I have a problem with the decomposition reaction of Nitric acid($\ce{HNO3}$) by light. My problem: is that reaction is a redox reaction? If it is please tell me the oxidation and reduction (balanced) reactions seperately.
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1$\begingroup$ Why do you think it's photochemical? Nitric acid is sold in clear colorless bottles. $\endgroup$– ZheJun 30, 2017 at 15:27
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$\begingroup$ @Zhe no, nitric acid is subjected to decomposition due to light. See my answer below. $\endgroup$– Nilay GhoshJun 30, 2017 at 18:34
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$\begingroup$ Interesting. Never seen the brown bottle before... Thanks! @NilayGhosh $\endgroup$– ZheJun 30, 2017 at 18:39
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1 Answer
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Generally, nitric acid is a strong oxidizing agent which get reduce to nitrogen oxides on reacting with metals which act as reducing agent thus forming a redox reaction. But in this case, only nitric acid is involved which get reduced to nitrogen oxides on reaction with air/light. So, this may be called oxidation reaction where nitric acid gets oxidized. From wikipedia article of nitric acid:
Nitric acid is subject to thermal or light decomposition and for this reason it was often stored in brown glass bottles:
$$\ce{ 4 HN^{V}O3 → 2 H2O + 4 N^{IV}O2 + O2}$$
This reaction may give rise to some non-negligible variations in the vapor pressure above the liquid because the nitrogen oxides produced dissolve partly or completely in the acid.
The nitrogen dioxide ($\ce{NO2}$) remains dissolved in the nitric acid coloring it yellow or even red at higher temperatures. While the pure acid tends to give off white fumes when exposed to air, acid with dissolved nitrogen dioxide gives off reddish-brown vapors, leading to the common name "red fuming acid" or "fuming nitric acid" – the most concentrated form of nitric acid at Standard Temperature and Pressure (STP). Nitrogen oxides ($\ce{NO_x}$) are soluble in nitric acid.
There are some old papers(here and here) which discuss the mechanism of nitric acid decomposition. Give a quick read on the papers and you will understand the mechanism of the reaction.
answered Jun 30, 2017 at 18:32