CHEMISTRY

TOPIC:  Electrochemistry – Part 1

                                     By Kingsley Idiagbor, B.Sc (Hon), PGDCs, NCE, MNSM

 

Introduction

Electrochemistry is the branch of chemistry that deals with the interrelationship of electrical currents and chemical reactions, and with the mutual transformation of chemical and electrical energy. It is also the study of chemical reactions that produce electrical and of the chemical phenomena that are caused by the action of currents or voltages (that is, the action of electricity on chemicals when electrical energy is transformed into chemical energy).

Electrode Potential

When a metal is dipped into a solution containing the ions of that metal, either of two processes could occur:

·         The metal could lose the appropriate number of electrons to become ionized and these ions will join those in the surrounding solution; OR

·         The ions in the surrounding solution could gain electron(s) to form the metallic atoms

Click on diagram for a larger view

When any of these processes occur, a potential difference would be set up between the metal rod and the ions in the surrounding solution.  This potential difference is what is known as electrode potential.

Standard Electrode Potential

It is, of course, impossible to obtain the electrical potential of a single half-cell and can only be measured for a complete circuit having two electrodes such as those of zinc metal and copper rod.. This can be achieved by arbitrarily assigning an electrode potential of 0.00V to a given cell. The electrode potentials of other half-cells can then be compared with this standard or reference.

The standard that is selected for this purpose is hydrogen, and the standard hydrogen half-cell is assigned a standard electrode potential (symbol E^θ) of 0.00 volts:

Thus:

H⁺_(aq) + e- → yH2_(g) E^θ = 0.00V

Platinum electrode is usually placed within a test tube filled with hydrogen gas or a solution of H+ ions, and hydrogen gas (H₂) bubbles over its surface. This allows H⁺ ions to form H₂ gas.

Thereafter, in measuring and comparing electrode potentials, we have to select the same standard conditions for all measurements. The standard conditions chosen are similar to those for thermo chemical measurements:

·         All solutions have a concentration of 1 mol dm-3.

·         Any gases are at a pressure of 1 atmosphere.

·         The temperature is 25 °C (77 °F/298 K).

·         When the half-cell does not include a metal, the electrode is made of platinum.

When the electrode potential of a metal ion/metal half cell in contact with one molar solution of its ions is measured using hydrogen electrode potential (i.e. 0.00V) as arbitrary standard, we have what is termed as Standard Electrode Potential. Standard electrode potential is denoted by the symbol, E^θ

When two solutions of zinc tetraoxosulphate(VI) and copper (II) tetraoxosulphate(VI) with each containing zinc plate and copper rod respectively are linked together by salt bridge and connected to a galvanometer, there is actually transfer of electrons.

Zinc metal loses two electrons which travel across the circuit to the copper rod and causes the pointer of the galvanometer (G) to deflect (indicating the presence of electric current). Thus, oxidation occurs at the zinc plate and is therefore acting as a reducing agent. Metals like zinc also transfer electron to the hydrogen electrode and has a negative value of standard electrode potential. Zinc, for instance, has standard electrode potential of -0.76V.

Copper metal, on the other hand, would receive electrons from hydrogen electrode and from an element above it in the electrochemical series.  Such a metal has a positive value of standard electrode potential. Thus, copper has standard electrode potential of +0.34V.

When the standard electrode potential of metal ions/metal half-cells are arranged in ascending order, Electrochemical series is the result.

FEATURES AND APPLICATIONS OF THE ECS

1. The elements that are found at the top of the series are usually strong reducing agents while those at the bottom are generally good oxidizing agents
2. The direction of electron flow in electrochemical cell is usually dictated by the relative position of the two metallic electrodes in the ECS. Thus in a cell with zinc and copper electrodes, the electrons would flow from the zinc (because it is higher up in the series) to copper. Zinc, which is higher up in the ECS would behave as the negative electrode or cathode. It is electron-rich electrode while copper which is lower down acts as the positive electrode or anode because it is electron-deficient.
3. Position of a metal in the ECS also determines its occurrence in nature. Elements found above hydrogen in the ECS are always found in their combined state because of their relatively higher reactivity. Whereas, those found below hydrogen occur most of the times in their native or free state.

(to be continued..)

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Copyright ©  Kingsley Idiagbor | 23 August 2005
All rights reserved. No part of this article may be reproduced, transmitted or stored in an electronic retrieval system except for academic purpose, without obtaining permission from the owner of the copyright. Violators may be liable to civil action under the applicable Federal laws.

 

 

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