3.1.3 (e, f) Disproportionation of Chlorine and Water Treatment

In a standard Redox reaction one element is oxidised and one is reduced - one loses electrons while the other gains them.

But if some atoms of an element are oxidised while some atoms of the same element are reduced it is called Disproportionation.

The above reaction of Chlorine with water is a syllabus requirement.

You should be able to write it when it's asked for.

You should be able to define disproportionation and show that this is an example by writing Oxidation Numbers on the Chlorines.

Cl_2(aq) + H₂O_(l) ⇌ HCl_(aq) + HOCl_(aq)

Oxidation No. Cl₂ = 0 - it's an element

Oxidation No. Cl in HCl = -1 - to balance the +1 H

Oxidation No. Cl in HOCl = -1 - to balance +1 H and -2 O

An examiner will want you to spell out that 0 → +1 is Oxidation and 0 → -1 is Reduction

OCl^- ions are a bleaching agent - which is how the test for Chlorine with damp blue litmus works.

They are still sometimes referred to as hypochlotrite ions but you should call then Chlorate (I).

The Litmus paper must be damp for the Chlorine to dissolve in it.

The HCl is acidic so blue Litmus paper will initially turn red.

But then it will go white as the HOCl bleaches it.

ClO- ions are also very good at killing microbes -hence the use of bleach to disinfect bathrooms

Chlorinating water has saved hundreds of thousands of lives that would otherwise have been affected by waterborne diseases like Cholera and Thyphoid.

What concerns some people are the "chlorinated hydrocarbons" that form when Methane from rotting matter reacts with Chlorine - most obviously CH₃Cl, Ch₂Cl₂ etc

There are suggestions that these may cause cancer - though the threat of death is much lower than in drinking non-chlorinated water.

Strictly speaking you are only meant to learn the reaction with cold dilute NaOH.

2NaOH + Cl₂ --> NaCl + NaOCl + H₂O

But you can work this one out if you've learned the reaction with water.

The Sodium Hydroxide is dissolved in water so the first thing that would happen when you bubbled Chlorine in Sodium Hydroxide solution is that the Chlorine would disproportionate.

Equation 1 Cl_2(aq) + H₂O_(l) ⇌ HCl_(aq) + HOCl_(aq)

So, the NaOH would react with the two acids HCl and HOCl to form two salts NaCl and NaOCl and two H₂Os

Equation 2 2NaOH + HOCl + HCl --> NaCl + NaOCl + 2H₂O

If we add the two equations together we get

Cl₂ + H₂O + 2NaOH + HOCl + HCl --> HCl + HOCl + NaCl + NaOCl + 2H₂O

2NaOH + Cl₂ --> NaCl + NaOCl + H₂O

Or, if you don't like that way of thinking about it you could just learn the equation.

Why is it worth knowing?

1. It's often on exams

2. NaOCl is one of the compounds in basic bleach.

1. (a) State why Chlorine is added to drinking water.

........................................................................................................................................................................(1)

(b) Write an equation for the reaction which occurs when Chlorine is bubbled into water.

Identify the substance which causes the resulting solution to be pale green.

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

Identity of substance ...........................................................................................................................................(2)

(c) Write an equation for the reaction which occurs when Chlorine is bubbled into an excess of cold aqueous Sodium Hydroxide.

.........................................................................................................................................................................(1)

(Total 4 marks)