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  4. Which one of the following pairs of substances on reaction will not evolve H2 gas?

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Q. Which one of the following pairs of substances on reaction will not evolve $H_{2}$ gas?

NTA AbhyasNTA Abhyas 2020The s-Block Elements

Solution:

$\text{Fe} + \text{dil} . \, \text{H}_{2} \text{SO}_{4} \rightarrow \text{FeSO}_{\text{4}} + \text{H}_{2} \uparrow$

$3 \mathrm{Fe}+4 \mathrm{H}_2 \mathrm{O}(\mathrm{g}) \rightarrow \mathrm{Fe}_3 \mathrm{O}_4+4 \mathrm{H}_2 \uparrow$

$\text{Cu} + \text{dil} \text{.} \, \text{HCl} \rightarrow $ No reaction

$2Na+C_{2}H_{5}OH \rightarrow 2C_{2}H_{5}ONa+H_{2}\uparrow$

Copper does not evolve $H_{2}$ from acid as it is below hydrogen in electrochemical series. The electrochemical series is built up by arranging various redox equilibria in order of their standard electrode potential (redox potential). The most negative $E ^\circ $ values are placed at the top of the electrochemical series, and the most positive at the bottom.

Metals at the top of the series are good at giving away electrons.