Sulfur forms different oxidation numbers from -2 to +6. Also hydrogen has different oxidation numbers, -1, 0, +1 . We can find oxidation numbers of sulfur and hydrogen in H2S by several methods.
In H2S molecule, oxidation numbers of sulfur and hydrogen are -2 and +1 respectively. Now in this tutorial, we are going to learn how those oxidation numbers are found in different ways.
First, we should have an idea about electronegativities of sulfur and hydrogen atoms.
According to the Pauling 's scale, electronegativity of sulfur (2.5) is higher than hydrogen (2.1) . So hydrogen atom is oxidized while sulfur atom is reduced in S-H bond.
There are two S-H bonds in H2S molecule. Due to higher electronegativity of sulfur, electrons of bond are attracted towards sulfur atom. So hydrogen looses its electrons and gets +1 oxidation state. So sulfur takes two electrons from two hydrogen atoms in two S-H bonds, therefore oxidation number of sulfur is -2.
We know in H2S, hydrogen atom is oxidized and it's oxidation number is +1 and there are two hydrogen atoms. Oxidation number of sulfur is unknown and take it as x. Then substitute those values to the algebraic equation.
According to the oxidation number, we can say, whether that atom can be oxidized or reduced or both occurred. Now, we are going to study that.
Take electronegativity values of sulfur and hydrogen. Sulfur's electronegativity value is 2.5 and hydrogen's it is 2.1 . When electronegativity is higher of an atom, that atom can attract the electrons of the bond and gets more electrons than the protons of the its nucleus. So sulfur has a negative oxidation number in H2S.
We know, sulfur can show different oxidation numbers. When sulfur is combined with metals, oxidation number of sulfur become -2 because sulfur takes two electrons from two sodium atoms. As an example, oxidation number of sulfur in sodium sulfide is -2.
When H2S reacts with oxygen (combustion), S in H2S is oxidized to +4 oxidation state, sulfur dioxide.
H2S reacts with sodium and produce sodium sulfide and hydrogen gas. Hydrogen at +1 oxidation state in the H2S is reduced to 0 oxidation state while sodium is oxidized from 0 to +1. So H2S is behaved as an oxidizing agent.
Both sulfur and oxygen has higher electronegativity values than hydrogen. Therefore, hydrogen is oxidized with both sulfur and oxygen. So hydrogen oxidation number is +1. Sulfur and oxygen have -2 oxidation number in H2S and H2O respectively.