Biochemistry - The Chemical Reactions of Living Cells Volume 1 - D. Metzler 1980

Bioenergetics of Biochemical Reactions
Thermodynamics
Standard States

To use thermodynamic data in chemical calculations, certain states of elements or compounds must be defined as standard. For instance, when discussing The change in Free energy during The conversion of one or more pure compounds into others, it is essential to specify the state to which these data refer (crystalline, liquid, gaseous, or in solution) and the pressure at which they were obtained (especially when gaseous substances are involved in the reactions). Standard pressure is conventionally taken as 1 atm. The standard states of elements correspond to pure crystalline, solid, or gaseous substances, such as C (graphite), S (orthorhombic crystalline), P (white crystalline), N2, O2, and H2 (gaseous). The Temperature must also be specified; most tabulated thermodynamic values correspond to 25°C.

In Chemical Reactions, it is usually much more important to know the changes in G or H—namely, ∆G and ∆Н—rather than their absolute values. When compiling tables of free energy, it is standard practice to set G to zero for all elements in their standard state. Consequently, the Standard Free Energy of formation, ∆G, for any substance can be determined by combining its constituent elements in the appropriate ratio (for example, by measuring ∆H during combustion and determining Entropy from heat capacity measurements). The standard Free energy of formation obtained in this manner is denoted as ∆G?



Last update: 06/08/2026

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