Question

The following electrochemical cell has been set up:
\[Pt(1)|F{e^{3 + }},F{e^{2 + }}(a = 1)|C{e^{4 + }},C{e^{3 + }}(a = 1)|Pt(2)\]
\[{E^o}(F{e^{2 + }},F{e^{3 + }}) = 0.77V\]
and \[{E^o}(C{e^{4 + }},C{e^{3 + }}) = 1.61V\]
If an ammeter is connected between two platinum electrodes, predict the direction of flow of current, will the current increase or decrease with time?

Answer 1

Hint: In order to know about the direction in which the current flows, we must first find which is anode and which is cathode. This is possible only when we know where oxidation takes place and where reduction takes place.

Complete answer:
The electrochemical cell representation is shown below.
\[Pt(1)|F{e^{3 + }},F{e^{2 + }}(a = 1)|C{e^{4 + }},C{e^{3 + }}(a = 1)|Pt(2)\]
In the electrochemical cell, there are two half cells: Right half-cell and left half-cell. The left half-cell undergoes oxidation reaction while the right undergoes reduction reaction.
In the left half-cell, \[F{e^{2 + }}\]undergoes oxidation to \[F{e^{3 + }}\]and hence the oxidation state changes from +2 to +3. Hence this cell is the anode. The reaction which takes place is
\[F{e^{2 + }} \to F{e^{3 + }} + {e^ - }\]
In the right half-cell, \[C{e^{4 + }}\]undergoes reduction to \[C{e^{3 + }}\]and hence oxidation state changes from +4 to +3. Hence this cell is the cathode. The reaction which takes place is
\[C{e^{4 + }} + {e^ - } \to C{e^{3 + }}\]
The overall electrochemical reaction is given by
\[C{e^{4 + }} + F{e^{2 + }} \to C{e^{3 + }} + F{e^{3 + }}\].
The cell potential of RHS and LHS are given. With the given value we can determine the net cell potential.
\[E_{cell}^o = E_{RHS}^o - E_{LHS}^o\]
\[E_{cell}^o = 1.61V - 0.77V\]
\[E_{cell}^o = 0.84V\]
As the net cell potential of the electrochemical cell is positive i.e. 0.84V, therefore cell reaction is spontaneous.
We know that current can flow from anode to cathode.

Therefore, the current flows from Pt (2) to Pt (1).

Additional information:
Difference between Electrochemical and Electrolytic cell

ELECTROCHEMICAL CELL	ELECTROLYTIC CELL
Cells which can produce electricity through chemical reactions.	Cells which can make a chemical reaction take place through electricity.
Cells that can convert chemical energy into electrical energy.	Cells which can convert electrical energy into chemical energy.
Anode is negatively charged	Anode is positively charged
Cathode is positively charged	Cathode is negatively charged.
Passage of electrons takes place from anode to cathode.	Passage of electrons takes place from cathode to anode.
Spontaneous chemical reaction can take place.	Non-spontaneous chemical reactions can take place.

Note: We must always remember that in an electrochemical cell the electricity will always pass from the anode to cathode. We should not confuse it with the electrolytic cell, where the passage takes place from cathode to anode.