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Enthalpy of formation or of reaction from enthalpies of combustion (Hess's law)

This type of question, asked 7 times: 14 Jun 2022, 25 Jun 2022, 25 Jul 2022, 6 Apr 2023, 22 Jan 2025, 28 Jan 2025, 4 Apr 2026 S2. All 7 questions below, oldest first.

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  1. Question 1

    14 Jun 2022

    124
    At 25∘C25^{\circ}C and 1 atm pressure, the enthalpies of combustion are as given below: The enthalpy of formation of ethane is

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  2. Question 2

    25 Jun 2022

    122
    At 25∘C25^{\circ}C and 1 atm pressure, the enthalpy of combustion of benzene (l) and acetylene (g) are −3268 kJ mol−1- 3268\text{ }kJ{\text{ }mol}^{- 1} and −1300 kJ mol−1- 1300\text{ }kJ{\text{ }mol}^{- 1}, respectively. The change in enthalpy for the reaction 3C2H2( g)→C6H6(l)3C_{2}H_{2}(\text{ }g) \rightarrow C_{6}H_{6}(l), is

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  3. Question 3

    25 Jul 2022

    52
    The enthalpy of combustion of propane, graphite and dihydrogen at 298 K298\text{ }K are: −2220.0 kJ mol−1-2220.0\text{ }kJ{\text{ }mol}^{-1}, −393.5 kJ mol−1-393.5\text{ }kJ{\text{ }mol}^{-1} and −285.8 kJ mol−1-285.8\text{ }kJ{\text{ }mol}^{-1} respectively. The magnitude of enthalpy of formation of propane (C3H8)\left( C_{3}H_{8} \right) is ____ kJ mol−1kJ\ mol^{-1}. (Nearest integer)

  4. Question 4

    6 Apr 2023

    147
    Consider the following data: Heat of combustion of H2( g)=−241.8 kJ mol−1H_{2}(\text{ }g) = - 241.8\text{ }kJ{\text{ }mol}^{- 1} Heat of combustion of C(s)=−393.5 kJ mol−1C(s) = - 393.5\text{ }kJ{\text{ }mol}^{- 1} Heat of combustion of C2H5OH(l)=−1234.7 kJ mol−1C_{2}H_{5}OH(l) = - 1234.7\text{ }kJ{\text{ }mol}^{- 1}. The heat of formation of C2H5OH(a)C_{2}H_{5}OH(a) is (−)( - )____ kJmol−1kJmol^{- 1} (Nearest integer)

  5. Question 5

    22 Jan 2025

    124
    Consider the following cases of standard enthalpy of reaction (ΔHr∘\left( \Delta H_{r}^{\circ} \right. in  kJmol−1)\left. \ kJmol^{- 1} \right)
    C2H6( g)+72O2( g)→2CO2( g)+3H2O(l)ΔH1∘=−1550C( graphite )+O2( g)→CO2( g)ΔH2∘=−393.5H2( g)+12O2( g)→H2O(l)ΔH3∘=−286\begin{matrix} & C_{2}H_{6}(\text{ }g) +\frac{7}{2}O_{2}(\text{ }g) \rightarrow 2CO_{2}(\text{ }g) + 3H_{2}O(l)\quad \Delta H_{1}^{\circ}= - 1550 \\ & C(\text{~graphite~}) +O_{2}(\text{ }g) \rightarrow CO_{2}(\text{ }g)\quad \Delta H_{2}^{\circ}= - 393.5 \\ & H_{2}(\text{ }g) +\frac{1}{2}O_{2}(\text{ }g) \rightarrow H_{2}O(l)\quad \Delta H_{3}^{\circ}= - 286 \end{matrix}
    The magnitude of ΔfH∘\Delta_{f}H^{\circ} of C2H6(g)C_{2}H_{6}(g) is ______\_\_\_\_\_\_ kJmol−1kJmol^{- 1} (Nearest integer).

  6. Question 6

    28 Jan 2025

    122
    Consider the following data : Heat of formation of CO2( g)=−393.5 kJ mol−1CO_{2}(\text{ }g) = - 393.5\text{ }kJ{\text{ }mol}^{- 1} Heat of formation of H2O(l)=−286.0 kJ mol−1H_{2}O(l) = - 286.0\text{ }kJ{\text{ }mol}^{- 1} Heat of combustion of benzene =−3267.0 kJ mol−1= - 3267.0\text{ }kJ{\text{ }mol}^{- 1} The heat of formation of benzene is  ___\ \_\_\_ kJmol−1kJmol^{- 1}. (Nearest integer)

  7. Question 7

    4 Apr 2026 S2

    46
    If 3.365 g of ethanol ( ll ) burnt completely in a bomb calorimeter at 298.15 K , the heat produced is 99.472 kJ . The ∣ΔHf ∘∣\left| \Delta H_{f}\ ^{\circ} \right| of ethanol at 298.15 K is ____\_\_\_\_ kJ mol−1kJ{\text{ }mol}^{- 1}. (Nearest integer) Given : Standard enthalpy of combustion of graphite =−393.5 kJ mol−1= - 393.5\text{ }kJ{\text{ }mol}^{- 1} Standard enthalpy of formation of water ( ll )=−285.8 kJ mol−1= - 285.8\text{ }kJ{\text{ }mol}^{- 1}. Molar mass in gmol−1{gmol}^{- 1} of C,H,OC,H,O are 12,1 and 16 respectively.