Question

The reaction of zinc with excess of aqueous alkali, evolves hydrogen gas and gives :

• A. $Zn ( OH )_2$
• B. $ZnO$
• C. $\left[ Zn ( OH )_4\right]^{2-}$
• D. $\left[ ZnO _2\right]^{2-}$

Answer 1

Answer: (D)

Zinc dissolves in excess of aqueous alkali
$Zn +2 OH ^{-}+2 H _2 O \rightarrow \underset{\text{Tetrahydroxozincate(II) ion}}{\left[ Zn ( OH )_4\right]^{2-}+ H _2 \uparrow}$
However, this reaction in NCERT is given as
$Zn +2 NaOH \rightarrow Na _2 ZnO _2+ H _2 \uparrow$
$ZnO _2^{2-}$ is anhydrous form of $\left[ Zn ( OH )_4\right]^{2-}$
So in aqueous medium best answer of this question is $\left[ Zn ( OH )_4\right]^{2-}$