Question

Why does bleach initially have a basic pH but an acidic pH upon exposure to air?

Cl2 + 2OH- <-->ClO- + Cl- + H2O

I thought that the chlorine gas concentration would decrease upon exposure to atmospheric air, since it would diffuse away. This would cause the equilibrium to shift toward the reactant side, producing more chlorine and more hydroxide ions. Shouldn't this cause the pH to increase over time though?

Answer 1

Well I'm not familiar with this reaction but I will try to answer you question to the best of my ability.

bleach usually consisting mainly of sodium hypochlorite

From wikipedia I got this equation,
4 NaClO + 2 H2O → 4 Na+ + 4 OH- + 2 Cl2 + O2
which shows the decomposition of sodium hypochlorite when in solution with water

Cl2 is your chlorine gas which will slowly enter the atmosphere
O2 will also leave as a gas
leaving you with sodium and hydroxide ions

Alright from what I can guess, the pH will start off more acidic because sodium hypochorite is less basic than sodium hydroxide.

There will be a lot of water around pushing the reactants towards products. Eventually all you will be left with is sodium hydroxide solution. As the constant loss of products will pull the reaction towards the products side as well as the large amount of water present to push the reaction forward.

This will result in a sodium hydroxide solution, and an increased pH and thus a more basic solution. Due to hydroxide ions being more acidic than sodium hypochlorite.

Hope this helps, if I'm incorrect please correct me.

Answer 2

First: 4NaClO + 2H2O --> 4Na+ + 4HO- + 2Cl2 + O2
Then: Cl2 + H2O --> HCl + HClO-
Also: HO- + Cl2 --> HCl + ClO-
The HCl is responsible for the delayed acidity and as the reaction produces it's own O2, it probably would occur even in the absence of air.