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When magnesium metal and hydrochloric acid react, hydrogen gas is released (and not chlorine gas). However, when metals such as copper is placed in nitric acid, NO(g) is released and not hydrogen. So, how do we know that chlorine gas is not released in the first reaction? Does this have to do with reduction potential?

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Nitric acid ($\ce{HNO3}$) is a strong oxidizing agent. The hydrogen gas ($\ce{H2}$) produced in the reaction with metal is oxidized to $\ce{H2O}$, in turn evolving nitric oxides like $\ce{NO2}$ and $\ce{NO}$. Even magnesium can be observed evolving nitric oxides when reacted with $\ce{HNO3}$.

Additionally, you know that chlorine gas isn't released by the reaction of $\ce{HCl}$ with magnesium because you would see it evolved from the solution as a pale, yellow-green gas. Instead, only clear hydrogen gas is evolved.

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  • $\begingroup$ Thanks for the answer. Though, my main intent of asking this question was to be able to predict which gases would be produced in an acid-metal reaction, so is there any theoretical way to know that Cl2 isn't produced in the reaction? Sorry for being unclear! $\endgroup$
    – 986
    Nov 30, 2015 at 6:21
  • $\begingroup$ It sounds like you are looking for a reduction potential table. $\endgroup$
    – ringo
    Nov 30, 2015 at 6:25
  • $\begingroup$ Thanks for the reply. I assume that Cl2 gas isn't produced because it has a high reduction potential to become Cl- ions - I think I get it now :) $\endgroup$
    – 986
    Nov 30, 2015 at 6:31

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