AS Chemistry 9701: Disproportionation of Chlorine in Water (Group 17)

Chlorine gas is used to disinfect water supplies. When chlorine gas is bubbled directly into pure water (rather than into sodium hydroxide solution), it sets up a reversible reaction, forming a mixture of hydrochloric acid and chloric(I) acid, $\text{HOCl}$ .

(a) Write a balanced equation, including state symbols and an equilibrium sign, for the reaction of chlorine gas with water. [2]

(b) State the oxidation number of chlorine in $\text{Cl}_2$ , in $\text{HCl}$ , and in $\text{HOCl}$ , and explain why this reaction is classified as a disproportionation reaction. [3]

(c) $\text{HOCl}$ is the active bactericidal species responsible for killing microorganisms in the treated water. Suggest one reason, in terms of the trend in volatility down Group 17, why chlorine (rather than bromine or iodine) is the halogen most commonly dosed directly as a gas into water supplies for large-scale disinfection. [2]

Part (a): Chlorine reacting with water

Chlorine dissolves in water and reacts reversibly to form a mixture of hydrochloric acid and chloric(I) acid:

$\text{Cl}_2\text{(g)} + \text{H}_2\text{O(l)} \rightleftharpoons \text{HCl(aq)} + \text{HOCl(aq)}$

The equilibrium sign shows that the reaction does not go to completion. A significant amount of dissolved $\text{Cl}_2$ remains in equilibrium with the two acids, which is why chlorine’s characteristic smell can still be detected in treated water.

Part (b): Oxidation numbers and disproportionation

Working out the oxidation number of chlorine in each species:

In $\text{Cl}_2$ , an element in its standard state, chlorine has oxidation number $\boxed{0}$ .
In $\text{HCl}$ , hydrogen is $+1$ and the compound is neutral overall, so chlorine is $\boxed{-1}$ .
In $\text{HOCl}$ , hydrogen is $+1$ and oxygen is $-2$ ; for the compound to be neutral, chlorine must be $\boxed{+1}$ .

Chlorine therefore starts at oxidation number $0$ in $\text{Cl}_2$ and, in the very same reaction, some chlorine atoms are reduced to $-1$ (in $\text{Cl}^-$ , within $\text{HCl}$ ) while others are oxidised to $+1$ (in $\text{HOCl}$ ). A reaction in which the same element is simultaneously oxidised and reduced is a disproportionation reaction, exactly the same pattern seen when chlorine reacts with sodium hydroxide.

Part (c): Why chlorine is used for large-scale water treatment

Down Group 17, the strength of the instantaneous dipole–induced dipole (van der Waals’) forces between molecules increases as the molecules get larger, so volatility decreases: chlorine is a gas, bromine a liquid, and iodine a solid at room temperature.

Because chlorine is a gas, it can be stored under pressure, piped, and bubbled directly into large volumes of water, where it disperses and mixes rapidly and evenly throughout the supply. Bromine, being a liquid, would need to be carefully metered in by other means and would mix and disperse more slowly and less evenly. Iodine, being a solid with very limited solubility in water, would dissolve far too slowly to be practical for treating large, continuously flowing volumes of water. Chlorine’s high volatility relative to bromine and iodine is therefore what makes it the most practical and effective choice for large-scale water disinfection.

Final answers

(a) $\text{Cl}_2\text{(g)} + \text{H}_2\text{O(l)} \rightleftharpoons \text{HCl(aq)} + \text{HOCl(aq)}$
(b) $\text{Cl}_2 = 0$ , $\text{Cl}$ (in HCl) $= -1$ , $\text{Cl}$ (in HOCl) $= +1$ ; chlorine is simultaneously oxidised and reduced, so this is disproportionation.
(c) Chlorine’s high volatility (it is a gas) allows it to be dosed and dispersed quickly and evenly through large volumes of water, unlike the liquid bromine or the poorly water-soluble solid iodine.