We are studying the chalcogens at school. I should prove (show) the acid nature of $\ce{H2S}$ by writing two chemical interactions.

I am really sorry if the terms I am using aren't the right ones. It's the first time I've ever used this part of the language. Which reactions should I write, and why? I think the first one must be neutralization:

$$\ce{H2S + 2 NaOH -> Na2S + 2 H2O}$$

  • 2
    $\begingroup$ How about the reaction with sodium carbonate? $\endgroup$
    – Waylander
    Nov 10, 2019 at 12:58
  • $\begingroup$ @Waylander, thank you for the response! I haven't seen this reaction in school, but it might be the reaction I am searching for. What are the reasons you wrote it? I mean what does it show? $\endgroup$ Nov 10, 2019 at 13:04
  • 1
    $\begingroup$ Do you know how an (Arrhenius) acid is defined? That may also suggest an equation which can illustrate the acidic behaviour of H2S. $\endgroup$ Nov 10, 2019 at 13:10
  • 1
    $\begingroup$ It will show that H2S will give a proton to carbonate, a reaction that is consistent with an acidic nature $\endgroup$
    – Waylander
    Nov 10, 2019 at 13:18
  • 1
    $\begingroup$ I suggest you do. It is difficult to describe something if you don't know what it is. $\endgroup$ Nov 10, 2019 at 13:37

1 Answer 1


As orthocresol suggested, to illustrate "classic" acidity as it's understood on a school level you unquestionably want to start with Arrhenius definition, which attributes acidic properties to any substance which increase concentration of hydronium $\ce{H3O+}$ ions in water.

That's said, the very first reaction you probably want to write is dissociation of $\ce{H2S}$ in water and demonstrating that it is a weak acid by showing the corresponding acid dissociation constants $K_\mathrm{a}$ (the larger the $\mathrm{p}K_\mathrm{a},$ the weaker the acid; data from [1, p. 5-87]):

$$ \begin{align} \ce{H2S + H2O &<=> HS- + H3O+} &\quad \mathrm{p}K_\mathrm{a1} &= 7.05\\ \ce{HS- + H2O &<=> S^2- + H3O+} &\quad \mathrm{p}K_\mathrm{a2} &= 19 \end{align} $$

Some reactions illustrating acidic properties of $\ce{H2S}$:

Reaction with sodium hydroxide

Note that the reaction

$$\ce{H2S(aq) + 2 NaOH(aq) -> Na2S(aq) + 2 H2O(l)}$$

is for complete neutralization (concentrated $\ce{NaOH}$ solution). If the reactants are taken in 1:1 ratio (diluted $\ce{NaOH}$ solution), sodium hydrosulfide is formed:

$$\ce{H2S(aq) + NaOH(aq) -> NaHS(aq) + H2O(l)}$$

Reaction with sodium carbonate

Sodium hydrosulfide is also formed when saturated $\ce{H2S}$ solution reacts with $\ce{Na2CO3}$:

$$\ce{H2S(aq) + Na2CO3(aq) -> NaHS(aq) + NaHCO3(aq)}$$

Reaction with ammonia

Reaction with aqueous ammonia solution yields exclusively in ammonium hydrogen sulfide $\ce{NH4HS},$ even when concentrated ammonia solution is used:

$$\ce{H2S(aq) + NH4OH(aq) -> NH4HS(aq) + H2O(l)}$$

Ammonium sulfide $\ce{(NH4)2S}$ doesn't form as ammonium $\ce{NH4+}$ $(\mathrm{p}K_\mathrm{a} = 9.25)$ is a much stronger acid than hydrosulfide $\ce{HS-}$ by a factor of about $10^{19 - 9.25}\approx\pu{6E9}.$

So far the reactions between well-soluble compounds that were carried in aqueous solutions where Arrhenius definition works fine. However, it cannot be used to explain acidity in non-aqueous media, where a Brønsted–Lowry definition of an acid comes in handy: the acid is a proton donor. This can be illustrated by the reaction with liquid ammonia:

$$\ce{H2S(g) + 2 NH3(l) ->[\pu{-40 °C}] (NH4)2S(s)}$$


  1. Haynes, W. M.; Lide, D. R.; Bruno, T. J., Eds, CRC Handbook of Chemistry and Physics: A Ready-Reference Book of Chemical and Physical Data; 97th Edn.; Taylor & Francis Group (CRC Press): Boca Raton, FL, 2016-2017. ISBN 978-1-4987-5429-3.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.