[sci0x]

I have a reaction vessel in which i heat cyclopropane gas to propene gas. Temperature is 700K and initial pressure is 0.5atm.

After 0 seconds there is 100% cyclopropane
After 108000 seconds (when reaction is over) there is 25% cyclopropane

How do I find the pressure at the end of the reaction?

[Dude]

This one seems straightforward.  Cyclopropane has an empirical formula of C3H6 and so does propylene.  Since the number of moles remains the same (its just a rearrangement and not dehydrogenation), the pressure will be the same regardless of the extent of reaction.  Slight variations in pressure might occur if a more complex gas calculation is used (other than the ideal gas law).

[GCT]

I have a reaction vessel in which i heat cyclopropane gas to propene gas. Temperature is 700K and initial pressure is 0.5atm.

After 0 seconds there is 100% cyclopropane
After 108000 seconds (when reaction is over) there is 25% cyclopropane

How do I find the pressure at the end of the reaction?

Write out the reaction, two hydrogens are given off.  You'll need to find the change in the total number of moles.

[ducthinh]

If the temperature and volume are constant , all gas is considered as ideal gas then ,I think, there will be no change in pressure as there is no change in the number of molecules in the vessel...

[Donaldson Tan]

Write out the reaction, two hydrogens are given off.  You'll need to find the change in the total number of moles.

cyclopropane and propene share the same molecular formula.

cyclopropane merely rearranges itself to form propene. since the product:reactant stoichiometric ratio is 1:1, then there is no change in number of moles in the chemical system.

Is this reaction exothermic or endothermic at 700K? You will need Cp (heat capacity) data and the standard heat of reaction to calculate the heat of reaction at 700K. You can find the heat of reaction data and Cp equations in terms of temperature from this website (http://www.cheric.org/kdb/). The equation below will help. Temperature T is expressed in units of kelvin FYI.

The heat of reaction at 700K will tell you if the temperature of your system will increase or decrease assuming adiabatic conditions (if your reaction vessel is well-insulated).

Once you have establised the temperature of the system, you may use PV = nRT to find the final pressure of the system.

[Donaldson Tan]

since we have calculated the heat of reaction

assume perfect gas, adiabatic condition, constant volume,

then Cp - Cv = R => Cv = Cp - R
(Cv is heat capacity at constant volume. R is universal gas constant)

dU = heat of reaction

assume zero heat of mixing,
final reaction mixture is 75% propene and 25% cylopropane,

then you arrive at the equation below


Your job is to find the value of T that corresponds to the above equation.

Once you find T, you can use Gay-Lussac's law to find the final Pressure.