For evaluation of spontaneity, two possible approaches can be chosen:

 - A reaction is spontaneous, if $\Delta_\mathrm{r} G^\circ =  -RT \ln{K} \lt 0$. 
   - This is to be interpreted as a system with all components in their standard state with unit thermodynamic activities undergoes spontaneously the net reaction in forward direction.
   - This is the evaluation of spontaneity of reaction regardless of the initial system state, as the implicit presumption.

 - A reaction is spontaneous, if $\Delta_\mathrm{r} G = \Delta_\mathrm{r} G^\circ + RT \ln{Q} = RT \ln{\left(\frac{Q}{K}\right)}\lt 0$. 
   - This is to be interpreted as a system with components in general states and component activities undergoes spontaneously the net reaction in forward direction.
   - This is the evaluation of spontaneity of reaction considering the initial system state.
   - If $Q \lt K$ then the equilibrium is in the forward direction of the reaction and the reaction is spontaneous in the latter sense regardless of if it is spontaneous in the former sense.