Question

Consider the reaction:             N₂O₄(g) ⇄ 2NO₂(g)   K_p = 80
In which of the following systems will the reaction proceed in a direction to use up some of the NO₂ (from right to left in the above equation).
                       

	Partial Pressure N2O4	Partial pressure NO2
X	0.0020 atm	0.400 atm
Y	0.0040 atm	0.800 atm
Z	0.0040 atm	0.300 atm

A. Y and Z only

B. Z only

C. Y only

D. X, Y, and Z

E. X and Y only

Answer: C

Why?

Answer 1

In order to determine the direction of reaction i.e whether it will proceed to left or right ,relation between reaction quotient (Q) and equilibrium constant (K) . If Q> K then reaction shifts in the left and more reactants are formed.

For the given reaction, , Kp= 80

N204 = 2N02

Q= [NO2]^2/ [N2O4]

case1 ) For X , N2O4 = .0020 atm and NO2 = .400 atm

Q= .400*.400/ .0020 = 80 . Q=Kp

case2) For Y , N2O4 = .0040 atm, NO2 = .800atm

Q = .800*,800/ .0040 = 160 , Q>Kp

case3) For Z , N2O4=.0040 atm , NO2= .300 atm

Q= .300*.300/ .0040 = 22.5 , Q< Kp

so. only in case 2 i,e in Y only Q>Kp therefore correct option is C