Chemical Forums
Chemistry Forums for Students => Inorganic Chemistry Forum => Topic started by: Argus on October 02, 2004, 04:54:02 PM
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How come some acids can disolve some things, but others cannot. e.g. HF can disolve glass, but carbonic acid cannot.
I cannot find this anywhere and it is bugging me.
Could it possibly be because the ion (not H+ or OH-) reacts and forms a film of percipitate over the substance and prevents further reaction?
In other words, what makes the ions (again not the H+ or OH-) so special?
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When you say dissolve, you really mean react. Yes, The ions play an important role. In the case of HF, F- reacts with silica in glass to produce silicon fluorides. Silica won't react with other usual ions.
Basically it depends on how reactive an element is with an ion.
In the case of copper, it is quite resistant to HCl acid because Cu is pretty stable in the zero oxidation state and Cl- is not a good oxidizer. But copper will react with HNO3 because nitric (NO3) can oxidize copper to Cu+2.
On the other hand, zinc can easily react with HCl and get oxidized not by Cl-, but by H+, because Zn is much more easily oxidized than Cu.
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Of couse.
I suppose I really meant why can't an ion such as CaCO3 2- react with SiO2 or why can't NO3 - oxidize Au, or why can't...?
Is one able to tell or explain why (or why not) a certain ion will react (or not), or can it only be determined experimentally?
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The type of reactions you are talking about are called metathasis. Metathasis reactions will fall under HSAB(hard-soft-acid-base) theory. Look in any Inorganic book for more details.
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In a nutshell, it's a matter of thermodynamics (energy) and kinetic (mechanistic) factors. Why certain ions react has to do with bond breaking and forming energies (thermodynamics), ionization energies, steric/size limitations (kinetic) and so on. So yes they can be explained but it may not be a simple explanation.
One can say for example that Si-O bonds are too strong to be replaced by Si-O bonds of carbonate, but Si-F bonds are energetically stronger. One can say, mechanistically, that F- is much smaller than CO3 and can penetrate the glass structure to attack Si.
One can say the ionization energy of Au is 890 kJ mol-1 vs 745 kJ mol-1 for Cu making it harder to oxidize.
(These explanations may be flawed but are used for illustration)