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Thermochemical Equations quiz #1

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  • For the freezing of water: H2O(l) → H2O(s), what are the signs of ΔH and ΔS?
    ΔH is negative (exothermic, heat released), and ΔS is negative (entropy decreases as liquid becomes solid).
  • Which reaction produces the largest amount of heat per mole of hydrogen?
    The reaction with the most negative ΔHrxn value per mole of H2 produces the largest amount of heat per mole of hydrogen.
  • What is required for a correctly written thermochemical equation?
    A correctly written thermochemical equation must be balanced and include the physical states of all reactants and products, along with the enthalpy change (ΔHrxn) for the reaction.
  • For which phase changes would q > 0?
    q > 0 (heat absorbed) for melting (fusion), vaporization, and sublimation.
  • What does a negative ΔHrxn of formation of carbon dioxide indicate about the reaction?
    The formation of CO2 from its elements is exothermic; heat is released during the reaction.
  • What is the enthalpy of vaporization if k = 3.2 × 10⁻³ at 0°C and 8.1 × 10⁻³ at 50°C?
    To find the enthalpy of vaporization, use the Clausius-Clapeyron equation relating vapor pressure (k) and temperature. ΔHvap can be calculated from the slope of ln(k) vs. 1/T.
  • What is the difference between the heat (q) of a reaction and the reaction enthalpy (ΔHrxn)?
    q is the heat exchanged under specific conditions, while ΔHrxn is the enthalpy change at constant pressure, representing the heat of reaction under standard conditions.
  • What is the change in enthalpy when gas is completely reacted according to the following reaction?
    The change in enthalpy is equal to the ΔHrxn value given for the complete reaction of the specified amount of gas, based on the balanced thermochemical equation.
  • Is the dissolution reaction of urea enthalpically favored?
    If ΔH for the dissolution of urea is negative, it is enthalpically favored; if positive, it is not. Typically, urea dissolution is endothermic (ΔH > 0), so it is not enthalpically favored.
  • What are the characteristics of a correctly written thermochemical equation?
    A correctly written thermochemical equation is balanced, includes physical states, and lists ΔHrxn. Example: 2H2(g) + O2(g) → 2H2O(l); ΔHrxn = -571.6 kJ.
  • What is the enthalpy of the overall chemical equation NO(g) + O(g) → NO2(g)?
    The enthalpy of the reaction is the ΔHrxn value associated with the formation of NO2 from NO and O, as given in the thermochemical equation.
  • Which equation is a correctly written thermochemical equation?
    A correctly written thermochemical equation is balanced, includes physical states, and provides ΔHrxn. Example: C(s) + O2(g) → CO2(g); ΔHrxn = -393.5 kJ.
  • Choose the thermochemical equation that illustrates ΔHf° for Li2SO4.
    The equation: 2Li(s) + S(s) + 2O2(g) → Li2SO4(s); ΔHrxn = ΔHf°(Li2SO4) illustrates the standard enthalpy of formation for Li2SO4.
  • What does the enthalpy of reaction (ΔHrxn) measure in a thermochemical equation?
    The enthalpy of reaction (ΔHrxn) measures the amount of heat absorbed or released during a chemical reaction, providing a quantitative value for the energy change associated with the reaction.