Standard States for Free-Energy Changes

We have already seen in Chapter 1 how the lowering of energy, which really means dispersal on the molecular level, is spontaneous in the thermodynamic sense. In this chapter, we are going to see how energetic considerations apply to metabolism. We are going to be comparing so many different processes that it will be useful to have a benchmark against which to make those comparisons. 

We can define standard conditions for any process and then use those standard conditions as the basis for comparing reactions. The choice of standard conditions is arbitrary. For a process under standard conditions, all substances involved in the reaction are in their standard states, in which case they are also said to be at unit activity. For pure solids and pure liquids, the standard state is the pure substance itself. For gases, the standard state is usually taken as a pressure of 1.00 atmosphere of that gas. For solutes, the standard state is usually taken as 1.00 molar concentration. Strictly speaking, these definitions for gases and for solutes are approximations, but they are valid for all but the most exacting work. 

What do standard states have to do with free-energy changes  

In this equation, the square brackets indicate molar concentrations, Ris the gas constant (8.31 J mol K ), and Tis the absolute temperature. The notation In refers to natural logarithms (to the base e) rather than logarithms to the base 10, for which the notation is log. This equation holds under all circumstances the reaction does not have to be at equilibrium. The value of AG under a given set of conditions depends on the value of AG and on the concentration of reactants and products (given by the second term in the equation). Most 

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