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.
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.
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.
Thank you.
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.
The reaction quotient (Q) and the equilibrium constant (K) are related concepts in chemistry. While they may appear similar in definition, there are some important differences between them.
1. Definition:
- Reaction quotient (Q): The reaction quotient is a measure of the relative concentrations of products and reactants at a particular point in a chemical reaction. It is calculated by substituting the actual concentrations (or partial pressures) of substances into the equilibrium expression without necessarily having reached equilibrium.
- Equilibrium constant (K): The equilibrium constant is a specific value that quantifies the ratio of the equilibrium concentrations (or partial pressures) of products and reactants at a given temperature. It is determined when the reaction has reached equilibrium.
2. Purpose:
- Reaction quotient (Q): The reaction quotient helps determine whether the reaction is at equilibrium or not. By comparing the calculated value of Q with the equilibrium constant (K), one can predict the direction in which the reaction will proceed to reach equilibrium.
- Equilibrium constant (K): The equilibrium constant expresses the extent of a chemical reaction at equilibrium and provides information about the relative concentrations of products and reactants.
3. Time of measurement:
- Reaction quotient (Q): The reaction quotient can be calculated at any point in time during a reaction, even before equilibrium is established. It allows us to evaluate the progress of a reaction non-equilibrium conditions.
- Equilibrium constant (K): The equilibrium constant is determined only when the reaction has reached equilibrium. It reflects the state of the reaction when rates of the forward and reverse reactions become equal.
4. Comparison:
- Reaction quotient (Q): By comparing Q to K, it is possible to determine the direction in which the reaction will proceed to reach equilibrium. If Q < K, the reaction will proceed in the forward direction to reach equilibrium. If Q > K, the reaction will proceed in the reverse direction to reach equilibrium. When Q = K, the reaction is already at equilibrium.
- Equilibrium constant (K): The equilibrium constant provides a quantitative measure of the balance between the forward and reverse reactions at equilibrium. It indicates the extent to which the reaction proceeds toward the products or the reactants.
In summary, while the reaction quotient (Q) and equilibrium constant (K) share a similar definition, they differ in terms of when and how they are calculated, as well as their specific purposes. Q helps assess the progress of a reaction towards equilibrium, while K quantifies the position of equilibrium once it is established.