[jakethepeguk1]

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

[AWK]

It dissolves in this acid very slowly, but dissolves

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

[jakethepeguk1]

Ok then,

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

Thanks

[BluRay]

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.

[jakethepeguk1]

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

[Schrödinger]

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.

[BluRay]

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.

. 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...

[jakethepeguk1]

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