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Hess's law: combine given thermochemical equations

This type of question, asked 7 times: 3 May 2023 S2, 4 May 2023 S2, 13 Apr 2026 S2, 17 Apr 2026 S1, 20 Apr 2026 S2, 12 May 2026 S1, 14 May 2026 S2. All 7 questions below, oldest first.

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  1. Question 1

    3 May 2023 S2

    55
    Find value of Q from following equations. (i) C(s)+O2(g)→CO2(g), ΔH=Q kJC(s)+O_2(g)\rightarrow CO_2(g),\, \Delta H=Q\text{ kJ} (ii) C(s)+12O2(s)→CO2(s), ΔH=−x kJC(s)+\frac{1}{2}O_2(s)\rightarrow CO_2(s),\, \Delta H=-x\text{ kJ} (iii) C(s)+12O2(g)→CO2(g), ΔH=−y kJC(s)+\frac{1}{2}O_2(g)\rightarrow CO_2(g),\, \Delta H=-y\text{ kJ}

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  2. Question 2

    4 May 2023 S2

    54
    Calculate heat required to convert 9 g of liquid water to water vapours from following equations. H2(g)+12O2(g)→H2O(g), ΔH=−57 kCalH_2(g)+\frac{1}{2}O_2(g)\rightarrow H_2O(g),\,\Delta H=-57\text{ kCal}; H2(g)+12O2(g)→H2O(l), ΔH=−68.3 kCalH_2(g)+\frac{1}{2}O_2(g)\rightarrow H_2O(l),\,\Delta H=-68.3\text{ kCal}

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  3. Question 3

    13 Apr 2026 S2

    5
    The enthalpy of combustion of cyclohexane, cyclohexene and H2\mathrm{H_2} are respectively −3920,−3800-3920, -3800 and −241 KJ mol−1-241\ \mathrm{KJ\,mol^{-1}}. The enthalpy of hydrogenation of cyclohexene is

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  4. Question 4

    17 Apr 2026 S1

    8
    At 25 ∘C25\ ^\circ\mathrm{C} standard enthalpy of combustion of H2\mathrm{H_2}, Cyclohexene and cyclohexane are -241, -3800 and -3920 kJ mol−1\mathrm{mol^{-1}} respectively. What is the heat of hydrogenation of cyclohexene?

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  5. Question 5

    20 Apr 2026 S2

    5
    The enthalpy of combustion of C, H2\mathrm{H_2}, and CH4\mathrm{CH_4} are −390-390, −285-285 and −890-890 kJ/mol. Find the enthalpy of formation of methane.

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  6. Question 6

    12 May 2026 S1

    7
    Calculate ΔfH\Delta_f H for C(s)+2H2(g)→CH4(g)\mathrm{C(s) + 2H_2(g) \rightarrow CH_4(g)} i. C(s)+O2(g)→CO2(g)\mathrm{C(s) + O_2(g) \rightarrow CO_2(g)}; ΔH1=−393.5\Delta H_1 = -393.5 kJ/mol ii. H2(g)+1/2 O2(g)→H2O(l)\mathrm{H_2(g) + 1/2\,O_2(g) \rightarrow H_2O(l)}; ΔH2=−285.8\Delta H_2 = -285.8 kJ/mol iii. CH4(g)+2O2(g)→CO2(g)+2H2O(l)\mathrm{CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(l)}; ΔH3=−890.3\Delta H_3 = -890.3 kJ/mol

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  7. Question 7

    14 May 2026 S2

    7
    Calculate the heat of formation of SO2\mathrm{SO_2} from the following data, i) S+3/2 O2(g)→SO3(g)\mathrm{S} + 3/2\,\mathrm{O_2}(g) \rightarrow \mathrm{SO_3}(g), ΔH=−5Y\Delta H = -5Y kJ ii) SO2+1/2 O2(g)→SO3(g)\mathrm{SO_2} + 1/2\,\mathrm{O_2}(g) \rightarrow \mathrm{SO_3}(g), ΔH=−3Y\Delta H = -3Y kJ

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