Ozone Story Flashcards
Define electronegativity.
Measures the attraction of a bonded atom for the pair of electrons in a covalent bond.
How is electronegativity measured?
Measured using the Pauling scale. It look at atoms within a molecule.
trend in electronegativity?
- increases across periods
- decreases down groups
(ignoring noble gases)
what are the most electronegative elements?
nitrogen, oxygen, fluorine, chlorine
electronegativity - polar covalent bonds
if the two atoms have different electronegativities the bonding e- will be pulled towards the more electronegative atom.
this causes the electrons to be spread unevenly across the bond, and so their will be a charge across the bond. (each atom has a partial charge)
in a polar bond the difference in electronegativity between the two atoms causes a dipole
- a dipole is a difference in charge between the two atoms caused by a shift in electron density in the bond
eg. HCl
the greater the difference in electronegativity….
….the more polar the bond
electronegativity - using the pauling scale
given values
find difference between the values
bonds are polar if difference is more than 0.4
what are the three types of intermolecular bonds?
- instantaneous dipole - induced dipole
- permanent dipole - permanent dipole
- hydrogen bonding
What is a permanent dipole?
A small charge difference across a bond, resulting from a difference in electronegativities of the bonded atoms.
Define an intermolecular force.
An attractive force between neighbouring molecules.
Describe intermolecular bonding: instantaneous dipole - induced dipole
all atoms form ID-ID bonds
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 than the other - INSTANTANEOUS DIPOLE
this dipole can INDUCE another temporary dipole in the opposite direction on a neighbouring atom
- a second dipole can induce a third
because electrons are constantly moving the dipole are being created and destroyed all the time
- even though the dipoles are changing the overall effect is for the atoms to be attracted to each other
Describe intermolecular bonding: permanent dipole - permanent dipole bonds
the slightly +ve and slightly -ve charges on polar molecules cause WEAK ELECTROSTATIC FORCES of attraction between the molecules
these are the PD-PD bonds
eg. H-Cl—–H-Cl—–H-Cl
+ - + - + -
happen as well as ID-ID (not instead of)
Describe intermolecular bonding - hydrogen bonds
only happens when hydrogen is covalently bonded to Fluorine, Nitrogen or Oxygen
- F, N, O are very electronegative so they draw the bonding electronns away from the hydrogen atom
the bond is so polarised, and hydrogen has such a high charge density (v. small) that the hydrogen atom forms weak bonds with LONE PAIRS on F, N, O atoms of other molecules
water and ammonia both have hydrogen bonding
organic molecules with -OH or -NH groups form hydrogen bonds
intermolecular bonding - why is ice less dense than water?
because of hydrogen bonding:
in ice water molecules are arranged so that there is the maximum number of hydrogen bonds
- the lattice structure formed in this way ‘wastes’ a lot of space
as the ice melts some of the hydrogen bonds are broken and the lattice breaks down
- allow molecules to ‘fill’ the spaces
this causes ice to be less dense than water
intermolecular bonding - how do hydrogen bonds affect how a substance behaves?
- hydrogen bonds are the strongest intermolecular bonds and have a huge affect on the properties of substances
- substances that form hydrogen bonds have high melting and boiling points because a lot of energy is needed to overcome the intermolecular bonds
hydrides of N, O, F usually have the highest boiling points because a lot of energy is needed to break the hydrogen bonds (N, O, F the most electronegative) - substances that form hydrogen bonds are also soluble in water
- can form hydrogen bonds with water molecules, allowing them to mix and dissolve
intermolecular bonding - boiling points of halogens trend?
as you go down the group the boiling points increase
this is because the Mr increases so number of shells of electrons increases and so the atomic/molecular size increases (so stronger ID-ID bonds)
What does PPM stand for?
Parts per million
Amount of gas particles in a sample containing 1 million particles
How can you calculate percentage composition from PPM?
To convert from ppm to %, ÷ by 10,000
What are haloalkanes?
an alkane with at least one halogen atom in place of a hydrogen atom
haloalkanes - naming
- longest part of carbon chain = last part of compounds name
- name and position of halogen atom at start
- chloro
- bromo
- iodo - if more then one of the same halogen atoms
- di
- tri
- tetra
haloalkanes - boiling points down group , why?
increase down group
boiling points depends on strength of intermolecular bonds
down group 7 atomic radius and no. of electron shells increase
so there are stronger ID-ID forces between molecules
more energy is needed to overcome them
haloalkanes - carbon-halogen bond
polar
fluorine, chlorine and bromine are much more electronegative than carbon –> so the bond is polar
the electronegtaive atom pulls electron density away from the carbon, so the carbon is e- deficient
—> this means it can be attacked by a nucleophile
what is a substitution reaction?
when a functional group in a compound is replaced by another functional group
what is a nucleophile?
an electron pair donor
it donates an electron pair to somewhere without enough electrons
Give example of nucleophiles.
- Ammonia
- Water
- OH- ions
What is nucleophilic substitution?
nucleophilic substitution = a nucleophile attacks a slightly +ve carbon and replaces the slightly -ve group
nucleophilic substitution - haloalkanes + hydroxide ions
conditions:
warm aq sodium hydroxide
reflux
mechanism:
the C-Br bond is polar
the slightly +ve carbon attracts the lone pair of electrons on OH-
the C-Br bond breaks heterolytically, a new bond forms between he C and OH- ion
general equation:
R-X + NaOH —-> ROH + NaX
nucleophilic substitution - haloalkane + water
conditions:
- warming a haloalkane with water
mechanism:
slightly +ve carbon attracts a lone pair on H2O
the C-Br bond breaks
an intermediate forms with an oxygen that has 3 bonds —> this is unstable so one O-H bond breaks
an alcohol is formed
general equation:
R-X + H2O —-> R-OH + HBr
nucleophilic substitution - haloalkanes + ammonia
conditions:
warm with excess ethanolic ammonia (ammonia dissolves in ethanol)
mechanism:
NH3 nucleophile attacks the slightly +ve carbon atom
forms an unstable intermediate —> R-N+H3
another ammonia molecule than removes a hydrogen group to leave an amine —> R-NH2 and forms ammonium (N+H4)
general equation:
R-Br + ammonia —> R-N+H3 + NH3 < —-> R-NH2 + N+H4
reactivity of haloalkanes
carbon-halogen bonds strength decides reactivity
C-F strongest —> has the highest bond enthalpy
reactivity increase down the group
stronger bond = slower reaction
How can you test the relative reactivities of the haloalkanes?
react the haloalkanes with water in the presence of silver nitrate
see which reacts the fastest.
- put a chloroalkane, bromoalkane, and iodoalkane in three different test tubes
- add silver nitrate solution and some ethanol
Ag+ + X- —> AgX (s)
the silver halide forms a precipitate
the precipitate forms fastest with iodoalkane - so must be the most reactive
What is heterolyic fission?
The uneven breaking of a covalent bond with both of the bonded
electrons going to one of the atoms, forming a cation (+ ion) and
an anion (– ion).
What is homolytic bond fission?
The breaking of a covalent bond with the pair of electrons in the bond being shared equally between each atom, forming two radicals.
What are the 3 stages of free radical chain reactions?
- Initiation
- Propogation
- Termination
Describe what occurs in the initiation stage?
Radicals are produced normally using visible light or UV in photochemical reactions. The bond breaks homolytically producing 2 radicals.
Describe what occurs in the propogation stage?
A radical reacts with a non-radical molecule forming a new radical which then goes on to react with other non-radicals. This is why it’s called a chain reaction.
Describe what occurs in the termination stage?
When 2 radicals react they from a non-radical molecule. This ends the chain reaction - termination.
Radical mechanism for reaction of alkanes with halogen - initiation
- sunlight provides enough energy to break the Cl-Cl bond - photodissociation
- the bond splits equally - homolytic fission
- the molecule form two highly reactive free radicals
Cl-Cl —UV—> Cl* + Cl*
Radical mechanism for reaction of alkanes with halogen - propagation
Cl* attacks a methane molecule, forming a hydrogen halide and a methyl radical
Cl* + CH4 –> HCl + *CH3
the new methyl radical, CH3, can attack another Cl2 molecule
CH3 + Cl-Cl —–> CH3Cl + Cl
the new Cl can attack another CH4 and so on until all the Cl2 and CH4 molecules are wiped out
Radical mechanism for reaction of alkanes with halogen - termination
if two free radicals join together they make a stable molecule
there are lots of possible termination reactions
eg.
* CH3 + Cl* —–> CH3Cl
*CH3 + *CH3 —> C3H6
what is ozone?
in the stratosphere
contains ozone molecules, O3
How is Ozone formed?
when UV radiation from the sun hits oxygen molecules
O2 + hv —-> O + O
O + O2 —-> O3
how is the ozone layer constantly being replaced?
O2 + O < —> O3
it is a reversible reaction
ozone is continuously being destroyed and replaced as UV hit the molecules
an equilibrium is set up so concentrations stay fairly constant
Why is ozone important?
protects us from UV radiation
- UVB can damage DNA in cells and cause skin cancer (also sunburn)
however in small amounts UV needed
- vitamin D
Ozone at ground level (troposphere) causes and effects
due to the effect of sunlight on mixtures of nitrogen dioxide and hydrocarbons
- from vehicle engine and power stations
in industrialised areas ozone mixes with solid particles of carbon to create PHOTOCHEMICAL SMOG
can cause respiratory problems
damages plants and materials
ozone itself is toxic to humans
How is Ozone broken down in the stratosphere
by CFCs (chlorofluorocarbons)
- haloalkanes that have all their hydrogens replaced by Cl and F atoms
the C-Cl bond can be broken down by high energy UV in the stratosphere to form chlorine radicals
- these act as catalysts in the breakdown of ozone
Describe the ease of photodissociation of haloalkanes
all haloalkanes have carbon and halogen atoms - UV can break these bonds
- the carbon-halogen bonds split homolytically to create two free radicals
the ease it is broken depends on the halogen
carbon-iodine most likely to break because it has the lowest bond enthalpy
carbon-fluorine lest likely to break because it has the highest bond enthalpy
How is the ozone layer destroyed by homogeneous catalysis
Chlorine free radicals formed when CFCs are broken down by high energy UV in the stratoshpere
- these free radicals are catalysts
INITATION - CCl3F3 (g) + hv —> CCl2F3 (g) + Cl (g)
These radicals react with ozone to form an intermediate ClO* and an oxygen molecule
PROPAGATION :
Cl* + O3 —> O2 + ClO*
ClO* + O —-> O2 + Cl* (the Cl* radical is regenerates and goes onto destroy more ozone)
the reaction will only terminate if two radicals react together
TERMINATION: Cl* + Cl* —> Cl2
OVERALL REACTION: O3 + O —-> 2O2 ….. and Cl* is the catalyst
what other free radical can destroy ozone?
NO from nitrogen oxides
eg. NO2 + hv —> NO* + O
