Electrolysis of water and aqueous solutions

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 ... 

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. 

Electrolysis of potassium iodide (Kl) solution. The electrolysis of aqueous Kl is similar to that of aqueous NaCI. The cathode reaction (left/ is the reduction of water to H2(g) and OH-, as shown by the pink color of phenolphthalein indicator in the water. The anode reaction fright) is the oxidation of l (aq) to Ijfaq), as shown by the brown color of the solution. 

Chemical reduction of an ore usually gives metal that is not pure enough for its intended use. Further refining of the metal removes undesirable impurities. Several important metals, including Cu, Ni, Zn, and Cr, are refined by electrolysis, either from an aqueous solution of the metal salt or from anodes prepared from the impure metal. To give one example, ions, obtained by dissolving ZnS or ZnO in acidic solution, can be reduced while water... 

Hiratsuka et al102 used water-soluble tetrasulfonated Co and Ni phthalocyanines (M-TSP) as homogeneous catalysts for C02 reduction to formic acid at an amalgamated platinum electrode. The current-potential and capacitance-potential curves showed that the reduction potential of C02 was reduced by ca. 0.2 to 0.4 V at 1 mA/cm2 in Clark-Lubs buffer solutions in the presence of catalysts compared to catalyst-free solutions. The authors suggested that a two-step mechanism for C02 reduction in which a C02-M-TSP complex was formed at ca. —0.8 V versus SCE, the first reduction wave of M-TSP, and then the reduction of C02-M-TSP took place at ca. -1.2 V versus SCE, the second reduction wave. Recently, metal phthalocyanines deposited on carbon electrodes have been used127 for electroreduction of C02 in aqueous solutions. The catalytic activity of the catalysts depended on the central metal ions and the relative order Co2+ > Ni2+ Fe2+ = Cu2+ > Cr3+, Sn2+ was obtained. On electrolysis at a potential between -1.2 and -1.4V (versus SCE), formic acid was the product with a current efficiency of ca. 60% in solutions of pH greater than 5, while at lower pH... 

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. 

Lithium, sodium, beryllium, magnesium, calcium, and radium are all made industrially by the electrolysis of their molten chlorides. These salts are all soluble in water, but aqueous solutions are not used for the electrolytic process. Explain why. 

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. 

There are two main approaches, depending whether the conversion of C02 is studied in aqueous or nonaqueous solutions. Formic acid is the main reaction product in electrolysis of aqueous solutions of C02. A problem in the utilization of C02 in aqueous solution derives from its low solubility in water at standard temperature and pressure. At the surface of the electrode, there are very small amounts of C02 available for the reaction to proceed. For aqueous solutions, in order to speed the reaction rate, the pressure must be increased. 

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 ... 

Electrolysis of brine The decomposition of brine, an aqueous solution of sodium chloride, is another process that is accomplished by electrolysis. Figure 21-19 illustrates the electrolytic cell and products of the electrolysis of brine. Two reactions are possible at the cathode, the reduction of sodium ions and the reduction of hydrogen in water molecules. 

---