Hydrogen Sulfide (H2S) Reactions

Hydrogen sulfide is a colorless gas which has a characteristic rotten egg odor. It dissolves in water for some extent. This gas can act as an oxidizing agent, a reducing agent, an acid and also a precipitant. Following example reactions can be used to describe above incidents. In this tutorial, we are going to learn different reactions of H2S.


Written by: Thishini Nimesha Edirisinghe (undergraduate), Department of Civil Engineering, University of Peradeniya


Reactions of H2S as an acid

H2S has the ability to dissolve in water for some extent. (Weak acid).Following reactions display how this process happens.

H2S + water reaction - acidic behavior of hydrogen sulfide

we can see that hydrogen sulfide has two H+ ions (protons) that can dissociate ( that can release ) in water. Hence it is known as a diprotic weak acid.


Following reaction is an example for H2S acidic property.


H2S and NaOH reaction

H2S and NaOH reaction
  • NaS is resulted only if there is plenty of (with extra base) NaOH in the reactants.
  • NaHS is produced when we have extra H2S in reactants.

Reactions of H2S with metals

Hydrogen sulfide reacts with metals and produce metal sulfide and hydrogen gas. Product formula of the metal sulfide changes according to the amount of reactants.

H2S and Na reaction


Reactions of H2S as a reducing agent

Hydrogen sulfide turns into sulfur by releasing electrons when it reacts with strong oxidizing agents. We can observe a turbidity after this reaction as sulfur particles are precipitating slowly.



Reactions of H2S as an oxidizing agent

As the oxidation number of sulfur in hydrogen sulfide is in its minimum state, it cannot reduce its oxidation number anymore. But in some occasions, it can act as an oxidizing agent. In this reaction hydrogen takes electrons and involve in reduction part.

  • If H2S reacts with strong metals such as Na
  • If H2S reacts with some metals which have lower activity such as Cu , Ag .

In such reactions, hydrogen gas and the metal sulfide can be obtained.

reactions of H2S as an oxidizing agent

The oxidation number of sulfur remains constant while hydrogen changes from +1 to 0.



Behavior of H2S as a precipitant

Most of the metal cations precipitate as metal sulfide in H2S medium except metal cations in S block and some cations including Al3+ and Cr3+ . By observing the colour of the metal sulfide we can identify unknown metals.

Some metal cations precipitate even in a low concentration of S2- medium. In this process we have to bubble H2S in acidic medium. Following are the some of the metal sulfides that are precipitated in acidic H2S medium and their colours given too.

Metal sulfides that are precipitated in acidic H2S medium and its color

Some metal cations precipitate in a high concentration of S2- medium. In this process we have to bubble H2S in basic medium.


Following are the some of the metal sulfides that are precipitated in basic H2S medium and its color.

metal sulfides that are precipitated in basic H2S medium and its color