Temperature equilibrium cell voltage

The temperature dependence of the equilibrium cell voltage forms the basis for determining the thermodynamic variables AG, A//, and AS. The values of the equilibrium cell voltage A%, and the temperature coefficient dA< 00/d7 which are necessary for the calculation, can be measured exactly in experiments. 

Eeq is the equilibrium cell voltage under the temperature and pressure working conditions... 

Fig. 2.6. Conversion diagram with two exemplary trajectories for identical temperature and cell voltage, but with different inlet conditions. Both end up in the same attractor, which is the intersecting point of the reforming and the oxidation equilibrium lines.

The thermodynamic data also determine the temperature coefficient of the equilibrium cell voltage or electrode potential according to the relation... 

Also the closely related temperature coefficient of the equilibrium cell voltage is slightly higher and results according to Eq. (13) to... 

For the reaction shown in Example 13.10 estimate the derivative of the equilibrium cell voltage with temperature (dEldT)p. We know it is a small number. How small ... 

Chapters 7 to 9 apply the thermodynamic relationships to mixtures, to phase equilibria, and to chemical equilibrium. In Chapter 7, both nonelectrolyte and electrolyte solutions are described, including the properties of ideal mixtures. The Debye-Hiickel theory is developed and applied to the electrolyte solutions. Thermal properties and osmotic pressure are also described. In Chapter 8, the principles of phase equilibria of pure substances and of mixtures are presented. The phase rule, Clapeyron equation, and phase diagrams are used extensively in the description of representative systems. Chapter 9 uses thermodynamics to describe chemical equilibrium. The equilibrium constant and its relationship to pressure, temperature, and activity is developed, as are the basic equations that apply to electrochemical cells. Examples are given that demonstrate the use of thermodynamics in predicting equilibrium conditions and cell voltages. 

A conductivity cell is set up using an yttria-stabilized zirconia electrolyte. At 900°C the equilibrium pressure in the cell was 1.02 x 10-10 atm, and the reference pressure outside the cell was 7.94 x 10 18 atm. (a) What is the cell voltage The temperature was dropped to 800°C and the reference pressure changed to 1.61 x 10-19 atm. The measured equilibrium voltage was 946 mV. (b) What is the equilibrium oxygen pressure in the cell [Data adapted from D-K. Lee et al., J. Solid State Chem., 178, 185-193 (2005).]... 

The oxidation rate depends not only on the gas composition and the temperature parameter, but also on the electric potential difference between the electronically conductive part of the anode electrode and the ionically conductive electrolyte. Defining the electric potential of the solid part of the anode electrode as zero potential, the reaction rate depends on the electric potential in the electrolyte, other hand, the reduction reaction rate depends on the electric potential difference at the cathode electrode, which is the difference between the given cell voltage, Uceii, and the electrolyte potential, equilibrium constants are determined by the... 

Fig. 2.5. Equilibrium lines of the reforming reaction (Eqrcf) at various temperatures and equilibrium lines of the oxidation reaction (Eq0x) at constant temperature and various cell voltages.

Due to their characteristic course, there is always one unique intersection point of both curves for any given temperature and voltage. At this intersection point, both reaction rates become zero. Once the gas has reached it, both reactions stop and the gas composition does not change anymore along the spatial coordinate, unless the temperature or the cell voltage is changed. Thus, this is a stationary point. Because the reactions always run towards their equilibrium, this stationary point is an attractor. 

Therefore such sensors are called Nernstian sensors. As a reference air with defined humidity is used. In reducing gases that are in chemical equilibrium (e.g., H2, H2Oj CO, C02 water gas) the oxygen partial pressure is determined by the mass law constant Kv and this in turn depends on the temperature. In the case of H2,H20-mixtures the cell voltage is obtained by insertion of a temperature function of log Kp into the Nernst equation... 

The Corning Ion-meter 135 and the Orion 811 are representative examples of pH-meters with and without a built-in microprocessor, respectively. The latter shows the cell voltage at equilibrium. Its temperature sensor automatically corrects the error due to the difference in temperature between the standards and the sample. When the potential difference between two standards varies significantly from the theoretical expectation, the Instrument shows the deviation. The microprocessor of the Ion-meter 135 allows the automatic use of different measurement programs (e.g. standard additions, sample addition, pH determination). Data can be stored In the memory for subsequent calculations. The microprocessor can also be used Independently. 

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