Answer to Question #76690 in Inorganic Chemistry for Arun Rawat

Question

5. a) Name the factors that affect the thermal stability of alkali metal salts. Explain the
stability trend observed in case of nitrates of these metals.
b) Why is the monomeric beryllium hydride unstable? Describe the structure of
polymeric beryllium hydride.

Answer 1

1. The thermal stability of a substance depends heavily on the structure of the substance which, in turn, is dependent on the bonds between atoms that hold that structure together.
Group 1 nitrates decompose to form products: MeNO2 + O2
Going down Groups 1, nitrates become more stable. Higher temperatures are needed to decompose them. Cations (positively charged ions) can affect anions such as NO3-. They can lower the energy needed to break a N–O bond.
This effect increases:
• the smaller the cation
• the greater the cation’s charge.
The small positive ions at the top of the Group polarise the nitrate more than the larger positive ions at the bottom.

2. The hydride of beryllium is covalent and polymeric (BeH2) and has a chainstructure containing chains with hydrogen bridges between berylliumatoms. Now, beryllium has only two valence electrons and hydrogen only one, it is apparent that there are not sufficient electrons to form so many electron pair bonds.