standard state
Summary
The standard state is the agreed reference condition for thermodynamic values: pure solids and liquids, gases at 1 bar (older texts use 1 atm), solutes at 1 M, usually tabulated at 298 K. Values there carry a ° sign (ΔG°, E°); ΔG° relates to K, not to the actual direction of a reaction.
Detail
Meaning: The standard state is the agreed reference condition for thermodynamic data. A pure solid or liquid is taken as the pure substance, a gas is at 1 bar partial pressure (older texts use 1 atm), and a solute is at 1 M. A degree sign marks values at these conditions: ΔH°, ΔS°, ΔG°, E°. The definition does not fix temperature, but tables usually give 298 K (25 °C). An element's standard state is its most stable form there, so ΔHf° of O₂(g) or graphite is zero.
Equation: ΔG = ΔG° + RT ln Q, and ΔG° = −RT ln K. ΔG is the free energy change (J/mol), R is 8.314 J/(mol·K), T is temperature (K), and Q and K are the reaction quotient and equilibrium constant (both unitless).
On the MCAT: Hess's law problems using tabulated ΔHf° values; predicting that a negative ΔG° means K > 1; cell potentials with 1 M ions, then the Nernst equation when concentrations differ; biochemistry ΔG°′, which uses pH 7 as its reference.
Common confusion: Standard state is not STP (0 °C and 1 atm), the condition behind the 22.4 L/mol molar volume of a gas. Also, ΔG° = 0 means K = 1, not equilibrium; only ΔG = 0 marks equilibrium, and ΔG (not ΔG°) sets the real direction.
Sources
- Chemistry: The Central Science
- Chemistry (Zumdahl)
- Physical Chemistry (Atkins)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.