength

E = ((6.63 x 10^-34) x (3.00 x 10^8)) / 242 x 10^-9
E = 8.219 x 10^-19 J

(8.219 x 10^-19) x (6.02 x 10^23) = 494, 783.8 J/mol^1
494, 783.8 /1000 / 495 KJ/mol^1

143
Q

What types of the electromagnetic radiation does the sun emit ?

A

Visible light
Infrared radiation
Ultraviolet radiation

144
Q

What is the equation that relates frequency to wavelength ?

A

Frequency = Speed of light / Wavelength

145
Q

Ozone is a molecule that absorbs ultraviolet light with a wavelength of 255 nm.

a) i. What frequency of ultraviolet radiation does ozone absorb ?

ii. What is the energy of the ultraviolet light that ozone absorbs ?

b) What is the concentration of ozone in ppm if its percentage composition by volume is 0.000, 021% ?

A

a) i. Frequency = Speed of light / Wavelength
Wavelength = 225 nm –> 225 x 10^-9 m
Frequency = (3.00 x 10^8) x (225 x 10^-9)
Frequency = 1.18 x 10^15 Hz

ii. Energy = ((speed of light) x (Planks constant)) / Wavelength
Energy = ((3.00 x10^8) x (6.23 x 10^-34)) / 225 x 10^-9
Energy = 7.80 x 10^-19 J

b) 0.000,021 in 100 units of air
0.000,000,021 in 1 unit of air
0.21 in 1,000,000 units of air.

146
Q

Calculate the energy absorbed when one molecule of H2O changes from its ground electronic level to the next level, given that the frequency of radiation absorbed is
1.80 x 10^15 Hz ?

A

Energy = Planks constant x Frequency
Energy = (6.63 x 10^-34) x (1.80 x 10^15)
Energy = 1.19 x 10^-18 J

Chemistry (8 decks)

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