# How do you calculate #K_"eq"# from #DeltaG^@#?

##### 1 Answer

Jul 18, 2017

You'd start from the expression for the **change in Gibbs' free energy**,

#DeltaG = DeltaG^@ + RTlnQ# where:

#Q# is the reaction quotient for the current state of the reaction.#R# and#T# are known from the ideal gas law.#RTlnQ# describes the shift in the free energy in reference to standard pressure and the chosen temperature (usually#25^@ "C"# for convenience).

At **chemical equilibrium**, the reaction has no tendency to shift in either direction, so the change in Gibbs' free energy is zero, i.e.

#DeltaG = 0#

Thus, with

#color(blue)(DeltaG^@ = -RTlnK_(eq))#

And usually the other kind of calculation of this kind is to solve for

#-(DeltaG^@)/(RT) = ln K_(eq)#

where

#"exp"(x) = e^x# .