Question

Given that,
\[N{{i}^{2+}}/Ni=0.25V,C{{u}^{2+}}/Cu=0.34V\]
\[A{{g}^{+}}/Ag=0.80V,Z{{n}^{2+}}/Zn=-0.76V\]
Which of the following reactions under standard condition will not place in the specified direction?
a.) \[N{{i}^{2+}}(aq)+Cu(s)\to Ni(s)+C{{u}^{2+}}(aq)\]
b.) \[Cu(aq)+2A{{g}^{+}}(aq)\to C{{u}^{2+}}(aq)+2Ag(s)\]
c.) \[Cu(s)+2{{H}^{+}}(aq)\to C{{u}^{2+}}(aq)+{{H}_{2}}(g)\]
d.) \[Zn(s)+2{{H}^{+}}(aq)\to Z{{n}^{2+}}(aq)+3{{H}_{2}}(g)\]

Answer 1

Hint: To know about which reaction is feasible and which reaction is not feasible, we should know about the electrochemical series.
Electrochemical series: A list that defines the arrangement of elements or atoms in order of their increasing or decreasing electrode potential values is called electrochemical series.

Complete step by step solution:
In the question it is given that \[N{{i}^{2+}}/Ni=0.25V,C{{u}^{2+}}/Cu=0.34V\]and \[A{{g}^{+}}/Ag=0.80V,Z{{n}^{2+}}/Zn=-0.76V\].
Before going to solve the question we should know the position of the given metals or ions in the electrochemical series.

Based on the given electrode potential we have to arrange them in an order.
\[\begin{align}
  & A{{g}^{+}}/Ag=0.80V \\
 & C{{u}^{2+}}/Cu=0.34V \\
 & N{{i}^{2+}}/Ni=0.25V \\
 & 2{{H}^{+}}/{{H}_{2}}=0V \\
 & Z{{n}^{2+}}/Zn=-0.76V \\
 & \\
\end{align}\]
The element or ion which has highest electrode potential always acts as anode and the element or ion which has less electrode potential acts as cathode.

Now coming to given options, option A,
\[N{{i}^{2+}}(aq)+Cu(s)\to Ni(s)+C{{u}^{2+}}(aq)\]
In the above reaction nickel is accepting electrons means nickel acts as cathode and copper is donating electrons and acts as anode. In the electrochemical series nickel is below when compared to copper. So, this reaction is feasible.

Coming to option B,
\[Cu(aq)+2A{{g}^{+}}(aq)\to C{{u}^{2+}}(aq)+2Ag(s)\]
In the above reaction silver is accepting electrons means acts as cathode and copper is donating electrons and acts as anode. In the electrochemical series copper is at below position when compared to silver. Means copper should act as cathode, but in the given reaction copper acts as anode. So, this reaction is not feasible.

Coming to option C,
\[Cu(s)+2{{H}^{+}}(aq)\to C{{u}^{2+}}(aq)+{{H}_{2}}(g)\]
In the above reaction copper is donating electrons means copper acts as anode and hydrogen ion is accepting electrons and acts as cathode. In the electrochemical series copper is above when compared to hydrogen ions. So, this reaction is feasible.

Coming top option D,
\[Zn(s)+2{{H}^{+}}(aq)\to Z{{n}^{2+}}(aq)+3{{H}_{2}}(g)\]
In the above reaction zinc is donating electrons means zinc acts as anode and hydrogen ion is accepting electrons and acts as cathode. In the electrochemical series zinc is at below position when compared to hydrogen ion. So, this reaction is not feasible.
So, the reactions in option B and option D are not feasible.

Note: Don’t be confused with the given electrode potential values, always we have to follow the electrochemical series to decide whether the reaction is possible or not. The electrode which has high potential value in the cell always acts as an anode and vice versa.