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Topic: chlorine in solution  (Read 9881 times)

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Offline plu

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chlorine in solution
« on: February 07, 2006, 09:17:46 AM »
Chlorine gas, when dissolved in water, produces an aqueous solution that
a) has a pH more acidic than 7, and has no redox properties.
b) has a pH more basic than 7, and is an oxidizing agent.
c) is neutral and is reducing agent
d) has a pH more acidic than 7 and is an oxidizing agent.
e) has a pH more acidic than 7 and is a reducing agent.

At first I believed the answer to be c.  However, wouldn't chlorine in solution be an oxidizing agent?  As well, how would chlorine in solution be either basic or acidic?
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Offline Borek

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Re:chlorine in solution
« Reply #1 on: February 07, 2006, 09:30:21 AM »
Bleach, disproportionation - these should push you in the right direction.
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Offline jdurg

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Re:chlorine in solution
« Reply #2 on: February 07, 2006, 03:57:40 PM »
Quote from: Borek on February 07, 2006, 09:30:21 AM
Bleach, disproportionation - these should push you in the right direction.

Chlorine in water is not bleach.  Chlorine in sodium hydroxide is bleach, however.   ;) ;D
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Offline jdurg

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Re:chlorine in solution
« Reply #3 on: February 07, 2006, 03:59:31 PM »
Another hint is that if you take a solution of chlorine dissolved in water and add it to some potassium bromide, elemental bromine will be formed.  That should give you a REALLY good clue as to whether or not chlorine water is an oxidizing or reducing agent.
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Offline Borek

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Re:chlorine in solution
« Reply #4 on: February 07, 2006, 04:24:37 PM »
Quote from: jdurg on February 07, 2006, 03:57:40 PM
Chlorine in water is not bleach.  Chlorine in sodium hydroxide is bleach, however.   ;) ;D

I have not stated that chlorine in water is bleach.

Difference is only quantitative. Qualitatively in both cases the same reaction takes place, just hydroxide is used to move equilibrium to the right. If plu understands how the bleach is made, he should have no problems with understanding what happens when chlorine is added to water and what will be the final pH.
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Offline jdurg

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Re:chlorine in solution
« Reply #5 on: February 07, 2006, 07:43:23 PM »
Quote from: Borek on February 07, 2006, 04:24:37 PM
I have not stated that chlorine in water is bleach.

Difference is only quantitative. Qualitatively in both cases the same reaction takes place, just hydroxide is used to move equilibrium to the right. If plu understands how the bleach is made, he should have no problems with understanding what happens when chlorine is added to water and what will be the final pH.

Not really.  HOCl is an acid, not a base.  The reason for the basicity of a sodium hypochlorite solution is because the free hypochlorite ion will take a hydrogen from water and form HOCl and OH-.  When chlorine gas reacts with water, it forms HCl and HOCl.  The HCl readily ionizes and gives the free H+ ions, but the HOCl remains bound as a fairly weak acid.  It is not the same as the reaction involving NaOH and Cl2.  When Cl2 is mixed with NaOH, you get the formation of NaOCl and a little bit of HCl.  The HCl is really pretty miniscule in comparison to the NaOCl, and the OCl- ions will grab H+ from water to form the weak acid HOCl.  This leaves behind free OH- ions which makes the solution basic.

So the reaction between chlorine and water isn't the same as that between chlorine and sodium hydroxide (to form bleach).  Chlorine and water would give a solution that is a pretty good oxidizer and has a pH less than seven.
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Offline Borek

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Re:chlorine in solution
« Reply #6 on: February 08, 2006, 04:16:27 AM »
Quote from: jdurg on February 07, 2006, 07:43:23 PM
Not really.  HOCl is an acid, not a base.  The reason for the basicity of a sodium hypochlorite solution is because the free hypochlorite ion will take a hydrogen from water and form HOCl and OH-.

While you have correctly described the hydrolyzis of the weak acid, I have no idea how it refers to my post :(

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When chlorine gas reacts with water, it forms HCl and HOCl.  The HCl readily ionizes and gives the free H+ ions, but the HOCl remains bound as a fairly weak acid.

Correct. It is disproportionation reaction I have mentioned. Amounts of HCl and HOCl are identical.

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It is not the same as the reaction involving NaOH and Cl2.  When Cl2 is mixed with NaOH, you get the formation of NaOCl and a little bit of HCl.

Write reaction equation describing how the amount of HCl can be different from the amount of HOCl created.

Quote
The HCl is really pretty miniscule in comparison to the NaOCl, and the OCl- ions will grab H+ from water to form the weak acid HOCl.  This leaves behind free OH- ions which makes the solution basic.

No need for hydrolyzis here - solution is already basic thanks to the NaOH present. Exactly opposite happens - in high pH HOCl behaves as every acid does - it gets neutralized lowering pH. That's why we refer to the bleach as the sodium hypochlorite solution. It is basic thanks to excess NaOH - it keeps hypochlorite stable. Remember your drinking/peeing/chlorine adventure?  ;)

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So the reaction between chlorine and water isn't the same as that between chlorine and sodium hydroxide (to form bleach).  Chlorine and water would give a solution that is a pretty good oxidizer and has a pH less than seven.

It is the same reaction. If you think they are different - write down both reactions equations. Remember in water all H2O, H+ and OH- are always present.
« Last Edit: February 08, 2006, 04:19:38 AM by Borek »
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Offline plu

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Re:chlorine in solution
« Reply #7 on: February 08, 2006, 07:50:29 AM »
If the solution contains OCl- ions and Cl- ions, how is it a good oxidizer?  Are not both of these ions primarily reducers?  Or is it due to the fact that the solution still contains a certain amount of undissociated Cl2?
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Offline jdurg

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Re:chlorine in solution
« Reply #8 on: February 08, 2006, 08:36:00 AM »
Quote from: plu on February 08, 2006, 07:50:29 AM
If the solution contains OCl- ions and Cl- ions, how is it a good oxidizer?  Are not both of these ions primarily reducers?  Or is it due to the fact that the solution still contains a certain amount of undissociated Cl2?

Very little OCl- exists in solution.  Still, the hypochlorite ion is a pretty potent oxidizer as it is not all that stable.  HOCl is also a pretty good oxidizer, hence why the solution is a good oxidizer.  
« Last Edit: February 08, 2006, 08:41:27 AM by jdurg »
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