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The reaction quotient is often asked for when the reaction is NOT YET at equilibrium. You will often need to solve for it when the problem states that there is an "initial concentration of ...", implying that it is not in equilibrium. Solving for Q is the same as solving for K and it is essential when trying to determine whether the reaction would favor a forward reaction (Q < K) or reverse reaction (Q > K). Let me know if you have any other questions, I hope I was able to help.
Lizette Noriega 1H wrote:Can someone explain the concept behind Q? When is it essential?
Like someone else said above, Q is basically a way for you to determine whether the reaction is at equilibrium. You calculate Q the same way that you calculate K. The only difference, is the concentration or pressure values you put in to calculate Q are not necessarily at equilibrium. This allows you to see
a) if the reaction is at equilibrium
b) which reaction is favored if the reaction is not at equilibrium (forward or reverse)
I hope this helps!
When a reaction isn't at equilibrium, you would try to find the reaction quotient, Q, in order to determine which direction the reaction will favor. When Q<K, the FORWARD (goes toward products) reaction is favored because the concentrations/partial pressures of the products are too low compared to reactants for equilibrium. When Q>K, the REVERSE (goes toward reactants) reaction is favored because the concentrations/partial pressures of the reactants are too low compared to products for equilibrium.
The reaction quotient Q is calculated in the same way with K using the same expression. However the difference is that when you look for Q the concentration/partial pressure values you plugged in are not yet at equilibrium. Reaction quotient Q is important since we can use it to determine which way the reaction is going to shift. If Q>K, it means that in order to reach equilibrium more reactants have to be formed and vice versa.
The reaction quotient is the rate of the equation when it’s not at equilibrium. This is important because you use it to determine which “direction” the reaction favors (towards products or reactants) by comparing it to the equilibrium constant K.
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