3 replies to this topic

[ChemEngUoBStudent]

Dear Members, I'm currently stuck on a thermodynamics question and would like a hint on how to continue with the question. Can anyone tell me how I should tackle this question?



For the equilibrium:
N2O4 <--> 2NO2
Estimate the mole fraction at equilibrium over the temperature range 200 to 400 K and for pressures 0.5, 5 and 50 bar

I'm thinking this should have something to do with Raoult's law and Dalton's law and maybe I'll need to find out the vapour pressures, but... within the range? Should I find vp at 200, 300 and 400?

Thanks in advance for the help :-)

[MrShorty]

What kind of equilibrium are they asking for? Raoult's law/Dalton's law would apply to a vapor-liquid equilibrium problem. However, the way you've stated it, it sounds more to me like a chemical equilibrium problem -- given a certain amount of N2O4 at the specified condition and an expression for the chemical equilibrium constant, determine how much dissociates into NO2.

[siretb]

Did you search, really? There are many entries if you google NO2 N2O4 equilibrium

http://chemed.chem.p...pages/21.1.html

There is an equilibrium. the K value is temperature dependent.

[kkala]

Both previous posts give enough help to solve the problem, a few more notes may be useful:
1. It concerns equilibrium of NO2g - N2O4g , Dalton's law of partial pressures can be used (not Rault's law). At 50 bar fugacities should be used rather than partial pressures, but I would neglect this for simplicity, assuming that the exercise aims at not precise quantitative results (but note the assumption).
2. Reference by Siretb gives Kp at four different temperatures for the equilibrium 2NO2g<->N2O4g. It is Kp=P(N2O4)/(P(NO2))^2. Assume an initial mole fraction of 1 for N2O4, changed to 1-x (x<1.0) after dissociation (at equilibrium). Total pressure P (0.5, 5, 50 bara) of NO2 - N2O4 mixture is given, so partial pressure of N2O4 is P(N2O4)=P(1-2*x) and P(NO2)=P*2*x; these introduced into above expression of Kp will create a second order equation (concerning x) that should be solved (0<x<1).
3. Yes, you have to estimate x for 0.5, 5, 50 bara at temperature of 200, (say) 300, 400 oK, 9 cases in total. If you do not find Kp values elsewhere, plot these by Siretb over temperature to estimate Kp at required temperatures. I would plot logKp (i.e. logarithmic scale of Kp) versus 1/T (T in oK), for hopefully better precision.
4. According to above, dissociation of N2O4 increases with temperatue (x increases) , as we can see from available Kp data by siretb. On the other hand increased pressure decreases N2O4 dissociation (Le Chatelier's principle). Results of the exercise should show it.