Galvanic cells, activity electromotive force

In writing the Etudes de dynamique chimique (1884), van t Hoff drew on Helmholtz s 1882 paper but especially on the work of August Horstmann, a student of Bunsen, Clausius, and H. Landolt.59 As has often been discussed, van t Hoffs was an ambitious and original synthesis of disconnected ideas and theories about opposing forces, equilibrium, active masses, work and affinity, electromotive force, and osmotic pressure. He demonstrated that the heat of reaction is not a direct measure of affinity but that the so-called work of affinity may be calculated from vapor pressures (the affinity of a salt for its water of crystallization), osmotic pressure (affinity of a solute for a solution), or electrical work in a reversible galvanic cell (which he showed to be proportional to the electromotive force). 

The electrode in the half-cell in which oxidation is occurring is said to be the anode (here, the zinc metal), whereas the other is the cathode (here, the platinum). In principle, we could connect any pair of feasible half-cells to form a galvanic cell the identity of the half-cells will determine which electrode will act as the anode, and which the cathode. The electromotive force (EMF, in volts) of the cell will depend on the identity of the half cells, the temperature and pressure, the activities of the reacting species, and the current drawn. An EMF will also be generated by a cell in which the two half cells are the chemically identical except for a difference in reactant activities (concentrations) this is called a concentration cell. 

It will be seen that the electromotive force E2 of the reversible galvanic cell which consists of an amalgam electrode (0.206 per cent Na) and of a hydrogen electrode in a solution of sodium hydroxide of unit mean activity equal 1.039 V. 

Electromotive force (emf) measurements are frequently used to determine activity coefficients of electrolyte solutions. Equation (136a) relates the emf to the activities of the reacting cell components. From concentration-dependent measurements the standard potential E° of the cell reaction and the activity coefficients can be obtained. As an example, according to Eq. (136a), the emf of the Galvanic cell... 

If the buried metal structure involves contact between different metals (such as mild steel, copper, bronze, brass, aluminum, zinc, lead, stainless steel, cast iron, etc.), it is possible that local galvanic cells can form in the contact areas (Figs. 8-9). Each mettd has its own tendency to corrode. An alternative way to express the metal reactivity is to look at the excess of its free energy (standard electrochemical metal potential) and predict the electromotive force emf) between metals in contact, as a general indication for the corrosion process (Table 2). The metal that has a more positive potential is nobler in the galvanic cell, and it is a cathode. The metal with a more negative potential is more active and acts as an anode in the corrosion cell, e.g., it suffers corrosion. 

Recall that MiSO iaq) and M2SOi aq) are in their ionic state in solution (aqueous). Both metals are externally connected to a electrical circuit in order to measure the potential difference between them. In other words, the circuit is used to measure the galvanic cell potential by a voltmeter (V). This measurable oeU potential is current/iesistance dependent. If current ceases to flow, then the cell potential is known as the open-circuit potential or standard potential (E°), which are illustrated in Figure 2.2 for pure metal reduction. The standard potential is also known as the electromotive force (emf) under equilibrium conditions unit activity, 25 °C, and 1 atm (101 kPa) pressure. 

On a purely thermodynamic basis the electromotive force of a galvanic cell cannot be objectively divided into the three potential jumps at the different phase boundaries If it is assumed, that separate ion activities (or, what comes to the same thing, diffusion potentials) are known, the dependence of one separate potential jump (Ss) on the ionic composition of the solution becomes accessible, except for an additive constant ... 

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