Topic 1.7: Group 7 (chlorine to iodine)

AS and A2 GCE Chemistry

1.7a Physical properties of the elements
They have characteristic odours and colours.

Task 1.7a.1
Task1.7a.2

1.7b Chemical tests for the elements
Chlorine turns blue litmus red then bleaches it.  Chlorine displaces bromine and iodine from bromide or iodide solutions.
Cl₂₍aq) + 2KBr(aq) ---> 2KCl(aq) + Br₂(aq)
Cl₂₍aq) + 2KI(aq) ---> 2KCl(aq) + I₂(aq)

Bromine is red if dissolved in an organic solvent.  Bromine displaces iodine from an iodide solution.
Br₂₍aq) + 2KI(aq) ---> 2KBr(aq) + I₂(aq)

Iodine turns starch black, is decolourised by thiosulphate solution and is brown in water but purple in organic solvents.

See reactions here
Task 1.7b
Describe what you would see and explain the observations for making the following mixtures 
(a) chlorine + sodium bromide, (b) iodine and sodium chloride (c) bromine and sodium iodide.

1.7c Hydrogen halides in water
Hydrogen chloride, hydrogen bromide and hydrogen iodide are all very soluble as water molecules pull hydrogen ions away from HCl.  
H₂O^D-  ..... H⁺
HCl is strongly acidic due to complete dissociation.
HX(aq) + H₂O(l) -----> H₃O⁺(aq) + X^-(aq)
Hydrogen ions H⁺ or   H₃O⁺ provide the acidity.
relative acid strength is HF <<  HCl < HBr < HI.  HI strongest as H-I bond is the longest weakest bond (299kJmol^-1 for H-I compared to 366kJmol^-1 for H-Br, 431kJmol^-1 for H-Cl and 562kJmol^-1 for H-F).  HF is also less dissociated because of the following interaction
H-F(aq) + F^-(aq) --> [F----H-F]^-(aq)  HF does attack glass but only because it is able to form the soluble ion SiF₆^2- and this is not an acid base reaction.
Task 1.7c State and explain the bond strengths in H-X.  Explain the relative acid strengths of (i) HBr and HCl. (ii) HCl and HF.

1.7d The identification of chloride, bromide and iodide ions
Adding aqueous silver nitrate to an aqueous halides mixed with nitric acid forms a silver halides.
Ag⁺(aq) + X^-(aq) -----> AgX(s)
Silver chloride is white, silver bromide is buff colour and silver iodide is yellow.  The silver halides are unstable in the presence of sunlight. They decompose forming silver (seen as dark specs) and the halogen, for example:
2AgI(s) ----> 2Ag(s) + I₂(s)
Silver chloride dissolves in dilute ammonia.  Precipitates of silver chloride and silver bromide react with  conc. ammonia solution to form the diammine silver(I)ion [Ag(NH₃)₂]⁺(aq) so the precipitates dissolve.  Silver iodide does not dissolve in even conc. ammonia solution.
Task 1.7d 
Write balanced chemical and ionic equations for the reactions of aqueous solutions of:
(i) sodium chloride and silver nitrate, (ii) lithium bromide and silver nitrate, (iii) potassium iodide and silver nitrate, (iv) silver chloride and ammonia.

1.7e Halide salts with concentrated sulphuric acid
Hydrogen chloride is more volatile than the sulphuric acid (or hydrogen sulphate). Therefore concentrated H₂SO₄ displaces hydrogen chloride from sodium chloride:
NaCl(s) + H₂SO₄(l) ----> NaHSO₄(s) + HCl(g).     displacement
HBr can similarly be displaced from KBr but some of the HBr is oxidised by the sulphuric acid.
KBr(s) + H₂SO₄(l) ----> KHSO₄(s) + HBr(g)         displacement
2HBr(g) + H₂SO₄(l) ----> Br₂(aq) + 2H₂O + SO₂(g)   oxidation of HBr
Conc. sulphuric acid also oxidises HI but the reaction goes further as HI is a better reducing agent.  Hydrogen sulfide has a rotten egg smell.
8HI(g) + H₂SO₄(l) ----> 2I₂(s) + H₂S(g) + 4H₂O(l)    oxidation of HI
Task 1.7e Describe observations and write balanced chemical equations for:
(i) lithium chloride + conc. sulfuric acid, (ii) sodium bromide + conc. sulfuric acid, 
(iii) potassium iodide + conc. sulfuric acid.

1.7f  The +1 and +5 oxidation states of chlorine
All the halogens except for fluorine exhibit a number of positive oxidation states. These are due to the promotion of electrons from p orbitals into vacant d orbitals.  Chloric (I) acid HClO forms salts called chlorate(I)'s e.g. NaClO. which contain the ion ClO^- .  Chloric acid (V) HClO₃ forms a series of salts called chlorate(V)'s.  For example sodium chlorate(V), NaClO₃ contains the ion ClO₃^-.  These salts and acids are all good oxidising agents.
Story 1.7f    
Task 1.7f  Write formulae for the following: sodium chlorate(I), potassium chlorate(V), sodium bromate(V) and potassium iodate(V).

1.7g Disproportionation of chlorine
Disproportionation is the simultaneous oxidation and reduction of atoms of the same element in the same reaction.  Chlorine (ox. no. = 0) disproportionates in water or alkali to form chloride (ox no. =-1) and chlorate(I) ions (ox. no. = +1).
Cl₂(g) + 2H₂O(l) ----> Cl^-(aq) + H₃O⁺(aq) + HClO(aq)
Cl₂(g) + 2OH^-(aq) -----> Cl^-(aq) + ClO^-(aq) + H₂O(l).
Further disproportionation of ClO^- can now occur on heating to form chlorate(V) ions.
3ClO^-(aq) -----> 2Cl^-(aq) + ClO₃^-(aq)
Task 1.7g  Write equations to show how chlorine reacts with water and with sodium hydroxide.  Explain how these are both example of disproportionation.  Explain how potassium bromate(I) might disproportionate in aqueous solution.

1.7h Halogens as oxidising agents and bromine extraction
Oxidising power of the halogens decrease down the group. 
Chlorine is therefore more strongly oxidising than bromine. 
Bromine is extracted from sea water at Amlwch in Anglesey in North Wales.
The raw materials are sea water (0.0065% by mass bromine) chlorine and sulphur dioxide.
Evaporation concentrates the salts containing bromides. Acid is added to the sea water to drop its pH to 3.5.  This prevents bromine reacting with water later in the process.
Chlorine gas is passed through the sea water when bromide ions are oxidised to bromine.
Cl₂(g) + 2Br^-(aq) -----> 2Cl^-(aq) + Br₂(aq)
Reactions of chlorine and bromine with water are prevented by the acidic conditions which force the following equilibria to the left.
Cl₂(aq) + H₂O <--> HClO(aq) +H⁺(aq) + Cl^-(aq)
Br₂(aq) + H₂O <--> HBrO(aq) +H⁺(aq) + Br^-(aq)
Air is blasted into the solution causing a stream of bromine gas in moist air to be formed.
The bromine gas formed is reduced with sulphur dioxide to form a solution of hydrobromic acid.
Finally this solution (now containing 13% bromine) is treated with chlorine again to convert the bromide to molecular bromine.  It is separated by distillation.
Task 1.7h Draw a flow chart showing bromine manufacture with formulae of all substances involved shown entering, exiting or flowing through the system.