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Standard enthalpy of formation Knowledge-Based Practice

Chemistry · Thermochemistry & Thermodynamics · Chemistry · Enthalpy

5 Chemistry practice questions on Standard enthalpy of formation. Questions that assess core concepts and understanding.

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Questions

  1. 1.

    Which of the following conditions is required for a reaction's standard enthalpy change to be classified as the standard enthalpy of formation (ΔHf∘\Delta H^\circ_f) of a compound?

    • A) The reaction must take place at absolute zero (0 K) and standard atmospheric pressure.
    • B) Exactly one mole of the compound must be formed from its constituent elements in their standard states.
    • C) The reactant elements must all be present as isolated gaseous atoms.
    • D) The reaction must be exothermic and release heat to the surroundings.
  2. 2.

    Given that ΔHrxn∘=−24.8 kJ\Delta H^\circ_{\text{rxn}} = -24.8\text{ kJ} for the reduction of iron(III) oxide represented by the equation Fe2O3(s)+3CO(g)→2Fe(s)+3CO2(g)\text{Fe}_2\text{O}_3(\text{s}) + 3\text{CO}(\text{g}) \rightarrow 2\text{Fe}(\text{s}) + 3\text{CO}_2(\text{g}), and the standard enthalpies of formation are ΔHf∘[CO(g)]=−110.5 kJ/mol\Delta H^\circ_f[\text{CO}(\text{g})] = -110.5\text{ kJ/mol} and ΔHf∘[CO2(g)]=−393.5 kJ/mol\Delta H^\circ_f[\text{CO}_2(\text{g})] = -393.5\text{ kJ/mol}, what is the standard enthalpy of formation of Fe2O3(s)\text{Fe}_2\text{O}_3(\text{s})?

    • A) +824.2 kJ/mol+824.2\text{ kJ/mol}
    • B) −283.0 kJ/mol-283.0\text{ kJ/mol}
    • C) −824.2 kJ/mol-824.2\text{ kJ/mol}
    • D) −873.8 kJ/mol-873.8\text{ kJ/mol}
  3. 3.

    Given the standard enthalpies of formation ΔHf∘[CH4(g)]=−74.8 kJ/mol\Delta H^\circ_f[\text{CH}_4(\text{g})] = -74.8\text{ kJ/mol}, ΔHf∘[CO2(g)]=−393.5 kJ/mol\Delta H^\circ_f[\text{CO}_2(\text{g})] = -393.5\text{ kJ/mol}, and ΔHf∘[H2O(l)]=−285.8 kJ/mol\Delta H^\circ_f[\text{H}_2\text{O}(\text{l})] = -285.8\text{ kJ/mol}, what is the standard enthalpy change for the combustion reaction CH4(g)+2O2(g)→CO2(g)+2H2O(l)\text{CH}_4(\text{g}) + 2\text{O}_2(\text{g}) \rightarrow \text{CO}_2(\text{g}) + 2\text{H}_2\text{O}(\text{l})?

    • A) +890.3 kJ+890.3\text{ kJ}
    • B) −604.5 kJ-604.5\text{ kJ}
    • C) −1039.9 kJ-1039.9\text{ kJ}
    • D) −890.3 kJ-890.3\text{ kJ}
  4. 4.

    Which of the following substances has a standard enthalpy of formation (ΔHf∘\Delta H^\circ_f) equal to zero at 298 K and 1 bar?

    • A) O3(g)\text{O}_3(g)
    • B) C(diamond)\text{C}(\text{diamond})
    • C) H2O(l)\text{H}_2\text{O}(l)
    • D) Br2(l)\text{Br}_2(l)
  5. 5.

    Which of the following thermochemical equations correctly represents a reaction where the standard enthalpy change of reaction, ΔHrxn∘\Delta H^\circ_{\text{rxn}}, is equal to the standard enthalpy of formation, ΔHf∘\Delta H^\circ_f, of the product?

    • A) 12N2(g)+32H2(g)→NH3(g)\frac{1}{2}\text{N}_2\text{(g)} + \frac{3}{2}\text{H}_2\text{(g)} \rightarrow \text{NH}_3\text{(g)}
    • B) N2(g)+3H2(g)→2NH3(g)\text{N}_2\text{(g)} + 3\text{H}_2\text{(g)} \rightarrow 2\text{NH}_3\text{(g)}
    • C) CO(g)+12O2(g)→CO2(g)\text{CO}\text{(g)} + \frac{1}{2}\text{O}_2\text{(g)} \rightarrow \text{CO}_2\text{(g)}
    • D) C(s, diamond)+O2(g)→CO2(g)\text{C}\text{(s, diamond)} + \text{O}_2\text{(g)} \rightarrow \text{CO}_2\text{(g)}

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