Gibbs energy half-reactions

Calculate the biological standard Gibbs energies of reactions of the following reactions and half-reactions ... 

This chapter is devoted to the important relationship between electrode potentials and the changes in Gibbs energy (AO ) for half-reactions and overall reactions. In discussions of the properties of ions in aqueous solution it is frequently more convenient to represent changes in Gibbs energy, quoted with units of k.I mol-1, in terms of electrode potentials, quoted with units of volts (V). The electrochemical series is introduced. The properties of the hydrated electron are described. 

Since tables of standard apparent reduction potentials and standard transformed Gibbs energies of formation contain the same basic information, there is a question as to whether this chapter is really needed. However, the consideration of standard apparent reduction potentials provides a more global view of the driving forces in redox reactions. There are two contributions to the apparent equilibrium constant for a biochemical redox reaction, namely the standard apparent reduction potentials of the two half-reactions. Therefore it is of interest to compare the standard apparent reduction potentials of various half reactions. 

Standard apparent reduction potentials are useful because they provide a more global view of reactivity than the standard transformed Gibbs energies of formation of reactants from which they can be calculated. It is also useful to think mechanistically in terms of half reactions because half reactions are connected with other half reaction only through electron transfer. 

The set of reactions that creates the complex redox environments of landfill leachate plumes consists of combinations of two half-reactions oxidation half-reaction and reduction halfreaction. Table 3 presents the most prominent overall redox reactions, along with their calculated Gibbs free energy change under standard conditions (AGo(W)). The lower (the more negative) the AGq(W), the more energy is gained. 

Taking thermodynamics into account, the electric potential of an individual electrode, E, can be related to the Gibbs energy, ArGi, of the corresponding electrochemical half-reaction as follows ... 

In an electrochemical cell a redox reaction occurs in two halves (see Topic B4). Electrons are liberated by the oxidation half reaction at one electrode and pass through an electrical circuit to another electrode where they are used for the reduction. The cell potential E is the potential difference between the two electrodes required to balance the thermodynamic tendency for reaction, so that the cell is in equilibrium and no electrical current flows. E is related to the molar Gibbs free energy change in the overall reaction (see Topic B3) according to... 

The Daniell cell is an example of a galvanic cell, in this type of electrochemical cell, electrical work is done by the system. The potential difference, between the two half-cells can be measured (in volts, V) on a voltmeter in the circuit (Figure 7.1) and the value of is related to the change in Gibbs energy for the cell reaction. Equation 7.9 gives this relationship under standard conditions, where is°ceu is the standard cell potential. 

Consequently, if we multiply any of the half-cell reactions in Table 14.6-1 by an integer or fractional constant, the standard-state Gibbs energy change will change by that same factor, but the standard-state half-cell potential will be unchanged.,... 

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