Question

CaCO2--> CaO+CO2
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Calculate the pressure of CO2 in an equilibrium process at (a)22 Celsius and (b) 755 Celsius . Assume delta H=177.8KJ/mol and delta S=160.5 J/mol*K for that temeprature range.


My question is do I have to subtract the two temperatures since it says "temperature range" and find delta G using delta H-TdeltaS and then use the equation G=-RTlnK to find K and for the temperature, plug in 22 and 755 separately.

Or do I find delta G for 22 Celsius, and then use G=-RTlnK to find K at that temperature and repeat for 755 C.

I started to do the second way.
for delta g=delta H-TdeltaS=130.4 J
G=-RTlnK
k=e^(-G/RT)=8.34*10^-24
Then I have to find the pressure (Kp)for CO2 which I don't know how to to do after I find K.

Answer 1

No, you don't subtract the two temperatures. The problem simply states that the delta H and delta S values are assumed to be constant "in that temperature range." The problem is two problems, one at 755 C and the other at 22 C. Don't forget to change T to Kelvin when you use it.
I think you have an error in the equation. You must have intended to write CaCO3 instead of CaCO2. CaCO3 is a solid. CaO is a solid. Therefore, Kp = partial pressure of CO2. So if you know Kp, you automatically know partial pressure of CO2.