Reaction between Ammonium chloride, Concentrated sulphuric acid and copper turnings

During a salt test, dry test for acid radical , mixed ammonium chloride with concentrated sulphuric acid and heated the mixture and added copper turnings to the mixture, the mixture on heating turned yellow.


Sulphur Dioxide was also evolved as detected by its odour.


Bubbling across the copper turnings were observed.


Wanted to know the chemistry behind the reaction, composition of the yellow compound.




A possible process expressed in ionic and summary formula forms:


\begin{align}
\ce{4 Cl- + 2 H+ + H2SO4 + Cu &-> CuCl4^2- + SO2 + 2 H2O}\\
\ce{4 NH4Cl + 3 H2SO4 + Cu &-> (NH4)2[CuCl4] + SO2 + 2 NH4HSO4 + 2 H2O }
\end{align}


As oxidative behavior of hot concentrated $\ce{H2SO4}$ is well known:


$$\ce{Cu + 5 H2SO4(l) -> Cu^2+(solv) + SO2(g) + 2 H3O+(solv) + 4 HSO4-(solv)}$$


 5

Poutnik's explanation is consistent with experimental observations and yellow color of chloro copper complex. This is a known complex of copper. The alternative proposed hypothesis in another answer suggesting the formation of $\ce{NOCl}$ or $\ce{NO}$ is not correct because it contradicts a well known analytical reaction employed in Kjeldahl method. Here nitrogen containing compounds (say meat, food) are digested with concentrated sulfuric acid at very high temperature, near the boiling point, and the end product is ammonium sulfate! So it is known that sulfuric acid will not oxidize ammonium ion to nitrogen oxide or other complexes even under extreme conditions.


 1

It might be [nitrosyl chloride](https://en.m.wikipedia.org/wiki/Nitrosyl_chloride). At elevated temperatures ammonia can be oxidized to [NO](https://en.m.wikipedia.org/wiki/Nitric_oxide), which can oxidize to [NO2](https://en.m.wikipedia.org/wiki/Nitrogen_dioxide), and further to [nitric acid](https://en.m.wikipedia.org/wiki/Nitric_acid).


In acidic solution, chlorine can combine with NO to form NOCl, a yellow gas.


Since essentially a mixture of aqua Regia results, ammonia and copper may both be subsequently oxidized.


It is certainly hoped this was run in a well ventilated hood.


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