Final answer:
There is insufficient information provided to calculate the change in enthalpy for the reaction of chlorine with carbon monoxide to produce phosgene, thus we cannot determine if it is endothermic or exothermic.
Step-by-step explanation:
The reaction of chlorine with carbon monoxide to produce phosgene can be determined as either endothermic or exothermic by calculating the change in enthalpy (ΔH) for the reaction. However, with the information provided in the question, there is insufficient information to calculate ΔH because the enthalpy changes for the individual reactants and products are not given. To determine if a reaction is endothermic or exothermic, one must know the enthalpy of formation for each reactant and product to perform Hess's Law calculations or have the reaction enthalpy directly provided.