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Microbial Electrochemical Technologies for Fuel Cell Devices
Published in Mu Naushad, Saravanan Rajendran, Abdullah M. Al-Enizi, New Technologies for Electrochemical Applications, 2020
S. V. Sheen Mers, K. Sathish-Kumar, L. A. Sánchez‐Olmos, M. Sánchez-Cardenas, Felipe Caballero-Briones
This equation is positive for a favorable reaction and directly produces a value of the emf for the reaction. The Nernst equation expresses the emf of a cell in terms of activities of products and reactants taking place in the cell reaction. The Nernst equation relates the potential of an electrochemical cell (half-cell or full-cell reaction) containing a reversible system with fast kinetics, and it is valid only at equilibrium and at the surface of the electrode. These would help to evaluate the potential of the cell under nonstandard conditions (i.e., Equation 5.12) and can be used to calculate the reduction potential at various pHs, temperatures, and concentrations of product/reactant. The reaction quotient (П) is the ratio of the activities of the products divided by the reactants raised to their respective stoichiometric coefficients: Π=productspreactantsr
Q
Published in Splinter Robert, Illustrated Encyclopedia of Applied and Engineering Physics, 2017
[atomic, chemical, nuclear] Reaction quotient, describing the transposition of states and energy configurations. The reaction quotient for a chemical process is defined as the ratio of the concentration of products to the concentration of reactants, for instance in a reaction with ingredients A; B; C; D at relative quantities a; b; c; d in reaction aA + bB ⇋ cC + dD, with respective (molar-) concentrations [A];[B];[C]; [D], proceeding as a [A] + b [B] ⇋ c[C] + d [D], which yields the reaction quotient Qc = CcDd/AaBb. This can provide the change in Gibbs free energy (ΔG) as (ΔG = ΔG0 + RT ln Qc), at temperature T with R = 8.3144621 (75) J/kmol the gas constant, applied to solution (see Figure Q.1).
Some Thermodynamic Concepts and Considerations
Published in Anthony Peter Gordon Shaw, Thermitic Thermodynamics, 2020
When the value of ξe is greater than zero but less than unity, equation 3.73 becomes quite useful. In that equation, ΔG° = G°p − G°r where G°p and G°r are the standard Gibbs energies at ξ = 1 and ξ = 0, respectively. At equilibrium, the slope of the Gibbs energy curve is zero, and a simplification can be written as equation 3.74. In this case, the reaction quotient is the equilibrium constant, which takes the same form. Equation 3.74 is especially important because the equilibrium composition can be determined from K once ΔG° is known.
Mass transport control of localised corrosion processes: in situ local probing and modelling
Published in Corrosion Engineering, Science and Technology, 2018
For each simulation, homogeneous chemical reactions must be considered, with their respective equilibrium constants. The reverse forward and backward reaction rate constants of these chemical reactions are imposed sufficiently high so as to maintain chemical equilibria in the whole domain. The fulfilment of local chemical equilibrium was checked a posteriori, comparing the reaction quotient of each reaction with its equilibrium constant. The electroneutrality conditionis applied to calculate the sodium ion concentration. The electroneutrality condition is also used to complete governing equations of the CETR model since, in the absence of charge excess, the divergence of the current density is null, i.e.
Plant wastes as alternative sources of sustainable and green corrosion inhibitors in different environments
Published in Corrosion Engineering, Science and Technology, 2023
Nnabuk Okon Eddy, Anduang O. Odiongenyi, Eno E. Ebenso, Rajni Garg, Rishav Garg
A corrosion concentration cell can be formed in a given metal if there is a difference in the concentration of the same electrolyte in different parts of the same metal, such that different electrode potentials occurred in the same metal at different positions. Concentration cells can be metal ions, oxygen or active–passive metal concentration cell. The thermodynamic feasibility of a reaction requires that the change in standard free energy is negative, which can customarily be linked to the electrode potential according to Equation (2): The Ecell in Equation (2) can be evaluated from the Nernst equation to obtain Equation (3) where . Q is the reaction quotient defined as the ratio of the lower concentration to the higher concentration. Based on Equations (2) and (3), the basic condition that can guarantee the feasibility of a reaction in a concentration corrosion cell requires the value of to be positive. Concentration corrosion cells have been noted for corrosion of some metals in the soil due to differences in (i) soil moisture, (ii) aeration and (iii) composition of the soil [14]. Differential temperature corrosion cells can be developed when a similar metal experiences different temperatures at different sections such as the corrosion process in a heat exchanger and in a condenser. In such cases, the section at higher temperature becomes the cathode while the one at lower temperature becomes the anode. This is because a cathode has a more positive value of Ecell than the anode, hence, the Nernst equation for the cathode takes the form, , which is more probable at a higher temperature. A corrosion cell can also be an electrolytic cell when an external source of electricity is applied. Such corrosion will also be controlled by the electrolytic processes but the direction or polarity will be a reversal of the direction in a galvanic cell.