Functions Galvanic cells

A student was given a standard Fe(s) Fe2+(aq) half-cell and another half-cell containing an unknown metal M immersed in 1.00 M MNO,(aq). When these two half-cells were connected at 25°C, the complete cell functioned as a galvanic cell with E = +1.24 V. The reaction was allowed to continue overnight and the two electrodes were weighed. The iron electrode was found to be lighter and the unknown metal electrode was heavier. What is the standard potential of the unknown MT/M couple ... 

FIGURE 2.2 Directions of current flow when the galvanic cell functions as a battery (a) and... 

Electrochemical measurements usually concern not a galvanic cell as a whole but one of the electrodes, the working electrode (WE). However, a complete cell including at least one other electrode is needed to measure the WE potential or allow current to flow. In the simplest case a two-electrode cell (Eig.l2.1a) is used for electrochemical studies. The second electrode is used either as the reference electrode (RE) or as an auxiliary electrode (AE) to allow current to flow. In some cases these two functions can be combined for example, when the surface area of the auxiliary electrode is much larger than that of the working electrode so that the current densities at the AE are low, it is essentially not polarized and thus can be used as RE. 

Fig. 8.1 Variation of the activity (logarithmic scale) or galvanic cell voltage as a function of the composition (schematic). The plateaux indicate multi-phase regions in which the activity is fixed according to Gibbs phase rule.

In-situ electrochemical techniques may be conveniently used to analyse the fundamental thermodynamic and kinetic parameters which are responsible for the performance of electrodes. The methods are nondestructive and may be applied to the actual galvanic cell. The data may be easily determined as a function of the discharge state. 

What is the function of a salt bridge in a galvanic cell ... 

The galvanic cell performs an electric work when the electrochemical reaction of the cell occurs. As work is not a state function, the amount of work which a system does depends on the path it takes for given initial and final states. Under what conditions does the amount of work the cell performs become maximum ... 

So far the spontaneous functioning of an electrochemical cell has been described, which corresponds to the transformation of energy obtained in a chemical reaction into electron movement, that is electrical energy. This type of cell is a galvanic cell. 

It follows that the contact potential between the two metals (difference between the work functions) is the principal factor determining the e.m.f. of galvanic cells consisting of two metals it is generally decidedly larger than the difference between the two electrode-electrolyte potentials, V -Vs, and VP— Vs. 

Polarization of the galvanic cell. The different phenomena of polarization of the anodic and cathodic reactions (activation, diffusion, convection, etc.), should be well known as a function of the evolution and change of the properties of the interface as a function of time. The polarization behavior of the cathodic and anodic reactions on the two electrodes should be examined (see Figure 6.5). In natural atmospheres, the cathodic reaction controls frequently the attack rate. The diffusion of oxygen is an important parameter to avoid control and polarization of the corrosion by the rate of the cathodic reaction (Figure 6.12).7... 

Dry pile Ritter was the first who constructed piles of galvanic cells with dry separators, e.g., of leather. The function of these piles is based on the small amounts of humidity in these separators. 

Nernst applied the electrical bridge invented by Wheatstone to the measurement of the dielectric constants for aqueous electrolytes and different organic fluids. Nemst s approach was soon employed by others for measurement of dielectric properties and the resistance of galvanic cells. Finkelstein applied the technique to the analysis of the dielectric response of oxides. Warburg developed expressions for the impedance response associated with the laws of diffusion, developed almost 50 years earlier by Fick, and introduced the electrical circuit analogue for electrolytic systems in which the capacitance and resistance were functions of frequency. The concept of diffusion impedance was applied by Kruger to the capacitive response of mercury electrodes. ... 

Explain why a galvanic cell is often used in an electrolytic cell. What function does the galvanic cell serve ... 

Figure 18-5 Cell potential in the galvanic cell of Figure 18-4b as a function of time. The cell current, which is directly related to the cell potential, also decreases with the same time behavior.

Relate the construction of a galvanic cell to how it functions to produce a voltage and an electrical current. 

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