They are both calculated with a similar formula, but there is one very important difference.
When you calculate K (equilibrium constant), you take the concentration values at equilibrium, that is, when the reaction is not proceeding anymore.
When you calculate Q, you take the concentration values at any point of time in the reaction, with concentrations that are not equal to the equilibrium concentrations.
So I can measure Q while I'm starting a reaction, or while I'm in the middle of an ongoing reaction, but K is the value of Q itself when the reaction reaches equilibrium.
Q can change depending on when you measure it and how far the reaction has completed, but K is a constant for a given reaction.
What is the difference between reaction quotient Q and equilibrium constant? According to books both are different but I just don't understand the difference bcz both have same definition.
4 answers
Thank you.
I would like to add something to this.
You can plug in the starting values for calculating Q and it tells you immediately if the reaction is at equilibrium (if Q = K) and if it isn't at equilibrium the value of Q tell you which way the reaction will go (to the left or to the right) to reach equilibrium if it isn't obvious from the beginning.
2. Arora, that is a really good explanation; however, I disagree with one part of your statement. This quote, "When you calculate K(equilibrium constant), you take the concentration values at equilibrium, THAT IS, WHEN THE REACTION IS NOT PROCEEDING ANYMORE." It's that last part that I'm being picky about. The reaction NEVER ceases. It is ALWAYS reacting. Equilibrium is reached when the concentrations are not changing. The forward reaction is still going; the reverse reaction is running, too. But the rates are such that the new change in concentrations for both reactants and products are not changing; i.e., no net change.
You can plug in the starting values for calculating Q and it tells you immediately if the reaction is at equilibrium (if Q = K) and if it isn't at equilibrium the value of Q tell you which way the reaction will go (to the left or to the right) to reach equilibrium if it isn't obvious from the beginning.
2. Arora, that is a really good explanation; however, I disagree with one part of your statement. This quote, "When you calculate K(equilibrium constant), you take the concentration values at equilibrium, THAT IS, WHEN THE REACTION IS NOT PROCEEDING ANYMORE." It's that last part that I'm being picky about. The reaction NEVER ceases. It is ALWAYS reacting. Equilibrium is reached when the concentrations are not changing. The forward reaction is still going; the reverse reaction is running, too. But the rates are such that the new change in concentrations for both reactants and products are not changing; i.e., no net change.
I always defined equilibrium as when the forward rate equaled the reverse rate. And if you upset it, by changing temp, that equilibrium point is likely to change, and reset the reactions and find a new point. That is mainly why Keq is temperature dependent. It is different at different temperatures
Arora: the explanation was good.
Arora: the explanation was good.
1 answer