For a reaction system at equilibrium, when the concentrations are substituted into the reaction quotient expression, the value calculated is K, the equilibrium constant.
K is constant for a reaction at a particular temperature.
The reaction quotient is shown for the general reaction:
aA + bB
cC + dD.
If, for a system at equilibrium, the concentration of a substance appearing in the reaction quotient is changed, the reaction quotient no longer equals K.
Increase concentration of reactant A in the reaction aA + bB cC + dD
Reaction in the forward direction is faster to consume sufficient added A until the reaction quotient again equals K.
The new equilibrium composition has
Decreasing concentration of reactant B in the reaction aA + bB cC + dD
Reaction is faster in the reverse direction to replace some of the B that has been removed until the reaction quotient again equals K.
The new equilibrium composition has
If a catalyst is added, the reaction quotient is unchanged, and there is no change in equilibrium composition.
K is constant for a reaction at a particular temperature.
| Kc = | [C]c[D]d |
| [A]a[B]b | |
| constant | reaction quotient |
aA + bB
cC + dD.If, for a system at equilibrium, the concentration of a substance appearing in the reaction quotient is changed, the reaction quotient no longer equals K.
The natural tendency is to restore equilibrium by net reaction in the direction that consumes the added substance until the reaction quotient again is equal to K.
Increase concentration of reactant A in the reaction aA + bB
cC + dDReaction in the forward direction is faster to consume sufficient added A until the reaction quotient again equals K.
The new equilibrium composition has
[A] higher (due to it having been added)
[C] and [D] higher (due to being the products of the reaction to restore equilibrium)
[B] lower (due to being consumed in the reaction to restore equilibrium)
[C] and [D] higher (due to being the products of the reaction to restore equilibrium)
[B] lower (due to being consumed in the reaction to restore equilibrium)
Decreasing concentration of reactant B in the reaction aA + bB
cC + dDReaction is faster in the reverse direction to replace some of the B that has been removed until the reaction quotient again equals K.
The new equilibrium composition has
[B] lower (due to it having been removed - only some of what was removed is replaced).
[C] and [D] lower (due to being reactants of the reaction to restore equilibrium).
[A] higher (due to being produced in the reaction to restore equilibrium.
[C] and [D] lower (due to being reactants of the reaction to restore equilibrium).
[A] higher (due to being produced in the reaction to restore equilibrium.
If a catalyst is added, the reaction quotient is unchanged, and there is no change in equilibrium composition.