Topic 4- The Atmosphere

Question 1

Name the 2 types of bond dissociation and what they create

Answer 1

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)

Question 2

What can happen when a molecule absorbs u.v?

Answer 2

1. Lower frequencies- electrons excited then re-emit the radiation and return to ground state
2. Higher frequencies- bonding elections excited and bond dissociation can occur
3. Highest frequencies- electrons gain sufficient energy to escape molecule and a molecular ion is produced

Question 3

What is a radical?

Answer 3

A species with an unpaired electron in its outer shell, it is made when a covalent bond is broken homolytically.

Question 4

How do radicals react?

Answer 4

Radicals react by gaining an electron from any species around it.

Question 5

Describe the reaction of chlorine and methane in the presence of light

Answer 5

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

Question 6

Give 3 features of a radical chain reaction

Answer 6

1. They occur in gas phase or a non-polar solvent
2. They are often initiated by heat or light
3. They usually go very fast

Question 7

How is ozone formed?

Answer 7

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)

Question 8

What happens when ozone absorbs radiation?

Answer 8

It undergoes photodissociation to create oxygen and an oxygen radical.

Question 9

What factors affect rate?

Answer 9

• temperature
• concentration of solutions
• pressure of gases
• catalysts
• surface area of solids
• intensity of radiation

Question 10

State the collision theory

Answer 10

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

Question 11

Why does increasing temperature increase the rate?

Answer 11

• 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

Question 12

Why does increasing concentration increase the rate?

Answer 12

• 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.

Question 13

What is causing ozone depletion?

Answer 13

Chlorine radicals, which come from chloromethane and chlorofluorocarbons.

Question 14

Describe the chlorine catalytic cycle

Answer 14

Cl (radical) + O3 — ClO + O2

Then…

ClO + O — Cl (radical) + O2

Chlorine is regenerated.

Overall:

O3 + O — O2 + O2

Question 15

Which other radicals react with ozone?

Answer 15

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)

Question 16

What types of energy are needed to change the behaviour in molecules?

Answer 16

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

Question 17

What is a homogeneous catalyst?

Answer 17

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.

Question 18

What were the main uses of CFCs?

Answer 18

• refrigerants and in air conditioning
• propellants in aerosols
• blowing agents in expanded plastics e.g. Polystyrene
• cleaning solvents

Question 19

What was the problem with CFCs?

Answer 19

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.

Question 20

What are chlorine reservoirs?

Answer 20

Molecules in the stratosphere that react with chlorine. Hydrogen chloride and ClONO2 ‘store’ chlorine.

Question 21

Why does the hole develop over the poles?

Answer 21

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.

Question 22

What does ozone depletion lead to?

Answer 22

• skin cancer
• cataracts
• death of plankton
• effects on food chains
• changes in temperatures and weather

Question 23

Why is methane bad in the troposphere?

Answer 23

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

Question 24

Not all carbon dioxide produced is going into the atmosphere, where does it go?

Answer 24

The oceans, carbon dioxide is fairly soluble in water.

Question 25

What is dynamic equilibrium?

Answer 25

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.

Question 26

What is Le Chatelier’s principle?

Answer 26

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.

Question 27

What are factors that effect the position of equilibrium?

Answer 27

• concentration of reactants and products
• temperature of the system
• pressure of any gases