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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: AdiDex on February 21, 2016, 02:02:29 PM

Title: Why Hg-Zn is used ??
Post by: AdiDex on February 21, 2016, 02:02:29 PM

Why we use Zinc Amalagam instead of Zinc only ??

I reached to the conclusion , We need a solvent for our reaction it should be Aprotic-Polar solvent (with high Boiling point) but Zinc will not get dissolved easily in it . Since Hg belongs to Zn group it will dissolved easily more over they are similar in their chemical properties .

We have aprotic Polar solvent -
DMSO - Boiling Point - 189 °C
DMF   - Boiling Point - 153 °C
Other aprotic solvents have generally lower temperature than 200 °C .

Where as Hg has Boiling point 356 °C . Moreover it is quite dense liquid so it entraps the reactants .

any other reasons ?? Above explanation is reasonable  ??

Moreover I got
http://www.orgsyn.org/demo.aspx?prep=CV6P0289 (http://www.orgsyn.org/demo.aspx?prep=CV6P0289)

In this, Hg is not used .

Title: Re: Why Hg-Zn is used ??
Post by: AWK on February 21, 2016, 03:52:51 PM

Neither Zn nor Zn amalgam practically do not dissolve in organic solvents.
For an efficient reduction you should use an activated zinc dust (read note 6 in your link to Clemmensen reduction).
Liquid Zn amalgam, when strongly mixed, works as good activated zinc dust.

Title: Re: Why Hg-Zn is used ??
Post by: AdiDex on February 22, 2016, 01:50:08 AM

So why in the literature Zn-Hg is mentioned ?? There should be some use of Hg .

Title: Re: Why Hg-Zn is used ??
Post by: AWK on February 22, 2016, 05:29:16 AM

Did you read how complex is preparation of activated zinc dust? Zink amalgam is always ready to use (though chemist should know how to work with Hg safely).

Title: Re: Why Hg-Zn is used ??
Post by: AdiDex on February 22, 2016, 10:46:29 PM

ok i've read it now . But i didn't understand what property of Hg makes it so special that it is ready to use ??

Title: Re: Why Hg-Zn is used ??
Post by: Arkcon on February 23, 2016, 06:41:40 AM

The amalgam is simply a convenient way of dispersing zinc, and the mercury is fairly inert to this reaction.

Title: Re: Why Hg-Zn is used ??
Post by: Enthalpy on February 23, 2016, 03:17:25 PM

My two-cents suggestion is that:

• Zinc develops spontaneously a hermetic oxide layer that makes it unreactive. This explains why zinc resists corrosion by rain and serves even to protect steel against corrosion, despite the redox potential should let zinc oxidize easily;
• The amalgam (ie with mercury) prevents the formation of this good oxide layer;
• Other methods spoil the existing oxide layer, for instance an electrochemical couple, say with copper.

Title: Re: Why Hg-Zn is used ??
Post by: AdiDex on February 26, 2016, 08:38:30 AM

i saw this video

https://www.youtube.com/watch?v=quBU3GH_CjY (https://www.youtube.com/watch?v=quBU3GH_CjY)

In this they dissovled Mercury in Aqua Regia first then they put mercury in it , then dissolved Zinc Then neutralized and filtered to get the Zn-Hg paste .

Why we can't dissolve Zinc directly ?? Is this due to Zinc spontaneously a hermetic oxide layer that makes it unreactive .