Electrolysis of water and aqueous

A DEEPER LOOK Electrolysis of Water and Aqueous Solutions... 

In Section 17.6 we discussed applications of electrolysis in the extraction and purification of metals from their ore sources. Here we examine the electrolysis of water and aqueous solutions. Consider first the electrolysis of water between inert electrodes such as platinum, for which the half-cell reactions are... 

Electrolysis of Water and Nonstandard Half-Cell Potentials Before we can analyze the electrolysis products of aqueous salt solutions, we must examine the electrolysis of water itself. Extremely pure water is difficult to electrolyze because very few ions are present to conduct a current. If we add a small amount of a salt that cannot be electrolyzed in water (such as Na2S04), however, electrolysis proceeds rapidly. A glass electrolytic cell with separated gas compartments is used to keep the H2 and O2 gases from mixing (Figure 21.25). At the anode, water is oxidized as the O.N. of O changes from —2 to 0 ... 

Non-aqueous processes have the advantage of avoiding the electrolysis of water and the related gas evolntion. 

Explosion Hazards. The electrolysis of aqueous solutions often lead to the formation of gaseous products at both the anode and cathode. Examples are hydrogen and chlorine from electrolysis of NaCl solutions and hydrogen and oxygen from electrolysis of water. The electrode reactions. 

We can recognize four main periods in the history of the study of aqueous solutions. Each period starts with one or more basic discoveries or advances in theoretical understanding. The first period, from about 1800 to 1890, was triggered by the discovery of the electrolysis of water followed by the investigation of other electrolysis reactions and electrochemical cells. Developments during this period are associated with names such as Davy, Faraday, Gay-Lussac, Hittorf, Ostwald, and Kohlrausch. The distinction between electrolytes and nonelectrolytes was made, the laws of electrolysis were quantitatively formulated, the electrical conductivity of electrolyte solutions was studied, and the concept of independent ions in solutions was proposed. 

When electrolyzing an aqueous solution, there are two compounds present water, and the dissolved electrolyte. Water may he electrolyzed as well as, or instead of, the electrolyte. The electrolysis of water produces oxygen gas and hydrogen gas, as shown in Figure 11.16. 

Chlorine (Cl ) and sodium hydroxide (NaOH) are two important chemicals produced by electrolysis. Chlorine and sodium hydroxide are generally among the top ten chemicals produced annually in the United States. The electrolysis of brine or aqueous NaCl solution is used to produce chlorine, sodium hydroxide, and hydrogen (Figure 14.14). The chloride ion in the brine solution is oxidized at the anode, while water is converted at the cathode according to the following reactions ... 

When an aqueous solution of either Na2S04 or H2SO4 is electrolyzed, H2 and O2 gases are collected at the electrodes. In the process of electrolysis of water, H2... 

The soluble bases are very limited in number and involve nondis-chargeable cations. Accordingly, when an aqueous solution of a base is electrolyzed, hydrogen gas is evolved at the cathode and, of course, the discharge of hydroxyl ions at the anode results in the liberation of gaseous oxygen. In such cases, the products are the same as those obtained by the electrolysis of water. Since a commercial application of the electrolysis of the strong base sodium hydroxide is described in the next section, no further discussion of this topic seems necessary at this juncture. 

Electrolysis of Water. Electrolysis of acidified water at Pt electrodes is commonly used to demonstrate the composition of water, and is a small-scale production method for generating hydrogen and oxygen. Very pure hydrogen of more than 99.99% can be formed from the electrolysis of warm aqueous solutions of barium hydroxide between nickel electrodes, via the following reaction ... 

Hindrance at the carbonyl group (e.g. 22c) favors )8,i8 -coupling. The radical anion derived from 22c is stable on a voltammetric time scale in DMF (n-Pr4NC104) and reacts slowly under preparative conditions giving 30% of the LHD, exclusively as the ( ) isomer [20]. When water or Li is added, the rate of reaction is increased. Electrolysis in a mixture of DMF and aqueous buffer (pH 9 or pH 13) gave an increased yield of the LHD (55-60%) predominantly as the ( ) isomer (< 5% meso). The ( ) isomer was also the product obtained by chemical reduction using Na in HMPA [20]. 

In this chapter, roughly one dozen quite different production processes will be summarized. For instance, the most important electrochemical process, the electrolysis of aqueous sodium chloride leading to chlorine, alkali, hydrogen, and chlorine oxygen compounds are incorporated (Sect. 5.2). Other processes are carried out in a small scale only, but have an unchallenged place in the technical chemistry, for instance, the generation of molecular fluorine (Sect. 5.3). Others are of limited importance nowadays, but may gain importance in the future, for example, the electrolysis of water (Sect. 5.4). 

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