Question

From their standard reduction potentials, which of the following metals would you expect to dissolve in \({\rm{HCl}}\)by the reaction\({\rm{M}} + n{{\rm{H}}^ + } \to {{\rm{M}}^{n + }} + \frac{n}{2}{{\rm{H}}_2}:{\rm{Zn}},{\rm{Fe}},{\rm{Co}},{\rm{Al}},{\rm{Hg}},{\rm{Cu}},{\rm{Pt}}\),\({\bf{Au}}\)?

(When the potential predicts that the element will not dissolve, it probably will not. If it is expected to dissolve, it may dissolve if some other process does not interfere. Predictions based on standard reduction potentials at \({\bf{2}}{{\bf{5}}^{^{\bf{o}}}}C\) are only tentative, because the potentials and activities in hot, concentrated solutions vary widely from those in the table of standard potentials.)

Short answer

\({\rm{Zn,}}\)\({\rm{Fe}},{\rm{Co}}\)and\({\rm{Al}}\) are dissolved in HCl because the reduction potentials of these metals are less than HCl. Thus they can reduce hydrogen and get dissolved in HCl.

Step-by-step solution

Definition of Hydrochloric acid.

Hydrochloric acid is a strong acid. It dissociates into \({{\rm{H}}^{\rm{ + }}}\,{\rm{and}}\,\,{\rm{C}}{{\rm{l}}^{\rm{ - }}}\) in aqueous solution.

\({\rm{HCl}}\,{\rm{ + }}{{\rm{H}}_{\rm{2}}}{\rm{O}} \to {{\rm{H}}_{\rm{3}}}{{\rm{O}}^{\rm{ + }}}{\rm{ + C}}{{\rm{l}}^{\rm{ - }}}\)

Determine the metals that dissolve in \({\rm{HCl}}\)by the reaction

• In this task we will determine which of the following metals ( \({\rm{Zn}},{\rm{Fe}},{\rm{Co}},{\rm{Al}},{\rm{Hg}},{\rm{Cu}},{\rm{Pt}}\) or \({\rm{Au}}\)) would we expect to dissolve in \({\rm{HCl}}\), from their standard reduction potentials by the reaction:
• When a metal reacts with \({\rm{HCl}}\)the hydrogen gets reduced from oxidation state +1 to oxidation state 0. Thus hydrogen gets reduced and the metal gets oxidized.
• \({\rm{M}} + n{{\rm{H}}^ + } \to {{\rm{M}}^{2 + }} + \frac{n}{2}{{\rm{H}}_2}\)
• The standard reduction potential for the reduction of hydrogen is zero.
• \({\rm{2}}{{\rm{H}}^{\rm{ + }}}{\rm{ + 2}}{{\rm{e}}^{\rm{ - }}} \to {{\rm{H}}_{\rm{2}}}\)\({{\rm{E}}_{{{\rm{H}}^{\rm{ + }}}{\rm{/}}{{\rm{H}}_{\rm{2}}}}} = 0.0\)
• So hydrogen can be reduced by only those metals whose standard reduction potential is less than zero. The elements having negative reduction potential are Fe, Co, Al and Zn. Therefore, these metals can dissolve in HCl.
• On the other hand, Hg, Cu, Pt and Au have positive values of reduction potential. Thus, they cannot reduce hydrogen and cannot get dissolved in HCl.