CHEMISTRY COMMUNITY

For Q<K, why is [R]>[P] and for Q>K, why is [P]>[R]?

When Q<K, [R]>[P] because [R] is the denominator in determining Q and a larger denominator will make Q smaller than K. The same logic works the other way around.

However, it is important to keep in mind that when referring to [R] and [P], the value is the concentrations raised to the power of their stoichiometric coefficients.

To understand how a reaction will proceed to reach equilibrium, it is important to understand what K and Q represent symbolically. Both represent the ratio of products to reactants in the reaction. However, K represents that ratio at equilibrium, while Q represents that ratio while not at equilibrium. Thus, if K is bigger than Q, it means there should be more products at equilibrium than there are now. Thus, the reaction must move forward. If K is less than Q, the reaction at equilibrium should have less products than it does now. So the reaction must move in reverse.

Giselle Littleton 1F wrote:For Q<K, why is [R]>[P] and for Q>K, why is [P]>[R]?

If Q is less than K, this implies that the reaction is proceeding in the forward direction, where reactants are being transformed into products. This means that there will be a higher concentration of reactants to products.

If Q is greater than K, this implies that the reaction is proceeding in the reverse direction, where products are being transformed into reactants. This means that there will be a higher concentration of products to reactants.

K is the fixed ration of the concentrations of the products to the reactants. If the Q, which means the ratio of the concentrations of products to reactants, is smaller than the K, than that means there aren't enough products in this reaction to be at equilibrium. So the concentrations of the products is smaller than that of the reactants.

For this problem we are only looking at the change in the P or R values of the Q equation, using K as a fixed ratio.

Your K value is the fixed ratio between the left and right sides when at equilibrium. While Q is also a ratio between the [R] and [P] but the ratio might not be at equilibrium.

This means that when K is larger than Q, there are more reactants readily available to become products. When Q is larger than K, there is more product readily available to become reactants.


K= Q=

So, if K>Q then the reaction is moving forward, towards the products. In order to reach equilibrium more product must form.
This makes [R]>[P] because if Q is small, then R must be larger than the P. This would cause the reaction to occur in the direction of the products, to negate the effects of a large concentration of reactants.

K= Q=
If K<Q the reaction is moving in reverse, towards the reactants. This implies that there is more product available than reactant and to reach equilibrium, the reaction must work in reverse.
This makes [P]>[R] because if Q is larger than K, the numerator P of the Q equation, must be larger than the R. When the amount of product is large, it is necessary for the reaction to occur in reverse so that equilibrium might be met.