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Answer to Question #17379 in Physical Chemistry for Mr Jenkins

Question #17379
You explain how pool chemicals react to a friend, they tell you, "But I don't understand why the sodium hypochlorite won't react any more with the chloride ions and the hydrogen ions." Knowing that this reaction displays an equilibrium, what would be a good response to their comment? When people sweat, their bodies often exude salts (including chloride ions) and acidic compounds (that produce hydrogen ions when dissolved in water). For this reason, people are asked in public pools to shower before jumping into the pool water.
Expert's answer
Cl2 reacts immediately with water:

Cl2 + H2O ⇋ HOCl + H+ + Cl-

The equilibrium constant is pH dependent and at the pH values of tap water and pool water (pH = 6.5-8.5) there is virtually no Cl2 present. Hypochlorous acid is a weak acid:

HOCl + H2O ⇋ OCl- + H3O+

such that at pH 7.5 [HOCl] ≈ [OCl]-
Both HOCl and OCl- are powerful oxidants destroying bacteria cells in seconds. HClO is however a significantly more powerful oxidant than OCl-. This is why sodium hypochlorite is used in pools as it is milder and easier to handle than Cl2 (which is also used with adjustment of pH):

OCl- + H2O ⇋ HOCl- + OH-

So hypochlorite ion does not react with Cl- at pH ≈ 7.5
Our bodies/bacteria give off cmpds with -NH2 groups (eg amino acids). These are converted by HOCl/OCl- to volatile NH2Cl and NHCl2 and give the pool water the distinctive "chlorine" smell. (I have smelled Cl2 and it is pretty much like heavily chlorinated pool water,however). OCl- slowly decomposes to chlorate ClO3- which is not a disinfectant:

3OCl- ⇋ 2Cl- + ClO3-

Anyway the OCl- levels gradually disappear over say three days.

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