Question

Explain why zinc dissolves in dilute HCl to liberate `H_(2)`(g) but from concentrated `H_(2)SO_(4)`, the gas evolved is `SO_(2)`.

Answer 1

In dilute HCl, zinc reacts as follows :
`Zn+2H^(+)(aq) to Zn^(2+)(aq)+H_(2)(g)`
Therefore, `H_(2)`(g) ion. Is liberated.
With concetration `H_(2)SO_(4)`, both `H^(+)` ions and `SO_(4)^(2-)` ions will be available. But `SO_(4)^(2-)` ions is a better oxidising agent than`H^(+)` ion. Therefore, the reaction will occur as follows :
`Zn(s) +SO_(4)^(2-)(aq)+4H^(+)(aq) to Zn^(2+)(aq)`