[Twickel]

Hi
I am very puzzled by the following questions


For the gas phase reaction, cyclopentene + I2 = cyclopentadiene + 2HI, we find that
ln Kp = 7.55 - (4817K / T)
 (where the symbol K on the right hand side refers to the units Kelvin). What are the changes in entropy and enthalpy?
This one I have no idea, I thought inK=-deltaG/RT.


At 298 K the equilibrium constant for the gas phase dissociation N2O4 ⇋ 2NO2 is 0.14. What are the equilibrium partial pressures of N2O4 at total pressures of 1 atm and 10 atm, where the total pressure is the sum of the partial pressures of NO2 and N2O4?

[Twickel]

So I'd like a push to get me started or if someone can work on them with me, it will be good.

[Twickel]

These are the possible answers

at total pressures of 1 atm and 10 atm, we have partial pressures of 0.85 atm and 9.3 atm, respectively.
at total pressures of 1 atm and 10 atm, we have partial pressures of 0.69 atm and 8.9 atm, respectively.
at total pressures of 1 atm and 10 atm, we have partial pressures of 0.1 atm and 1 atm, respectively.
at total pressures of 1 atm and 10 atm, we have partial pressures of 0.57 atm and 6.7 atm, respectively.


Is it the seocnd one, let me show you what I did and tell me if it is write

Kp= 0.14. PN2O4=0.69 tjen PNO2=  0.69 = 0.31. Kp= PNO^2/0.69= 0.14

Is that correct?

[Dan]

Is it the seocnd one, let me show you what I did and tell me if it is write

Kp= 0.14. PN2O4=0.69 tjen PNO2=  0.69 = 0.31. Kp= PNO^2/0.69= 0.14

Is that correct?

I cannot follow what you are doing there. You have also stated that 0.69 = 0.31, which cannot be true can it?

Start by writing an expression for Kp. Here's an example:

For A + B  C + D

Kp = [P(C).P(D)]/[P(A).P(B)]

Now you do this for N₂O₄  2NO₂

[Twickel]

Sorry about that, I meant 1-0.69

Well Kp in this will Kp= P(NO₂)^2/P(N₂O₄)

I am not sure if I should add in the reference state, which would make every partial pressure be divided by atm.

[Dan]

Ok, how did you get P(N₂O₄) = 0.69?

[Twickel]

Well the question is multiple choice and one of the options is at 1atm PN2O4 is 0.69. I thought this was correct because PNO2 equals 1-0.69. And subbing these values into the Kp fraction is 0.14. But I am not sure if that is correct.

[Dan]

Well the question is multiple choice and one of the options is at 1atm PN2O4 is 0.69. I thought this was correct because PNO2 equals 1-0.69. And subbing these values into the Kp fraction is 0.14. But I am not sure if that is correct.

Ok. That is probably not the best way to approach the question, because you will be completely stuck if it's not multiple choice next time. What would you do if the question was not multiple choice?

Let's recap, we know:

Kp = P(NO₂)²/P(N₂O₄)                (eq. 1)

Now, can you write an expresstion for total pressure, P, in terms of the partial pressures of P(NO₂)² and P(N₂O₄)? This will be (eq. 2).

You can now substitute (eq. 2) into (eq. 1) and solve the quadratic equation.