TBR Gen Chem Section 3 Passage 2

[Psychodramandy]

Hi everyone! This is my first time asking a question regarding MCATs on SDN so I hope I'm doing this right! I am confused with a particular question:

Reaction:
H2(g) + Br2(l) → 2 HBr(g)

Mixing 3.0 atm. HBr(g) with 1.5 atm. H2(g) over Br2(l) shows which of the following in time?

A . A large increase in the partial pressure of HBr(g)
B. A small increase in the partial pressure of HBR (g)
C . A large increase in the partial pressure of H2 (g)
D. A small increase in the partial pressure of H2(g)

I understand that we're suppose to use the reaction quotient and compare it with Keq. The Keq here is (2.72)^2/1.36 [This is given in the passage] while Q is (3)^2/1.5. Since Q is greater than K we know that the reaction proceeds to the left. However - how do we know whether it is a large increase in H2 or small increase in H2? TBR states that Q and K are similar so it's a small increase in partial pressure. What is the range that Q and K are similar so that it'll be only small increase and what is the range that Q and K are not similar so that there'll be a large increase?

Thanks for the help in advance!

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[aldol16]

I understand that we're suppose to use the reaction quotient and compare it with Keq. The Keq here is (2.72)^2/1.36 [This is given in the passage] while Q is (3)^2/1.5. Since Q is greater than K we know that the reaction proceeds to the left. However - how do we know whether it is a large increase in H2 or small increase in H2? TBR states that Q and K are similar so it's a small increase in partial pressure. What is the range that Q and K are similar so that it'll be only small increase and what is the range that Q and K are not similar so that there'll be a large increase?

There's really no hard and set rule. Pre-meds tend to like memorizing things and the point is that memorization doesn't really accomplish much. So in this case, your Q/K ratio is just about 1. That's close enough to be unarguably small.

 

[BerkReviewTeach]

Keq is (2.72 x 2.72)/1.36 = 2.72 x (2.72/1.36) = 2.72 x 2 = 5.44. Qrx is (3 x 3)/1.5 = 9/1.5 = 6. As Aldol said, there is no steadfast rule, but 5.44 is so close to 6.00 that a small shift in the reaction will get Qrx from 6.00 to 5.44.