Chemical potential from galvanic cells

The electrical potentials of galvanic cells resulting from differences in chemical potentials are already familiar from numerous examples discussed in this book In particular the potential of cells of the general type... 

A potential developed when a current/ flows in an electro-chemical cell. It is a consequence of the cell resistance R and is given by the product IR. It is always subtracted from the theoretical cell potential and therefore reduces that of a galvanic cell and increases the potential required to operate an electrolysis cell. 

Ozkaya (76) studied conceptual difficulties experienced by prospective teachers in a number of electrochemical concepts, namely half-cell potential, cell potential, and chemical and electrochemical equilibrium in galvanic cells. The study identified common misconceptions among student teachers from different countries and different levels of electrochemistry. Misconceptions were also identified in relation to chemical equilibrium, electrochemical equilibrium, and the instrumental requirements for die measurement of cell potentials. Learning difficulties were attributed mainly to failure of students to acquire adequate conceptual understanding, and the insufficient explanation of the relevant... 

Another way of determining the solvent transport by emf measurements has been proposed by C. Wagner The two half cells contain two solvent mixtures of similar composition which are both saturated with a sparingly soluble salt, e.g. a silver salt AgX. Though the chemical potential of is the same throughout the galvanic cell with transport, the emf is different from zero since the chemical potential of the solvent is different in the two half cells and A moles of the non-aqueous solvent component are transported into the cathode compartment per Faraday. 

In voltaic cells, it is possible to carry out the oxidation and reduction halfreactions in different places when suitable provision is made for transporting the electrons over a wire from one half-reaction to the other and to transport ions from each half-reaction to the other in order to preserve electrical neutrality. The chemical reaction produces an electric current in the process. Voltaic cells, also called galvanic cells, are introduced in Section 17.1. The tendency for oxidizing agents and reducing agents to react with each other is measured by their standard cell potentials, presented in Section 17.2. In Section 17.3, the Nernst equation is introduced to allow calculation of potentials of cells that are not in their standard states. 

Only potential differences between chemically identical forms of matter are easily measurable, so the two terminals of a galvanic cell must be made of the same material. The cell potential E, also called the electromotive force (emf), is the potential difference between the terminals when they are not connected. Connecting the terminals reduces the potential difference due to internal resistance within the galvanic cell. The drop in the emf increases as the current increases. The current from one terminal to the other through the load (or resistance) flows in the direction opposite the electron flow. Since electrons in tire anode have higher potential energy than those in the cathode, electrons flow through the load from the anode to the cathode. 

Even in a galvanic cell with a salt bridge, there is some leakage of ions across the liquid junction, which causes the battery to lose its chemical potential over time. Commercial cells use an insoluble salt to prevent this from happening. 

IR drop The potential drop across a cell due to resistance to the movement of charge also known as the ohmic potential drop. Irreversible cell An electrochemical cell in which the chemical reaction as a galvanic cell is different from that which occurs when the current is reversed. 

Volta s greatest contribution was, however, the discovery, in 1796. of the voltaic pile, which consisted of a series of units, each made from sheets of dissimilar metals such as zinc and silver separated by wet doth. Volta showed that metals could be arranged in au "electromotive series so that each became positive when placed in contact with the one next below it in the series. Although, as has already been mentioned, Volta considered that the source of the electric energy was at the surface of contact of, the metals, this theory was thrown in doubt when it was discovered that chemical action accompanied the operation of the pile. It is of interest that the question of the seat of the potential of the galvanic cell is not, even today, finally settled. Many improvements of the voltaic pile were made. It is, of course, the precursor of the modern galvanic cell. 

We have seen previously how a galvanic cell may produce current from a chemical reaction. Similarly we shall see in this section how the opposite reaction may be used to make a chemical reaction occur. Such a process is called electrolysis which involves the addition of current in order to make the chemical reaction occur having otherwise a negative cell potential. This means that the reaction will not take place spontaneously. We looked briefly into this principles in the example with the lead battery earlier in this chapter but in this section we will go deeper into the phenomenon of electrolysis. 

Further we looked at galvanic cells where it was possible to extract electrical energy from chemical reactions. We looked into cell potentials and standard reduction potentials which are both central and necessary for the electrochemical calculations. We also looked at concentration dependence of cell potentials and introduced the Nemst-equation stating the combination of the reaction fraction and cell potentials. The use of the Nemst equation was presented through examples where er also saw how the equation may be used to determine equilibrium constants. 

Table 1 gives the phase compositions of the alloys used in our investigation, the chemical reactions responsible for the flow of the current in the galvanic cells, and the equations describing the changes in the isobaric-isothermal potential due to the formation of 1 mole of a given germanlde from the pure components. 

A battery operates on the principle of a Galvanic cell a chemical reaction is used to produce electricity. The materials that are involved in the reaction form the electrodes and the reaction takes place by the passage of ions through an electrolyte. The formation of ions during the chemical reaction involves the transfer of electrons to or from the electrodes. In a galvanic cell these are not allowed to pass through the electrolyte but must travel around an external circuit, driven by a potential difference created between the electrodes. It is the electron movement through the external circuit that can be used to do work. 

For electrochemical systems, electrochemical potentials jliip, T) (Section II) are used instead of chemical potentials. Under the action of driving forces, both chemical reactions (e.g., reaction in a galvanic cell) and charge transport (e.g., electron transport outside the cell from the anode to the cathode) may take place. The scheme... 

In Eqs. (122) and (123), M(Hg) is an alkali metal amalgam electrode, MX the solvated halide of the alkali metal M at concentration c in a solvent S, and AgX(s)/Ag(s) a silver halide-silver electrode. Equation (124) is the general expression for the electromotive force " of a galvanic cell without liquid junction in which an arbitrary cell reaction 0)1 Yi + 0)2Y2 + coiYi + , takes place between k components in v phases. In Eq. (124) n is the number of moles of electrons transported during this process from the anode to the cathode through the outer circuit, F the Faraday number, and the chemical potential of component Yi in phase p. Cells with liquid junctions require the electromotive force E in Eq. (124) to be replaced by the quantity E — Ej), where Ey> is the diffusion potential due to the liquid junction. The standard potential E° for the cell investigated by Eq. (122) is given by the relationship... 

---