JEE Main 2023 · 30 January · morning · Single correct · +4 / −1
Lithium aluminium hydride can be prepared from the reaction of :
- (a) LiCl and Al₂H₆
- (b) LiH and Al(OH)₃
- (c) LiH and Al₂ Cl₆
- (d) LiCl, Al and H₂
Answer and solution
Answer (c) LiH and Al₂ Cl₆
Solution
Chemistry ma’am · 4 steps
Hydride comes from LiH, aluminium comes from its chloride, and forming LiCl drives the reaction forward.
- What you need. LiAlH₄ contains Li⁺ and the [AlH₄]⁻ ion. So you need a source of hydride ions (H⁻) and a source of aluminium that can accept them.
- Pick the reagents. LiH is an ionic hydride, so it supplies H⁻. Aluminium chloride, which exists as the dimer Al₂Cl₆, is a Lewis acid, so its Al accepts the hydride ions. The reaction is carried out in dry ether.
8LiH + Al₂Cl₆ →(dry ether) 2LiAlH₄ + 6LiCl
- Why it works. Each Cl on aluminium is swapped for an H, and the chloride leaves as LiCl. The atoms balance: 8 Li, 8 H, 2 Al and 6 Cl on each side.
- So. Lithium aluminium hydride is made from LiH and Al₂Cl₆, which is option (c).
The idea to keep. Complex hydrides like LiAlH₄ and NaBH₄ are made by reacting an ionic hydride with the metal halide. The halide ends up as the alkali metal salt, here LiCl.
Answer from the NTA final answer key. Solution by PrepWiser.