[Chevalier]

Help. I've been trying to do this question for quite some time.

The first stage in polymerization is  dimerization — the bonding of two identical molecules.
Tetrafluoroethylene, a gas, dimerizes at high temperature to form octafluorocyclobutane, C4F8,
another gas. A sample of pure tetrafluoroethylene was introduced to a 1.00 liter reaction vessel at 175°C at  time  t = 0. The pressure inside the reaction vessel was monitored as a function of time and the  following data were obtained:

Time (s)           0     25   50    75   100  125
Pressure (atm) 9.19 8.19 7.55 7.10 6.77 6.52

Define the rate of reaction in terms of the disappearance of tetrafluoroethylene.
(a) Calculate the pressure and concentration of C2F4 and C4F8 at each of the times listed above.
Hint: Let x be the partial pressure of reacted C2F4. Use stoichiometry to write an equation for
the total pressure in the vessel, which is a sum of the partial pressures of C2F4 and C4F8. Use
this equation to find the partial pressures of the two gases at each time.

I'm legitimately stumped.

I have done Ptotal=PC2F4 + C4F8 as the question reccomends, and have set the pressure of reacted C2F4 to x.

I assume that Ptotal= The value in the table.
PC2F4= Initial C2F4 pressure- x(as it's reacted, and thus the amount has decreased)
PC4F8= Initial C4F8 pressure + x(what has reacted has now formed C4F8, and thus has increased).

I then attempted to apply PV= nRT to find the initial pressure. However, I was unable to determine the amount of moles and thus unable to solve.

My second method involved formulating the classic Products/Reactants= Kp value equation. However, the Kp value was not given, and thus I doubt they intend its use.


Please assist me, thank you very much. Maybe its because I'm sick, but I just can't seem to figure this out.

[Borek]

I then attempted to apply PV= nRT to find the initial pressure.

Initial pressure is given, isn't it?

However, I was unable to determine the amount of moles and thus unable to solve.

1L of gas at 175°C and at 9.19 atm. Just plug and chug.

PC2F4= Initial C2F4 pressure- x(as it's reacted, and thus the amount has decreased)
PC4F8= Initial C4F8 pressure + x(what has reacted has now formed C4F8, and thus has increased).

It can't be x in both cases, as that would mean neglecting stoichiometry.

[Chevalier]

Thank you so much for your reply. My biggest oversight was the initial temp.


So, initial and total = value in table?

For first one:

9.19=(9.19-2x)[C2F4] + (9.19-x)[C4F8]

The x is 2x because of C2F4------>C4F8(taking into account stoichi as you encouraged above).

Please correct me where I may have gone wrong or confirm. Thanks!

[Borek]

So, initial and total = value in table?

I am not sure what you mean. At t=0 total=initial.

9.19=(9.19-2x)[C2F4] + (9.19-x)[C4F8]

This is off. You combine so good ideas with some bad ones. Either try to elaborate what is what, or think it over.

[Chevalier]

Thank you very much for the guided inquiry, but I wouldn't mind a hint as to where I'm going wrong.

I don't know if the [C2F4] was interprested to be the concentrations, but it was just stating what is what. Is it that x shouldn't be 2x? When I "work it out", the values I find for both add up to Ptotal.

I thought and still don't see what I'm not seeing with the method I used above.

A completely different way I thought of  is:

There are a total of 3 moles, and we know all gases exert the same pressure. So taking C2F4 as 2/3 and C4F8 as 1/3

(2/3) * 9.16 = C2F4
(1/3) * 9.16 = C4F8

Is that completely good ideas?

[Borek]

Where did you got 2/3 and 1/3 from? I can't think of any reason to use these numbers.

Total pressure is a sum of partial pressures exerted by both gases. At the beginning there are n moles of tetrafluoroethylene (you can calculate n). After that at any moment total pressure is proportional to n-x (left tetrafluoroethylene) plus x/2 - number of moles of produced dimer. x is the only unknown here, as both total pressure at t and n are known.