Reaction Quotient Predictions

[IsabelMurillo3K]

Hey guys!
I was wondering if someone could explain how we can use Q to predict the direction of a nonequilibrium reaction?
Thank you!

[Aanya Tanti 3C]

If Q>Kc then there is too much product being formed (forward reaction) and the rxn will shift to the left. If Q<Kc not enough product is being formed and rxn will shift to the right.

[JonathanM]

Hey! I know someone has answered this question but I thought I could add how I think about it and remember which way it goes. If Q>K, that means that if the denominator of both were the same, the numerator of Q would have to be bigger than K's, and so to reach K, products have to be converted into reactants. I do the same thing but the other way around when Q<K. If you hold the denominators constant, the numerator of Q is smaller than K, and so reactants have to be converted to products to reach equilibrium. Hope this helps!

[Mina Tadros 3L]

If Q>K, then the reaction will progress towards the reactants since more products formed. If Q<K, the reaction will progress towards the products since there are more reactants present.

[Nathan Lao 2I]

I like to write the value of K in comparison to Q. The reason is that you can essentially visualize the greater than/less than sign as an arrow that points to which side it will shift.

K < Q means it will shift towards reactant (I visualize it as K <-- Q, arrow points left so reaction shifts to the left)
K > Q means it will shift towards products (I visualize it as K --> Q, arrow points right so reaction shifts to the right)

This is what works for me, even though most TAs/UAs write Q first.

[Kiana Tashakori 1D]

You could use Q to predict the direction of the reaction because if its greater than Kc, then the reaction will shift towards the reactants because this means that too much product is formed. If Q is less than Kc, then the reaction will shift towards the products since not enough is being formed.

[Valerie Doan 3I]

Just a reminder because I saw a lot of textbook problems on this concept in conjunction with ICE tables. Remember if you're given the non-equilibrium concentrations, calculate for Q first to determine which way the rxn will proceed. Then, do the ICE table making sure to label the correct molecules as decreasing concentration (-x) or increasing concentration (+x). We have done a lot of problems in which the reactant is losing concentration while the products start initially with zero concentration and then increase, but we have to make sure to compare Q with K first to determine direction.