Electrochemistry III Electrolysis

Chapter 17 emphasizes the principles associated with obtaining electrical energy from electron-transfer reactions in solution. This chapter emphasizes what happens when electrical energy is applied to solutions in the operation of electrolytic cells. The oxidation and reduction processes that take place in an electrolytic cell are called electrolysis. We focus on determining what products are obtained and how much energy is required. 

At the positive electrode to which the Cl ions have migrated, Cl ions give up electrons to the electrode according to the half-reaction 

If proper mechanical arrangements are provided, the Na is collected (as a vapor at the temperature of molten NaCI) in the absence of air at the negative electrode, and Cl2 gas is collected at the positive electrode. Different mechanical provisions must be made if the metal is produced as a liquid or a solid, but the principle is the same in every case. If we look at the completed circuit, we see that electrons have come from the power supply to the negative electrode and have gone to the power supply from the positive electrode, with a hi directional flow ofions within the cell. 

The situation is more complicated when the electrolytic cell contains aqueous 

we can, see that all ions lying abi readilv than H+ at same concentration. In IO-tJ a concentration we can use in the Nemst equation 

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For chemists, the second important application of electrochemistry (beyond potentiometry) is the measurement of species-specific [e.g., iron(III) and iron(II)] concentrations. This is accomplished by an experiment in which the electrolysis current for a specific species is independent of applied potential (within narrow limits) and controlled by mass transfer across a concentration gradient, such that it is directly proportional to concentration (/ = kC). Although the contemporary methodology of choice is cyclic voltammetry, the foundation for all voltammetric techniques is polarography (discovered in 1922 by Professor Jaroslov Heyrovsky awarded the Nobel Prize for Chemistry in 1959). Hence, we have adopted a historical approach with a recognition that cyclic voltammetry will be the primary methodology for most chemists. 

Refs. [i] Aurbach D, Weissman (1999) Nonaqueous electrochemistry an overview. In Aurbach D (ed) Nonaqueous electrochemistry. Marcel Dekker, New York, pp 1-52 [ii] Blomgren GE (1999) Physical and chemical properties of nonaqueous electrolyte solutions. In Aurbach D (ed) Nonaqueous electrochemistry. Marcel Dekker, New York, pp 53-58 [iii] Izutsu K (2002) Electrochemistry in nonaqueous solutions. Wiley-VCH, Weinheim, pp 3-24 [iv] Lund H (2001) Practical problems in electrolysis. In Lund H, Hammerich O (eds) Organic electrochemistry, 4th edn. Marcel Dekker, New York, pp 223-292 [v] Linden D (1994) Handbook of batteries, 2nd edn. McGraw-Hill, New York, Appendix A... 

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