2.6.3 Redox Reactions of the Halogens i.e. Group VII elements

Introduction Redox comes from reduction and oxidation Oxidation Is Loss of electrons Reduction Is Gain of electrons Remember this by OIL RIG Halogens are powerful oxidising agents They remove [take] electrons from other elements to become negative ions

Reaction of chlorine gas with potassium iodide Experiment 1 Reaction of chlorine gas with potassium iodide

Safety Precautions KMnO4 – powerful oxidising agent and irritant HCl - corrosive Cl2 - poisonous KI – irritant Wear safety glasses and rubber gloves

Apparatus Test tubes Potassium manganate(VII) crystals Conc. Hydrochloric acid Potassium iodide solution Litmus paper [blue]

Method Pour some KI solution into a test tube. Place some potassium manganate(VII) crystals [KMnO4] in a test tube Add some conc. HCl Note what happens. Pour the chlorine gas into the potassium iodide and shake.

Results A greenish yellow gas is produced when the conc. HCl is dropped onto the KMnO4 this gas is chlorine [Cl2] MnO4- + HCl = Mn2+ + H2O + Cl2 It is heavier than air It has a characteristic smell [swimming pool] It turns blue litmus red - then bleaches it When chlorine is poured into KI solution turns from colourless to brown as iodine is released.

Cl2(g) + 2 KI(aq) = 2 KCl(aq) + I2(aq) Another way of writing the equation is Cl2(g) + 2 I-(aq) = 2 Cl- (aq) + I2(aq) the chlorine has displaced the Iodine and so is more reactive The chlorine has been reduced [gained an electron to become Cl -] The iodide ion I- in KI has been oxidised [lost an electron to become I2 ] The chlorine is an oxidising agent.

Experiment 2 Reaction of Cl2 with KBr

Make chlorine as in last experiment MnO4- + HCl = Mn2+ + H2O + Cl2 Put some KBr solution into a test tube Pour the Cl2 into the test tube of KBr Note what happens The KBr solution turns from colourless to red/brown as bromine is released

Cl2(g) + 2 KBr(aq) = 2 KCl(aq) + Br2(aq) the chlorine has displaced the Bromide ion and so is more reactive The chlorine has been reduced [gained electron to become Cl-] The bromide ion Br- in KBr has been oxidised [lost a electron to become Br2.] The chlorine is an oxidising agent. Iodine is darker than Bromine

Conclusions F- Cl- Br- [O2-] I- [SO42-] More Reactive Non-metals can also be arranged in a reactivity series. Less Reactive

Experiment 3 Reaction of Cl2 with Fe2+

Apparatus KMnO4 crystals Conc. HCl Iron (II) sulphate solution FeSO4

Method Make up a solution of iron(II)sulphate it is pale green in colour Make some chlorine gas mix the gas and the iron(II)sulphate solution Note what happens the solution turns brown 2 Fe2+ + Cl2 = 2 Fe3+ + 2 Cl- Green Brown the Fe2+ has been oxidised to Fe3+

Key Points to remember OIL RIG F most reactive More reactive element displaces less reactive from solution

Reaction of Cl2 with Na2SO3 Experiment 4 Reaction of Cl2 with Na2SO3

Apparatus KMnO4(s) Conc. HCl Sodium sulphite solution [Na2SO3] Barium Nitrate Solution

Make up a solution of sodium sulphite Test it with Barium nitrate solution A white precipitate forms Add HCl and the precipitate re-dissolves This tells us the solution is a sulphite Take a fresh sample of the solution react it with some chlorine Add barium nitrate solution Note what happens

A white precipitate forms Add Hydrochloric acid to see what happens to the precipitate The precipitate does not re-dissolve the sulphite has been oxidised to a sulphate Cl2 + SO32- + H2O = 2 Cl- + SO42- + 2H+ look at oxidation numbers S goes from +4 to +6 so oxidised by Cl2