Predicting the Products of Electrolysis

Electrolysis, the splitting (lysing) of a substance by the input of electrical energy, is often used to decompose a compound into its elements. Electrolytic cells are involved in key industrial production steps for some of the most commercially important elements, including chlorine, copper, and aluminum. The first laboratory electrolysis of H2O to H2 and O2 was performed in 1800, and the process is still used to produce these gases in ultrahigh purity. The electrolyte in an electrolytic cell can be the pure compound (such as H2O or a molten salt), a mixture of molten salts, or an aqueous solution of a salt. The products obtained depend on several factors, so let s examine some actual cases. 

Electrolysis of Molten Salts and the Industrial Production of Sodium Many electrolytic applications involve isolating a metal or nonmetal from a molten binary ionic compound (salt). Predicting the product at each electrode is simple if the salt is pure because the cation will be reduced and the anion oxidized. The electrolyte is the molten salt itself, and the ions move through the cell because they are attracted by the oppositely charged electrodes. 

Consider the electrolysis of molten (fused) calcium chloride. The two species present are Ca and Cl , so Ca ion is reduced and CF ion is oxidized  

Metallic calcium is prepared industrially this way, as are several other active metals as well as the halogens CI2 and Bt2. 

Another important application is the industrial production of sodium, which involves electrolysis of molten NaCl. The sodium ore is halite (largely NaCl), which is obtained either by evaporation of concentrated salt solutions (brines) or by mining vast salt deposits formed from the evaporation of prehistoric seas. 

Predicting the products of an electrolysis reaction is in some cases relatively straightforward and in other cases more complex. We cover the simpler cases first and follow with the more complex ones. 

Pure Molten Salts Consider the electrolysis of a molten salt such as sodium chloride, shown in Eigure 18.23 . Na and CF are the only species present in the cell. The chloride ion cannot be further reduced (—1 is its lowest oxidation state), so it must be oxidized. The sodium ion cannot be fiirther oxidized (+1 is its highest oxidation state), so it must be reduced. Thus, we can write the half-reactions  

Although the reaction as written is not spontaneous, it can be driven to occur in an electrolytic cell by an external power source. We can generalize as follows  

Mixtures of Cations or Anions What if a molten salt contains more than one anion or cation For example, suppose our electrolysis cell contained both NaCl and KCl. Which of the two cations would be reduced at the cathode In order to answer this question, we must determine which of the two cations is more easily reduced. Although the values of electrode potentials for aqueous solutions given in Table 18.1 do not apply to molten salts, the relative ordering of the electrode potentials does reflect the relative ease with which the metal cations are reduced. We can see from the table that the reduction of Na is listed above the reduction of K+ that is, Na+ has a more positive electrode potential. 

NaCI In the electrolysis of a pure molten salt, the anion (in this case CF) is oxidized and the cation (in this case Na ) is reduced. 

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Predict the products of electrolysis of a solution of copper(n) sulfate if carbon electrodes are used instead of those made from copper as referred to in the purification of copper section. 

On the basis of the preceding generalizations, it is possible to predict the products of electrolysis of aqueous solutions of simple salts. It is not... 

Notes In the molten salt mixture, Na is reduced in preference to Ca ". In aqueous salt solutions, care must be taken in predicting the products of electrolysis because of the possibility of oxidizing or reducing water instead of either the anion or cation, respectively. 

Predicting the products of electrolysis and the potential difference needed to generate products... 

Predicting the Products of Electrolysis 720 Purifying Copper and Isolating Aluminum 724... 

The Electrolysis of Aqueous Sodium Chloride and Overvoltage An additional complication that we must consider when predicting the products of electrolysis is overvoltage—an additional voltage that must be applied in order to get some nonspontaneous reactions to occur. We can demonstrate this concept by considering the electrolysis of a sodium chloride solution, shown in Figure 18.25 t. In order to predict the product of the electrolysis, we consider the two possible oxidation half-reactions ... 

Predicting the Products of Electrolysis Reactions (18.8) Example 18.9 For Practice 18.9 Exercises 91-96... 

In this part of Chapter 12, we study electrolysis, the process of driving a reaction in a nonspontaneous direction by using an electric current. First, we see how electrochemical cells are constructed for electrolysis and how to predict the potential needed to bring electrolysis about. Then, we examine the products of electrolysis and see how to predict the amount of products to expect for a given flow ot electric current. 

To predict the products of an electrolysis involving an aqueous solution, you must examine all possible half-reactions and their reduction potentials. Then, you must find the overall reaction that requires the lowest external voltage. That is, you must find the overall cell reaction with a negative cell potential that is closest to zero. The next Sample Problem shows you how to predict the products of the electrolysis of an aqueous solution. 

Predict the products of the electrolysis of a 1 mol/L solution of sodium chloride. 

In Investigation 11-B, you will build an electrolytic cell for the electrolysis of an aqueous solution of potassium iodide. You will predict the products of the electrolysis, and compare the observed products with your predictions. 

The principle described in this section is very useful, but it must be applied with some caution. For example, in the electrolysis of an aqueous solution of sodium chloride, we should be able to use values to predict the products. Of the major species in the solution (Na, Cl, and HjO), only Cl and HjO can be readily oxidized. The half-reactions (written as oxidization processes) are... 

SECTION 20.9 An electrolysis reaction, which is carried out in an electrolytic cell, employs an external source of electricity to drive a nonspontaneous electrochemical reaction. The current-carrying medium within an electrolytic ceU may be either a molten salt or an electrolyte solution. The products of electrolysis can generaUy be predicted by comparing the reduction potentials associated with possible oxidation and reduction processes. The electrodes in an electrolytic ceU can be active, meaning that the electrode can be involved in the electrolysis reaction. Active electrodes are important in electroplating and in metaUuigical processes. 

Sei e-Test 12.13A Predict the products resulting from the electrolysis of l M AgNOj(aq). 

Predict the likely products of electrolysis of an aqueous solution from standard potentials (Example 12.1 I). 

Predict the products that would be expected from the electrolysis of 1.0 mol/L Nal. Use the nonstandard E values for water. 

The rate of electrodeposition is dependent on a number of factors, and these are predictable to only a limited degree. However, the thickness of the diffusion layer must be minimized to obtain a rapid electrolysis. This is accomplished by vigorously stirring and by the use of electrodes with large surface areas. An increase in temperature enhances the rate of electrolysis because it increases the mobility of the electroactive species. The use of high ionic concentrations minimizes the iR drop between electrodes and also improves the electrolysis rate. The orientation and geometry of the electrodes is important to insure a uniform and adherent plate. Depolarizers frequently are introduced to prevent formation of interfering products from the counter electrode (see Table 3.5). 

A comparison of the E°s would lead us to predict that the reduction (it) would be favored over that of (i). This is certainly the case from a purely energetic standpoint, but as was mentioned in the section on fuel cells, electrode reactions involving 02 are notoriously slow (that is, they are kinetically hindered), so the anodic process here is under kinetic rather than thermodynamic control. The reduction of water (iv) is energetically favored over that of Na+ (iii), so the net result of the electrolysis of brine is the production of Cl2 and NaOH ( caustic ), both of which are of immense industrial importance ... 

Kekul6 1 advanced a theory based upon the phenomena of decomposition and from this deduced certain formulae which make it possible to predict the nature of the products resulting from the electrolysis of monobasic and dibasic acids of the fatty-acid series. Since, however, the reaction is influenced by the slightest variation of conditions, his formulae hold good only in the case of the decomposition of perfectly pure substances, a condition seldom met with in practice. 

Predict the products liberated at the anode and cathode of an electrolysis cell with a given aqueous electrolyte composition (Section 17.7, problems 63-64). 

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