Chloroalkanes and CFC'S

Question 1

What is a free radical

Answer 1

A particle with an un unpaired electron

Question 2

How are free radicals formed?

Answer 2

When a covalent bond splits equally, giving one electron to each atom, homolytic fission

Question 3

What makes particles very reactive

Answer 3

unpaired electrons

Question 4

How are free radicals identified?

Answer 4

Cl•

Question 5

What is formed when halogens and alkanes react together?

Answer 5

Halogenoalkanes

Question 6

What type of reactions form halogenoalkanes

Answer 6

Photochemical reactions

Question 7

Describe a free radical substitution reaction

Answer 7

> Started by ultraviolet light
A hydrogen atom is replaced by either chlorine or bromine

Question 8

How many stages are free radical substitution reactions grouped into? and name them

Answer 8

3
>Initiation reactions
>Propagation reactions
>Termination reactions

Question 9

Describe each free radical substitution reaction

Answer 9

> Initiation reactions: Free radicals are produced, sunlight provides enough energy to break Cl-Cl bond, Photodissociation
Cl2-> (UV) 2Cl

> Propagation reactions: Free radicals are used up and created in a chain reaction until all the molecules are used up

> Termination Reactions: Two free radicals join together and they make a stable molecule, the two unpaired electrons form a covalent bond

Question 10

What determines which product will be in excess in a free radical substitution reaction?

Answer 10

If there is excess of a halogen, di, tri, tetra e.g. tetrachloromethane

If there is excess of the alkane the product will be mostly e.g Chloromethane

Question 11

What are chlorofluorocarbons?

Answer 11

Halogenoalkane molecules where all of the hydrogen atoms have been replaced by chlorine and fluorine atoms

Question 12

What ions destroy the ozone layer and why?

Answer 12

Chlorine free radicals because they as a catalyst for forming oxygen molecules

Question 13

What is the symbol for ozone

Answer 13

O3

Question 14

How is ozone formed?

Answer 14

• When an oxygen molecule is broken down into two free radicals by ultraviolet radiation. They attack other oxygen molecules forming the ozone

Question 15

What is the function of the ozone?

Answer 15

Absorbs ultraviolet radiation from the sun because ultraviolet radiation can cause skin cancer and skin burn

Question 16

Describe how holes are created are in the ozone and give the reaction steps.

Answer 16

> Chlorine free radicals are formed in the upper atmosphere when C-Cl bonds in CFCs are broken down by ultraviolet radiation.

> These chlorine free radicals are catalysts

> They react with ozone to form an intermediate (ClO•) and an oxygen molecule

Question 17

What was the use of CFC’s and why are CFC’s banned?

Answer 17

> Once used as coolant gas in fridges, as solvents and as propellants in aerosols

> Advantages couldn’t outweigh the environmental problems they were causing

> Alternatives of CFC’s are HFC’s (hydrofluorocarbons) and hydrocarbons

Question 18

why is ultraviolet light needed for a initiation reaction to occur

Answer 18

it provides energy to break covalent bonds in chlorine to from chlorine free radicals

Question 19

how do greenhouse gases contribute to global warming

Answer 19

Absorb infrared radiation
molecules have polar bonds

Question 20

What is the equation for how ozone is formed

Answer 20

• O2 -> (UV) O• + O•
• O2 + O• -> O3

Question 21

Explain with the aid of equations how chlorine atoms act as a catalyst in the decompositions of ozone into oxygen

Answer 21

• Ultraviolet radiation
• C-Cl bond breakage
• Cl• + O3 -> ClO• + O2
• ClO• + O3 -> Cl• + 2O2
• 2O3-> 3O2
• Cl• provides an alternative route, activation energy

Question 22

Give the formula for the decomposition of ozone into oxygen

Answer 22

2O3 -> 3O2

Question 23

Is the decomposition of the ozone catalysed by chlorine molecules?

Answer 23

No

Question 24

Are ozone holes regions of the upper atmosphere where there is a reduced concentration of ozone?

Answer 24

Yes

Question 25

State the benefit to life on earth of ozone in the upper atmosphere

Answer 25

• Absorbs (harmful) ultraviolet

Question 26

Suggest one reason why the use of CFC’s was not restricted until several years after Rolland and Molina published their research

Answer 26

• Lack of evidence that the ozone was being depleted
• Lack of alternatives to CFC’s
• commercial interest to continue to use CFCs
• Hard to obtain international agreement

Question 27

Chloroethane reacts with potassium hydroxide in the presence of propan-1-ol to form ethene.

State the role of potassium hydroxide and the role of propan-1-ol in the reaction

Answer 27

• KOH- Base
• porpan-1-ol- acts as a solvent

Question 28

Explain why chlorine radical can cause the decomposition of many molecules of ozone

Answer 28

• Cl radical is regenerated and causes a chain reaction in the decomposition of ozone