Phosgene is prepared from the reaction of carbon monoxide and chlorine in the presence of a catalyst: CO(g) + Cl2(g) ⇌ COCl2(g) ΔH = -9.93 kJ mol^-1: Which of the following sets of conditions would produce the highest yield of phosgene? (A) High temperature, high pressure (B) Low temperature, low pressure (C) Low temperature, high pressure (D) High temperature, low pressure - HSC - SSCE Chemistry - Question 10 - 2004 - Paper 1

To determine the conditions that yield the highest amount of phosgene, we must consider the principles of chemical equilibrium and thermodynamics, specifically Le Chatelier's principle and the reaction's enthalpy change.

1. Understanding the Reaction: The reaction is exothermic as indicated by the negative enthalpy change (ΔH = -9.93 kJ mol^-1). According to Le Chatelier’s principle, for exothermic reactions, increasing the temperature will shift the equilibrium to favor the reactants, thus reducing the yield of phosgene. Therefore, high temperature is not desirable for maximizing yield.

2. Considering Pressure: The reaction involves a decrease in the number of gas moles (2 moles of reactants to 1 mole of product). Increasing the pressure favors the side with fewer gas moles. Hence, high pressure is advantageous for yield.

3. Evaluating Options:

   • (A) High temperature, high pressure: High temperature is not favorable.
   • (B) Low temperature, low pressure: Low temperature is favorable, but low pressure is not optimal for yield.
   • (C) Low temperature, high pressure: This combination is optimal because it favors phosgene production.
   • (D) High temperature, low pressure: Again, high temperature is not desirable.

4. Conclusion: The best conditions for maximizing phosgene yield are low temperature and high pressure, which is option (C).