Previous Page: How Acidic...
One of the ways acids were originally classified was by their characteristic reactions. Did they react with metals and if they did were there consistent observations? Did they do anything interesting when an alkali was dropped in them? Over time folks who liked to watch for these things drew certain conclusions, of which the following are deemed to be particularly important:
You might like to note that the reactions above are generic and ignore the fact that 'oxidising acids' notably concentrated sulfuric and nitric acids, react with metal to give much more interesting products. For example, concentrated nitric acid reast with copper metal to give salt, water AND a particularly unfriendly gas, N2O4.
In the above discussion the term salt has occurred several times; it is not, of course, referring solely to NaCl, so called table salt. In this context the term salt refers to any ionic compound resulting from a reaction involving acids. So if you mix solutions of HCl and NaOH you get aqueous NaCl and HOH (or H2O). NaCl is the salt. On the other hand, if you mix HNO3 and CuO you get Cu(NO3)2 and H2O. In this case Cu(NO3)2 is the salt.
As it turns out, each of the typical reactions listed above follows a pattern, as set out in this table. In all cases the resulting salt is CuCl2(aq).
|metal or compound||example|
|pure metal||Cu(s) + 2HCl(aq) → CuCl2(aq) + H2(g)|
|hydroxide||Cu(OH)2(s) + 2HCl(aq) → CuCl2(aq) + 2H2O(g)|
|oxide||CuO(s) + 2HCl(aq) → CuCl2(aq) + H2O(g)|
|carbonate||CuCO3(s) + 2HCl(aq) → CuCl2(aq) + H2O(g) + CO2(g)|
|hydrogen carbonate||Cu(HCO3)2(s) + 2HCl(aq) → CuCl2(aq) + 2H2O(g) + 2CO2(g)|
|sulfite||CuSO3(s) + 2HCl(aq) → CuCl2(aq) + H2O(g) + SO2(g)|
The key thing about this table is that any metal and any acid can be substituted, with the only caveat being always be sure to balance the final equation.
For example, how would you derive the equation for the reaction between potassium hydroxide and sulfuric acid? Well, having been told it involves and acid, refer to the table; in particular row 2: metal hydroxides. Substituting potassium for copper and sulfate for chloride you would get, initially:
KOH(s) + H2SO4(aq) → K2SO4(aq) + H2O(g).
This, however, requires balancing, giving
2KOH(s) + H2SO4(aq) → K2SO4(aq) + 2H2O(g).
Only the ratios and the salt, now potassium sulfate, change.
Likewise, consulting the table suggests copper oxide and nitric acid give
CuO(s) + 2HNO3(aq)→ Cu(NO3)2(aq) + H2O(g).
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