[greentea11]

eqm at room temp
2NO2 <--> N2O4

The info I have been given are the pressures of NO2 and N2O4 at 298k and 305k

The first part of the question is to find the eqm constant

I'm familiar with being given concentrations, or that you can use partial pressure

Just unsure how to use the pressure itself to work out the eqm constant?
any help appreciated

[Rutherford]

Assume the number of moles of NO₂, let it be 1 mole e.g. Using the ideal law equation you can calculate the volume. Calculate the concentrations for both 298K and 305K, then make a system of two equations to calculate the reacted amount of NO₂ and the equilibrium constant (K_c).

[greentea11]

Assume the number of moles of NO₂, let it be 1 mole e.g. Using the ideal law equation you can calculate the volume. Calculate the concentrations for both 298K and 305K, then make a system of two equations to calculate the reacted amount of NO₂ and the equilibrium constant (K_c).

thank you

 I've calculated the volumes, and thus concentration at both 298k and 305k for both NO2 and N2O4

and then Kc = [N2O4]/[NO2]^2

not quite sure how to get either of these concentrations,
I did the concentration at 305K NO2 - conc at 298k NO2 to get conc of  NO2 at eqm?
doesn't lead to correct answer when used in the above equation
where can I get [N204] at equilibrium from?

[Rutherford]

Could you write the whole problem, and the pressure values? Are initial or equilibrium pressures given?

[greentea11]

NO2 is in equilibrium with its dimer at room temperature
2NO2 (g) <--> N2O4 (g
use the following data for pressures of NO2 and N2O2 at equilibrium to calculate a. equilibirum constant for reaction b. standard molar gibbs free energy c. enthalpy d/ entropy of reaction at 298k

pressure in Torr mmHg
298K NO2 46 (converted to 6132.8Pa)
298k N2O4 23 (3066.4)

305K NO2 68 (9065.9)
305K N2O4 30 (3999.7)

[Rutherford]

Okay, no concentrations needed. Kp should be calculated-for both temperatures. It's easy to do it with given pressures (just calculate the partial pressures). For Gibbs energy, what expression is connecting Gibbs energy and the equilibrium constant? Use Van't Hoff equation for enthalpy calculation. Entropy is connected with Gibbs energy and enthalpy by one equation.

[greentea11]

The answer only gives 1 kp value, so why do you calculate it for 298k and 305k?

[Rutherford]

So you can use them in the Van't Hoff equation.

[greentea11]

oh i see, for the enthalpy part later on

Still doing part a

so I was calculating partial pressures

P = Pt X

Pt is total pressure
X is mole fractions where

X = n1 / n1 + n2

but for e.g using the torr values, at 298k,

Partial pressure NO2 = (46+23) * (2/1+2)
which gives 46, the value in the table
Am I calculating that wrong or is the table already values of partial pressure?

[greentea11]

OK got part A!

taking those values as partial pressures I wondered why it wasn't working - turns out just needed to convert into atmospheres then it worked. sorry for confusion!

EDIT: I've finished the question without any more bumps, the help was much appreciated!