3.1.3 (b,c,d) Redox reactions and reactivity of Halogens and their compounds

So when the syllabus says that you should know about the s²p⁵ electron configuration of Halogens they are just saying what type of orbitals these 7 electrons are in.

But you already knew that by now.

And you knew in Year 10 that Halogen atoms need to gain 1 electron to complete their outer shells.

You must be getting sick of OILRIG by now.

So you should realise that all Halogens are Oxidising Agents because they all need to gain electrons and this involves taking electrons from some other atom - thereby oxidising it.

As always at A level there is a trend in how well the elements in the groups can gain electrons.

And, as always, its a competition between Atomic Radius & Shielding versus Nuclear Charge.

This is an experiment you may well do, or have already done.

The first tube: Adding Chlorine water to KCl is a waste of time. Chlorine won't displace Chloride. The tube stays colourless (or at least very pale yellow)

The next two tubes: When KBr and KI are added to Chlorine water there is a colour change in both tubes.

We should expect this because Chlorine is smaller than Br or I, so capable of taking electrons from Br^- or I^- ions.

This means that Chlorine will be converted to Chloride ions while Bromine and Iodine will be made from Bromide and Iodide ions.

Cl_2(aq) + 2I^-_(aq) → 2Cl^-_(aq) + I_2(aq)

_{Observations: Slightly green/yellow solution goes brown}

Cl_2(aq) + 2Br^-_(aq) → 2Cl^-_(aq) + Br_2(aq)

_{Observations: Slightly green/yellow solution goes orange.}

_{(note that Group 1 halide solutions have no colour )}

The next three tubes; Adding Bromine water to KBr is a waste of time. Bromine won't displace Bromide. The tube stays orange (Bromine)

When KCl and KI have Bromine water added there is a colour change in one tube only.

We should expect this because Bromine is smaller than I but not Cl.

So, Bromine is capable of taking electrons from Br^- ions but not from Cl^- ions.

This means that in the Br₂/KI mixture Bromine will be converted to Bromide ions while and Iodine will be made from Iodide ions.

But there will be no reaction in the Br₂/KCl mixture

Br_2(aq) + 2I^-_(aq) → 2Br^-_(aq) + I_2(aq)

_{Observations: Orange solution goes brown}

The last three tubes: Adding Iodine water to KI is a waste of time. Iodine won't displace Iodide. The tube takes the colour of the Iodine - brown if concentrated enough, yellow if very dilute

When KCl and KBr have Iodine water added there is no colour change in either tube .

We should expect this because Iodine is larger than Br and Cl, so not capable of taking electrons from either ions.

So there will be no reaction in the I₂/KCl mixture or the I₂/KBr mixture

Both tubes takes the colour of the Iodine - brown if concentrated enough, yellow if very dilute.

Organic solvents that can't Hydrogen-bond - like cyclohexane - float on water.

Halides dissolve well in water because they are charged.

Ahlogens aren't so they tend to move into the organic layer where they can interact with the solvent via van der Waals forces.

The only halogen that changes colour when this happens is Iodine - which is brown in water but purple in most organic solvents.

This doesn't make any difference to the equations you should write, just the observations if Iodine is made.

1.

(a) (i) Describe what you would observe when an aqueous solution of Bromine is added to an aqueous solution containing Iodide ions. Write an equation for the reaction occurring.

Observation ...........................................................................................................................................................

Equation ...............................................................................................................................................................

(ii) Explain why Bromine does not react with aqueous Chloride ions.

...........................................................................................................................................................................

.........................................................................................................................................................................(3)

(b) Describe what you would observe when aqueous silver nitrate is added to separate aqueous solutions of Potassium Fluoride and Potassium Bromide.

Observation with KF(aq) ..........................................................................................................................................

Observation with KBr(aq) ......................................................................................................................................(2)

1. (a) (i) Observation: brown solution or black solid (1)

purple wrong

Equation: Br₂ + 2I^- --> I₂ + 2Br^- (1)

Allow NaI, KI

(ii) Br₂ is a weaker oxidising agent than Cl₂ (1) (or converse)

OR Br₂ is less reactive than Cl₂

penalise Cl, Br, Cl^-, Br^-etc

(b) Observation with KF (aq): no change (1) (or colourless)

Observation with KBr(aq): cream/off white ppt (or solid) (1)