Question

Account for the following :
(a) Aluminium sulphide gives a foul odour when it becomes damp.
(b) Although aluminium is above hydrogen in the electrochemical series, still it is stable in moist air.

Answer 1

(a) Aluminium sulphide is a thiol compound that itself has a foul smell and can be detected just by smelling. When damp, it reacts with water in the moisture and releases hydrogen sulphide which is also a thiol and a poisonous gas that emits an extremely foul rotten egg smell. Hence, it is very important to place aluminium sulphide in sealed containers to prevent its reaction with moisture.  

The equation for the above reaction is:

${ Al }_{ 2 }{ S }_{ 3 }\quad +\quad { 6H }_{ 2 }O\quad \xrightarrow [ \quad  ]{ \quad  } \quad { 3H }_{ 2 }S\quad +\quad 2Al(OH)_{ 3 }$

(b) Although, aluminium is placed higher than hydrogen in the highly reactive metals group of electrochemical series, aluminium is more stable in moist air. This is because it rapidly develops a thin and protective layer of a few millimetres thickness of aluminium oxide and this prevents the metal to further react with water in the moist air.

${ 2HI }_{ (g) }\quad \rightleftharpoons \quad { H }_{ 2(g) }\quad +\quad { I }_{ 2(g) }$