It is often required to forecast the future course of a reaction by analysing the relationship between the starting activity or concentration and the current state of the reaction.
Solving for the net change requires knowledge of the relationship between the reaction quotient, Qc, and the equilibrium constant, Kc.
This connection allows one to determine with great accuracy whether a reaction will shift to the left or right in order to reach chemical equilibrium.
K and Q in Equilibrium
Using an ICE table and the natural progression of the reaction, either from left to right or right to left, Kc can be used to determine the equilibrium concentrations of a reaction.
However, this is a problem if you are unable to predict the direction in which the reaction will proceed.
Fortunately, there is a straightforward connection between Kc and the reaction quotient, Qc. At any given time, the ratio of products to reactants can be expressed as the reaction quotient, Q.
The rate of a reaction can be calculated using only the known initial concentrations or initial activity of all the reactants and products.
Relationship Between K and Q
Remembering these connections is simple if you picture the > or as the alligator’s gaping maw. If Q is placed before K, the alligator will “eat” in the direction that the reaction changes.
LeChatelier’s Principle on Equilibrium
If we apply a modification to a chemical system, we can predict its reaction thanks to LeChatelier’s Principle.
But there is a far more precise technique to quantify these shifts and figure out if a response has reached equilibrium. The Reaction Quotient is the essential quantity that serves this purpose.
As time goes on, you’ll see that the Reaction Quotient closely resembles a concept you may be familiar with called the Equilibrium Constant. The two may appear similar, yet there are significant differences between them.
The Reaction Quotient
Numerator of the reaction quotient includes product activities (C,D), with each activity raised to the power that corresponds to its stoichiometric constant.
(e.g., product C’s activity is raised to the power of “1,” while product D’s activity is raised to the power of “2”).
Each reactant’s activity is multiplied by its stoichiometric constant in the denominator of the reaction quotient, so the activity of reactant.
A is multiplied by 2 in order to account for its stoichiometric role, while the activity of reactant B is multiplied by 1.