The reaction Gibbs energy

To explore metabolic processes, we need a measure of the driving power of a 

Case study 4.1 Binding of oxygen to myoglobin and hemoglobin 144 

Case study 4.4 The fractional composition of a solution of lysine 166 

To keep our ideas in focus, we consider two important processes. One is the isomerism of glucose-6-phosphate (1, G6P) to fructose-6-phosphate (2, F6P), which is an early step in the anaerobic breakdown of glucose Case study 4.3)  

These two reactions are specific examples of a general reaction of the form 

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The equilibrium oxygen pressure over the series of iron oxides at 1100°C is shown in the figure, (a) What phases are involved (b) What does the sloping part of the plot indicate (c) Write equations for each of the formation reactions taking place, (d) Estimate the value of the reaction Gibbs energy, AGr per mole of product, at 1100°C for each reaction. The value of AGr for the formation of... 

Figure 2.6 Flow does a 4 kj mol-1 error bar in the reaction Gibbs energy affect the value of the equilibrium constant Curve b represents the accurate data curves a and c show the upper and lower limits,...

An electrical potential difference between the electrodes of an electrochemical cell (called the cell potential) causes a flow of electrons in the circuit that connects those electrodes and therefore produces electrical work. If the cell operates under reversible conditions and at constant composition, the work produced reaches a maximum value and, at constant temperature and pressure, can be identified with the Gibbs energy change of the net chemical process that occurs at the electrodes [180,316]. This is only achieved when the cell potential is balanced by the potential of an external source, so that the net current is zero. The value of this potential is known as the zero-current cell potential or the electromotive force (emf) of the cell, and it is represented by E. The relationship between E and the reaction Gibbs energy is given by... 

In a fuel cell, an electrolyte is used to divide the strongly exothermic chemical reaction H2+V2O2 HjO into two electrochemical reactions a reduction and an oxidation reaction. In this way, the conversion of the reaction Gibbs energy, A,G, into electrical energy is possible, and the only reaction product is water (if pure... 

For any chemical reaction the reaction Gibbs energy is written... 

The energy that is bound in one mole ofwater is given by its enthalpy of formation. It differs from the reaction Gibbs energy by the product of the thermodynamic temperature and the entropy of reaction. According to the second fundamental theorem of thermodynamics, a part of the enthalpy of reaction can be applied as thermal energy with a maximum of AQr = TASr which is the amount of energy that corresponds to the entropy of reaction ASr at the thermodynamic temperature T(see Eq. (5.15)). 

Thermodynamically, this reaction is a nonspontaneous reaction with the reaction Gibbs energy (A,G°) > 0 at temperatures higher than 130 °C (Lide, 1995, pp. 5—68). Therefore, this reaction needs to be done under electrolysis conditions. 

The relatively low positive value of the reaction Gibbs energy, as well as the observed escape of gaseous BF3, indicate the probability of reaction (20) in the melt. 

In a fuel cell, the difference in reactant gas compositions at the two electrodes leads to the formation of a difference in Galvani potential between anode and cathode, as discussed in the section Electromotive Force. Thereby, the Gibbs energy AG of the net fuel cell reaction is transformed directly into electrical work. Under ideal operation, with no parasitic heat loss of kinetic and transport processes involved, the reaction Gibbs energy can be converted completely into electrical energy, leading to the theoretical thermodynamic efficiency of the cell. 

Application of a negative overpotential, < 0, shifts the reaction Gibbs energy of... 

On dividing through by dn, we obtain the reaction Gibbs energy, A,G ... 

The reactants and products in a biological cell are rarely at equilibrium, so we need to know how the reaction Gibbs energy depends on their concentrations. 

Note that Q has the form of products divided by reactants, with the activity of each species raised to a power equal to its stoichiometric coefficient in the reaction because activities are dimensionless quantities, Q is a dimensionless quantity. We can now write the overall expression for the reaction Gibbs energy at any composition of the reaction mixture as... 

I I 1. The reaction Gibbs energy, ArG, is the slope of a plot of Gibbs energy against composition. 

Provided the work is done reversibly at constant temperature and pressure, we can equate this electrical work to the reaction Gibbs energy and obtain eqn 5.12. 

Equation 5.12 shows that the sign of E is opposite to that of the reaction Gibbs energy, which we should recall is the slope of a graph of G plotted against the composition of the reaction mixture (Section 4.1). When the reaction is spontaneous in the forward direction, Afi < 0 and E > 0. When A,G > 0, the reverse reaction is spontaneous and < 0. At equilibrium A,G = 0 and therefore Eceu = 0 too. 

Our next step is to see how Eceii varies with composition by combining eqn 5.12 and eqn 4.6, showing how the reaction Gibbs energy varies with composition ... 

The reaction Gibbs energy, AjG, with electron transfer becoming more efficient as the reaction becomes more exergonic. For example, efficient photo-oxidation of S requires that the reduction potential of S be lower them the reduction potential of Q. 

For historical reasons, G°, is also defined to be zero for the pure elements in their reference states. For other substances, G° is the reaction Gibbs energy for making the substances in their standard state from the constituting elements in their reference states. This is a point that one should be aware of, because its implication is that when using tabulated values, generally... 

As the rate constant depends on the equiKbrium constant following the Bronsted equation and the equihbrium constant in term is a function of the reaction Gibbs energy, it would be first instructive to consider the kinetic features of the cluster-size effect based on surface thermodynamics of sohds. 

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