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Chapter 21: Q21.115CP (page 922)

Compare and contrast a voltaic cell and an electrolytic cell with respect to each of the following:

(a) Sign of the free energy change

(b) Nature of the half-reaction at the anode

(c) Nature of the half-reaction at the cathode

(d) Charge on the electrode labelled 鈥渁node鈥�

(e) Electrode from which electrons leave the cell

Short Answer

Expert verified

(a) The sign of is negative and positive in voltaic cells and electrolytic cells respectively.

(b) The half-reaction at anode is spontaneous and non- spontaneousin voltaic cells and electrolytic cells respectively.

(c) The half-reaction at anode is spontaneous and forced in voltaic cells and electrolytic cells respectively.

(d) The charge of anode is negative and positive in voltaic cells and electrolytic cells respectively.

(e) The electrons flow from anode to cathode and from cathode to anode in voltaic cells and electrolytic cells respectively.

Step by step solution

01

Concept Introduction

An electrolytic cell is an electrochemical device that employs electrical energy to help a non-spontaneous redox reaction take place. Electrolytic cells are electrochemical cells that can be used to electrolyze a variety of substances.

A galvanic cell or voltaic cell is an electrochemical cell that converts the chemical energy of spontaneous redox reactions into electrical energy.

02

Free Energy Change

(a)

In voltaic cells,鈭�G掳 is negative, because the reaction is spontaneous and energy is released into the environment. In electrolytic cells, energy is taken up by the system to make the system work, therefore its鈭�G掳 is positive, and the reaction is non-spontaneous.

Therefore, energy change is negative in voltaic cell and positive in electrolytic cell.

03

Half-reaction at Anode

(b)

In both voltaic and electrolytic cells, oxidation occurs in the anode. However, in the voltaiccell, the half-reaction in the anode occurs spontaneously, while in electrolytic cells, electrons areremoved from the anode through an external power source.

Therefore, half-cell reactions in the anode in voltaic cells occur spontaneously, while in electrolytic cells, they are non-spontaneous.

04

Half-reaction at Cathode

(c)

In both voltaic and electrolytic cells, reduction occurs at the cathode. Electrons from the cathode are supplied to the cathode involtaic cells and reduction takes place spontaneously. In electrolytic cells, electrons from the external power source are directed to the cathode to force the reduction to happen.

Therefore, in voltaic cells, reduction takes place spontaneously, while in electrolytic cells, electrons are forced into the cathode to make reduction happen.

05

Charge on anode

(d)

In voltaic cells, anode is the negative electrode because electrons come from it. In electrolytic cells, anode is the positive electrodebecause electrons are removed from it.

Therefore, in voltaic cells, anode is the negative electrode, while it is the positive electrode in electrolytic cells.

06

Electrons leaving Cell

(e) In voltaic cells, electron flows from anode to cathode, while in electrolytic cells, electrons move from cathode to anode.

Therefore, in voltaic cells, electrons flow from anode to cathode, while the reverse is true for electrolytic cells.

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