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D and F Block Elements question

2022 · 29 Jul · Shift 1 · Q4
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  5. /2022 · 29 Jul · Shift 1 · Q4

D and F Block Elements question

2022 · 29 Jul · Shift 1 · Q4

JEE MainChemistryD and F Block ElementsMCQ+4 / −1
The reaction of zinc with excess of aqueous alkali, evolves hydrogen gas and gives :
  1. A
    Zn(OH)2\mathrm{Zn}(\mathrm{OH})_{2}Zn(OH)2​
  2. B
    ZnO\mathrm{ZnO}ZnO
  3. C
    [Zn(OH)4]2−\left[\mathrm{Zn}(\mathrm{OH})_{4}\right]^{2-}[Zn(OH)4​]2−
  4. D
    [ZnO2]2−\left[\mathrm{ZnO}_{2}\right]^{2-}[ZnO2​]2−
View written solutionFree

Correct answer: C

  1. Identify the nature of zinc

Zinc is an amphoteric metal, which means it reacts with both acids and strong bases.

  1. Reaction of zinc with aqueous alkali

When zinc reacts with excess aqueous alkali such as NaOH or KOH, hydrogen gas is evolved and a zincate complex is formed.

The reaction is:

Zn+2OH−+2H2O→[Zn(OH)4]2−+H2↑\mathrm{Zn} + 2\mathrm{OH}^- + 2\mathrm{H_2O} \rightarrow [\mathrm{Zn}(\mathrm{OH})_4]^{2-} + \mathrm{H_2}\uparrowZn+2OH−+2H2​O→[Zn(OH)4​]2−+H2​↑

So, in aqueous excess alkali, the product is tetrahydroxozincate(II) ion.

  1. Check the options

  • A: Zn(OH)2\mathrm{Zn}(\mathrm{OH})_2Zn(OH)2​
    This may form initially, but in excess alkali it dissolves further due to amphoteric behavior.

  • B: ZnO\mathrm{ZnO}ZnO
    This is not the product in aqueous excess alkali.

  • C: [Zn(OH)4]2−[\mathrm{Zn}(\mathrm{OH})_4]^{2-}[Zn(OH)4​]2−
    Correct. This is the zincate complex formed in aqueous solution.

  • D: [ZnO2]2−[\mathrm{ZnO}_2]^{2-}[ZnO2​]2−
    This representation is sometimes used for zincate in non-aqueous/simplified form, but for aqueous alkali, the correct species is [Zn(OH)4]2−[\mathrm{Zn}(\mathrm{OH})_4]^{2-}[Zn(OH)4​]2−.

  1. Final answer

Therefore, the correct option is:

C  :  [Zn(OH)4]2−\boxed{\mathrm{C}\; :\; [\mathrm{Zn}(\mathrm{OH})_4]^{2-}}C:[Zn(OH)4​]2−​
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