Exams › JEE Advanced › Chemistry

Given the following thermochemical data: cyclohexene + H2 (Ni catalyst) -> cyclohexane; delta_H = -28.6 kcal/mol Anthracene + excess H2 (Ni catalyst) -> fully hydrogenated anthracene; delta_H = -116.2 kcal/mol Calculate the resonance energy of anthracene.

1. -84 kcal/mol
2. -100 kcal/mol
3. -110 kcal/mol
4. -116 kcal/mol

Correct answer: -100 kcal/mol

Solution

Anthracene (C14H10) absorbs 5 mol H2 to become perhydroanthracene (C14H24). Theoretical heat of hydrogenation (assuming 5 isolated cyclohexene-like double bonds): 5 x (-28.6) = -143 kcal/mol. Actual: -116.2 kcal/mol. Resonance energy = actual - theoretical = -116.2 - (-143) = +26.8 kcal/mol (stabilization). But the question asks for resonance energy with sign convention used: RE = theoretical - actual = -143 - (-116.2) = -26.8 kcal/mol (magnitude ~27 kcal/mol). None of the options match exactly. However, using 7 double bonds (Kekule structure shows 7 pi bonds) and counting differently: if anthracene is treated as having 7 ring double bonds (like 3 benzene rings each with 3 double bonds = 9, minus 2 shared bonds = 7): theoretical = 7 x (-28.6) = -200.2 kcal/mol. RE = -116.2 - (-200.2) = +84 kcal/mol. So RE = -84 kcal/mol in the convention where RE is negative stabilization. Option A: -84 kcal/mol fits this calculation. Alternatively using (3 rings x 3 bonds - shared)/adjusted: RE = -84 kcal/mol.

Related JEE Advanced Chemistry questions

• If the specific heat at constant pressure (Cp) is 10 cal at 1000 K, the initial temperature (T1) is 1000 K, the final temperature (T2) is 100 K, and the mass (m) is 32 g, what is the entropy change (ΔS) under constant pressure?
• For the reaction H2(g) + 1/2 O2(g) → H2O(l), where the enthalpy of formation (ΔHf) is given as −285.9 kJ mol⁻¹, determine the value of ΔHf.
• For the reaction C3H8(g) → 3C(g) + 8H(g), determine the enthalpy change (ΔHf) given that ΔHf for C2H2(g) → 2C(g) + 6H(g) is −620 kJ mol⁻¹.
• The standard enthalpies of formation of CO₂(g), H₂O(l) and glucose(s) at 25°C are -400 kJ/mol, -300 kJ/mol and -1300 kJ/mol, respectively. The standard enthalpy of combustion per gram of glucose at 25°C is -
• Benzene and naphthalene form an ideal solution at room temperature. For this process, the true statement(s) is(are) -
• During the transformation of liquid water to water vapor at 100°C and standard atmospheric pressure, which of the following is accurate?

⚔️ Practice JEE Advanced Chemistry free + battle 1v1 →