Question

Explain the physical states, volatility and chemical properties of hydrides of group 16 elements.

Answer 1

(1) Physical state : Hydride of oxygen, H₂O is a colourless odourless liquid while the hydrides of other group 16 elements are colourless poisonous gases with unpleasant odours.

(2) Volatility : Volatility of hydrides increase from H O to H S and then decreases. 

H₂O < H₂S > H₂Se > H Te 

At ordinary temperature H₂O is a liquid while all other hydrides are gases.

(3) Thermal stability :

• The thermal stability of hydrides decreases in the order of H₂O > H₂S > H₂Se > H₂Te.
• Since atomic size increases from O to Te, the tendency to form hydride bond, M-H decreases. Hence M-H bond in O-H is the strongest and in Te-H the weakest. Therefore thermal stability decreases from H₂O to H₂Te.

(4) Acidic character :

• The hydrides of group 16 elements are weakly acidic and acidic strength increases from H₂O to H₂Te.

Since M-H bond strength decreases, bond enthalpy decreases, acidic character increases.

(5) Reducing power :

• Except H₂O, all hydrides of group 16 elements are reducing agents. 
• Reducing power increases from H₂S to H₂Te.
• Reducing power of the hydrides is due to their less stability and tendency to dissociate, which increases from H₂S to H₂Te.