Effect of Concentration on Voltage

Click Coached Problems for a self-study module on voltaic cells under non-standard conditions. 

To this point, we have dealt only with standard voltages, that is, voltages when all gases are at 1 atm pressure and all species in aqueous solution are at a concentration of 1M. When the concentration of a reactant or product changes, the voltage changes as well. Qualitatively, the 

Voltage will increase if the concentration of a reactant is increased or that of a product is decreased. Either of these changes increases the driving force behind the redox reaction, making it more spontaneous. 

When a voltaic cell operates, supplying electrical energy, the concentration of reactants decreases and that of the products increases. As time passes, the voltage drops steadily. Eventually it becomes zero, and we say that the cell is dead. At that point, the redox reaction taking place within the cell is at equilibrium, and there is no driving force to produce a voltage. 

To obtain a quantitative relation between cell voltage and concentration, it is convenient to start with the general expression for the free energy change discussed in Chapter 16  

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The net cell reaction is an equilibrium reaction. The cell voltage directly relates to the spontaneity of a reaction. The effect of concentration on cell voltages can easily be explained by Le ChMelier s principle. Let us consider the following reaction ... 

Figure 15.1. SEM micrographs showing the effect of concentration on the electrospun Poly (vinyl phenol) nanofibers from 10, 20, 30 and 40 w/v% at 15cm and applied voltage of 15 kV.

The effects of H2S on cell voltage are reversible if H2S concentrations are present at levels below that required to form nickel sulfide. 

Based on the present understanding of the effect of sulfur on MCFCs, and with the available cell components, it is projected that long-term operation (40,000 hr) of MCFCs may require fuel gases with sulfur " levels of the order 0.01 ppm or less, unless the system is purged of sulfur at periodic intervals or sulfur is scrubbed from the cell burner loop (76). Sulfur tolerance would be approximately 0.5 ppm (see Table 6-3) in the latter case. Considerable effort has been devoted to develop low-cost techniques for sulfur removal, and research and development are continuing (80,81). The effects of H2S on cell voltage are reversible if H2S concentrations are present at levels below which nickel sulfide forms. 

In order to confirm the possible interaction of ethanol and crocin on NMDA receptors, we also performed whole-cell patch recording with primary cultured hippocampal neurons and measured membrane currents induced by the application of NMDA in a voltage-clamped condition. Application of 100 pM NMDA induced an inward current of 100.2 9.8 pA (n=10) at a holding potential of -60 mV. The NMDA-induced inward current was not affected by 10 pM CNQX (data not shown), but was completely abolished by 30 pM APV, supporting the fact that the response was mediated by NMDA receptors. Ethanol inhibited NMDA-induced currents in a concentration-dependent manner. Crocin (10 pM) had no effect on NMDA-induced currents by itself (data not shown), but attenuated the inhibitory effect of ethanol on NMDA-induced currents. The concentration-effect curve for ethanol was shifted to the right by the presence of crocin [22]. 

The effect of illumination on the I-V curves of both p-Si and n-Si is shown in Fig. 3.13. The anodic reaction kinetics of/ -type silicon are not affected by illumination because the reaction consumes holes which are the majority carriers and their concentration is little affected by illumination. For n-Si during anodization, tbe interface is reversely biased and in order to sustain the reaction, either holes have to be generated or electrons have to be injected from the electrolyte. Thus, in the dark, an extra voltage, Vexj above that which is required for anodization of p-Si, is needed to drive the current. For example, about 100 V is required for 7mA/cm initially in NMA. The extra voltage needed to anodize -Si diminishes with increasing illumination intensity and at sufficient light intensity the anodic current becomes identical to that for p-Si. The quantum yield of illuminated anodization is low a value as low as 1% has been found for the anodization of silicon under illumination. ... 

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