Consider the following isomerization process: CH2=CH-CH2-CH=CH2(g) → CH3-CH=CH-CH=CH2(g). Which of t — Thermodynamics and Thermochemistry Chemistry Question

💡 Solution & Explanation

Let's analyze the isomerization from an isolated diene (1,4-pentadiene) to a conjugated diene (1,3-pentadiene):<br>(a) Conjugated dienes are more stable than isolated dienes due to resonance energy/delocalization. Since the product is more stable, the process is exothermic (ΔH < 0). Thus, Statement A is correct.<br>(b) If we assume the localized bond energies are identical on both sides (same number of C-H, C-C, and C=C bonds), the theoretical ΔH is 0. The actual ΔH change is solely due to the resonance stabilization of the conjugated product. Hence, the enthalpy change of the reaction is equal to the resonance enthalpy of the product. Thus, Statement B is correct.<br>(c) Since the product (conjugated diene) is more stable (lower energy state) than the reactant, its complete combustion will release less heat than the reactant. Thus, the magnitude of the enthalpy of combustion of the product is less than that of the reactant. Thus, Statement C is correct.<br>(d) Hydrogenation of 1,3-pentadiene (product) releases less heat because the conjugated double bonds are stabilized by resonance, unlike the isolated double bonds in 1,4-pentadiene (reactant). Thus, the magnitude of enthalpy of hydrogenation of the product is less, not greater. Thus, Statement D is incorrect.<br>Therefore, the correct statements are A, B, and C.