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Topic: Why does HCl dissolve ZnO but not SnO2?  (Read 17572 times)

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Offline jakethepeguk1

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Why does HCl dissolve ZnO but not SnO2?
« on: February 04, 2010, 05:55:40 AM »

Hello everyone,

I was wondering if anyone knew the answer to this question. Im trying to figure out why dilute HCl is able to dissolve ZnO (i have deposited it as a film), but conc HCl cannot dissolve SnO2 on its own. It only dissolves (or etches) in the presence of Zn (so HCl+Zn+SnO2). I've been thinking about it for a long time and cant quite figure an answer. It seems to be something to do with the standard electrode potentials of each material, but also to do with the binding energies of the electrons? Im not sure. If anyone can quantify any rates/energies/potentials, to make it understandable for me, this would be really useful. Im thinking that SnO2 is more stable than SnCl4 whereas ZnCl2 is more stable that ZnO, hence the preferential reactions in each, but if anyone could elaborate on this that would be great.

Thanks everyone

Offline AWK

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Re: Why does HCl dissolve ZnO but not SnO2?
« Reply #1 on: February 04, 2010, 06:38:39 AM »
It dissolves in this acid very slowly, but dissolves

http://en.wikipedia.org/wiki/SnO2
AWK

Offline jakethepeguk1

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Re: Why does HCl dissolve ZnO but not SnO2?
« Reply #2 on: February 04, 2010, 08:29:44 AM »
Ok then,

Why does it dissolve more slowly than ZnO? Id like a quantitative description if possible.

Thanks

Offline BluRay

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Re: Why does HCl dissolve ZnO but not SnO2?
« Reply #3 on: February 04, 2010, 03:09:56 PM »
If SnO2 would (easily) dissolve in HCl (we are talking of water solutions), it should form (appreciable amounts of) Sn++++, which is very acid and having a large charge would combine immediately with water forming again SnO2 (unless the solution is *extremely* acid or complexant, so that the ion can be stabilized forming a complex, as in the case of SnF6-- or, less stable, SnCl6--).
Actually SnO2 is one of those compounds which are very difficult to dissolve at cold with non complexants acids.

Offline jakethepeguk1

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Re: Why does HCl dissolve ZnO but not SnO2?
« Reply #4 on: February 05, 2010, 09:30:13 AM »
Im still confused... So what is the quantitative difference between the two? Is it just on charge? Or anything to do with electron binding energies?

Does it mean that SnO2 with Sn4+, is more stable than ZnO with Zn2+ just because of the charge. Would you then expect TiO2, for example to be more or less stable in an acid than SnO2? They are both 4+ metal ions. However you can not etch TiO2 with HCl even with Zn present, unlike SnO2.

Help. Im still very confused.

Thanks




Offline Schrödinger

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Re: Why does HCl dissolve ZnO but not SnO2?
« Reply #5 on: February 05, 2010, 12:45:02 PM »
Hey guys

I tried to come up with an explanation using Pearson's concept of hard and soft acids and bases. See http://www.meta-synthesis.com/webbook/43_hsab/HSAB.html for more on the same.

H+ and Sn4+ are hard acids, while Zn2+ is a borderline case. Refer link above.

Hard acids react with hard bases and soft acids react with soft bases.
2 chemicals of the same type can't react together. This sounds logical.
Both H+ and Sn4+ being hard acids(same property) find it difficult to react with each other, while Zn2+ being not-so-hard an acid reacts with H+ more readily than Sn4+.

This was just an unverified attempt on the problem, so please correct me if I am wrong.

"Destiny is not a matter of chance; but a matter of choice. It is not a thing to be waited for; it is a thing to be achieved."
- William Jennings Bryan

Offline BluRay

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Re: Why does HCl dissolve ZnO but not SnO2?
« Reply #6 on: February 05, 2010, 02:59:39 PM »
Im still confused... So what is the quantitative difference between the two? Is it just on charge? Or anything to do with electron binding energies?

Does it mean that SnO2 with Sn4+, is more stable than ZnO with Zn2+ just because of the charge
This is one of the explanations. However we are talking about the stability of stannic dioxide in presence of strong acids. With strong bases is different...Also stannic dioxide cannot properly be defined as a ionic compound (it's ~ half ionic and half covalent), while zinc oxide is.

Quote
. Would you then expect TiO2, for example to be more or less stable in an acid than SnO2?
Ti4+ ion is slightly smaller than Sn4+ ion but this fact only is not enough to establish the greater stability of titanium oxide. How ions are put in the crystal? How is their bond? What kind of hydrated ions they would form in water or with an acid? What kind of salt? Which stability of all these compounds? Chemistry it's not like algebra...

Offline jakethepeguk1

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Re: Why does HCl dissolve ZnO but not SnO2?
« Reply #7 on: February 08, 2010, 12:01:07 PM »
Thanks for the reply.

Am i correct to say that ZnO is more ionic than SnO2 which is more ionic than TiO2 (Im basing these assumptions on Fajans rules), hence the trend. Maybe in a very broad way i can base my understanding of the differences on this trend (although broadly its probably incorrect). I understand now that there are many factors involved which i may or may not ever understand. Its certainly not easy...

Thanks guys for your help though

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