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You calculate Q when you are not sure whether the reaction is at equilibrium. If you already know the reaction’s K value, you can use the same formula for K at any time, even when the reaction is not at equilibrium, and call the calculated value Q. Its value relative to K (whether it is less than K or greater than K) helps you to predict the future dynamics of the reaction. For example, if you calculate a Q that is less than K, you can infer that the reaction will then favor product formation over reactant formation to get closer to to the equilibrium state.
Q uses the same equation as K, but it can be calculated at any point of the reaction, whereas K can only be calculated at equilibrium. You can then compare Q to K to find out where the reaction is relative to equilibrium and which way the reaction might favor.
Both Q and K use the same equation during calculation. However, K is used at equilibrium while Q can be used at any point during a reaction. It is important understand the fact that Q can be changed due to changes in pressure and concentration but K does not. K shifts only when there is a change in temperature.
For Q, we can plug in the concentrations of reactants/products at any instant of time. By comparing Q with K, we can predict whether the reaction will move in the forward or reverse direction. If Q< K the reaction will proceed in the forward direction. If Q>K, the reaction will proceed in the reverse direction and if Q=K the reaction is already at equilibrium.
The Reaction Quotient (Q) is similar to the Equilibrium Constant (K) in that they are calculated the same way. However, Q only equals K when at equilibrium, it is somewhat like a "temporary" K value. You would need to calculate Q whenever the system is not in equilibrium, or you are unsure whether or not the system is stable.
You calculate the reaction quotient (Q) when you are determining which direction the reaction shifts. Q=[Products]/[Reactants] just as K. If Q<K then the reaction shifts right to form towards products and if Q>K then the reaction shifts left to form towards the reactants.
Q is calculated whenever the equation is not at equilibrium, so if there is a change or anything similar. Also, Q is calculated whenever you are trying to see the result of the reaction (i.e. if the reaction favors the reactants or products).
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