For each reaction, wheter the sign of `Delta S` (entropy) is positive or negative ?
(a) `Ni(s)+4CO(g)rarr Ni(CO)_(4)(g)` (b) `4Fe(s)+3O_(2)(g)rarr 2Fe_(2)O(s)`
(c )`H_(2)(g)+S(s)rarr H_(2)S(g)` (d) `2NO(g)+O_(2)(g)rarr 2NO_(2)(g)`

Explanation

(a) Five molecules are organised into only one, so the disorder decreases and `Delta S` in negative.
(b) Seven molecules, three of which is are gases, become only two solid molecules thus `Delta S lt 0`.
(c )Two molecules, one of which is solid, from a gaseous molecule. Since only a heteronuclear gas results, disorder increases and `Delta S` is positive.
(d) Three gas molecules produce two gas molecules disorder decreases, thus, `Delta S lt 0`.