Reaction Quotient Definition

[Blake Ballew 1H]

When exactly is the reaction quotient utilized? Seeing as chemical equilibrium indicates that the ratio of products and reactant should always be k how do we have a reaction quotient "Q" different than that value? Is this value of "Q" only utilized when a product of reactant is added to a previously existing k?

[Stuti Pradhan 2J]

The reaction quotient is used when the reaction is not at equilibrium. At equilibrium, the ratio or products to reactants is the K value, but when the reaction is not at equilibrium, there can be more products or reactants, so the ratio would no longer equal the K value. When a product or a reactant is added to a reaction that is already at equilibrium, then finding the reaction quotient, or Q, will indicate in what direction the reaction will proceed to return to equilibrium. If Q<K then more products will be formed and if Q>K then more reactants will be formed. Q can also be used before the reaction has reached equilibrium to understand how the reaction will proceed.

Hope this helps!

[Sameer Chowdhury 3C]

Unless the problem tells us the reaction is at equilibrium, we'd be using Q. K is only used when it the reaction is at equilibrium. If we are given K and are asked to find Q after a change has occurred in the reaction we can know what direction the reaction will move, either towards reactants or products.

[Hana Sigsbee 3B]

we find Q instead of K when the reaction is not at equilibrium. Q can be used to compare to a given K to find if the reaction favors products or reactants.

[Sandy Lin 1L]

The difference between Q and K is that K is the equilibrium constant, so it is calculated when the system is at equilibrium. Q, the reaction quotient is found during any stage of the reaction. They are calculated by the same equation, and this is why when you compare the two values, you can determine the direction of a reaction. If you find a Q value that is equaled to K, then you know the system is at equilibrium. If Q < K then there are more reactants than products (shift to the right), and if Q > K then there are more products than reactants (shift to the left).

[Charmaine Ng 2D]

We find Q when we don't know whether the reaction is at equilibrium or not, and we use K when we are certain that the reaction is at equilibrium. Q is just a measure of the ratio when we don't know!

[Kaihan_Danesh_2J]

Q is used when the reaction is no longer in equilibrium. For example, a change in pressure due to compression will result in the reaction shifting a certain way. For that instant where one way is favored, we use Q to determine which way is favored. And we do this by comparing Q to K because inevitably anything that shifts the equilibrium in a system will eventually lead back to a concentration ratio equal to K—this of course applies to when changes are made that do not affect K, but only temporarily shift the reaction's equilibrium.

[Kelly Tran 1J]

The reaction quotient Q is used when you are calculating the ratio of the concentration of products not at equilibrium to the concentration of reactants not at equilibrium. In other words, you are calculating Q when the reaction is not at equilibrium.