Help please!! TBR gen chem equilibrium

[cdistel]

In the BR gen chem, i am having a hard time understanding the relationship between Q (reaction quotient) and Keq. For example,

N2O4 (g) <----> 2NO2 (g)

It says if you add N2O4, you have more reactants and the system will shift to the right, i get that, but it says the value of K is greater than Q, WHY is that????

Then it says if you add more NO2 it shifts to the left (makes sense to relieve the stress of the system) but again now the value of K is less than Q, for the love of pete can someone explain this lol, spent the better half of my morning searching high and low for this answer. this will help me out a ton thanks!!

---

Members don't see this ad.

 

[nuD]

K is the equilibrium constant and K = [NO2]^2/[N2O4] at equilibrium. Q is Q = [NO2]^2/[N2O4], but at any point in the system. So Q is equal to K when the system is at equilibrum. Otherwise, Q is not equal to K, and the value of Q gives some indication as to which direction the system will move to go towards equilibrium.

So in your example, let's pick some arbitary numbers and say that at equilibrium, [NO2] = 1 and [N2O4] = 1. So then K = 1^2/1 = 1 (which is a constant).

Now add in some N2O4, and let's say it increases the concentration of N2O4 to [N2O4] = 2 immediately after adding it. So Q right after adding the extra N2O4 is, Q = [NO2]^2/[N2O4] = 1^2/2 = 0.5

Eventually, the system reaches equilibrium and Q = K = 1. Now add in some NO2 and let's say the concentration of [NO2] increases to 2. So immediately after adding the NO2, Q becomes, Q = 2^2/1 = 4. So Q > K and the reaction will shift to the reactants.

As you know, you can use Le Chatelier's principle to figure out which way the system will shift in the above examples. Saying that Q is less than or greater than K is just showing that mathematically. Hope this helps!

 

[BerkReviewTeach]

K is the equilibrium constant and K = [NO2]^2/[N2O4] at equilibrium. Q is Q = [NO2]^2/[N2O4], but at any point in the system. So Q is equal to K when the system is at equilibrum. Otherwise, Q is not equal to K, and the value of Q gives some indication as to which direction the system will move to go towards equilibrium.

So in your example, let's pick some arbitary numbers and say that at equilibrium, [NO2] = 1 and [N2O4] = 1. So then K = 1^2/1 = 1 (which is a constant).

Now add in some N2O4, and let's say it increases the concentration of N2O4 to [N2O4] = 2 immediately after adding it. So Q right after adding the extra N2O4 is, Q = [NO2]^2/[N2O4] = 1^2/2 = 0.5

Eventually, the system reaches equilibrium and Q = K = 1. Now add in some NO2 and let's say the concentration of [NO2] increases to 2. So immediately after adding the NO2, Q becomes, Q = 2^2/1 = 4. So Q > K and the reaction will shift to the reactants.

As you know, you can use Le Chatelier's principle to figure out which way the system will shift in the above examples. Saying that Q is less than or greater than K is just showing that mathematically. Hope this helps!

Excellent explanation NuD!

The main reason to know the Q vs. K relationship is in case they make it one of the answer choices. Le Chatelier's perspective is much simpler to grasp, but you never know when the answer choices might be something like this.

a) The reaction will shift to the right to reach equilibrium
b) There is an excess of reactants compared to equilibrium conditions
c) deltaG for the reaction conditions as given is a negative number
d) The magnitude of Q is greater than the magnitude of K.

Knowing which way the reaction will shift for each of the four choices will allow you to use POE by finding the three statements that are saying the same thing, thus isolating the best answer (the one that's different here).