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CHEMISTRY AS > * Ozone story > Flashcards

* Ozone story Flashcards

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1
Q

What is electronegativity

A

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

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2
Q

Describe the trends of electronegativity

A
  • electronegativity is measured using the Pauling scale
  • the higher the electronegativity value, to more electronegative the element
  • fluorine = most electronegative
  • oxygen, chlorine and nitrogen = also very electronegative
  • electronegativity increases across periods and decreases down groups (ignoring noble gases)
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3
Q

What is the electronegativity of hydrogen

A
  • 2.2
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4
Q

What is the electronegativity of carbon

A
  • 2.6
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5
Q

What is the electronegativity of nitrogen

A
  • 3.0
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6
Q

What is the electronegativity of chlorine

A
  • 3.2
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7
Q

What is the electronegativity of oxygen

A
  • 3.4
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8
Q

What is the electronegativity of fluorine

A
  • 4.0
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9
Q

Are covalent bonds of diatomic gases polar

A
  • covalent bonds in homonuclear, diatomic gases (H2,Cl2) are non-polar because the atoms have equal electronegativities, so the electrons are equally attracted to both nuclei
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10
Q

How can you tell if a covalent bond is polarised

A
  • in a covalent bond, the bonding electrons sit in orbitals between two nuclei, if both atoms have similar or identical electronegativities, the electrons will sit roughly midway between the two nuclei and the bond will be non-polar
  • some elements like carbon and hydrogen have pretty similar electronegativities, so bonds between them are non-polar
  • if the bond is between two atoms with different electronegativities, the bonding electron will be pulled to towards the more electronegative atom. This causes the electrons to be spread unevenly, and so there will be a charge across the bond (each atom has a partial charge - one atom is slightly positive and the other is slightly negative) the bond is said to be polar
  • in a polar bond the difference in electronegativity between two atoms causes a dipole
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11
Q

What is a dipole

A
  • difference in charge between two atoms caused by a shift in electron density in the bond
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12
Q

How do you determine whether a molecule is polar?

A
  • Whether a molecule is polar or not depends on its shape and the polarity of its bonds
  • A polar molecule has an overall dipole, which is just a dipole caused by the presence of a permanent charge across the molecule
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13
Q

is hydrogen chloride polar

A
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14
Q

is carbon dioxide polar

A
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15
Q

Is CHCl3 or CHF3 polar?

A
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16
Q

What are intermolecular bonds?

A
  • intermolecular bonds are forces between molecules
  • they are much weaker than covalent, ionic or metallic bonds
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17
Q

What are the types of intermolecular bonding?

A
  • instantaneous dipole-induced dipole
  • permanent dipole-permanent dipole
  • hydrogen bonding
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18
Q

What forms instantaenous dipole-induced dipole bonds?

A
  • instantaneous dipole-induced dipole bonds cause all atoms and molecules to be attracted to each other
  • electrons in charge clouds are always moving really quickly. at any particular moment, the electrons in an atom are likely to be more to one side that the other. at this moment the atom would have a temporary (or instantaneous) dipole
  • the dipole can induce another temporary dipole in the opposite direction on a neighbouring atom. The two dipoles are then attracted to each other.
  • The second dipole can induce another dipole in a third atom
  • because the electrons are constantly moving, the dipoles are being constantly created and destroyed
  • even though the dipoles keep changing, the overall effect is for the atoms to be attracted to each other
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19
Q

Describe what affects intermolecular bonds in organic molecules

A
  • the shape of an organic compounds molecules affect the strength of the intermolecular bonds
  • For example alkanes have covalent bonds inside the molecules. between the molecules there are instantaneous dipole-induced dipole bonds, which hold them all together
  • the longer the carbon chain, the stronger the instantaneous dipole-induced dipole bonds - because there’s more molecular surface contact and more electrons to interact
  • so as the molecule gets 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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20
Q

How does the mass of the atom or molecule affect the intermolecular bonds?

A
  • not all instantaneous dipole-induced dipole bonds are the same strength - larger molecules have larger electron clouds so stronger instantaneous dipole-induced dipole bonds. Molecules with greater surface areas also have stronger instantaneous dipole-induced dipole forces because they have a bigger exposed electron cloud
  • for a liquid to boil, the intermolecular bonds need to be overcome. You need more energy to overcome a stronger intermolecular bonds - so liquids with stronger instantaneous dipole-induced dipole bonds will have higher boiling points
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21
Q

Describe and explain the trend of boiling points with halogens with reference to intermolecular bonds

A
  • as you go down the group, the instantaneous dipole-induced dipole bonds (and their boiling points) increase. This is because as the Mr increases, the number of shells of electron increases, and so the atomic/molecular size increases
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22
Q

What molecules form permanent dipole-permanent dipole bonds?

A
  • The δ+ and δ- charges on polar molecules cause weak electrostatic forces of attraction between molecules
  • these are known as pemanent dipole-permanent dipole bonds
  • permanent dipoles happen as well as (not instead of) instantaneous dipole-induced dipole bonds
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23
Q

Describe an experiment that could be used to decide if a liquids molecules are polar

A
  • if you put an electrostatically charged rod next to a jet of a polar liquid, like water, the liquid will move towards the rod
  • its because polar liquids contain molecules with permanent dipoles
    • doesnt matter if rod is negatively or positively charged, the polar molecules in the liquid can turn around so the oppositely charged end is attracted to the rod
  • work out if theyre polar and if theyre likely to form permanent dipole-permanent dipole bonds
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24
Q

Which is the strongest type of intermolecular bonding

A
  • hydrogen bonding
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25
Q

when can a hydrogen bond be formed

A
  • hydrogen bonding only happens when hydrogen is covalently bonded to fluorine, nitrogen or oxygen
  • fluorine, nitrogen and oxygen are very electronegative, so they draw the bonding electrons away from the hydrogen atom
  • the bond is so polarised, and hydrogen has such a high charge density because its so small, that the hydrogen atoms form weak bonds with lone pairs of electrons on the fluorine, nitrogen, or oxygen atoms of other molecules
  • water and ammonia have hydrogen bonding
  • organic molecules that form hydrogen bonds often contain -OH or -NH groups (alcohols, amines)
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26
Q

how do hydrogen bonds explain why ice is less dense than 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 uses a lot of space
  • as the ice melts, some of the hydrogen bonds are broken and the lattice breaks down - allowing molecules to fill the spaces between
  • this effect means that ice is much less dense than water - which is why ice floats
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27
Q

how do hydrogen bonds affect how a substance behaves

A
  • hydrogen bonds are the strongest type of intermolecular bonds and have a huge effect on the properties of the substance
  • substances that form hydrogen bonds have high melting points because a lot of energy is needed to overcome the intermolecular bonds
  • hydrides of nitrogen, oxygen and fluorine have the highest boiling points compared to other hydrides in their groups, because of the extra energy needed to break the hydrogen bonds
  • substances that from hydrogen bonds are also soluble in water, this is because they can form hydrogen bonds with the water molecules allowing them to mix + dissolve
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28
Q

butan-1-ol and butan-2-ol have the same molecular formula but the boiling point of butan-1-ol is higher. explain why

A
  • because of their shape
  • due to strength of instantaneous dipole-induced dipole bonds between molecules
  • butan-1-ol is less branched than butan-2-ol, so the surface contact is greater. molecules can pack closer together, allowing it to form stronger instantaneous dipole-induced dipole bonds. So butan-1-ol has a higher boiling point
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29
Q

Describe an experiment on how you can tell the strength of different intermolecular bonds

A
  • when liquid evaporates they take in heat (its an endothermic process) and so the temperature around them decreases. The more easily a substance evaporates, the faster its rate of evaporation will be, and so the surrounding temperature will also decrease at a faster rate. You can use this temperature change to investigate how easily a liquid evaporates, and so what type of intermolecular bonds its likely to form
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30
Q

How do particles react?

A
  • particles in liquids and gases are always moving and colliding with each other
  • they don’t react every time though - only when the conditions are right
  • a reaction wont take place unless
    • they collide in the right direction, need to be facing each other the right way
    • the collide with at least a certain minimum amount of kinetic energy
    • this is called collision theory
31
Q

What is the activation enthalpy

A
  • minimum amount of kinetic energy particles need to collide
  • need this much energy to break the bonds to start the reaction
32
Q

Draw and label an enthalpy profile diagram

A
33
Q

Describe the distibution of energy in a gas

A
  • graph of numbers of molecules in a gas with different kinetic energies = boltzmann distribution
34
Q

how does increasing the temperature make a reaction go faster?

A
  • if you increase the temperature, the particles will on average have more kinetic energy and will move faster
  • so, a greater proportion of molecules will have the activation energy and be able to react, this changes the shape of the boltzmann distribution curve
  • 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.
35
Q

how do concentrations affect the rate of reaction

A
  • increasing concentration of reactants of a solution means the particles are closer together on average
  • if theyre closer, theyll collide more often, more collisions mean more chances to react
36
Q

how does pressure affect the rate of reaction

A
  • increasing pressure of a gas speeds up the reaction because particles are closer together on average, they collide more often, and so have more chances of successful collisions
37
Q

how does catalysts affect the rate of reaction

A
  • catalysts can speed up reactions by lowering the activation enthalpy by providing an alternative pathway for the bonds to be broken and remnade. If the activation enthalpy is lower, more particles will have enough energy to react
38
Q

how does temperature affect the rate of reaction (2)

A
  • increasing the temperature gives the particles more energy, so that theyre more likely to react when they collide
  • and because theyre moving faster, they collide more frequently
39
Q

how can you use experiments to find out the rate of a reaction

A
  • to measure the rate of a reaction you can either monitor the loss of a reactant or the formation of a product at regular intervals throughout the reaction.
  • many methods:
    • measuring volume of gas produced
    • measuring loss of mass as a gas is produced
    • measuring the change in pH during a reaction
    • measuring a temperature change
    • taking samples at regular intervals and analysing them by titration
  • data can be used to plot a graph
  • time on x-axis, change measure on y-axis
  • rate always fastest at the beginning of the reation - as the reaction continues, the concentration of reactants will decrease (as they form products) so there will be less frequent collisions between reactant particles and the rate will decrease
40
Q

What are the two types of catalysts

A
  • catalysts increase the rate of reaction. They do this by providing an alternative reaction pathway with a lower activation enthalpy. The catalyst is chemically unchanged at the end of the reaction
  • there are two types of catalyst - homegeneous and heterogeneous
41
Q

What is a homogeneous catalyst?

A
  • catalyst thats in the same state as the reactants
    • so if reactants are gases - catalyst is a gas
42
Q

How do homogeneous catalysts work?

A
  • homogeneous catalysts speed up reactions by forming one or more intermediate compounds with the reactants. The products are then formed from the intermediate compounds
  • the activation enthalpy needed to form the intermediates (and to form products from intermediates) is lower than to make products directly from reactants
  • if a reaction is sped up with a homogeneous catalyst, its enthalpy profile will have two humps
  • catalyst is reformed again and carries on catalysing the reaction
43
Q

What are haloalkanes

A
  • an alkane with at least one halogen atom in place of a hydrogen atom
44
Q

how do you name a haloalkane

A
45
Q

Describe the trend of boiling points of haloalkanes

A
  • the boiling points of the haloalkanes depend on the strength of their intermolecular bonds - the stronger the bonds between the molecules, the higher the boiling point
  • as you go down group 7 - atomic radius of the halogen atoms, and the number of electron shells that they have, increases
  • this leads to stronger instantaneous dipole-induced dipole forces between molecules - you have to put in more energy to overcome them
  • so boiling point of the haloalkanes increases down the group
46
Q

is the carbon-halogen bond polar

A
  • fluorine, chlorine and bromine are much more electronegative than 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 is 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
47
Q

What is a substitution reaction

A
  • when a functional group in a compound is replaced by another functional group. so in nucleophilic substitution, a nucleophile attact a δ+ carbon and replaces the δ- atom or group
48
Q

how do haloalkanes undergo nucleophilic substitution

A
  • haloalkanes react with hydroxide ions by nucleophilic substitution
  • you have to use warm aqueous sodium hydroxide and do the reaction under reflux
  • the general formula for the equation is:
    • R-X + NaOH –> ROH + NaX
49
Q

How does water act as a nucleophile

A
  • warming a haloalkane with water also results in a nucleophilic substitution reaction
50
Q

how do haloalkanes form amines

A
  • amines are organic compounds, they’re based on ammonia (NH3) 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 (nucleophilic substitution)
51
Q

which haloalkane is the most reactive?

A
  • iodoalkane
  • C-F bond = most polar
  • C-I not polar at all - but most reactive
  • reacting halolakanes with water in the presence of silver nitrate
    • to compare reactivities of haloalkanes, need to see which reacts fastest, put chloroalkane, bromoalkane and iodoalkane in three different test tubes. Add some silver nitrate solution to each (this contains the water) and some ethanol (as a solvent)
    • silver halide compound = insoluble - forms precipitate
    • Ag+(aq) + X-(aq) –> AgX(s)
    • iodoalkane forms fastest - so is most reactive
52
Q

explain how iodoalkanes are the most reactive but non-polar

A
  • carbon-halogen bond strength determines reactivity
  • despite being most polar, the C-F bond is the strongest, 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 overall reaction will be
53
Q

What is bond fission

A
  • breaking of a covalent bond
  • a single covalent bond is a shared pair of electrons between two atoms. it can break in two ways
54
Q

What are the two types of bond fission

A
55
Q

How do radicals react?

A
  • radicals take part in chain reactions
  • chain reaction happen if a product causes more reactions to take place
  • radical chain reactions have three main stages
56
Q

What are the three main stages of radical chain reactions

A
  • initiation reaction: free radicals are produced
  • propagation: free radicals react with molecules and produce new radicals, these go on the react with more molecules, producing more radicals, this is the chain part of the reaction
  • termination: two radicals react together to form a stable molecule
57
Q

How do halogen radicals form haloalkanes

A
  • halogens react with alkanes in photochemical reactions
  • photochemical reactions means started by light
  • a hydrogen atom is substituted (replaced) by a chlorine or bromine,
  • This is a radical chain reaction
    • chlorine and methane to form chloromethane
    • CH3 + Cl2 + uv –> CH3Cl + HCl
58
Q

Describe the initiation stage of forming a haloalkane from a halogen radical

A
  • sunlight provides enough energy to break the Cl-Cl bond - this is photodissociation
  • the bond splits equally and each atom gets one electron - homolytic fission
  • the molecule forms two highly reactive free radicals, Cl•
59
Q

Describe the propagation stage of forming a haloalkane with a halogen radical

A
  • Cl• attacks a methane molecule, forming a haloalkane and a methyl radical
  • The new methyl free radical, CH3• can attack another Cl2 molecule
  • The new Cl• can attack another methane molecule and so on, until all of the radicals are terminated
60
Q

Describe the termination stage of forming a haloalkane using a halogen radical

A
  • if two free radicals join together, they make a stable molecule
61
Q

How do oxygen radicals react with O2

A
  • the ozone layer is in a layer of the atmosphere called the stratosphere
  • it contains most of the atmospheres ozone molecules, O3
  • ozone is formed when UV radiation from the sun hits oxygen molecules
  • if the right amount of UV radiation is absorbed by an oxygen molecule, the oxygen molecule splits into separate atoms or free radicals. the free radicals then combine with other oxygen molecules to form ozone molecules
62
Q

How is the ozone layer constantly being replaced

A
  • UV radiation can also reverse the formation of ozone
  • so the ozone layer is continuously being destroyed and replaced as UV radiation hits the molecules. An equilibrium is set up, so the concentrations stay fairly constant
63
Q

How does the ozone layer protect against UV radiation

A
  • ozone layer protects us from the most of the harmful effects of the suns ultraviolet radiation
  • when ozone breaks down it absorbs high energy UV radiation
    • ozone removes all of UVC and 90% of UVB
  • high energy UV can damage the DNA in cells and causes skin cancer, and cataracts, age faster
64
Q

How does ozone occur at ground level

A
  • ozone (O3) also occurs in the troposphere - lowest part of the atmosphere
    • due to 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 large amounts
  • in industrial areas and cities with lots of cars, the ozone is mixed with solid particles of carbon and many other substances to create an air pollutant called photochemical smog
  • photochemical smog can cause respiratory problems, can be dangerous for anmials and plants
    • ozone itself is toxic to humans
65
Q

Describe what CFCs are

A
  • chlorofluorocarbons (CFCs) are haloalkanes that have all their hydrogen atoms replace by chlorine and fluorine atoms
  • the C-Cl bond can be broken down by high energy UV radiation in the stratosphere to form chlorine radicals - these act as catalysts in the breakdown of ozone
  • further down in the troposphere only a few CFCs get broken down by the UV because most of the high energy UV has been absorbed by the ozone layer.
66
Q

Which halogen bond is most likely to be broken by UV radiation

A
  • all haloalkanes contain bonds between carbn atoms and halogen atoms. Ultraviolet radiation can break these bonds - the carbon-halogen bond splits homolytically to create two free radicals
  • carbon-iodine most easily broken - because most reactive - due to lowest bond enthalpy
67
Q

how is the ozone layer being destroyed by catalysts

A
  • ozone being destroyed by CFCs
  • chlorine free radicals, Cl• are formed when CFCs are broken down by high energy UV radiation in the stratosphere
  • these free radicals are catalysts, they react with ozone to form an intermediate ClO•, and an oxygen molecule
  • reaction can only terminate if two radicals react together
  • Cl• is the catalyst
68
Q

What other free radicals can also destroy ozone?

A
  • other free radicals can destroy ozone too
  • NO from nitrogen oxides
    • NO2(g) + hv –> NO•(g) + O(g)
    • other halogen free radicals from haloalkanes
69
Q

how are gases measured by percentage composition?

A
  • the composition of the atmosphere is sometimes measured by its percentage concentration by volume of dry air (no water vapour)
  • nitrogen - 78%
  • oxygen - 21%
70
Q

How are gases measured by ppm?

A
  • the major gases in the atmosphere are normally given by percentages of the total volume
  • but some gases are present in such small amounts that its not convenient to write their quantities in percentages
  • ppm - parts per million
  • to switch from % to ppm
    • /100
    • x 1, 000, 000
  • atmosphere has 0.1 ppm carbon dioxide
  • 0.3 ppm nitrous oxide
71
Q

What radiation does the atmosphere, and earth absorbs

A
  • the sun gives out electromagnetic radiation
  • electromagnetic radiation is energy thats transmitted as waves, with a spectrum of different frequencies
  • sun mainly gives out visible radiation (light) and infrared radiation (heat)
  • earths atmosphere absorbs some of the suns infrared radiation and most of the ultraviolet radiation
  • the earths surface also absorbs radiation from the sun and is warmed
  • it then re-emits radiation, mostly as infrared
  • the earth emits much lower frequency than the Sun
72
Q

How does radiation contribute to electrons energy?

A
  • electron in molecules have fixed energy levels that can jump between - these are called quantised energy levels
  • when UV 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 break - forming free radicals
73
Q

What are the equations linking energy, wavelength, speed of light and plancks constant

A

CHEMISTRY AS (24 decks)

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