Advanced A Level Chemistry Notes on Group 7/17 Halogens - ozone formation & depletion due to CFCs

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Advanced Level Inorganic Chemistry Periodic Table Revision Notes - Part 9. Group 7/17 The Halogens

9.10 Ozone, CFC's and halogen organic chemistry links

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All my Group 7/17 halogens advanced chemistry level revision notes

All my advanced A level inorganic chemistry revision notes

GCSE Level Group 7 Halogens revision notes

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What is the connection between halogen compounds such as CFCs are the destruction of the ozone layer? Ozone layer depletion in terms of the free radical reactions involving CFCs chlorine atoms. At the end of the page are links to the chemistry of organic halogen compounds.

PLEASE NOTE KS4 Science GCSE/IGCSE/O Level GROUP 7 HALOGENS NOTES are on a separate webpage

Pd s block d blocks and f blocks of metallic elements p block elements
Gp1 Gp2 Gp3/13 Gp4/14 Gp5/15 Gp6/16 Group7/17 Gp0/18


2 3Li 4Be ZSymbol, z = atomic or proton number

highlighting position of Group 7/17 Halogens

outer electrons ns2np5

5B 6C 7N 8O 9F


3 11Na 12Mg 13Al 14Si 15P 16S 17Cl


4 19K 20Ca 21Sc 22Ti 23V 24Cr 25Mn 26Fe 27Co 28Ni 29Cu 30Zn 31Ga 32Ge 33As 34Se 35Br


5 37Rb 38Sr 39Y 40Zr 41Nb 42Mo 43Tc 44Ru 45Rh 46Pd 47Ag 48Cd 49In 50Sn 51Sb 52Te 53I


6 55Cs 56Ba 57-71 72Hf 73Ta 74W 75Re 76Os 77Ir 78Pt 79Au 80Hg 81Tl 82Pb 83Bi 84Po 85At


7 87Fr 88Ra 89-103 104Rf 105Db 106Sg 107Bh 108Hs 109Mt 110Ds 111Rg 112Cn 113Uut 114Fl 115Uup 116Lv 117Uus



9.10 Ozone, CFC's and halogen organic chemistry links

Abbreviations used: CFC = chlorofluorocarbon;  HCFC = hydrochlorofluorocarbon;  HFC = hydrofluorocarbon

CFCs, Ozone and Free Radicals

Since these notes where written I've done a more detailed page on the free radical chemistry of ozone formation and destruction in catalytic cycles

  • CFCs – what is so good about them? (before we get into the problems they cause!)

    • A CFC is a covalently bonded relatively small molecule of carbon, chlorine and fluorine atoms (chlorofluorocarbon).

  • If enough energy is supplied by heat or by visible/uv electromagnetic radiation, or the is weak enough, a covalent bond can break in two ways. This illustrated with the molecule chloromethane CH3Cl.

  • The bond breaks unevenly where the electron bond pair can stick with one fragment and a positive and negative ion form.

    • e.g. CH3Cl ==> CH3+ + Cl   (called heterolytic bond fission)

    • shows what happens to the molecule, or
  • The bond breaks evenly, where the bonding pair of electrons are equally divided between two highly reactive fragments called free radicals.

    • Free radicals are characterised by having an unpaired electron not involved in a chemical bond.

    • The . means the 'lone' electron on the free radical, which is not part of a bond anymore, and wants to pair up with another electron to form a stable bond – that's why free radicals are so reactive!

    • e.g.  CH3Cl ==> CH3. + .Cl    (called homolytic bond fission)

    •   shows what happens to the molecule.

    • Homolytic bond fission can occur by molecules hit by uv photons i.e. ultraviolet electromagnetic radiation of quite high energy – great enough to cause homolytic bond fission.

  • The chemistry of free radicals is important in the current environmental issue of ozone layer depletion.

  • Chlorofluorocarbons (CFC's for shorthand) are organic molecules containing carbon, fluorine and chlorine

  • e.g.  dichlorodifluoromethane has the formula CCl2F2 (shown above).

  • They are very useful low boiling organic liquids or gases, until recently, extensively used in refrigerators and aerosol sprays e.g. repellents.

  • They are relatively unreactive, non–toxic and have low flammability, so in many ways they are 'ideal' for the job they do.

  • However it is their chemical stability in the environment that eventually causes the ozone problem but first we need to look at how ozone is formed and destroyed in a 'natural cycle'. This presumably has been in balance for millions of years and explains the uv ozone protection in the upper atmosphere – the stratosphere.

  • Ozone is formed in the stratosphere by free radical reactions.

    • 'ordinary' stable oxygen O2 (dioxygen) is split (dissociates) into two by high energy ultraviolet electromagnetic radiation (uv photon energy 'wave packets from Planck's Equation E = uv) into two oxygen atoms (which are themselves radicals) and then a 'free' oxygen atom combines with an oxygen molecule (dioxygen) to form ozone (trioxygen).

      • O2 + uv ==> 2O. then O. + O2 ==> O3 

    • The ozone is a highly reactive and unstable molecule and decomposes into dioxygen when hit by other uv light photons. The oxygen atom radical can do several things including ...

      • O3 + uv ==> O2 + O. 

    • This last reaction is the main uv screening effect of the upper atmosphere and the ozone absorbs a lot of the harmful incoming uv radiation from the Sun.

    • If the ozone levels are reduced more harmful uv radiation reaches the Earth's surface and can lead to medical problems such as increased risk of sunburn and skin cancer and it also accelerates skin aging processes.

    • There is strong evidence to show there are 'holes' in the ozone layer with potentially harmful effects, so back to the CFC problem for some explanations and solutions!

  • The chemically very stable CFCs diffuse up into the stratosphere and decompose when hit by ultraviolet light (uv) to produce free radicals, including free chlorine atoms, which themselves are highly reactive free radicals.

    • e.g. CCl2CF2 ==> CClF2. + Cl.  (note the C–Cl bond is weaker than the C–F bond, so breaks first)

  • The formation of chlorine atom radicals is the root of the problem because they readily react with ozone and change it back to much more stable ordinary oxygen.

    • (i) O3 + Cl. ==> O2 + ClO. bye bye ozone! and no uv removed in the process!

    • and then (ii) ClO + O ==> Cl + O2 , which means the 'destructive' Cl atom free radical is still around!

    • The two reactions above involving chlorine atoms are known as a catalytic cycle because the chlorine atoms from CFC's etc. act as a catalyst in the destruction of ozone.

    • So, if you add up (i) + (ii) you get

    • O3 + O ==> 2O2    and is an example of the catalytic cycle

    • (i)     X▪  +  O3  ===>  XO▪  +  O2
      (ii)    XO▪  +  O  ===>  X▪  +  O2
      (iii)     O  +  O3  ===>  2O2
    • X can be Cl, NO or even OH, e.g. Cl from CFCs, NO from road vehicles, OH from atmospheric water.

    • See extra note on NO cycle

  • Therefore many countries are banning the use of CFCs, but not all despite the fact that scientists predict it will take many years for the depleted ozone layer to return to its 'original' O3 concentration and alternatives to CFC's are already being marketed.

    • BUT at least the ozone layer is recovering thanks to some world–wide co–operation and the work of chemists in developing less environmentally harmful alternatives.

  • Alternatives to CFCs i.e. HFCs and HCFCs

    • The idea is to use replacement compounds that are less harmful to the ozone layer.

    • The molecules listed below contain C–H bonds and are broken down in the lower troposphere before they reach the ozone layer in the stratosphere.

    • Hydrochlorofluorohydrocarbons (a HCFC is composed of hydrogen, chlorine, fluorine and carbon atoms)

      • e.g. CH3CFCl2 1,1–dichloro–1–fluoroethane

    • Hydrofluorocarbons (a HFC is composed of hydrogen, fluorine and carbon atoms)

      • e.g. CH2FCF3 1,1,1,4–tetrafluoroethane

    • Alkanes (composed of hydrogen and carbon atoms)

      • e.g. butane CH3CH2CH2CH3

      • but they are flammable!

    • However, all of these molecules are greenhouse gases and will contribute to global warming!

    • Index of all my notes on halogenoalkanes

CHLOROALKANES (halogenoalkanes)

  • Alkanes are usually not very reactive unless burned! BUT they will react with reactive chemicals like chlorine when heated or subjected to uv light to form chlorinated hydrocarbons.
    • Despite the reactivity of chlorine you still need something extra to initiate the reaction.
    • A substitution reaction occurs and a chloro–alkane is formed e.g.
    • a hydrogen is swapped for a chlorine and the hydrogen combines with a chlorine atom
    • ethane + chlorine ==> chloroethane + hydrogen chloride
    • C2H6 + Cl2 ===> C2H5Cl + HCl
    • + Cl2 ==> + HCl
    • Chloro–alkanes are useful solvents in the laboratory or industry but though their vapours can be harmful.
    • Index of all my notes on halogenoalkanes

Halogen – alkene addition reaction

doc b oil notes Used as a test for alkenes: Hydrocarbons are colourless. Bromine dissolved in water or trichloroethane solvent forms an orange (yellow/brown) solution. When bromine solution is added to both an alkane or an alkene the result is quite different. The alkane solution remains orange – no reaction. However, the alkene decolourises the bromine as it forms a colourless dibromo–alkane compound – see equations below.

Ex 1. doc b oil notes doc b oil notes doc b oil notesarrow doc b oil notes.... or

  doc b oil notes doc b oil notes doc b oil notes doc b oil notes doc b oil notes

ethene + bromine ==> 1,2–dibromoethane

Ex 2. doc b oil notes doc b oil notes doc b oil notes doc b oil notes doc b oil notes.... or

  doc b oil notes doc b oil notes doc b oil notes doc b oil notes doc b oil notes

propene + bromine ==> 1,2–dibromopropane

Ex 3. alkenes structure and naming (c) doc b doc b oil notes doc b oil notes doc b oil notes (c) doc b

cyclohexene + bromine ==> 1,2–dibromocyclohexane

Index of all my notes on halogenoalkanes


Advanced A level chemistry: Organic Halogen Compound page links


PLEASE NOTE GCSE Level GROUP 7 HALOGENS NOTES are on a separate webpage

INORGANIC Part 9 Group 7/17 Halogens sub–index: 9.1 Introduction, trends & Group 7/17 data * 9.2 Halogen displacement reaction and reactivity trend  * 9.3 Reactions of halogens with other elements - halides * 9.4 Reaction between halide salts and conc. sulfuric acid * 9.5 Tests for halogens and halide ions * 9.6 Extraction of halogens from natural sources * 9.7 Uses of halogens & compounds * 9.8 Oxidation & Reduction – more on redox reactions of halogens & halide ions * 9.9 Volumetric analysis – titrations involving halogens or halide ions * 9.10 Ozone, CFC's and halogen organic chemistry links * 9.11 Chemical bonding in halogen compounds * 9.12 Miscellaneous aspects of halogen chemistry

Advanced Level Inorganic Chemistry Periodic Table Index: Part 1 Periodic Table history Part 2 Electron configurations, spectroscopy, hydrogen spectrum, ionisation energies * Part 3 Period 1 survey H to He * Part 4 Period 2 survey Li to Ne * Part 5 Period 3 survey Na to Ar * Part 6 Period 4 survey K to Kr and important trends down a group * Part 7 s–block Groups 1/2 Alkali Metals/Alkaline Earth Metals * Part 8  p–block Groups 3/13 to 0/18 * Part 9 Group 7/17 The Halogens * Part 10 3d block elements & Transition Metal Series * Part 11 Group & Series data & periodicity plots

Group numbering and the modern periodic table

The original group numbers of the periodic table ran from group 1 alkali metals to group 0 noble gases (= group 8). To account for the d block elements and their 'vertical' similarities, in the modern periodic table, group 3 to group 0/8 are numbered 13 to 18. So, the halogen elements are referred to as group 17 at a higher academic level, though group 7 is still used, usually at a lower academic level.


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