Which gases(or gas) are evolved in the leaching of Galena(PbS) with conc. Nitric acid? [closed]








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I want to know the gases produced(primarily nitrogen oxides) in leaching of Galena in nitric acid and the effect of concentration of nitric acid on the gases evolved.


I found papers about the efficiency of leaching in different concentrations of nitric acid but none mentioned which gases are produced.




The answer is hydrogen sulfide with dilute nitric acid, otherwise little to none using the concentrated acid.


Here is a source: ["Leaching of Lead Sulfide with Nitric Acid"](https://www.tandfonline.com/doi/abs/10.1080/12269328.1999.10541133?journalCode=tges20), quoting:



> 
> Galena, lead sulfide, is leached with nitric acid in order to prepare higher value-added products such as lead nitrate, lead carbonate, lead basic carbonate, etc. Leaching behavior has varied with the concentration of nitric acid. At the lower concentration of nitric acid, galena is dissolved into lead nitrate and hydrogen sulfide. However, at the higher concentration of nitric acid, galena is not dissolved but it is changed into lead sulfate as precipitate. The leaching efficiency is strongly dependent upon temperature and nitric acid concentration. The optimal conditions are found to be 40°C and 4–8 N nitric acid where about 95% of lead can be dissolved from lead sulfide.
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I hope this helps.


For those students interested in the advanced underlying chemistry surrounding the action of hydrogen sulfide on nitric acid and associated nitric oxides, see this source: ["Potential Biological Chemistry of Hydrogen Sulfide (H2S) with the Nitrogen Oxides"](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3798156/).


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Adding to the previous answer (with a fume hood recommendation) is that $\ce{NO2}$ and $\ce{SO2}$ can also [evolve](https://www.chemistryscl.com/reactions/sulfur-and-nitric-acid/index.php#:%7E:text=nitric%20acid%20is%20an%20oxidizing,H2SO4) using the concentrated acid at higher temperatures.


It is really interesting that $\ce{H2S}$ (gas) can be produced with dilute $\ce{HNO3}$ conditions because hydrogen carries a very slightly stronger positive charge than $\ce{Pb^2+}$, $\ce{NO3-}$ serve as counter ions, and the oxidation rate of sulfur is very low.


With more concentrated nitric acid at higher temperatures, the oxidation of sulfur rate becomes higher than the evolution of $\ce{H2S}$ gas as:


$\ce{HNO3 -> NO2 (g) + H2O}$
  
$\ce{ [S] atoms -> $\ce{SO2}$ (g) -> SO4^2-}$


resulting in $\ce{PbSO4}$ salt. The loss of $\ce{H2S}$ and $\ce{SO2}$ during the reaction explains the less than 2:1 ratio of $\ce{S}$ to $\ce{Pb}$ after the galena leaching reaction is complete.


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