Chloroalkanes and CFC'S
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What is a free radical
A particle with an un unpaired electron
How are free radicals formed?
When a covalent bond splits equally, giving one electron to each atom, homolytic fission
What makes particles very reactive
unpaired electrons
How are free radicals identified?
Cl•
What is formed when halogens and alkanes react together?
Halogenoalkanes
What type of reactions form halogenoalkanes
Photochemical reactions
Describe a free radical substitution reaction
> Started by ultraviolet light A hydrogen atom is replaced by either chlorine or bromine
How many stages are free radical substitution reactions grouped into? and name them
3 >Initiation reactions >Propagation reactions >Termination reactions
Describe each free radical substitution reaction
> 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
What determines which product will be in excess in a free radical substitution reaction?
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
What are chlorofluorocarbons?
Halogenoalkane molecules where all of the hydrogen atoms have been replaced by chlorine and fluorine atoms
What ions destroy the ozone layer and why?
Chlorine free radicals because they as a catalyst for forming oxygen molecules
What is the symbol for ozone
O3
How is ozone formed?
• When an oxygen molecule is broken down into two free radicals by ultraviolet radiation. They attack other oxygen molecules forming the ozone
What is the function of the ozone?
Absorbs ultraviolet radiation from the sun because ultraviolet radiation can cause skin cancer and skin burn
Describe how holes are created are in the ozone and give the reaction steps.
> 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
What was the use of CFC’s and why are CFC’s banned?
> 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
why is ultraviolet light needed for a initiation reaction to occur
it provides energy to break covalent bonds in chlorine to from chlorine free radicals
how do greenhouse gases contribute to global warming
Absorb infrared radiation molecules have polar bonds
What is the equation for how ozone is formed
• O2 -> (UV) O• + O• • O2 + O• -> O3
Explain with the aid of equations how chlorine atoms act as a catalyst in the decompositions of ozone into oxygen
• Ultraviolet radiation • C-Cl bond breakage • Cl• + O3 -> ClO• + O2 • ClO• + O3 -> Cl• + 2O2 • 2O3-> 3O2 • Cl• provides an alternative route, activation energy
Give the formula for the decomposition of ozone into oxygen
2O3 -> 3O2
Is the decomposition of the ozone catalysed by chlorine molecules?
No
Are ozone holes regions of the upper atmosphere where there is a reduced concentration of ozone?
Yes