Chemistry and biochemistry use a different standard state. For example, the standard hydrogen ion concentration is the one at pH = 0 in one case, and the one at neutral pH in the other case.

Which quantities change when choosing a different standard state, and which quantities have special values at standard state?


Quantities that are dependent on the standard state

When calculating the equilibrium constant K or the reaction quotient Q from a set of observed concentrations, these concentrations are given relative to the standard concentration of the species. For example, in aqueous solutions the solvent water has a different standard concentration (that of pure water) than solute (roughly 1 M). Therefore, the value of K and Q depend on choice of standard state.

Whenever a thermodynamic quantity has a Plimsoll symbol appended, it refers to a system at standard state, e.g. $\Delta_r G^\circ, \Delta_r H^\circ, S_f^\circ, \Delta H_f^\circ$ or to the substance at standard state. The choice of standard state changes the value of these quantities. For the biochemical standard state, the quantity is appended with a prime, e.g. $\Delta_r G^\circ {'}$ to indicate that difference.

Quantities that have special values at standard state

At standard state, activities are equal to one. Consequently, the reaction quotient Q is also equal to one.

$$Q = 1 \ \ \ \ \text{(at standard state)}$$Also, and this is a bit of a tautology but useful in deriving some relationships, when concentrations are at standard state (or more generally if $Q = 1$), quantities are equal to the standard quantities. Take for example the Gibbs energy of reaction:

$$\Delta_r G = \Delta_r G^\circ + R T \ln Q$$

When concentrations are at standard state, this reduces to:

$$\Delta_r G = \Delta_r G^\circ \ \ \ \ \text{(at standard state)}$$

Of course, that is the idea in calling $\Delta_r G^\circ$ the standard Gibbs energy of reaction.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.