Question

Consider phosgene, COCl2(g). It is an important intermediate in the manufacture of some plastics. It can decompose into carbon monoxide and chlorine gases. Its ΔHf° is -220.1 kJ/mole. a) Write the balanced chemical equation for the decomposition of phosgene including an equilibrium arrow and the states in ( ). b) Write the equilibrium constant expression for this decomposition. c) At 500°C, after the reaction has reached equilibrium, Pcarbon monoxide = 0.413 atm, Pchlorine = 0.237 atm, and Pphosgene = 0.217. Calculate Kp at 500°C. d) Initially, a flask is charged with 0.689 atm phosgene, 0.250 atm chlorine, and 0.333 atm carbon monoxide. After equilibrium is established at 500°C, the new pressure on phosgene is 0.501 atm. Calculate the pressures of chlorine and carbon monoxide at equilibrium. e) [For the first part, you cannot use the equilibrium shortcut; both Kp and the concentrations are too large to be considered negligible…quadratic equation ahead…] i) Initially a second flask contains 0.750 atm of chlorine, 0.750 atm carbon monoxide, and 0.500 atm phosgene. What is the pressure of each gas when equilibrium is established? ii) Suppose enough phosgene is added to the flask to raise the pressure temporarily to 2.00 atm. According to Le Chatelier’s Principle, in which direction will the reaction shift to get back to equilibrium? f) According to Le Chatelier’s Principle, explain which direction the reaction will shift to get back to equilibrium if: i) the reaction is transferred into a flask that is larger in volume? ii) helium gas is added? iii) the temperature is increased? iv) a platinum-iridium-magnesium-palladium (PIMP) catalyst is added?

Answer 1

a) Balanced equation for decomposition of phosgene,

COCl2(g) <==> CO(g) + Cl2(g)

b) Equilibrium constant Kc = [CO][Cl2]/[COCl2]

c) Kp = [pCO].[pCl2]/[pCOCl2]

Feed the values from above,

Kp = (0.413 x 0.237)/(0.217) = 0.451

d) Initial,

[COCL2] = 0.689 atm

[Cl2] = 0.250 atm

[CO] = 0.333 atm

at equilibrium,

[COCl2] = 0.501 atm

change in pressure = 0.689 - 0.501 = 0.188 atm

So equilibrium pressure for,

[CO] = 0.333 + 0.188 = 0.521 atm

[Cl2] = 0.250 + 0.188 = 0.438 atm

e) i) Initial,

[Cl2] 0.750 atm

[CO] = 0.750 atm

[COCL2] = 0.500 atm

let x be the change in concentration at equilibrium,

Kp = 0.451 = (0.750 - x)(0.750 - x)/(0.500 + x)

0.5625 - 1.5x + x^2 = 0.225 + 0.451x

x^2 - 1.95x + 0.337 = 0

x = 0.19 atm

So the pressure of each gas at equilibrium,

[Cl2] 0.750 - 0.19 = 0.56 atm

[CO] = 0.750 - 0.19 = 0.56 atm

[COCL2] = 0.500 + 0.19 = 0.69 atm

(ii) If enough phoshgene is added to raise pressure to 2 atm, the reaction equilibrium would shift to right according to LeChatellier's principle.

(iv) When reaction is transferred to larger volume flask, the equilibrium will shift to right.

(v) He gas is added : equilibrium shifts to the right

(vi) Temperature is increased : exothermic reaction, equilibrium will shift to left.

(vii) catalysts is added : equilibrium shifts to right