In school practicals introducing the reactions of metals with acids, it seems to be standard (as in this one from the RSC) to use both hydrochloric and sulfuric acid, but with metals which react to both or neither (e.g. magnesium, zinc and copper). It would be nice to be able to make the point that different things react with different acids, but I'm at a loss for what might fit the bill. Is there anything?
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1$\begingroup$ One example is lead. Lead reacts with HCl but not H2SO4.See this:-chemistry.stackexchange.com/questions/27954/… $\endgroup$– Nilay GhoshSep 10, 2016 at 10:43
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3 Answers
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OK, let's clean things up.
Reaction of an acid with a metal is generally a redox reaction. The oxidizing agent are protons generated in the process of dissociation of the acid (let's forget about anions for a sec). The property that decides whether a metallic element can be oxidized by protons is its standard reduction potential. In general, any metal with a negative reduction potential will react with any Brønsted acid (one that produces protons), no matter whether it is a weak (e.g., acetic) or strong acid (e.g., hydrochloric). And if a metal has a positive reduction potential, it won't react with acids. Therefore, if we do not consider other factors, there are no examples to make the point that different acids react differently with a given metal, in fact they all should behave in the same way.
But what makes Chemistry fun are those 'other factors'. The anions produced in the dissociation process of an acid can also act as an oxidizer. For example, nitrate ions produced from nitric acid will oxidize copper, so the metal will dissolve in nitric acid. Sulfate ions are also oxidizing agents, but much weaker than nitrate ions, so copper won't dissolve in diluted sulfuric acid, but a hot and concentrated sulfuric acid will do the trick. And hydrochloric acid in the absence of some other oxidants will not attack copper at all. Another factor to consider is the passivation of a metal. A thin, invisible layer of an oxide or salt can protect a metal from an acid attack. Aluminum, which is a very reactive metal, will not dissolve in oxidizing acids (e.g., nitric), because it will develop a protective layer of aluminum oxide on its surface. Non-oxidizing acids, on the other hand, will easily do the job. Metal salts that are insoluble in water can also create a passivation layer.
Regarding the original question, there are some examples for sulfuric and hydrochloric acid, where they will behave differently due to the formation of insoluble salts. One example is thallium - it dissolves in sulfuric acid but not in hydrochloric acid. An opposite is barium - it dissolves in hydrochloric acid but not in sulfuric acid.
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$\begingroup$ Thanks, good answer! I do actually have a little barium, stored under paraffin, so I could use that, but I'm not sure if it's worth it. Doesn't it react with water anyway? I also found that zinc doesn't react noticeably with acetic acid, unlike magnesium, but I'm not sure I want to deal with the vinegar smell. I guess this is why teachers usually just go with hydrochloric and sulphuric acid with metals where it doesn't make much difference which you use! $\endgroup$– OolongSep 6, 2016 at 11:35
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1$\begingroup$ @Oolong Unfortunately barium reacts with water quite vigorously. Plus it is toxic (its soluble salts). Thallium is also dangerously toxic. Zinc does react with acetic acid, it all depends on concentration. In general weak acids and diluted acids react slowly. I think you can do a nice experiment where you show how the concentration of an acid changes its reactivity. Or you can go for the classic: show why some metals are called 'noble' because they do not react with 'normal' acids, but react with nitric acid or aqua regia. $\endgroup$– vapidSep 7, 2016 at 6:47
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I think you will be astounded to learn about aqua regia. Also, have a look at the reactivity series to learn more about the reactivity of various metals. To make your point that different metals react differently, you could reference this reactivity series and show the trend of certain metals and their ions to lose and/or gain electrons.
Following on what @Hanry said, in aqua regia, nitric acid is used to partially dissolve the gold, whereas hydrochloric acid is used to "take advantage" of chloride ions by using them to form a complex with chloride anions to form a complex of tetrachloroaurate(III) anions, thereby allowing nitric acid to continuously dissolve a small amount of gold as the HCl continuously "grabs" that ever-so-small amount from solution, causing it to be captured in the chloride complex. This cycle continues until the gold or either acid has been used up.
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Usually when we use these acids, we use them for the H+ (protons) that they will donate to react. $$\ce{HAcid \ce{->} \ce{H^+} + Acid^- 2x\ce{H^+} + Metal^0 \ce{->} x\ce{H_2} + Metal^{+x}}$$ This is why it seems that different acids react the same way. However, if you wanted them to react differently, you might try to take advantage of the different non-acid parts ($\ce{SO4^2-}$ and $\ce{Cl-}$).
So for sulfuric acid and hydrochloric acid, $\ce{SrSO4}$ will form, but $\ce{SrCl2}$ will stay in solution. Therefore, sulfuric acid will react with strontium ion, while hydrochloric acid will not. (Note: strontium metal will react with both.)
Edit: I may have misread your question. If you are looking for a metal element, then one metal element that resists reaction would be Au; it is commonly known that it will react with aqua regia (Nitric and Hydrochloric acid), but resists other lone acids. (www.cracksat.net/sat2/chemistry/question-644-answer-and-explanation.html)
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