Finding equilibrium constant at different temperatures

Question

I am given the reversible reaction N2O(g) + NO2(g) <--> 3NO(g).I am also told that Kc=1.4*10^-10 at 200.0 degrees Celsius. Given that 300 mL of NO at 800 torr and 25 degrees Celsius are added to a flask with a volume of 4L, I am told to find the equilibrium concentrations of N2O and NO2 at 25 degrees Celsius.

I am super confused about the Kc information given because I was under the impression that the equilibrium constant will change with a change in temperature, so I am not sure the 200-degree information is helpful.

I have been able to use PV=nRT to calculate the number of moles (I think?) And I calculated 0.0129 initial moles of NO(g). I have no clue how to continue :( Any help would be greatly appreciated!!

Answer 1

You are correct that Kc will change with the temperature. The direction of change will depend on whether the reaction is endothermic or exothermic. In the present problem, we have no thermodynamic data, so we can't predict the direction of change. Are you sure you aren't given any other data, such as ∆H of reaction? With ∆H, we could use ln (K2/K1) = -∆H/R (1/T2 - 1/T1). This would give us the value of Kc @ 25ºC which we could use to find equilibrium concentrations of N₂O and NO₂.

And for what it's worth, I do agree with your calculation of 0.0129 moles for the initial concentration of NO.