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Question

What is the mechanism of following reactions in the preparation of hydrogen:

Zn+2NaOH=Na2ZnO2 +H2

2Al+2NaOH+2H2O=2NaAlO2 +3H2

Sn+2NaOH+H2O=Na2ZnO3+ 2H2

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Solution

1

When a base reacts with a metal, then a metal salt and hydrogen gas are formed. When sodium hydroxide solution is heated with zinc, then sodium zincate and hydrogen gas are formed.

Zn(s) + 2 NaOH(aq) → Na2ZnO2(aq) + H2 (g)

Zinc Sodium hydroxide Sodium zincate Hydrogen

(Base) (Salt)

In the salt formed by the reaction between a base and a metal, the metal is present as a part of the negative ion.In the sodium zincate salt (Na2ZnO2), the zinc metal is present as a part of the negative ion, zincate ion (ZnO22-).


2 ) Aluminum reacts with sodium hydroxide to form a salt called sodium aluminate(NaAlO2) and hydrogen gas. We usually think that when a metal reacts with an acid,it forms salt and hydrogen gas. This case aluminium acts as a amphoteric metal which basically means that it has both acidic and basic properties. Its oxide reacts to an acid or base to form salt and water. It is known as neutralisation reaction.

3) this is an oxidation- reduction (redox) reaction


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