Spontaneous change chemical potential

Chemical work and chemical energy are defined in an analogous way. The intensity factor here is the chemical potential of a molecule or combination of molecules. This is stated as free enthalpy G (also known as Gibbs free energy ). When molecules spontaneously react with one another, the result is products at lower potential. The difference in the chemical potentials of the educts and products (the change in free enthalpy, AC) is a measure of the driving force of the reaction. The capacity factor in chemical work is... 

It is important to be able to write the fundamental equation for a system of chemical reactions in terms of components because components are involved in the criterion for spontaneous change and equilibrium. We have seen earlier (Section 2.3) that this is done by eliminating one chemical potential from the fundamental equation with each independent equilibrium condition of the form v /q = 0 to obtain... 

Diffusion occurs spontaneously from a region of higher chemical potential mi to a region of lower chemical potential P-2- and the direction of flow is the same as the direction of decreasing chemical potential. The total Gibbs energy change for such a system is expressed by... 

It is assumed that the tendency of a molecular mixture to interact can be analyzed as a function of the chemical (quantum) potential energy field and some action variable that reflects mass ratios or amounts of substance. Spontaneous chemical change occurs as the chemical potential of a system decreases, i.e. while Ap < 0, and ceases when Ap = 0, at equilibrium. The quantity here denoted by Ap, also known as the affinity, a of the system, is the sum over all molecules, reactants and products... 

The condition for a spontaneous change — a decrease in chemical potential — has important implications for discussing fluxes from one region to another. In particular, we can use the chemical potential difference between... 

We need to understand the concepts of Gibbs free enei gy (G) and chemical potential i/i) in order to know the direction of spontaneous change of a reaction or system. These concepts can also be used to define or predict the most stable (equilibrium) assemblage and gas, fluid, or rock compositions expected in a system at a given pressure and temperature. Some phases and aqueous species in a system may be out of equilibrium with that system. Free-energy calculations permit us to decide which substances are out of equilibrium, and, therefore, which concentrations may be governed by chemical kinetics. 

The chemical potential concept provides a useful way to think about the tendency for spontaneous chemical change in complex environmental systems involving gases, liquids, and solids (cf. Wood and Fraser 1976 Stumm and Morgan 1981). In a particular phase, the chemical potential, /U, of component i is related to the activity of i through the expression... 

The most important application for the chemical potential fj. is that it enables us to predict whether a change of substances can happen spontaneously or not (the role of possible inhibitions is left aside). 

Equilibria and rates should be clearly distinguished. Equilibrium is the end point of any spontaneous process, whether chemical or physical, in which the driving forces (potentials) for changes are balanced and there is no further tendency to change. Chemical equilibrium is the final state of a reaction at which no further changes in compositions occur at a given temperature and pressure. As an example of a physical process, let us consider the absorption of a gas into a liquid. 

To calculate standard cell potentials from the half-cell potentials in Table 9-1, there are four principles that we must know (1) When we reverse the direction of the chemical reaction, we change the sign of the potential. (2) If we multiply the coefficients in the equation by some number, we do NOT change the potential. Potential is an intensive property, and does not depend on the quantity of reagents. (3) When we add chemical equations for half-cells, we add the corresponding potentials. (4) A positive potential for a complete cell reaction means that the reaction proceeds spontaneously in the direction of the equation, and a negative potential means that the reaction goes spontaneously in the opposite direction. 

In other words, increments of are now in moles rather than degree of disorder and the rate of change of each constituent in a reaction is its (dimensionless) stoichiometric coefficient. This may be quite opaque on first reading, but will become almost trivial when understanding dawns. If for example we have a mole of A that wants to change spontaneously into 5 moles of B, the reaction is A = 5B, and (dnA/d ) = — 1 (dn /d ) = 5. This just says that for every mole of A that disappears, 5 moles of B must appear. This fairly obvious relation now allows us to link with the chemical potentials. From (5.39) we have... 

The driving force for nucleation and crystal growth from solution can be expressed as the positive difference between the chemical potential of a species in a supersaturated solution and that of a saturated solution. Once a solution becomes supersaturated with respect to a particular mineral, 3-D nucleation may occur. For the spontaneous formation of an apatite nucleus (for the sake of discussion we will refer to fluorapatite) in solution the change in the Gibbs free energy (AG) of the reaction... 

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