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Topic: Br2 solubility  (Read 20234 times)

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Offline kle89

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Br2 solubility
« on: April 20, 2008, 10:50:57 PM »
I just recently did this experiment where i had to test these halogens solubility in acetone, water and cyclohexane. Now for Br2 i got nearly full solubility in cyclo and acetone and not in H2O, but i don't know what the reasoning is exactly. Does it have to do with polarity? If it did then i wouldve expected water to be a fairly decent solvent compared with cyclohexane (or maybe i pulled a lab error [oops!] and mislabled them) thanks in advance.

Offline Alpha-Omega

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Re: Br2 solubility
« Reply #1 on: April 21, 2008, 02:53:03 AM »
Halogens readily form a variety of covalent compounds with non-metals, where a filled octet is achieved by sharing pairs of electrons. 

As covalent molecules with non-polar bonds (e.g., Br2, Cl2, I2), the halogens dissolve rather poorly in water.  This is especially the case for  iodine, I2 (although in the presence of iodide ions, the solubility of iodine in water is enhanced by the formation of I3- species). 

Halogens dissolve readily in organic solvents, such as carbon tetrachloride (tetrachloromethane), ether (CH3OCH3) and ethanol (C2H5OH). 

On the other hand, alkali metal halides (and many other metal halides) are soluble in water. Additionally, the binary compounds with hydrogen, HX, e.g. hydrogen chloride, are covalent diatomic molecules that are gases at room temperature.  The H-X bond is polar; thus in the presence of water, hydrogen halides dissolve readily and ionize to give strong hydrohalic acids (H3O+ + Cl-), e.g. hydrochloric acid.

Nonmetal halides are generally hydrolyzed to a hydrogen halide and to an oxy-acid
containing the other element.

When pure bromine is added to water, it forms hypobromous acid and hydrobromic acid (HBr):

Br2(l) + H2O(l) → HOBr(aq) + HBr(aq)

When bromine dissolves in water, it partially disproportionates.

Br2 +H2O <-----> HOBr + H+ + Br-

The equilibrium constant for this reaction at 25°C is 7.2×109 M2 . Light catalyzes the decomposition of hypobromous acid to hydrogen bromide and oxygen.

HOBr ----(hν)--> HBr + 1/2O2

In the dark, hypobromous acid decomposes to bromic acid and bromine. Bromic acid is relatively unstable and decomposes slowly to give bromine and oxygen.

5HOBr ------> HBrO2 + 2Br2 + 2H2O

4HBrO3 ---> 2Br2 + 5 O2 + 2H2O


In alkaline solution, bromine reacts rapidly to produce hypobromite.

Br2 + 2OH- <--------> Br- + BrO- + H2O K = 2 x 108

It is necessary to maintain this reaction below 0°C to minimize the disproportionation of hypobromite to bromate and bromide.

3BrO- ----> 2Br- + BrO3-

References: 

Kirk-Othmer Encyclopedia of Chemical Technology Copyright © 2001 by John Wiley & Sons, Inc.  Baruch Grinbaum, IMI(TAMI) Institute for Research and Development Mira Freiberg, Dead Sea Bromine Group

Wikipedia



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