JEE Main 2022 · 29 July · morning · Single correct · +4 / −1
The reaction of zinc with excess of aqueous alkali, evolves hydrogen gas and gives :
- (a) Zn(OH)₂
- (b) ZnO
- (c) [Zn(OH)₄]²⁻
- (d) [ZnO₂]²⁻
Answer and solution
Answer (c) [Zn(OH)₄]²⁻
Solution
Chemistry ma’am · 3 steps
Zinc is amphoteric, so excess alkali carries it past the hydroxide into the soluble zincate ion.
- Zinc is amphoteric. Zinc and its hydroxide react with acids and with bases. In alkali, zinc reduces water to H₂ gas and is oxidised to Zn²⁺.
- Excess alkali matters. With only a little alkali, white Zn(OH)₂ precipitates. In excess alkali this hydroxide takes up two more OH⁻ ions and dissolves as the tetrahydroxozincate(II) ion.
Zn + 2OH⁻ + 2H₂O → [Zn(OH)₄]²⁻ + H₂ ↑
- So. The product in solution is [Zn(OH)₄]²⁻, for example as Na₂[Zn(OH)₄] with NaOH. The answer is (c).
The idea to keep. Amphoteric metals like Zn, Al, Sn and Pb dissolve in excess aqueous alkali as soluble hydroxo complexes, such as [Zn(OH)₄]²⁻ and [Al(OH)₄]⁻, and give off H₂.
Answer from the NTA final answer key. Solution by PrepWiser.