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Enthalpy of reaction from bond enthalpies

This type of question, asked 6 times: May 2021 S1, 2 May 2023 S2, 3 May 2023 S2, 11 Apr 2026 S1, 11 Apr 2026 S2, 13 Apr 2026 S1. All 6 questions below, oldest first.

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  1. Question 1

    May 2021 S1

    62
    What is enthalpy of formation of NH3_3, if bond enthalpies are (N≡N) = 941 kJ/mol, (H-H) = 436 kJ/mol and (N-H) = 389 kJ/mol?

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  2. Question 2

    2 May 2023 S2

    84
    Calculate enthalpy change for following reaction.
    H2C=CH2(g)+H2(g)⟶H3C−CH3(g)\text{H}_2\text{C}=\text{CH}_{2(g)} + \text{H}_{2(g)} \longrightarrow \text{H}_3\text{C}-\text{CH}_{3(g)}
    [The bond energy of C-H\text{C-H}, C-C\text{C-C}, C=C\text{C=C} and H-H\text{H-H} is 414, 347, 615 and 435 kJ respectively]

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  3. Question 3

    3 May 2023 S2

    98
    Given that C(g)+4H(g)→CH4(g), ΔH∘=−1665 kJC(g)+4H(g)\rightarrow CH_4(g),\,\Delta H^{\circ}=-1665\text{ kJ}. What is bond energy per mole of C-H bond?

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  4. Question 4

    11 Apr 2026 S1

    6
    Calculate the enthalpy change for the following reaction, using given bond energy (kJ/mol) (C-H = 414414, H-O = 463463, H-Cl = 431431, C-Cl = 326326 and C-O = 335335) CH3OH(g)+HCl(g)→CH3Cl(g)+H2O(g)\mathrm{CH_3OH}_{(g)} + \mathrm{HCl}_{(g)} \rightarrow \mathrm{CH_3Cl}_{(g)} + \mathrm{H_2O}_{(g)}

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  5. Question 5

    11 Apr 2026 S2

    7
    The bond dissociation enthalpy of H2\mathrm{H_2}, Cl2\mathrm{Cl_2} and HCl are 434,242434, 242 and 431 kJ mol−1431\ \mathrm{kJ\,mol^{-1}} respectively. Calculate the enthalpy of formation of HCl.

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  6. Question 6

    13 Apr 2026 S1

    8
    Calculate the enthalpy change of the reaction, H2(g)+Cl2(g)→2HCl(g)\mathrm{H_2(g) + Cl_2(g) \rightarrow 2HCl(g)} if bond energies (kJ mol−1\mathrm{mol^{-1}}): H-H = 436, Cl-Cl = 242, H-Cl = 431

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