Q.(a) What are hydrides? What are the different types of hydrides? [2]

Hydrides are binary compounds of hydrogen with other elements, classed as ionic, covalent (molecular), or metallic (interstitial); H2O2 oxidises Fe2+ to Fe3+ in acidic medium.

(a) Hydrides and their types:

A hydride is a binary (two-element) compound formed between hydrogen and another element. Hydrides are classified into three main types based on the nature of the bond formed:

1. Ionic (saline) hydrides: formed by the reaction of hydrogen with highly electropositive s-block metals (alkali and alkaline earth metals). Hydrogen exists as the hydride ion, H-. Examples: NaH, CaH2, LiH.
2. Covalent (molecular) hydrides: formed with p-block nonmetals, where hydrogen shares electrons covalently. These are further sub-classified by the octet rule as:
   • Electron-deficient hydrides (fewer than 8 electrons around the central atom), e.g. B2H6.
   • Electron-precise hydrides (exactly 8 electrons, complete octet), e.g. CH4.
   • Electron-rich hydrides (central atom has lone pairs beyond a complete octet), e.g. NH3, H2O.
3. Metallic (interstitial) hydrides: formed by d-block and f-block transition metals, where H atoms occupy interstitial spaces (voids) within the metal's crystal lattice. These are typically non-stoichiometric (variable, non-whole-number composition), e.g. TiH1.5-1.8, PdH0.6-0.8.

(b) H2O2 as an oxidising agent in acidic medium: …