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From the Beryllium article in Wikipedia:

Beryllium dissolves readily in non-oxidizing acids, such as $\ce{HCl}$ and diluted $\ce{H2SO4}$, but not in nitric acid or water as this forms the oxide.

I think it would be helpful to have a definition of 'non-oxidizing acid'. Is it an established term in chemistry? Am I right to understand that it is an acid that can only act through its $\ce{H+}$ moiety, and not by the reduction of its conjugate base part (as in the reaction between $\ce{HNO3}$ and $\ce{Cu}$)?

There's a little obscure page titled oxidizing acid on Wikipedia.

The trouble is, until I came across this term, I imagined all acids to be oxidizing by definition. It's not very intuitive.

P.S.

Related: Weird Wikipedia Section on Oxidizing Acids

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    $\begingroup$ Yeah, basically that's it. $\endgroup$
    – Mithoron
    May 11, 2016 at 19:26
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    $\begingroup$ "All acids are oxidising by definition" - this is not true - an acid (as far as Arrhenius was concerned) is simply something that releases H+ into solution. It may or may not then go on to oxidise something. HCl + NaOH is not a redox reaction; HCl is an acid but it is not oxidising anything here. HCl + Na is a redox reaction, and yes H+ is acting as an oxidising agent, but that is not because HCl is an acid. It is because Na is so easy to oxidise that H+ can oxidise it. $\endgroup$
    – orthocresol
    May 11, 2016 at 20:19
  • $\begingroup$ Hydroiodic acid is certainly not oxidizing. $\endgroup$
    – Curt F.
    May 12, 2016 at 10:57

3 Answers 3

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A non-oxidising acid is an acid in which there is no stronger oxidant than $\ce{H+}$ at $\mathrm{pH}\ 0$.

Or, in a slightly different wording, a non-oxidising acid would be one that cannot dissolve any noble metal at $1~\mathrm{M}$ concentration, but should (in the absence of effects like overpotential) dissolve any non-noble metal at that concentration.

Nitric acid has the strong oxidation agent $\ce{NO3-}$ present, which will reduce to $\ce{NO}$ or $\ce{NO2}$ depending on the concentration. It is able to oxidise noble metals such as silver.

Hydrochloric acid, on the other hand, has no other oxidising agent. The chloride ion is already fully reduced. Therefore, it is not able to oxidise noble metals such as copper.

Some acids are more strongly oxidising if they are concentrated. Sulphuric acid is one. Only in higher concentrations will reduction of sulphate to $\ce{SO2}$ take place and make the acid oxidising. In lower concentrations, only $\ce{H+}$ takes part in oxidations.

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  • $\begingroup$ Jan, are there any cases where the oxidising power of the anion and the H+ (in an acid with acidic hydrogens) are roughly or exactly equal? Such acids would oxidise with both the proton and the anion, and would be of an intermediate class of acids between oxidising and non-oxidising. $\endgroup$
    – harry
    May 9, 2021 at 12:40
  • $\begingroup$ Also, isn't pH=0 and concentration=1M only for strong acids? $\endgroup$
    – harry
    May 9, 2021 at 12:57
  • $\begingroup$ @harry Good point. The standard potential assumes a $\pu{1M}$ concentration of all dissolved ions (including $\ce{H+}$ which maybe doesn't work as well with weak acids. I would have to research what is done in those cases but I am too lazy. As for oxidising anions, I am not aware off the top of my head of any anion that would have an equal oxidising power, but do check the electrochemical series. $\endgroup$
    – Jan
    May 20, 2021 at 8:32
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As an add-on, I'll post an excerpt from the 6th edition of "Chemistry: The Molecular Nature of Matter" by Jespersen et al.

Kudos to Freddy who posted it in the chatroom.

enter image description here

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    $\begingroup$ Basically my answer but backed by a textbook ;) $\endgroup$
    – Jan
    May 13, 2016 at 11:20
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    $\begingroup$ @Jan - a lot of good stuff gets dissociated in the chat, why not bring it here, by a salt bridge, as it were; lest the half-cells become unbalanced. $\endgroup$
    – CowperKettle
    May 13, 2016 at 11:36
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    $\begingroup$ I thought this would be more of a metallic bridge but yeah. Not even complaining. Upvoted it towards sportsmanship ;) $\endgroup$
    – Jan
    May 13, 2016 at 11:37
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There is really nothing more to say than what's in the Wikipedia article referred to by the OP and the comments. "Nonoxiding acids" are those acids that react with metals by displacement of hydrogen, such as $\ce{HCl}$ reacting with zinc. "Oxidizing acids" like $\ce{HNO3}$ contain more strongly oxidizing components that react instead of the solvated $\ce{H^+}$, or possibly convert wiuld-be displaced hydrogen back to solvated $\ce{H^+}$.

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  • $\begingroup$ you can, and should, use $\ce{}$: $\ce{H2O + H+ -> H3O+}$ gives $\ce{H2O + H+ -> H3O+}$ Package documentation $\endgroup$
    – orthocresol
    May 12, 2016 at 10:38
  • $\begingroup$ I went with "solvated $H^+$" because the concept could be applied to nonaqueous solvents. $\endgroup$
    – Oscar Lanzi
    May 12, 2016 at 10:44
  • $\begingroup$ It was just an example of the formatting syntax, I was not commenting on any of the things you wrote $\endgroup$
    – orthocresol
    May 12, 2016 at 10:53
  • $\begingroup$ Ok, point taken. $\endgroup$
    – Oscar Lanzi
    May 12, 2016 at 11:01

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