Section 5 The Ozone Story Flashcards
(20 cards)
Define electronegativity and state electronegativity trend across periods and groups.
The ability of an atom to attract the binding electrons in a covalent bond.
Electronegativity increases across periods and decreases down groups (ignoring the noble gases)
Why may a molecule containing polar bonds not be a polar molecule?
If the polar bonds are arranged so they point in opposite directions , they’ll cancel each other out.
Which elements can hydrogen bonding occur when hydrogen is bonded to them?
Fluorine, nitrogen, oxygen
Which factors can affect rate of reaction?
Increasing concentration: particles are closer together so collide more frequently.
Increasing pressure: particles closer together so there are more frequent collisions.
Presence of a catalyst: offer alternative reaction route with lower activation enthalpy.
Temperature: particles have more energy, so collide more frequently.
What are homogeneous catalysts and how do they speed up chemical reactions?
Catalyst that is in same state as reactants.
- A homogeneous catalyst speeds up reactions by forming intermediate compounds with reactants.
- The products are then formed from intermediate compounds.
- The activation enthalpy needed to form intermediates, and then form products from them, is lower than what is needed to make products directly from the reactants.
What methods can be used to measure rate of reaction?
Volume of gas produced
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
Boiling point of haloalkanes trend?
Increase down the group.
As atomic radius increases as well as number of electron shells. This leads to stronger id-id forces.
Test to compare reactivities of the haloalkanes?
Put a chloro, bromo, and iodoalkane into 3 separate test tubes. Add silver nitrate solution ( this contains the water) and ethanol.
Ppt forms fastest with the iodoalkane, so this must be the most reactive.
Carbon-halogen bond strength trend?
Decreases down group due to increasing atomic size which weaken the attraction between the carbon and halogen atoms.
What is the difference between homolytic and heterolytic bond fission?
Homolytic: forms two uncharged ‘radicals’ which are particles that have an unpaired electron. This makes them very reactive.
Heterolytic: forms a cation and an anion.
What is formed in
1. Initiation reactions
2. Propagation reactions
3. Termination reactions
- Free radicals are produced
- Free radicals react with molecules and produce new radicals. These then react with more molecules, producing more radicals.
- Two radicals react to form a stable molecule.
Initiation of chlorine molecule.
Sunlight provides energy to break Cl-Cl bond- photodissociation
The bond splits equally and each atom keeps one electron - homolytic fission. This forms two radicals.
Propagation involving chlorine radicals.
Cl radical attacks methane molecule, forming a hydrogen halide and a methyl radical.
New methyl radical can then attack another Cl2 molecule.
The new Cl radical can attack another methane molecule and so on. Until all Cl2 or CH4 are wiped out.
Termination of chlorine radicals
If 2 radicals join together, they make a stable molecule.
There are lots of possible termination reactions e.g. chlorine radical + methyl radical or methyl radical + methyl radical
How is ozone formed?
O2 + UV > 2O
O2 + O > O3
UV can also reverse the reaction:
O3 + UV > O2 + O
So, the ozone layer is constantly being destroyed and replaced.
Why is the ozone layer in the stratosphere important?
It protects from harmful UV radiation. It does this by absorbing high energy UV when it is broken down.
Why is ozone harmful at ground level?
It mixes with solid carbon particles and other substances to form photochemical smog. This can cause respiratory problems. Ozone itself is toxic to humans.
How do chlorofluorocarbons (CFCs) cause the breakdown of ozone?
- Chlorine radicals formed when CFCs are broken down by UV radiation. Initiation:
CCl3F + UV > .CClF + Cl. - These radicals are catalysts and react with ozone. Propagation.
Cl. + O3 > O2 + ClO.
ClO. + O > O2 + Cl. - Reaction terminates when two radicals react. Termination.
Cl. + Cl. > Cl2
Overall reaction: O3 + O > 2O2 catalysed by Cl.
How to convert % into ppm?
ppm = % × 10,000
How can E= hv and v= c/λ be combined to make a new equation for energy?
E= hc/λ
E= energy in joules
h= plancks constant
c= speed of light m s-¹
λ= wavelength in m
