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Question: Which of the following reactions occurs at cathode? A.\[2{\text{O}}{{\text{H}}^ - } \to {{\text{H}...

Which of the following reactions occurs at cathode?
A.2OH−→H2O+O+2e−2{\text{O}}{{\text{H}}^ - } \to {{\text{H}}_2}{\text{O}} + {\text{O}} + 2{{\text{e}}^ - }
B.Ag→Ag++e−{\text{Ag}} \to {\text{A}}{{\text{g}}^ + } + {{\text{e}}^ - }
C.Fe2+→Fe+3+e−{\text{F}}{{\text{e}}^{2 + }} \to {\text{F}}{{\text{e}}^{ + 3}} + {{\text{e}}^ - }
D.Cu2++2e−→Cu{\text{C}}{{\text{u}}^{2 + }} + 2{{\text{e}}^ - } \to {\text{Cu}}

Explanation

Solution

Always reduction occurs at cathode. During the process of reduction, the oxidation state of the element decreases and the loss of electrons occurs.

Complete step by step solution:
During reduction the element reduces, that is the oxidation state of the element decreases. The elements gain electrons in order to decrease its oxidation state. Let us see in which of the example the oxidation state of the element is decreasing:
2OH−→H2O+O+2e−2{\text{O}}{{\text{H}}^ - } \to {{\text{H}}_2}{\text{O}} + {\text{O}} + 2{{\text{e}}^ - }
In hydroxide ion oxygen has an oxidation number equal to −2 - 2 whereas if we see in the product side it converts into elemental oxygen which has zero charge. Hence the oxidation state of hydroxide ion occurs because Oxidation State increases from −2 - 2 to 0.
Ag→Ag++e−{\text{Ag}} \to {\text{A}}{{\text{g}}^ + } + {{\text{e}}^ - }
Silver is in zero oxidation state or neutral state changes to oxidation state of plus one. That means the oxidation state increases from 0 to 1 and hence the process occurring is oxidation.
Fe2+→Fe+3+e−{\text{F}}{{\text{e}}^{2 + }} \to {\text{F}}{{\text{e}}^{ + 3}} + {{\text{e}}^ - }
Similarly here also the oxidation state is increasing or we can say loss of electrons is occurring it means oxidation is taking place.
Cu2++2e−→Cu{\text{C}}{{\text{u}}^{2 + }} + 2{{\text{e}}^ - } \to {\text{Cu}}
In this one copper iron is gaining the electrons also the oxidation state of copper decrease is from plus 2 to 0. That is reduction is occurring.

Hence, the correct option is D.

Note:
There are two types of cell electrochemical cell and electrolytic cell. Electrochemical cell is the one in which chemical energy gets converted into electrical energy and in an electrolytic cell the reverse process occurs, that is the electrical energy gets converted into chemical energy. In both the cells there are two types of electrode that are named as cathode and anode. In both of the cells reduction will always take place at cathode and oxidation will always take place at anode irrespective of the charge carried by cathode and anode.