Topic 4- The Atmosphere Flashcards
Name the 2 types of bond dissociation and what they create
Homolytic fission- creates free radicals (needs high E situations)
Heterolytic fission- creates ions (does not always need high E e.g with halogenoalkanes and nucleophiles)
What can happen when a molecule absorbs u.v?
- Lower frequencies- electrons excited then re-emit the radiation and return to ground state
- Higher frequencies- bonding elections excited and bond dissociation can occur
- Highest frequencies- electrons gain sufficient energy to escape molecule and a molecular ion is produced
What is a radical?
A species with an unpaired electron in its outer shell, it is made when a covalent bond is broken homolytically.
How do radicals react?
Radicals react by gaining an electron from any species around it.
Describe the reaction of chlorine and methane in the presence of light
Initiation- Cl-Cl is broken in u.v to form 2 chlorine radicals. (Photodissociation)
Propogation- chlorine radical reacts with methane to form hydrogen chloride and a methyl radical
Methyl radical reacts with chlorine to form CH3Cl and a chlorine radical.
Termination- Cl+Cl— Cl2
CH3 + Cl — CH3Cl
CH3 + CH3 — C2H6
Give 3 features of a radical chain reaction
- They occur in gas phase or a non-polar solvent
- They are often initiated by heat or light
- They usually go very fast
How is ozone formed?
Oxygen undergoes photodissociation to create 2 oxygen radicals.
These radicals react with oxygen to form ozone. (They can also react with ozone to create 2O2 or with each other to create oxygen again)
What happens when ozone absorbs radiation?
It undergoes photodissociation to create oxygen and an oxygen radical.
What factors affect rate?
- temperature
- concentration of solutions
- pressure of gases
- catalysts
- surface area of solids
- intensity of radiation
State the collision theory
Reactions occur when particles collide, the collision must have a minimum combined energy for a reaction to occur. Therefore not every collision is successful.
Activation energy= minimum energy of collision
Why does increasing temperature increase the rate?
- increasing the temperature increases the energy of the particles
- therefore the particles, when they collide, are more likely to have at least the minimum activation energy in their collisions
- therefore there are more successful collisions and the rate increases
Why does increasing concentration increase the rate?
- there are more particles per given volume
- therefore there are more collisions per unit time in this volume
- at the same temperature the same proportion of collisions will be successful, so if the number of particles per volume and therefore number of collisions per volume increase the number of successful collisions will increase, increasing the rate.
What is causing ozone depletion?
Chlorine radicals, which come from chloromethane and chlorofluorocarbons.
Describe the chlorine catalytic cycle
Cl (radical) + O3 — ClO + O2
Then…
ClO + O — Cl (radical) + O2
Chlorine is regenerated.
Overall:
O3 + O — O2 + O2
Which other radicals react with ozone?
Hydroxyl radicals- HO
(Come from water in the stratosphere)
Nitrogen monoxide- N2O
(Made in car engines and released by bacteria in the soil and oceans)
What types of energy are needed to change the behaviour in molecules?
To move electrons between energy levels- u.v and visible
To increase vibrational energy- infra red
To increase rotational energy- microwaves
To translate molecules- any part of the EM spectrum
What is a homogeneous catalyst?
Homogeneous catalysts are in the same state as the reactants, they catalyse a reaction by forming a reaction intermediate that then breaks down to give the product, the catalyst is then reformed.
What were the main uses of CFCs?
- refrigerants and in air conditioning
- propellants in aerosols
- blowing agents in expanded plastics e.g. Polystyrene
- cleaning solvents
What was the problem with CFCs?
They are too unreactive and so have plenty of time to reach the stratosphere. Once in the stratosphere u.v radiation causes them to photodissociate to form radicals- which cause ozone depletion.
What are chlorine reservoirs?
Molecules in the stratosphere that react with chlorine. Hydrogen chloride and ClONO2 ‘store’ chlorine.
Why does the hole develop over the poles?
In the arctic winter the temperatures are very low (-80) and a vortex of air forms. This caused clouds to form made of solid particles which are isolated by the vortex. Chlorine reservoir molecules are adsorbed onto particles in the clouds and react. Chlorine is released as a gas but stays dissolved in the vortex. When sunlight returns in spring the vortex breaks up and chlorine is released.
What does ozone depletion lead to?
- skin cancer
- cataracts
- death of plankton
- effects on food chains
- changes in temperatures and weather
Why is methane bad in the troposphere?
Methane is a greenhouse gas. Greenhouse gases absorb infra red radiation emitted by the Earth. Some i.r is re-emitted in all directions.
Most: i.r radiation increases the vibrational energy of the molecules, so bonds vibrate more. This vibrations transfers to other molecules in the air, and so they move faster and have more kinetic energy. So the temperature of the air is raised
Not all carbon dioxide produced is going into the atmosphere, where does it go?
The oceans, carbon dioxide is fairly soluble in water.
What is dynamic equilibrium?
In a dynamic equilibrium the rate of the forward reaction is equal to the rate of backward reaction. There are a set of concentrations that do not change.
What is Le Chatelier’s principle?
If a system is at equilibrium and a change is made on any of the conditions then the system responds to counteract the change as much as possible.
What are factors that effect the position of equilibrium?
- concentration of reactants and products
- temperature of the system
- pressure of any gases
