A hypothetical gas-phase reaction converts reactants R into products P in a single elementary step. The average total energy required to break all bonds in the reactants is 1580 kJ mol⁻¹ of reaction, while the total energy released upon forming all bonds in the products is 1760 kJ mol⁻¹ of reaction. A chemist proposes modifying R so that one particular bond to be broken is weakened by 50 kJ mol⁻¹ (all other bond energies remain unchanged).
Answer the following questions about both the original and the modified reaction.
(a) Calculate the enthalpy change, ΔHorig, for the original reaction. State whether the reaction is exothermic or endothermic.
(b) Determine the new total energy required to break the reactant bonds after the 50 kJ mol⁻¹ weakening. Then calculate the modified enthalpy change, ΔHmod.
(c) Compare ΔHmod to ΔHorig. Quantify and explain how weakening a single bond in the reactants affects the thermochemistry of the reaction.
(d) Sketch qualitatively (no exact numerical axis needed) a potential-energy diagram for the original and modified reactions on the same set of axes. Label reactants, products, and relative enthalpy changes. Explain any differences in the profiles.