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Dumb newbie with a ?

Dumb newbie with a ? - Chemistry Forum

Dumb newbie with a ? - Chemistry Forum. Discuss chemical reactions, chemistry.


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  #1  
Old 07-27-2003, 04:40 AM
Dave
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Default Dumb newbie with a ?



I am studying chem on my own with a book I bought from a used bookstore, and
ran into something today that I don't understand. Supposedly, if you react
HCl with solid sodium hypochlorite (which doesn't even exist in solid form,
from what I understand) you get HOCl and NaCl (I think). But I would expect
one to get NaCl, H20 and Cl2, according to the following:

NaOCl(s) + 2HCl(aq) -> H2O + NaCl(aq) + Cl2(g)

Is there really something like HOCl? What would that be called? Hydrogen
hypochlorite?

I've caught this book in a number of other (minor) errors, mostly using
constants slightly different from those listed in the book and a couple of
rounding errors, but this really looks like a screw-up. Am I wrong? I've
never heard of anything like HOCl.

Thanks for any help.

Dave
[Only registered users see links. ]





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  #2  
Old 07-27-2003, 05:06 AM
Buckleys
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Dave wrote:

Hypochlorous acid. Exists in solution only, in equilibrium with
hypochlorite ion and dissolved chlorine gas, and is why swimming pool pH
should be properly maintained.

Rob.
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  #3  
Old 07-27-2003, 05:55 AM
Bruce Hamilton
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"Dave" <[Only registered users see links. ].net> wrote:


Hypochlorous acid. Once you know the name you can search for details.

Here's an old sci.chem post to get you started. You may need to ensure that you
have a fixed pitch font to see the greaph correctly ( should be two sine curves
).

From: [Only registered users see links. ] (Bruce Hamilton)
Subject: Re: There's a Nobel in This One, was Re: Hypochlorous acid
Date: 1998/05/12
[ big snip ].

Well, I believe the " Bleaching Agents ( Survey ) " monograph
in the Kirk Othmer Encyclopedia of Chemical Technology " should
provide answers to most of the questions in these threads. Some
relevant snippets are below....

Chlorine-containing Bleaching Agents

There are four classes: chlorine, hypochlorites, N-chloro compounds,
and chlorine dioxide. The first three are classified as "available
chlorine" types, and are subject to the following equilibria, with
rapid rates in aqueous solution.

Cl2 (g) <==> Cl2 (aqueous) (1)
Cl2 (aqueous) + H2O <==> HOCl + H+ + Cl- (2)
HOCl <==> H+ + OCl- (3)
RR'NCl + H2O <==> HOCl + RR'NH (4)

Equilibrium constant at 25C for (2) = 3.94 x 10^-4M
Equilibrium constant at 25C for (3) = 2.88 x 10^-8M

The available chlorine is the equivalent amount of Cl2 required to
produce the oxidant via the above equations. Available chlorine is
also often defined as the chlorine liberated by the action of
dilute acids. In solutions, the concentration of available chlorine
in the form of hypochlorite or hypochlorous acid is called
free-available chlorine.

From the equations, above pH 9.5, more than 99% of available chlorine
is present as hypochlorite ions. The ratio of hypochlorous acid to
hypochlorite ion increases with decreasing pH until pH 5.5, below which
less than 1% of the available chlorine will be hypochlorite ions.

Below pH 6, Cl2 may be present, and the amount increases with decreasing
pH and increasing total available chlorine. With an available chlorine
concentration of 0.1%, Cl2 begins to appear about pH 4, and becomes
dominant about pH 2.5. With an available chlorine concentration of 10%,
Cl2 begins to appear about pH 6, and becomes dominant about pH 4.5.

For a closed system at 25C containing aqueous solution of 0.5 mass %
available chlorine with equimolar chloride, the following major species
concentrations apply;-

Mole
Fraction
1.0 | o o +
|x Cl2 o HOCl o + NaOCl
0.9 | x +
| o o
0.8 | x +
| o o
0.7 | x +
| o o
0.6 | x +
| o o
0.5 | x +
| o o
0.4 | x +
| o o
0.3 | x +
| o o
0.2 | x +
| o o
0.1 | x +
|o x + o
0.0 |____ ____ ____ ____ _x__ __+_ ____ ____ ____ ____o____ ____
| | | | | | | | | | | | |
0 1 2 3 4 5 6 7 8 9 10 11 12
pH

Other species that may be present in minor concentrations are Cl3-
from Cl(aq), and H2OCl+ and Cl2O from HOCl. Solutions of available
chlorine bleaches decompose on standing, with the decomposition
depending on several factors.
Hypochlorous acid; 3HOCl --> HClO3 + 2HCl
Hypochlorite anions; 3OCl- --> ClO3- + 2Cl-

The solutions are most stable above pH 11 where tthe decomposition
rate is nearly independent of pH. In this region, the decomposition
has a second-order dependance on the hypochlorite concentration, and
the rate increases with increasing ionic strength - thus concentrated
solutions will decompose faster than dilute solutions. The activation
energy is high, so the decomposition rate also quickly increases with
temperature. High pH ( >11 ) solutions with <6% available chlorine
have sufficient life at ambient temperatures to be consumer products.

Below pH 11, the decomposition also becomes dependent on pH, with the
rate increasing rapidly as pH decreases to 7 ( where the decomposition
rate peaks ), and then the rate decreases as the pH drops to 3, so that
at pH < 3 the rate is slow, but still faster than at pH 11.
Decomposition is also catalysed by :-
1. Trace concentrations of metals such as Co, Ni, and Cu, via
2OCl- --> O2 + 2Cl-,
2. UV light
3. Acids, via equations (1) and (2).


Sodium Hypochlorite

Usually produced as an aqueous solution by chlorination of bases
( sodium hydroxide is the most common, but also sodium carbonate )
using either chlorine gas or in-situ electrolysis of alkaline salt
solutions. Cl2 + 2NaOH <==> NaOCl + NaCl + H2O
The final pH of retail solutions is adjusted to above 11 to maximise
stability, and the salt is usually not removed. If low salt solutions
are required for specific applications, the starting solution will
be increased to above 20.5% NaOH, and some salt will precipitate, and
the precipitate is filtered off to produce 12 - 15% NaOCl solutions
with only half the salt content.

Solutions with very low salt concentrations are produced by reacting
high purity hypochlorous acid with metal hydroxides. Salt-free
hypochlorous acid solutions heve been economically made from steam
and chlorine ( US patent 4,584,178 ). These solutions may have
sufficient stability at 0C to be sold for industrial use.

Bruce Hamilton
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  #4  
Old 07-28-2003, 09:53 AM
Dave
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Buckleys <[Only registered users see links. ].au> wrote in message
news:[Only registered users see links. ].au...
and
react
form,
expect
Hydrogen
of
I've

Of course. Hypochlorous acid. That's what HOCl would be. Thank you. Lots
to learn still.

Dave
[Only registered users see links. ]



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  #5  
Old 07-28-2003, 09:54 AM
Dave
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Default Dumb newbie with a ?

Thanks for the detailed post, Bruce. I've already printed it out, and will
study it over the next couple of days.

Really appreciate the help.

Dave
[Only registered users see links. ]

Bruce Hamilton <[Only registered users see links. ].nz> wrote in message
news:[Only registered users see links. ]...
and
react
form,
expect
Hydrogen
that you
curves


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