Cell potential reaction free energy

Cell Potentials and Free-Energy Changes for Cell Reactions... 

Later in this chapter, we ll see that cell potentials, like free-energy changes, depend on the composition of the reaction mixture. The standard cell potential E° is the cell potential when both reactants and products are in their standard states—solutes at 1 M concentrations, gases at a partial pressure of 1 atm, solids and liquids in pure form, with all at a specified temperature, usually 25°C. For example, E° for the reaction... 

Cell potentials, like free-energy changes (Section 17.10), depend on temperature and on the composition of the reaction mixture—that is, on the concentrations of solutes and the partial pressures of gases. This dependence can be derived from the equation... 

The half-reactions are Cr(s) Cfr (aq) + 2 e and Cr X q) + e Cr X q)- The standard cell potential E° = 0.50 V With this positive cell potential, the free energy will be negative, so the reaction is spontaneous. Therefore chromium(ni) and solid chromium cannot both exist in the same solution at equilibrium. 

A voltaic cell is based on Ag (aq)/Ag(s) and e (aq)/Fe (aq) half-cells, (a) What is the standard emf of the cell (b) Which reaction occurs at the cathode, and which at the anode of the cell (c) Use S values in Appendix C and the relationship between cell potential and free-energy change to predict whether the standard cell potential increases or decreases when the temperature is raised above 25°C. 

As a result of the combination of Eqs. (20) and (21), the reaction free energy, AG, and the equilibrium cell voltage, A< 00, under standard conditions are related to the sum of the chemical potentials //,. of the substances involved ... 

Before dealing with various important applications of the electrochemical series with some practical examples, a break is given here to present a more detailed elaboration on the hydrogen electrode, reference electrodes, and some of the theoretical and general aspects pertaining to electrode potentials and free energy changes involved with cell reactions. 

EXAMPLE 12.3 Estimating the reaction free energy from a cell potential... 

One application of Eq. 2 is the determination of a reaction free energy—a thermodynamic quantity—from a cell potential, an electrical quantity. Consider the chemical equation for the reaction in the Daniel cell (reaction A) again. For this reaction, n = 2 because 2 mol of electrons migrate from Zn to Cu and we measure E = 1.1 V. It follows that... 

Cell potential and reaction free energy are related by Eq. 2 (AGr = -nFE) and their standard values by Eq. 3 (AGr° = —nFE°). The magnitude of the cell potential is independent of how the chemical equation is written. 

In this chapter we have seen that the standard reaction free energy is related to the standard cell potential of a galvanic cell by... 

As the reactants in a chemical reaction are used up and the concentrations of products increase, AGr changes until it reaches 0 at equilibrium. Because the cell potential is proportional to the reaction free energy (Eq. 2), it follows that E also changes as the reaction proceeds. We already know how A Gr varies with composition ... 

Relation between reaction free energy, AGr, and cell potential, E... 

The work done and reaction free energy are given by the product of the total charge, - F, and the cell potential, AGr = = -nFE. 

The standard reaction free energy can be obtained from the standard cell potential (emf) and vice... 

Calculate the thermodynamic potential of the following cell and the free energy change associated with the cell reaction. 

Other thermodynamic quantities can be derived from electrochemical measurements now that we have linked the potential difference across the cell to the free energy. For example, the entropy change in the cell reaction is given by the temperature dependence of AG.-... 

When the electrodes are in their standard states, the free energy change is called the standard reaction free energy, AG°r, and the cell voltage is just the standard cell potential, E°. In this case ... 

Redox Potential and Free Energy. The concept of redox potential, derived from the above experimental setup, has been an invaluable aid in chemistry. The concept is intimately associated with the free energy of an oxidation-reduction reaction, because the reaction in a galvanic cell is reversible and electric energy is made available for useful work. Thus the redox potential becomes a direct measure of the free energy (cf. Chapt. V-2), except that it is expressed in different units. It must always be remembered, however, that the redox potential invariably refers to the reaction with gaseous hydrogen. That is the zero point of the redox scale. 

Cell Volta.ge a.ndIts Components. The minimum voltage required for electrolysis to begin for a given set of cell conditions, such as an operational temperature of 95°C, is the sum of the cathodic and anodic reversible potentials and is known as the thermodynamic decomposition voltage, is related to the standard free energy change, AG°C, for the overall chemical reaction,... 

Temperature, K Enthalpy of reaction (AH ), kj/mol Free energy of reaction (AG ), kJ/mol Equilibrium cell potential (E ), V... 

Cell Potential and Reaction Gibbs Free Energy... 

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