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Why does nitric acid behaves differently from the other common acids, (e.g. hydrochloric and sulphuric acids) when it reacts with a metal, such as magnesium.
Igloo replies …
The short answer to this is that nitric acid - even in its dilute form - is an oxidising agent, whereas hydrochloric and sulphuric acids are not (sulphuric acid only acts as an oxidising agent when it is concentrated).
When hydrochloric acid reacts with a metal, e.g. magnesium, there is an interaction between the hydrogen ions of the acid and the metal, producing magnesium ions and hydrogen gas:
Mg + 2H+(aq) -> Mg2+(aq) + H2(g)
and that’s the end of it!
However, although nitric acid also contains hydrogen ions, and can behave as an acid (just like hydrochloric), its nitrate ion can also oxidise the hydrogen – which might otherwise have been formed – to water, whilst itself being reduced to oxides of nitrogen (e.g. NO2).
e.g. Mg + 4H+ + 2NO3– -> Mg2+(aq) + 2H2O(l) + 2NO2(g) *
In other words, unlike reactions with hydrochloric and sulphuric acids, where only the hydrogen ions get involved, nitric acid invokes both its hydrogen and nitrate ions.
If you are an A2 student you will find Table 6.1. of the Nuffield Data Book useful, since in this table there are listed several relevant processes involving the nitrate ion as an oxidising agent. It would be highly instructive to try to balance a few alternative equations to the one I have marked with a star.
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updated: 27 August 2003
