Introductory Electrochemistry and Related Applications
by
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1.0 Introduction
Electrochemistry is concerned with the study of the mutual inter-conversion of chemical and electrical energies. A system in which electrochemical process takes place has three distinct features as illustrated in Fig. 1. These features are the electrolyte, the electrodes in contact with the electrolyte and the external circuit connecting the electrodes.
- Electrolytes
- Electrolytic Solution
Electrolyte is very necessary in any electrochemical process. It is a conductor of the second class. Electrolytes may be defined as those compounds that dissociate in solution to produce ions, that is, negative and positive ions. These ions conduct electricity in solution by their movement. In an electric field, the positive ions migrate to the negative poles and the negative ions migrate to the positive poles. Electrolytes need not necessarily be only those compounds that dissociate in solution. A molten salt that conducts electricity is also an electrolyte.
Electrolytes may be classified by their conductivity in solution. Strong electrolytes are those compounds that dissociate completely in water into ions and are good conductors of electricity. Examples of strong electrolytes and their ionization are as follows:
KCl ®
K+ + Cl-
Weak electrolytes are compounds that do not dissociate appreciably in solution. They exist in solution mainly in their molecular form. They are poor conductors of electricity. Some known weak electrolytes and their dissociation in water are shown for CH3COOH, HCN and NH3. CH3COOH
HNO3 ®
H+ + NO3-
HCN ⇄ H+ + CN-
NH3 + H2O ⇄ NH4+ + OH-
As stated above, when an electrolyte is dissolved in a solvent such electrolyte may dissociate completely, partially or not at all. The degree of dissociation is equal to the ratio between the number of molecules dissociated, n, and the total number of molecules of the electrolyte dissolved in the solvent, N. This may be represented mathematically thus:
a
= n/N = n/n+na.
na is the number of undissociated molecule. If a solute does not dissociate into ions at all, then
a = 0, na = N and n = 0. In this case, the solute may be regarded as a non-electrolyte. An example of a non-electrolyte is sucrose, which exists, in its molecular form in solution. If the ratio of n to N is close to unity, then the solute may be referred to as a strong electrolyte. If n << N and 0<a<<1, the solute may be regarded as a weak electrolyte. The degree of dissociation of an electrolyte is related to the dissociation constant, K. Consider an electrolyte, HX, which dissociates in water, thus:HX ⇄ H+ + X- The dissociation constant may be represented by the well-known equilibrium constant expression: [H+][X-] If we represent the undissociated HX by Ca and the dissociated ions as C+ and C- for the cation and the anion, respectively, the resulting dissociation equilibrium constant will be 1.1 [C+][C-] The square brackets denote concentration. However, in the above equation, [C+] = [C-]. The degree of dissociation, as earlier stated, is
1.2 C = Ca + C+
Combining equations 1.1 and 1.2 will give equation 1.3 1.3 K = (
This equation can be rewritten as 1.4 α
If α<0.1 or K/C<0.01, then equation 1.4 is simplified to 1.5 α
K = _________
[HX]
K = _________
[Ca]
= Ca +
Ca = C - aC
= a2C/1-a
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About the Book
This monograph is an introductory text of electrochemistry for students of Science, Technology and Engineering at the very elementary level (Second year University students and their equivalent counterparts in Technical and Teachers Training Colleges). It may also be used as a refresher for upper level students at these institutions. The science of electrochemistry has undergone very dramatic changes in the past fifty years and its slow development at an earlier stage was due mainly to the fact that Nernstian equilibrium treatment of the subject does not tell the whole truth about electrochemical systems and their reactions. Application of kinetics to the study of electrochemistry has accelerated the pace and its eventual recognition as an important branch of science in its own right. Despite this recognition, elementary textbooks at the first Year University degree program levels have tended to treat this branch of science with cursory and utmost brevity. The unfortunate consequence of this is that students are not fully aware of the important technology that goes along with electrochemistry. These students are, perhaps, aware only of the Nernstian relationship and just the vague notion of accumulator cells. The aim of this monograph therefore is both to enlighten and to heighten the inquisitive and, I dare say, enthusiastic minds of the young students who may wish to know more about the important fundamentals and application of this branch of science. Chapters one and two of this short monograph are merely a revision and recapitulation of the basic information. Chapter three discusses the application of the electrochemical theory. Different battery systems, their manufacture and assembly are discussed. Fuel cell technology is also discussed. The very important analytical techniques, potentiometry/ion selective electrode is briefly mentioned. Electrosynthesis of organic compounds, popularized by Manuel Baizer, after his development of electrohydrodimerization of acetonitrile to adiponitrile (a process known as the "Monsanto Process) is discussed. Electroplating and corrosion, important aspects in which electrochemical knowledge is vital, are also discussed. Application of electrochemical knowledge far transcends these aforementioned fields. They go into electrorefining, electrowinning, electromachining, electrodeposition, electroforming, etc. The discussion of these latter applications are beyond the scope of this monograph and their mention only serves to stress the very importance and extent to which electrochemical knowledge can be applied. The writing of this monograph was necessitated by the very welcome inquisitiveness of my first year degree students at the Federal University of Technology, Yola, Nigeria, whose ferocious thirst for knowledge was an incentive. I sincerely hope that their knowledge of this branch of science will be both widened and accelerated for a future pursuit after reading this monograph.
About the Author
Maurice O. Iwunze obtained a Ph.D. degree in chemistry from Baylor University. While there he worked on the mechanisms of electrochemical oxidation of organic compounds, solubilized in micelles and microemulsion systems. After working briefly at the Federal University of Technology, Yola, Nigeria, he went to the University of Connecticut, Storrs as a research fellow working on advanced electrochemical techniques in bicontinuous microemulsion. He has been on the faculty of the department of chemistry at Morgan State University since 1990 where he continues to do research in the area of electrochemical application in the detoxification of organic pollutants and electron transfer reactions in organized media including sol-gel matrices.