Reaction Quotient
The same ratio as the equilibrium constant but evaluated with the concentrations actually present at any moment.
Formula / equation
Q = [C]^c[D]^d / [A]^a[B]^b
Unit
Various
In depth
Comparing Q with K predicts the direction of change: if Q is less than K the reaction proceeds forward, if greater it proceeds in reverse, and if equal the system is already at equilibrium. The comparison also determines whether a precipitate will form.
Examples in the real world
Mixing reagents to give Q = 0.5 when K = 54 means the reaction will run strongly forward.