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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: cliverlong on May 12, 2008, 10:51:32 AM

Title: Acidic nature of Silicon Dioxide ???
Post by: cliverlong on May 12, 2008, 10:51:32 AM
Hi,

   My textbook (Facer, A2 Chemistry - used for UK EdExcel specification) states on page 36 that Silicon Dioxide is a weakly acidic oxide because of the following reaction

SiO2 + 2NaOH <> Na2SiO3 + H2O (where <> represents the symbol "reversible")

Now, OK, I can see that the reaction is silicon dioxide plus an alkali gives water and a compound (sodium silicate?) that looks "salt-like"

However, everything I have read to date about acids (ignoring Lewis) states an acid is a proton donor.

My problem: where is the H+ in SiO2 ? Because there isn't an H+, how can SiO2  be an acid - even if its reaction with an alkali generates a salt  ?  ???

Salts, acids, bases  ::)


Clive
Title: Re: Acidic nature of Silicon Dioxide ???
Post by: tflint89 on May 12, 2008, 03:39:30 PM
I think the Lewis acid/base theory has a great deal of pertinence in this case!
Title: Re: Acidic nature of Silicon Dioxide ???
Post by: Faisal on May 12, 2008, 04:05:53 PM
Quote from: cliverlong on May 12, 2008, 10:51:32 AM
Hi,

SiO2 + 2NaOH <> Na2SiO3 + H2O (where <> represents the symbol "reversible")

There is a reaction in between....btw your doing A-level edexcel...I am doing AS...maybe u can help me
Title: Re: Acidic nature of Silicon Dioxide ???
Post by: DevaDevil on May 12, 2008, 04:16:29 PM
look at the reaction; in particular NaOH, a strong base.

Acting as a Bronsted acid can be by proton donation, or by hydroxide neutralization.
The latter is what happens in this case. As there is less OH- after reaction, the pOH will be higher, and thus the compound less basic. in other words, the SiO2 acted as an acid.
Title: Re: Acidic nature of Silicon Dioxide ???
Post by: Faisal on May 12, 2008, 04:30:44 PM
I thought SiO3 simply becomes HSiO3- in a solution and there is the H+ to make it a bronsted lowry acid..
Title: Re: Acidic nature of Silicon Dioxide ???
Post by: cliverlong on May 12, 2008, 04:30:58 PM
Quote from: tflint89 on May 12, 2008, 03:39:30 PM
I think the Lewis acid/base theory has a great deal of pertinence in this case!
I was afraid it was relevant.

The little I have read on Lewis acids made me think the definition of acids and bases is so broadened and that the concept applies to so many reactions that the "distinctive" nature of acids and bases is lost in the Lewis definition


Ta

Clive
Title: Re: Acidic nature of Silicon Dioxide ???
Post by: cliverlong on May 12, 2008, 04:33:53 PM
Quote from: DevaDevil on May 12, 2008, 04:16:29 PM
look at the reaction; in particular NaOH, a strong base.

Acting as a Bronsted acid can be by proton donation, or by hydroxide neutralization.
The latter is what happens in this case. As there is less OH- after reaction, the pOH will be higher, and thus the compound less basic. in other words, the SiO2 acted as an acid.
I haven't read anywhere that "hydroxide neutralisation" of substance A defines A as an acid.

I see where you are coming from, because I used to think all bases were hydroxides and vice versa.

Can you provide a link for this definition of base "neutralisation" of an acid, where the acid does not contain a proton?


Thanks

Clive
Title: Re: Acidic nature of Silicon Dioxide ???
Post by: cliverlong on May 12, 2008, 04:35:02 PM
Quote from: Faisal on May 12, 2008, 04:30:44 PM
I thought SiO3 simply becomes HSiO3- in a solution and there is the H+ to make it a bronsted lowry acid..
You may well be right - but I'm not seeing the connection with the equation I wrote.

Can you clarify for me?


Ta,

Clive
Title: Re: Acidic nature of Silicon Dioxide ???
Post by: Faisal on May 12, 2008, 04:52:44 PM
First of all SiO2 has to be dissolved in the solution so therefore it is:
SiO3+H2O->H+ + SiO4-(similar to SO3 dissolving in water)
This is can be written as HSO4-
It is this which reacts with NaOH and neutralizes...hope tht clarifies
Title: Re: Acidic nature of Silicon Dioxide ???
Post by: cliverlong on May 12, 2008, 04:57:54 PM
Quote from: Faisal on May 12, 2008, 04:52:44 PM
First of all SiO2 has to be dissolved in the solution so therefore it is:
SiO3+H2O->H+ + SiO4-(similar to SO3 dissolving in water)
This is can be written as HSO4-
It is this which reacts with NaOH and neutralizes...hope tht clarifies
The question is about SiO2 not SiO3

SiO2 does not dissolve in water - only in heated, strong alkali.

I don't see any connection between Silicon (Si Group 4) and Sulphur (S Group 6) as your argument suggest is relevant.

I think your argument is incorrect for both these reasons.

Please use sub-scripts and superscripts in equations on this forum as relevant - it makes them easier to read.


Regards


Clive
Title: Re: Acidic nature of Silicon Dioxide ???
Post by: DevaDevil on May 12, 2008, 05:02:40 PM
silicon dioxide is relatively insoluble in water; 0.12 g/L


And:
" Neutralization is the reaction between an acid and a base, producing a salt and neutralized base; for example, hydrochloric acid and sodium hydroxide form sodium chloride and water:
        HCl(aq) + NaOH(aq) → H2O(l) + NaCl(aq) "
Source: e.g. Wiki

In this case substitute SiO2 for HCl, and you see different products. However, the principle is the same. The SiO2 acts as the acid in the neutralization reaction.


Title: Re: Acidic nature of Silicon Dioxide ???
Post by: Faisal on May 12, 2008, 05:08:16 PM
Oh right...SiO2
this is the rx I think it shud be...
SiO2+H2O->SiO3- + 2H+ which can be written as H2SiO3
Title: Re: Acidic nature of Silicon Dioxide ???
Post by: Faisal on May 12, 2008, 05:08:59 PM
Maybe cuz its insolube thts y it forms only a weak acid....