Voltage half-cell

The two estimates for the first or a parameter of the parabolic fit are the intercepts on the voltage axis of Eig. 3-1, so both procedures arrive at a standard potential of the silver-silver chloride half-cell of 0.2225 V. The accepted modem value is 0.2223 V (Barrow, 1996). 

Standard, reduction potentials are determined by measuring the voltages generated in reaction half-cells (Figure 21.2). A half-cell consists of a solution containing 1 M concentrations of both the oxidized and reduced forms of the substance whose reduction potential is being measured, and a simple electrode. 

The standard voltage for a given cell is that measured when the current flow is essentially zero, all ions and molecules in solution are at a concentration of 1M, and all gases are at a pressure of 1 atm. To illustrate, consider the Zn-H+ cell. Let us suppose that the half-cells are set up in such a way that the concentrations of Zn2+ and H+ are both 1 M and the pressure of H2(g) is 1 atm. Under these conditions, the cell voltage at very low current flow is +0.762 V. This quantity is referred to as the standard voltage and is given die symbol fi°. 

Standard half-cell voltages are ordinarily obtained from a list of standard potentials such as those in Table 18.1 (page 487). The potentials listed are the standard voltages for reduction half-reactions, that is,... 

The Nernst equation can also be used to determine the effect of changes in concentration on the voltage of an individual half-cell, E or Consider, for example, the half-reaction... 

In chemistry, the most important use of the Nemst equation lies in the experimental determination of the concentration of ions in solution. Suppose you measure the cell voltage and know the concentration of all but one species in the two half-cells. It should then be possible to calculate the concentration of that species by using the Nemst equation (Example 18.7). 

A voltaic cell consists of two half-cells. One of the half-cells contains a platinum electrode surrounded by chromium(III) and dichromate ions. The other half-cell contains a platinum electrode surrounded by bromate ions and liquid bromine. Assume that the cell reaction, which produces a positive voltage, involves both chromium(III) and bromate ions. The cell is at 25°C. Information for the bromate reduction half reaction is as follows ... 

In foe anode half-cell, hydrogen gas at 1.0 atm is bubbled over a platinum electrode dipping into a solution that has a pH of 7.0. The other half-cell is identical to the first except that the solution around the platinum electrode has a pH of 0.0. What is the cell voltage ... 

This shows that the voltage of a given cell may be thought of as being made up of two parts, one part characteristic of one of the half-reactions and one part characteristic of the other halfreaction. Chemists call these two parts half-cell potentials, a term that emphasizes the relation between voltage and potential energy. The halfcell potentials are symbolized °. 

We would like to measure the contribution each half-reaction makes to the voltage of a cell. Yet every cell involves two half-reactions and every cell voltage measures a difference between their half-cell potentials. We can never isolate one half-reaction to measure its E°. An easy escape is to assign an arbitrary value to the potential of some selected half-reaction. Then we can combine all other half-reactions in turn with this reference half-reaction and find values for them relative to our reference. The handiest arbitrary value to assign is zero and chemists have decided to give it to the half-reaction... 

Since concentration variations have measurable effects on the cell voltage, a measured voltage cannot be interpreted unless the cell concentrations are specified. Because of this, chemists introduce the idea of standard-state. The standard state for gases is taken as a pressure of one atmosphere at 25°C the standard state for ions is taken as a concentration of 1 M and the standard state of pure substances is taken as the pure substances themselves as they exist at 25°C. The half-cell potential associated with a halfreaction taking place between substances in their standard states is called ° (the superscript zero means standard state). We can rewrite equation (37) to include the specifications of the standard states ... 

Now if we combine a Zn-Zn+2 half-cell in its standard state with a H2-2H+ half-cell in its standard state, the voltage (potential) we measure (0.76 volt) is the value assigned to the halfreaction ... 

Chemists have determined a large number of these half-cell potentials. The magnitude of the voltage is a quantitative measure of the tendency of that half-reaction to release electrons in comparison to the H2-2H+ half-reaction. If the sign is positive, the half-reaction has greater tendency to release electrons than does the H2-2H+ half-... 

A half-cdl consisting of a palladium rod dipping into a 1 M Pd(NOj)2 solution is connected with a standard hydrogen half-cell. The cell voltage is 0.99 volt and the platinum dectrode in the hydrogen half-cell is the anode. Determine E° for the reaction... 

The reaction may be regarded as taking place in a voltaic cell, the two half-cells being a C12,2C1 system and a Fe3+,Fe2+ system. The reaction is allowed to proceed to equilibrium, and the total voltage or e.m.f. of the cell will then be zero, i.e. the potentials of the two electrodes will be equal ... 

In galvanic cells it is only possible to determine the potential difference as a voltage between two half-cells, but not the absolute potential of the single electrode. To measure the potential difference it has to be ensured that an electrochemical equilibrium exists at the phase boundaries, e.g., at the electrode/electrolyte interface. At the least it is required that there is no flux of current in the external and internal circuits. 

The potentials of the metals in their 1 mol U salt solution are all related to the standard or normal hydrogen electrode (NHE). For the measurement, the hydrogen half-cell is combined with another half-cell to form a galvanic cell. The measured voltage is called the normal potential or standard electrode potential, E° of the metal. If the metals are ranked according to their normal potentials, the resulting order is called the electrochemi-... 

It was mentioned earlier that the equilibrium cell voltage A%, is equal to the difference between the equilibrium potentials of its half-cells e.g., for the Daniell element,... 

The voltaic cell with the highest voltage will be the one connecting the K+/K half-cell with the F2/F half-cell E° ... 

Therefore, the Nernst equation predicts that the voltage of a standard half-cell equals E°. 

QB For this cell because the electrodes are identical, the standard electrode potentials are numerically equal and subtracting one from the other leads to the value c°dl = 0.000 V. However, because the ion concentrations differ, there is a potential difference between the two half cells (non-zero nonstandard voltage for the cell). [Pb2+] = 0.100 M in the cathode compartment. The anode compartment contains a saturated solution of Pbl2. We use the Nemst equation (with n = 2) to determine [Pb2+] in the saturated solution. 

While the voltage of the cell represents the potential difference between the two terminals of the battery, in reality it relates to the separation in energy between the two half-cells. We call this separation the emf where the initials derive from the archaic phrase electromotive force. An emf is defined as always being positive. 

Table 7.3 Temperature voltage coefficients for various cells and half cells...

Electrochemical cells comprise a minimum of two half-cells, the energetic separation between them being proportional to the cell emf. Since this energy is usually expressed as a voltage, we see that the energy needs to be measured electrically as a voltage. 

The first difference between these two batteries is the voltage they produce a watch battery produces about 3 V and a lead-acid cell about 2 V. The obvious cause of the difference in emf are the different half-cells. The electrode potential E is the energy, expressed as a voltage, when a redox couple is at equilibrium. 

---