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I read that silver in the presence of hydrogen sulfide corrodes to form silver sulfide and hydrogen.

$$\ce{Ag + H2S -> Ag2S + H2}$$

But in the reactivity series silver is placed much below hydrogen.

So, how is this displacement reaction taking place at all?

Reference:

Silver to Black - and Back. JCE

Single Replacement Reactions. Chemistry LibreTexts.

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    $\begingroup$ Are you aware the reactivity serie applies to hydrated ions ? $\endgroup$
    – Poutnik
    Commented Oct 11, 2020 at 15:45
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    $\begingroup$ I don't quite believe this happens. Oxygen is a part of the process too. $\endgroup$
    – Ivan Neretin
    Commented Oct 11, 2020 at 15:51
  • $\begingroup$ @Shub Please note ChemSE uses mhchem to ease writing chemical equations / formulae. It functions well in questions, answers, comments; but because it is something special, you should not use it to format the title of a question. Its scope is described on mhchem.github.io/MathJax-mhchem. $\endgroup$
    – Buttonwood
    Commented Oct 11, 2020 at 16:29

2 Answers 2

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Though this is looked at as a displacement reaction, this reaction here is more of an redox reaction. Let me elaborate..

(Ag) is less reactive than (H) in reactivity series meaning (Ag) can't displace (H). But when you look at it as redox reaction,

2Ag + H2S → Ag2S + H2

Mechanism:

  1. Ag atoms reacts with H2S gas ​

  2. Ag atoms undergo oxidation (lose electrons) and becomes Silver ions (Ag+).

  3. The electrons lost by Ag are gained (reduced) by Sulphur atoms in H2S and becomes Sulphide ions (S2-).

  4. The Ag+ ions and S2- ions react to form Ag2S, which is deposited as a dark layer on the surface. This is the process of tarnishing.

So, in this reaction, Ag does not displace H. Instead, Ag+ ions react with S2- ions to form Ag2S, leading to the tarnishing of silver.

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  • $\begingroup$ Consider formatting guides for texts and formulas/equations/expressions, including mhchem \ce{} specifics. $\endgroup$
    – Poutnik
    Commented Mar 20 at 7:25
  • $\begingroup$ thank you sir . $\endgroup$
    – Anjankumar
    Commented Mar 20 at 7:34
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    $\begingroup$ It is rather $\ce{4 Ag + 2 H2S + O2 -> 2 Ag2S + 2 H2O}$, similarly as $\ce{2 Cu + 4 CH3COOH + O2 -> 2 (CH3COO)2Cu + 2 H2O}$ $\endgroup$
    – Poutnik
    Commented Mar 20 at 12:50
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    $\begingroup$ Cu and Ag will react with H2S in an anaerobic setting because of the extreme insolubility of the sulfides. H2S reacts with the copper in Monel metal rather than the nickel. $\endgroup$
    – jimchmst
    Commented Mar 21 at 22:20
  • $\begingroup$ @jimchmst that preference may be a kinetic one. Copper assumes an oxidation state of +1 in the sulfide versus nickel +2, and the lower oxidation state compound tends to form faster due to lower charge ions being less tightly held and diffusing faster. $\endgroup$
    – Oscar Lanzi
    Commented Mar 22 at 15:37
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We do indeed have an exothermic reaction between hydrogen sulfide and silver, which goes against the usual EMF series. The EMF series applies specifically to species in water solution, so we should not be surprised that it does not hold when we go to gas-phase reactions (even with steam, let alone with hydrogen sulfide).

We compare the standard enthalpy changes associated with oxidation of silver by water vapor and by hydrogen sulfide (technically, we should be using free energies but the enthalpies make the point and are easier to come by). Using the data from this table:

$\ce{2Ag + H2O -> Ag2O + H2}; \Delta H =+210.7\text{ kJ/mol}$

$\ce{2Ag + H2S -> Ag2S + H2}; \Delta H =-11.2\text{ kJ/mol}$

The reaction with water comes nowhere near being thermodynamically favorable, whereas the reaction with hydrogen sulfide releases energy and thus may be favored.

This selective reaction of silver with hydrogen sulfide may be explained in tetms of molecular-orbital structure. Sulfur atoms are large with valence orbitals having multiple nodes, which leads to poor covalent overlap and thus weak bonding with hydrogen. Therefore some metals that cannot break the hydrogen-oxygen bonds of water, or do so only with difficulty, may break the hydrogen-sulfur bonds of hydrogen sulfide. Silver is one of several late transition and post-transition metals with this selectivity; these are the chalcophile metals that often occur in nature as sulfides.

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  • $\begingroup$ At least 9 out of 10 downvotes come with no alternatives or suggestions for improvements. Here's your chance to stand out from the crowd. $\endgroup$
    – Oscar Lanzi
    Commented Mar 22 at 1:13

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