Sciencemadness Discussion Board

Help with stoichiometry of a complex reaction: sulfuric acid and bleach

Cou - 18-2-2015 at 15:42

This is the website I read: http://www.ehow.com/about_6521382_sulfuric-acid-chlorine-ble...

The reaction between H2SO4 and NaClO has many middle steps, but this kind of reaction stoichiometry is out of my knowledge. How can I find out how much 98% sulfuric acid I need to completely react with a given amount of 8.25% bleach? And is the gas pure Cl2?

blogfast25 - 18-2-2015 at 16:26

Quote: Originally posted by Cou  
And is the gas pure Cl2?


Not really. Search for it on this forum, there are much better ways to prepare chlorine.

Brain&Force - 18-2-2015 at 16:26

Why don't you want to use HCl for making chlorine? It's a lot more efficient.

And I wouldn't trust eHow for chemistry at all...

blogfast25 - 18-2-2015 at 16:39

Quote: Originally posted by Brain&Force  
And I wouldn't trust eHow for chemistry at all...


What do you mean? It's Chemistry 101! ;)

Cou - 18-2-2015 at 17:08

Quote: Originally posted by Brain&Force  
Why don't you want to use HCl for making chlorine? It's a lot more efficient.

And I wouldn't trust eHow for chemistry at all...

Even 31% HCl vs 98% H2SO4?

Metacelsus - 18-2-2015 at 17:27

Yes, it will give a cleaner product. Acidifying hypochlorite salts gives hypochlorous acid, which will react (comproportionate) with hydrochloric acid to form chlorine gas. Without chloride ions, a substantial amount of Cl2O may be formed, which will eventually decompose into chlorine and oxygen.

Cou - 18-2-2015 at 18:36

Quote: Originally posted by Cheddite Cheese  
Yes, it will give a cleaner product. Acidifying hypochlorite salts gives hypochlorous acid, which will react (comproportionate) with hydrochloric acid to form chlorine gas. Without chloride ions, a substantial amount of Cl2O may be formed, which will eventually decompose into chlorine and oxygen.


Oooo... Hope that Cl2O isn't a carcinogen. I didn't get exposed to more than 20 PPM, even while not wearing my respirator.

[Edited on 19-2-2015 by Cou]

subsecret - 18-2-2015 at 19:08

I use TCCA and 31% HCl to make chlorine. For most purposes, run the gas through a tube stuffed with cotton or fiberglass, which should trap acid mists.

Darkstar - 18-2-2015 at 19:12

Quote: Originally posted by Cou  

Even 31% HCl vs 98% H2SO4?


In bleach, Cl2, HClO and ClO- are all in a sort of pH-dependent, three-way equilibrium with one another. When the pH is low, Cl2 dominates. When the pH is high, like your bleach, it's primarily ClO- ions. When the pH is closer to neutral, HClO forms. Since HClO and HCl are in equilibrium with Cl2 and H2O, adding additional protons using an acid shifts the equilibrium towards Cl2 production. The beauty of using hydrochloric acid is that you're not just adding more protons, but more chloride ions, too.

HClO + HCl ⇌ Cl2 + H2O

If you want the more technical version, HCl is the better acid because HClO is also in equilibrium with Cl2O. When you add an acid to bleach, the first thing that happens is the ClO- ions get protonated to give HClO. If the HClO is protonated a second time, it's then activated for nucleophilic attack on its chlorine. If a chloride ion attacks, Cl2 and H2O are produced. However, if the oxygen on another HClO/ClO- molecule attacks, H2O and Cl2O are produced instead. By using HCl, you are adding a bunch of chloride ions, greatly increasing the probability that it will be a chloride ion that ends up being oxidized.

In my opinion, though, HCl + TCCA (trichloroisocyanuric acid. can be purchased in bulk quantities from pool supply stores) is a FAR superior method for Cl2 production.
Quote: Originally posted by blogfast25  
Quote: Originally posted by Brain&Force  
And I wouldn't trust eHow for chemistry at all...


What do you mean? It's Chemistry 101! ;)


Probably because you see things like this:
Quote:

In water solutions, hypocholorite (HClO) and chlorine (Cl2) reach an equilibrium that is dependent on the pH of the solution. In an acidic solution, the equilibrium favors chlorine in the following fashion: Hypochlorous acid partially breaks down into the hypochlorite anion (OCl?) and hydrogen cation (H+). Hypochlorous acid is a strong oxidant, so the remaining hypochlorous acid in the solution oxidizes the hypochlorite anion producing the irritating and toxic chlorine gas (Cl2).


ClO- + HClO + H+ ⇌ Cl2 + ?

That doesn't even make sense. I get that these articles are written for beginners and those just getting into chemistry, but come on. Keeping it simple by calling protons H+ instead of H3O+ is one thing, but this is just wrong. What most likely happens is the formation of a hypochloronium ion, followed by nucleophilic substitution by an aqueous choride ion to give water and Cl2 (as mentioned above).

ClO- + H+ ⇌ HClO
HClO + H+ ⇌ H2OCl+
H2OCl+ + Cl- ⇌ Cl2 + H2O


[Edited on 2-19-2015 by Darkstar]

Molecular Manipulations - 19-2-2015 at 06:19

Quote: Originally posted by Cou  


Oooo... Hope that [insert any chemical ever] isn't a carcinogen.

I doubt it, but it might give you metal fume fever!
But really, do you not have hydrochloric acid?

Metacelsus - 19-2-2015 at 11:49

If you have sulfuric acid, you can easily make it from salt.

Cou - 19-2-2015 at 14:53

Quote: Originally posted by Molecular Manipulations  
Quote: Originally posted by Cou  


Oooo... Hope that [insert any chemical ever] isn't a carcinogen.

I doubt it, but it might give you metal fume fever!
But really, do you not have hydrochloric acid?

Sorry, I'm a chemophobe... just because someone is chemophobic doesn't mean they can't be an amateur chemist :p

Texium - 19-2-2015 at 20:18

Quote: Originally posted by Cou  
Quote: Originally posted by Molecular Manipulations  
Quote: Originally posted by Cou  


Oooo... Hope that [insert any chemical ever] isn't a carcinogen.

I doubt it, but it might give you metal fume fever!
But really, do you not have hydrochloric acid?

Sorry, I'm a chemophobe... just because someone is chemophobic doesn't mean they can't be an amateur chemist :p
Umm, sorry, but it kind of does mean that... if you use reasonable safety procedures whenever necessary and know what you're doing, you have nothing to worry about. You can't go very far when you limit yourself so much.
I've observed that you often tend to post rather distressed and dramatic posts regarding both lab safety and legal issues in multiple threads. Chill out dude. You get overly worked up about this stuff. Respect chemistry, but do not fear it.

AJKOER - 19-3-2015 at 12:37


An interesting paper: "Effect of chloride ion on the kinetics and mechanism of the reaction between chlorite ion and hypochlorous acid." Available from: http://www.researchgate.net/publication/23141635_Effect_of_c... . On page 6, to quote:

"The most interesting steps are the chloride-catalyzed ones (kr1 and kr6). Both are appearing when HOCl is one of the reactant. For the interpretation of the chloride catalysis, we should start from the plausible assumption of Eigen and Kustin, that the hydrolysis of chlorine in fact is a two-step process, namely:

Cl2 + H2O = Cl2OH- + H+ (R8)
Cl2OH- = HOCl + Cl- (R9)

.... Assuming Cl2OH-to be the reacting species, the third-order V′1 = k′1 [HClO2][HOCl][Cl-] equation can be replaced by V′1= k′1KR9eq[HClO2][Cl2OH-]. Similar transformation applies for kr6. It seems to be a reasonable assumption that these equilibria and the increased reactivity of Cl2OH-are responsible for the appearance of chloride catalysis in this and in many other redox reactions of the oxychlorine species. As chloride ion is one of the product of the reaction, it is in fact an autocatalyst. The difference between the rate of the uncatalyzed and autocatalyzed path is not very high, thus the typical autocatalytic feature of the product formation does not appear on the experimental curves. "