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Enthalpy of a reaction or of formation from bond enthalpies

This type of question, asked 3 times: 13 Apr 2023, 4 Apr 2024, 3 Apr 2025. All 3 questions below, oldest first.

Try each one first, then open its answer.

  1. Question 1

    13 Apr 2023

    54
    A2+B2→2AB⋅ΔHf0=−200kJmol−1A_{2}+B_{2}\rightarrow 2AB \cdot \Delta H_{f}^{0}= - 200kJmol^{- 1} AB,A2AB,A_{2} and B2B_{2} are diatomic molecule. If the bond enthalpies of A2,B2A_{2},B_{2} and ABAB are in the ratio 1:0.5:1, then the bond enthalpy of A2A_{2} is_________ kJmol−1kJmol^{- 1} (Nearest integer)

  2. Question 2

    4 Apr 2024

    51
    The enthalpy of formation of ethane (C2H6)\left( C_{2}H_{6} \right) from ethylene by addition of hydrogen where the bond energies of C−H,C−C,C=C,H−HC - H, C - C, C = C, H - H are 414 kJ,347 kJ,615 kJ414\text{ }kJ, 347\text{ }kJ, 615\text{ }kJ and 435 kJ435\text{ }kJ respectively is ______ kJkJ.

  3. Question 3

    3 Apr 2025

    49
    Given :
    ΔHsub⊖[C( graphite )]=710 kJ mol−1ΔC−HH⊖=414 kJ mol−1ΔH−HH⊖=436 kJ mol−1ΔC=CH⊖=611 kJ mol−1\begin{matrix} & \Delta H_{sub}^{\ominus}\lbrack C(\text{~graphite~})\rbrack = 710\text{ }kJ{\text{ }mol}^{- 1} \\ & \Delta_{C - H}H^{\ominus} = 414\text{ }kJ{\text{ }mol}^{- 1} \\ & \Delta_{H - H}H^{\ominus} = 436\text{ }kJ{\text{ }mol}^{- 1} \\ & \Delta_{C = C}H^{\ominus} = 611\text{ }kJ{\text{ }mol}^{- 1} \end{matrix}
    The ΔHf⊖\Delta H_{f}^{\ominus} for CH2=CH2{CH}_{2} = {CH}_{2} is ____ kJmol−1kJ{mol}^{- 1} (nearest integer value)