Which of the following combination will produce H_(2) gas ?

1.	Which of the following combination will produce H_(2) gas ?
Answer» Cu metal and conc. `HNO_(3)` ZN metal and NaOH (aq) Au metal and NaCN (aq) in the presence of air Fe metal and conc. `HNO_(3)` Solution :NITRIC being a strong oxidising AGENT, oxidises initially Cu to `CUO` which further reacts with `HNO_(3)` to form `Cu(NO_(3))_(2)` with the evolution of `NO_(2)` gas. However , `H_(2)` gas is not produced. Fe becomes passive in conc. `HNO_(3)` DUE to the formation of an impervious layer of iron oxide on its surfacewhich prevents the further action. Au reacts with NaCN in presence of air to form soluble complex of dicyanoaurate (I) `underset("Dicyanoaurate "(I))(4Au + 8 NaCN+ 2H_(2)O + O_(2) to 4Na [Au(CN)_(2)]+4NaOH`) Only Zn reacts withe NaOH solution to evolve `H_(2)` gas `Zn+2NaOH overset(Delta)to underset("Sod. zincate")(Na_(2)ZnO_(2))+H_(2)`

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