Phosgene is prepared from the reaction of carbon monoxide and chlorine in the presence of a catalyst: $$CO(g) + Cl_2(g) \rightleftharpoons COC_2(g)$$ $$\Delta H = -9.93 \text{ kJ mol}^{-1}$$ 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 would produce the highest yield of phosgene, we can apply Le Chatelier's Principle and consider the exothermic nature of the reaction (as indicated by the negative enthalpy change, (\Delta H)).

• Pressure Consideration: Increasing the pressure will favor the side of the reaction with fewer moles of gas. In this case, both reactants and product are in gaseous form, but based on stoichiometry, an increase in pressure will favor the formation of the product (phosgene).

• Temperature Consideration: Since the reaction is exothermic, lowering the temperature will favor the production of phosgene, as the system will seek to produce heat in order to counteract the temperature decrease.

Given these points, the condition that provides low temperature and high pressure (Option C) is the most favorable for maximizing the yield of phosgene.