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Average bond enthalpy from the enthalpies of formation of the molecule and its gaseous atoms

This type of question, asked 3 times: 2021 Paper 26, 28 Jan 2025, 21 Jan 2026 S2. All 3 questions below, oldest first.

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  1. Question 1

    2021 Paper 26

    53
    The average S−FS - F bond energy in kJmol−1kJmol^{- 1} of SF6SF_{6} is (Rounded off to the nearest integer) [Given: The values of standard enthalpy of formation of SF6( g),S(g)SF_{6}(\text{ }g),S(g) and F(g)F(g) are - 1100, 275 and 80 kJ mol−180\text{ }kJ{\text{ }mol}^{- 1} respectively.]

  2. Question 2

    28 Jan 2025

    50
    The formation enthalpies, ΔHfΘ\Delta H_{f}^{\Theta} for H(g)H_{(g)} and O(g)O_{(g)} are 220.0 and 250.0 kJ mol−1250.0\text{ }kJ{\text{ }mol}^{- 1}, respectively, at 298.15 K , and ΔHf⊖\Delta H_{f}^{\ominus} for H2O(g)H_{2}O_{(g)} is −242.0 kJ mol−1- 242.0\text{ }kJ{\text{ }mol}^{- 1} at the same temperature. The average bond enthalpy of the O−HO - H bond in water at 298.15 K is ____\_\_\_\_ kJmol−1kJmol^{- 1} (nearest integer).

  3. Question 3

    21 Jan 2026 S2

    31
    Consider the following data: ΔfHΘ(\Delta_{f}H^{\Theta}( methane, g)=−XkJmol−1) = - XkJmol^{- 1} Enthalpy of sublimation of graphite =YkJmol−1= YkJmol^{- 1} Dissociation enthalpy of H2=ZkJmol−1H_{2}= ZkJmol^{- 1} The bond enthalpy of C−HC - H bond is given by:

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