In: Chemistry
For the reaction below, Kp = 29.95 at 800 K. Calculate the equilibrium partial pressures of the reactants and products if the initial pressures are P_PCl5 = 0.3900 atm and P_PCl3 = 0.4300 atm. Assume Cl2 is 0 atm. PCL5 (g) > PCl3 (g) + Cl2 (g)
Let's write the reaction once again:
PCl5 ----------------------> PCl3 + Cl2
i) 0.39 0.43 0
eq) 0.39 - x 0.43+x x
Kp = pCl2 pPCl3 / pPCl5
Now, let's solve for x:
29.95 = (0.43 + x) (x) / (0.39 - x)
29.95 (0.39 - x) = x (0.43 + x)
11.681 - 29.95x = x2 + 0.43x --------> At this point, we can rearrange the equation to a 2nd grade equation:
x2 + 0.43x + 29.95x - 11.681 = 0
x2 + 30.38x - 11.681 = 0
x = -30.38 (30.382 + 4x1x11.681)1/2 / 2x1
x = -30.38 31.14 / 2
x1 = 0.38
x2 = -34.76
The value we are going to take would be 0.38 so:
pCl5 = 0.39 - 0.38 = 0.01 atm
pCl3 = 0.43 + 0.38 = 0.81 atm
pCl2 = 0.38 atm
This wuld be the correct values, and it's just one way to solve this problem.
The other way is doing in a longer way by calculating Kc,applying this expression: Kc = Kp (RT)n then with the ideal gas equation PV = nRT or P = cRT calculate c to all the gases, and doing later the same procedure as before to calculate x, and then converting again into pressure. You can choose the way you like the most.