Potential of half-cell

The half-cell potential is thus related with the concentration of the active ion and, therefore, the measurement of half-cell potential can be used to study the progress of reaction. For example, the formation of complex between Fe3+ and F can be studied by measuring the potential of half-cell Fe2+/Fe3+ at various time intervals. Since the complex formation changes the concentration of Fe3+, the potential of half-cell Fe2+/Fe3+ will also change. 

Although potentials of half-cell electrode reactions cannot be measured, their consideration is extremely useful. For example, consider the two cells in series, as indicated in reaction (X),... 

By convention, the standard potentials of half-cells are expressed as reduction potentials (i.e., for the reduction reaction), such that larger negative values indicate a greater tendency to lose electrons. 

The potentials of half-cells that are constructed using amalgams are important whenever one is interested in performing reactions in a media in which hydrogen evolution at the metal-solution interface might interfere with the reaction of interest. Consequently, half-cell potentials for alkali metal amalgams are important to synthetic chemists and electrochemists alike. The standard potential for a metal amalgam in contact with a solution of its monovalent cation ( °(M" /M(Hg))) can be related to that of the pure metal in contact with its monovalent cation (EJ ) as shown in Eq. (14)... 

Equation (5) or (11) can be applied directly to half-cell reactions such as (6) and (7) and the resulting potentials obtained will be identical to those obtained from the overall reactions (9) and (10) because of the definition of the SHE as the universal standard. A selection of standard potentials of half-cell reactions is shown in Table 1 [5]. By international convention, electrode reactions in thermodynamic tables are always written as reduction reactions, so the more noble metals have a positive standard potential. Lists such as that in Table 1 are also called electromotive force series or tables of standard reduction potentials. 

One of the methods by which the formation constants of metal complexes can be determined involves the measurement of electrode potentials of half cells of the type M" + ne M in a series of solution of varying ligand concentrations. 

Here rev is the reversible potential of the half cell measured with respect to an arbitrary reference. Normally, we measure the reversible potential of half cells by using the standard hydrogen electrode as the point of reference. Equation (2.158) then becomes ... 

The IB Chemistry data booklet contains standard electrode potentials of half-cells recorded in... 

From knowledge of cell potential it is thus possible to predict whether a reaction would proceed spontaneously or not. From knowledge of potential of half cells, and the cell potentials, it is possible to predict whether a metal is an anode or cathode or whether a corrosion reaction is likely to take place or not. An understanding of electrode potential is of fundamental importance to the understanding of corrosion mechanism. This is illustrated in typical problems given below. 

The electrode potentials of half-cells are measured in relation to that of a standard hydrogen electrode, which is assigned an electrode potential of zero under standard conditions (solution concentrations of 1 M, gas pressures of 1 atm, and a temperature of 25 °C). 

To compare the potentials of half cells a reference had to be deflned. For this reason it was decided arbitrarily that the potential of the hydrogen electrode in a 1M acidic solution should be equal to 0 V at a temperature of 25 °C and a pressure of 101.3 kPa. These conditions are called standard conditions [6]. 

Calculating cell potential The tendency of a substance to gain electrons is called its reduction potential. The reduction potential of a half-cell reaction is expressed in volts (V). Chemists measure the reduction potentials of half-cells against the standard hydrogen electrode, which has a reduction potential defined as 0 V at 25°C, 1.0 atm pressure, and IM hydrogen ion concentration. 

In assembling cells for making thermodynamic measurements, one should try not to combine half-cells in a manner that results in a junction potential. Figure 9.7 is a schematic representation of the Daniell cell, which is one with a junction potential. The half-cell reactions are... 

Identify the two half-reactions, (b) Determine the potential of this cell, (c) Identify the anode and cathode, (d) Redraw the sketch to show the direction of electron flow and the molecular processes occurring at each electrode. 

Analytical methods based upon oxidation/reduction reactions include oxidation/reduction titrimetry, potentiometry, coulometry, electrogravimetry and voltammetry. Faradaic oxidation/reduction equilibria are conveniently studied by measuring the potentials of electrochemical cells in which the two half-reactions making up the equilibrium are participants. Electrochemical cells, which are galvanic or electrolytic, reversible or irreversible, consist of two conductors called electrodes, each of which is immersed in an electrolyte solution. In most of the cells, the two electrodes are different and must be separated (by a salt bridge) to avoid direct reaction between the reactants. 

Calculate the standard potentials of the cells formed by combination of each of the following pairs of half-cells ... 

For the first two types, a table of metals relating their ease of oxidation to each other is useful in being able to predict what displaces what. Table 6.1 shows the activity series for metals, which lists the metal and its oxidation in order of decreasing ease of oxidation. An alternative to the activity series is a table of half-cell potentials, as discussed in Chapter 16. In general, the more active the metal, the lower its potential. 

In the discussion of the Daniell cell we indicated that this cell produces 1.10 volts. This voltage is really the difference in potential between the two half-cells. There are half-cell potentials associated with all half-cells A list of all possible combinations of half-cells would be tremendously long. Therefore, a way of combining desired half-cells has been developed. The cell potential (really the half-cell potentials) depends on concentration and temperature, but initially we ll simply look at the half-cell potentials at the standard temperature of 298 K (25°C) and all components in their standard states (1 M concentration of all solutions, 1 atmosphere pressure for any gases, and pure solid electrodes). All the half-cell potentials are tabulated as the reduction potentials, that is, the potentials associated with the reduction reaction. The hydrogen half-reaction has been defined as the standard and has been given a value of exactly 0.00 V. All the other half-reactions have been measured relative to it, some positive and some negative. The table of standard reduction potentials provided on the AP exam is shown in Table 16.1 and in the back of this book. 

Measurements of the cell potential are essential and require a voltmeter (potentiometer). These measurements may be taken from different combinations of half-cells, or from measurements before and after changes of some aspect of the cell were made. 

Predict whether the cell potentials of galvanic cells depend on the electrodes and electrolytes in the half-cells. Give reasons for your prediction. 

In section 11.1, you learned that a cell potential is the difference between the potential energies at the anode and the cathode of a cell. In other words, a cell potential is the difference between the potentials of two half-cells. You cannot measure the potential of one half-cell, because a single half-reaction cannot occur alone. However, you can use measured cell potentials to construct tables of half-cell potentials. A table of standard half-cell potentials allows you to calculate cell potentials, rather than building the cells and measuring their potentials. Table 11.1 includes a few standard half-cell potentials. A larger table of standard half-cell potentials is given in Appendix E. 

The numerical values of cell potentials and half-cell potentials depend on various conditions, so tables of standard reduction potentials are true when ions and molecules are in their standard states. These standard states are the same as for tables of standard enthalpy changes. Aqueous molecules and ions have a standard concentration of 1 mol/L. Gases have a standard pressure of 101.3 kPa or 1 atm. The standard temperature... 

In this section, you learned that you can calculate cell potentials by using tables of half-cell potentials. The half-cell potential for a reduction half-reaction is called a reduction potential. The half-cell potential for an oxidation half-reaction is called an oxidation potential. Standard half-cell potentials are written as reduction potentials. The values of standard reduction potentials for half-reactions are relative to the reduction potential of the standard hydrogen electrode. You used standard reduction potentials to calculate standard cell potentials for galvanic cells. You learned two methods of calculating standard cell potentials. One method is to subtract the standard reduction potential of the anode from the standard reduction potential of the cathode. The other method is to add the standard reduction potential of the cathode and the standard oxidation potential of the anode. In the next section, you will learn about a different type of cell, called an electrolytic cell. 

Refer to the table of half-cell potentials to determine if MnOz can oxidize Br to Br2 in acidic solution under standard conditions. 

A number of detailed thermodynamic comparisons of half-cells containing alkali metal and alkali metal amalgams are available. For example, Cogley and Butler examined cell potentials as a function of amalgam concentration for the cell shown below [22]. 

Notice that when the silver reaction is multiplied by 2 its E° value stays the same. The potential of a half-cell does not depend on the coefficients of the equation because the potential of the cell is independent of the quantities of reactants. Adding the two half reactions gives the overall reaction and the cell voltage ... 

Thus, h becomes more positive as the concentration of the oxidized form of the compound increases. h is influenced by pH since pH affects the standard potential of a number of half-cells. The above equation becomes ... 

Subtraction of reduction (or of oxidation) potentials of half-reactions having the same number of electrons gives the emf of the corresponding cell reaction, e.g. 

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