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

9.2 Halogen displacement reactions, reactivity trend & 9.3 Reaction of halogens with other elements

The halogen displacement reactions are fully described and explained including the oxidation state changes and the group reactivity trend. The reaction of halogens with other elements including both metals and non-metals is also described.

PLEASE NOTE KS4 Science GCSE/IGCSE/O 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 * 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 * All 11 Parts have their own sub-indexes near the top of the pages


 9.2 The halogen displacement reaction and Group 7 reactivity trend

A few drops of chlorine water, bromine water and iodine water are added in turn to aqueous solutions of the salts  potassium chloride (KCl), potassium bromide (KBr) and potassium iodide (KI). Three combinations produce a reaction (and three don't!). You can get 'simple' observations from the diagrams! A darkening effect compared to a water blank confirms a displacement reaction has happened. Chlorine displaces bromine from potassium bromide and iodine from potassium iodide.  Bromine only displaces iodine from potassium iodide and the least reactive iodine cannot displace chlorine or bromine from their salts.

Halogen added KCl solution KBr solution KI solution BLANK of water
chlorine Cl2 VERY pale green solution 1. orange-reddish brown solution 2. brown solution-black precipitate VERY pale green solution
bromine Br2 orange-reddish brown solution orange-reddish brown solution 3. brown solution-black precipitate orange-reddish brown solution
iodine I2 dark brown solution dark brown solution dark brown solution dark brown solution

On the basis that the most reactive element displaces a least reactive element the reactivity order must be:

chlorine > bromine > iodine

The word, 'molecular' symbol and ionic equations for the 1 - 3 DISPLACEMENT REACTIONS on the diagram are given below.

1. chlorine + potassium bromide ==> potassium chloride + bromine

Cl2(aq) + 2KBr(aq) ==> 2KCl(aq) + Br2(aq)

Cl2(aq) + 2Br-(aq) ==> 2Cl-(aq) + Br2(aq)

2. chlorine + potassium iodide ==> potassium chloride + iodine

Cl2(aq) + 2KI(aq) ==> 2KCl(aq) + I2(aq)

Cl2(aq) + 2I-(aq) ==> 2Cl-(aq) + I2(aq)

3. bromine + potassium iodide ==> potassium bromide + iodine

Br2(aq) + 2KI(aq) ==> 2KBr(aq) + I2(aq)

Br2(aq) + 2I-(aq) ==> 2Br-(aq) + I2(aq)

The halogen molecule is the electron acceptor (the oxidising agent) and is reduced by electron gain to form a halide ion.

The oxidation state of the halogen in the halogen molecule changes (reduces) from 0 to -1, electron gain, reduction

The halide ion is the electron donor (the reducing agent) and is oxidised by electron loss to form a halogen molecule

The oxidation state of the halogen in the halide ion changes (increases) from -1 to 0, electron loss, oxidation

chlorine molecule + bromide ion ==> chloride ion + bromine molecule

Advanced Inorganic Chemistry Page Index and Linksionically the redox equations are written ...

1.  Cl2(aq) + 2Br-(aq) ==> 2Cl-(aq) + Br2(aq)

because the potassium ion, K+, is a spectator ion, that is, it does not take part in the reaction. The other two possible reaction equations involving (ii) chlorine + iodide and (iii) bromine + iodide, are similar to the example above.

2.  Cl2(aq) + 2I-(aq) ==> 2Cl-(aq) + I2(aq)

3.  Br2(aq) + 2I-(aq) ==> 2Br-(aq) + I2(aq)

Explaining the Reactivity Trend of the Group 7 Halogen

Period 2 halogen: (c) doc b  F [2.7] + e- ==> (c) doc bF- [2.8]-

Period 3 halogen: (c) doc b Cl [2.8.7] + e- ==> (c) doc bCl- [2.8.8]-

Period 4 halogen: Br [2.8.18.7] + e- ==> Br- [2.8.18.8]-

Period 5 halogen: I [2.8.18.18.7] + e- ==> I- [2.8.18.18.8]-

  • When a halogen atom reacts, it gains an electron to form a singly negative charged ion e.g. Cl + e-  ==> Cl- which has a stable noble gas electron structure like argon. (2.8.7 ==> 2.8.8)

  • As you go down the group from one Group 7 halogen element down to the next .. F => Cl => Br => I ...

    • the atomic radius gets bigger due to an extra filled electron shell,

    • the outer electrons are further and further from the nucleus and are also shielded by the extra full electron shell of negative electron charge,

    • therefore the outer electrons are less and less strongly attracted by the positive nucleus as would be any 'incoming' electrons to form a halide ion (or shared to form a covalent bond).

  • SO, this combination of factors means to attract an 8th outer electron is more and more difficult as you go down the group, so the element is less reactive as you go down the group, i.e. less 'energetically' able to form the X- halide ion with increase in atomic number.

Advanced Inorganic Chemistry Page Index and Links


9.3 Reactions of halogens with other elements


Reaction with hydrogen H2

  • (c) doc bHalogens readily combine with hydrogen to form the hydrogen halides which are colourless gaseous covalent molecules. (Halogen compounds - covalent bonding details revision notes)

  • e.g. hydrogen + chlorine ==> hydrogen chloride

  • H2(g) + Cl2(g) ==> 2HCl(g)

  • The hydrogen halides dissolve in water to form very strong acids with solutions of pH1 e.g. hydrogen chloride forms hydrochloric acid in water HCl(aq) or H+Cl-(aq) because they are fully ionised in aqueous solution even though the original hydrogen halides were covalent! An acid is a substance that forms H+ ions in water.

  • Bromine forms hydrogen bromide gas HBr(g), which dissolved in water forms hydrobromic acid HBr(aq). Iodine forms hydrogen iodide gas HI(g), which dissolved in water forms hydriodic acid HI(aq). Note the group formula pattern.

  • The mechanism of the direct combination of chlorine and bromine with hydrogen is a classic case of a free radical chain reaction.

    • initiation: X2 ==> 2X.

      • Homolytic bond fission by heat or light to give two halogen free radicals.

    • propagation: X. + H2 ==> HX + H. and H. + X2 ==> HX + X.

      • Two steps of product + radical to continue chain

    • termination: H. + X. ==> HX or 2H. ==> H2 or 2X. ==> X2

    • Three possible ways of ending a chain sequence.

Advanced Inorganic Chemistry Page Index and Links


Reaction with Group 1 Alkali Metals Li Na K etc.

  • (c) doc bAlkali metals burn very exothermically and vigorously when heated in chlorine to form colourless crystalline ionic salts e.g. NaCl or Na+Cl-. This is a very expensive way to make salt! Its much cheaper to produce it by evaporating sea water!

  • e.g. sodium + chlorine ==> sodium chloride

  • 2Na(s) + Cl2(g) ==> 2NaCl(s)

  • The sodium chloride is soluble in water to give a neutral solution pH 7, universal indicator is green. The salt is a typical ionic compound i.e. a brittle solid with a high melting point. Similarly potassium and bromine form potassium bromide KBr, or lithium and iodine form lithium iodide LiI.  Again note the group formula pattern.

  •  (Halogen compounds - ionic bonding details revision notes)

Advanced Inorganic Chemistry Page Index and Links


Reaction with other metals

  • Reaction of element with chlorine

    • Burns when heated strongly in chlorine gas to form the white* solid aluminium chloride on heating in chlorine gas.

      • aluminium + chlorine ==> aluminium chloride

      • 2Al(s) + 3Cl2(g) ==> 2AlCl3(s)

      • * It is often a faint yellow in colour, due to traces of iron forming iron(III) chloride.

      • Aluminium chloride is a curious substance in its behaviour. The solid, AlCl3, consists of an ionic lattice of Al3+ ions, each surrounded by six Cl- ions, BUT on heating, at about 180oC, the thermal kinetic energy of vibration of the ions in the lattice is sufficient to cause it break down and sublimation takes place (s ==> g).

      • In doing so, the nature of the aluminium-chlorine bond changes and the co-ordination number of the aluminium changes from six to four to form the readily vapourised covalent dimer molecule, Al2Cl6, shown above.

  • You can also prepare iron(III) chloride in the same way.

    • iron + chlorine ==> iron(III) chloride(brown solid)

    • 2Fe(s) + 3Cl2(g) ==> 2FeCl3(s)

  • If the iron is repeated with bromine the reaction is less vigorous, with iodine there is little reaction, these reactions also illustrate the halogen reactivity series.

Advanced Inorganic Chemistry Page Index and Links


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

WHAT NEXT?

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 * 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


keywords phrases formula oxidation states balanced symbol equations:


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