Final answer:
The reaction in question is endothermic since energy is listed as a reactant, indicating that it is absorbed. Endothermic reactions require energy input to break bonds, as opposed to exothermic reactions that release energy.
Step-by-step explanation:
The reaction given is 653 kJ + PCl3 → P + Cl2; since energy is written as a reactant, this indicates that energy is absorbed by the reaction. Therefore, this reaction is endothermic. In an endothermic reaction, the system absorbs energy from the surroundings, typically as heat, which is required to break the bonds of the reactants.
As a comparison, exothermic reactions release energy, which is shown as a product in the chemical equation. For example, the reaction CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) + 213 kcal is exothermic because the energy is produced and listed on the product side.