Activities in Ideal Solutions

Let us assume that in a sufficiently diluted solution molecules of the dissolved components are so disconnected between themselves by the solvent that their interaction may be disregarded. To such solutions are applicable the laws of infinitely diluted solutions, which are analogous to the laws of ideal gas solution. 

According to the theory of state of ideal gas solution, change of the free enthalpy of each component i at constant temperature depends only on its partial pressure according to Equation 

It enables the determination of the change in free enthalpy of individual component as function of change in its partial pressure and temperature. If we take standard free enthalpy of a component AZp 298, for some tentative origin of coordinates, its relative free enthalpy may be computed under any nonstandard conditions. Let us assume Z = AZp 298, and equate p. with partial pressure of component at standard pressure of 1 bar. Then the deflection of free enthalpy from its values xmder standard conditions is equal 

Free enthalpy, according to Equation (1.55), depends on temperature and pjp° ratio. The ratio of partial pressures of the component i in the solution and in its standard state is called thermodynamic concentration, or relative activity, and more often simply activity. Then free enthalpy and chemical potential of the component i under nonstandard conditions are calculated from  

In its substance it shows how much energy of the component i under the solution conditions differs from the energy of its formation imder standard conditions. The concept of thermodynamic concentration was introduced by Gilbert Newton Lewis (1875-1946) in 1907 for diluted gas solutions and later expanded for other solutions. In this connection, determination of the activity values depends on the nature and state of the component. In this respect gas, non-polar hydrophobic and polar hydrophilic components should be distinguished. 

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Introducing chemical potentials for biochemical substrates needs to be done with caution when considering, for example, molecular crowding and signaling molecules with limited copy numbers (Parsegian et al., 2000). This simple chemical system is for cellular metabolic networks, and concentrations replace activities in ideal solutions. 

Parsegian et al., 2000). This simple chemical system is for cellular metabolic networks and concentrations replace activities in ideal solutions. 

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