What is the change in oxidation number of S in the following...

In $H_2S$, let the oxidation number of S be $x$.

Assuming the typical oxidation number of H is $+1$:

$2(+1) + x = 0$

$x = -2$

In the products, sulfur is formed in its standard elemental state as S.

The oxidation number of any element in its free uncombined state is $0$.

Thus, the oxidation number of S changes from $-2$ to $0$.

Answer: $-2$ to $0$