Stoichiometry of Thermochemical Equations
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Questions and Answers

What is the significance of the sign of ΔHrxn in a thermochemical equation?

  • It indicates the reactants of the reaction.
  • It indicates the catalyst used in the reaction.
  • It indicates whether the reaction is exothermic or endothermic. (correct)
  • It indicates the stoichiometry of the reaction.

    • What is the relationship between the magnitude of ΔH and the amount of substance?

  • The magnitude of ΔH is inversely proportional to the amount of substance.
  • The magnitude of ΔH is proportional to the square of the amount of substance.
  • The magnitude of ΔH is independent of the amount of substance.
  • The magnitude of ΔH is proportional to the amount of substance. (correct)

    • What is the purpose of a thermochemical equation?

  • To include ΔHrxn and the phase states of the chemicals participating in the reaction. (correct)
  • To calculate the entropy of the reaction.
  • To balance the chemical equation.
  • To determine the activation energy of the reaction.

    • How do the coefficients of the balanced equation affect the value of ΔHrxn?

    <p>The coefficients of the balanced equation do not affect the value of ΔHrxn.</p> Signup and view all the answers

    What is the unit of ΔHrxn in a tabulated data?

    <p>kJ/mol</p> Signup and view all the answers

    How can the value of ΔHrxn be used in a calculation?

    <p>As a mole ratio.</p> Signup and view all the answers

    What is the first step in the production process described in the text?

    <p>The oxidation of ammonia</p> Signup and view all the answers

    What formula is used to calculate the standard enthalpy change of a reaction?

    <p>DHrxn = SmDH°f (products) - SnDH°f (reactants)</p> Signup and view all the answers

    What is the energy needed to overcome the attraction between nuclei and shared electrons called?

    <p>Bond energy</p> Signup and view all the answers

    In a thermochemical equation, what does DH represent?

    <p>Standard enthalpy change</p> Signup and view all the answers

    What do higher bond energies indicate?

    <p>Stronger bonds</p> Signup and view all the answers

    Which law states that the total enthalpy change for a reaction is independent of the intermediate steps?

    <p>Hess's Law</p> Signup and view all the answers

    What is needed to find qsoln in the context of the text?

    <p>Mass of the solution and change in temperature</p> Signup and view all the answers

    When finding qsoln, why is the heat capacity of the calorimeter ignored?

    <p>It has a negligible effect on the final calculations</p> Signup and view all the answers

    Why are volumes not additive in the context of the text?

    <p>Intermolecular forces change during mixing and reaction</p> Signup and view all the answers

    In calorimetry measurements, why is a 0.500 M solution not dilute enough to be likened to pure water?

    <p>It contains NaCl and NaOH which affect properties</p> Signup and view all the answers

    Why is 4.184 J/(g K) only an approximation for heat capacity?

    <p>Intermolecular forces in the solution change during the reaction</p> Signup and view all the answers

    What does the negative of the heat of the reaction (qrxn) represent?

    <p>Heat of the surroundings</p> Signup and view all the answers

    What is the implication of the subscript P in thermodynamic changes?

    <p>It indicates a constant pressure transformation</p> Signup and view all the answers

    What is the purpose of calorimetry measurements?

    <p>To measure the thermal effects of a reaction at constant pressure</p> Signup and view all the answers

    Why is it necessary to convert the unit of qrxn from kJ to Cal?

    <p>Because Cal is a more common unit of energy in chemistry</p> Signup and view all the answers

    What is the significance of the value of ΔT in the calculation of qrxn?

    <p>It represents the temperature change of the reaction</p> Signup and view all the answers

    What is the relationship between qsys and qrxn?

    <p>qsys is the negative of qrxn</p> Signup and view all the answers

    What is the implication of a negative qrxn value?

    <p>The reaction is exothermic</p> Signup and view all the answers

    What is the primary role of the radiator in the context of the text?

    <p>To release heat to the surroundings</p> Signup and view all the answers

    In the given problem, why is work denoted as negative when the gases push the pistons outward?

    <p>Because the system loses energy to the surroundings</p> Signup and view all the answers

    Which term represents the change in energy for the system in the given scenario?

    <p>ΔE</p> Signup and view all the answers

    What is the purpose of defining system and surroundings in thermodynamics problems?

    <p>To choose signs for 'q' and 'w'</p> Signup and view all the answers

    In a pressure-volume work scenario, what does a positive value for work indicate?

    <p>The surroundings are doing work on the system</p> Signup and view all the answers

    What is the significance of negative values for both 'q' and 'w' in the given problem?

    <p>Both heat and work are being lost by the system</p> Signup and view all the answers

    What is the purpose of writing balanced formation equations in chemistry?

    <p>To calculate the enthalpy change of reactions</p> Signup and view all the answers

    In the context of formation equations, why are all substances written in their standard states?

    <p>To allow comparison of enthalpy values reliably</p> Signup and view all the answers

    What is the significance of ΔHf° in the context of chemical reactions?

    <p>It quantifies the standard enthalpy change of formation</p> Signup and view all the answers

    Why is it important to determine ΔH°rxn from ΔH°f values in chemistry?

    <p>To evaluate the enthalpy change for a specific reaction</p> Signup and view all the answers

    What role does the decomposition of reactants to elements play in determining ΔH°rxn?

    <p>It serves as the reverse of the formation reactions of reactants</p> Signup and view all the answers

    How does the re-combination of elements to form products relate to the formation reactions of products?

    <p>It mirrors the formation reactions of products</p> Signup and view all the answers

    Study Notes

    Bond Energy and Enthalpy of Reaction

    • The energy needed to overcome the attraction between nuclei and shared electrons is called bond energy.
    • Stronger bonds have higher bond energies.

    Thermochemical Equations

    • A thermochemical equation is a balanced equation that includes ΔHrxn and the phase states of the chemicals participating in the reaction.
    • The sign of ΔH indicates whether the reaction is exothermic (ΔH < 0) or endothermic (ΔH > 0).
    • The magnitude of ΔH is proportional to the amount of substance.

    Calculating ΔHrxn from ΔHf Values

    • ΔHrxn can be calculated using the equation: ΔHrxn = ∑mΔHf(products) - ∑nΔHf(reactants)
    • This equation is derived from the concept that any reaction can be thought of as happening in two steps:
      • The decomposition of reactants to elements (reverse of formation reactions of reactants).
      • The re-combination of elements to form products (formation reactions of products).

    Stoichiometry of Thermochemical Equations

    • The coefficients of the balanced equation need not be the lowest integers.
    • ΔHrxn is proportional to the amount of substance.

    Calorimetry at Constant Pressure

    • Most reactions in chemistry happen in solution, liquid, or solid state.
    • Thermal effects are measured at constant pressure, i.e., in baric equilibrium with the surrounding air.
    • Thermodynamic changes at constant pressure are called isobaric transformations (or isobaric paths).

    Energy Transfer as Heat and Work

    • Energy can be transferred as heat (q) or work (w).
    • Heat is transferred as energy flows from a higher-temperature body to a lower-temperature body.
    • Work is done when a force is applied over a distance.

    Formation Equations and ΔHf°

    • A formation equation is a balanced equation that describes the formation of a compound from its elements.
    • ΔHf° is the enthalpy change for the formation of 1 mole of a compound from its elements.

    Practice Problems

    • Example problems are provided to practice calculating ΔHrxn from ΔHf values, writing formation equations, and determining the heat of a reaction.

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    Ch 6 Thermochemistry: Energy Flow and Chemical Change PDF

    Description

    Learn about stoichiometry in thermochemical equations, which involve balancing equations and understanding the heat of reaction (ΔH). Discover how to determine if a reaction is exothermic or endothermic based on the sign of ΔH.

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