5: Thermodynamics
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Questions and Answers

Which of the following topics will not be covered in Lecture 6?

  • Electron Configurations
  • Gibbs Free Energy (correct)
  • The Wave Nature of Light
  • Ionization Energy

    • The Second Law of Thermodynamics states that energy can be created or destroyed.

    False

    Name two topics discussed in Lecture 5.

    Entropy and Gibbs Free Energy

    The law that relates internal energy to heat and work is known as the _______.

    <p>First Law of Thermodynamics</p> Signup and view all the answers

    Match the following concepts with their descriptions:

    <p>Internal energy = The total energy contained within a system Enthalpy = The heat content of a system at constant pressure Exothermic = A process that releases heat Endothermic = A process that absorbs heat</p> Signup and view all the answers

    What is the due date for Problem Set 4?

    <p>Tomorrow before Exercise #5</p> Signup and view all the answers

    The Study Center offers help every Thursday from 18:00 to 20:00.

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

    When do Prof. Norris and Brisby hold their office hours?

    <p>Thursdays from 17:00 to 18:00</p> Signup and view all the answers

    What does entropy measure?

    <p>Number of microstates consistent with a macroscopic state</p> Signup and view all the answers

    Entropy decreases with an increase in the number of molecules in a system.

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

    What is the Boltzmann constant?

    <p>1.38 × 10−23 J/K</p> Signup and view all the answers

    The change in entropy, ∆S, is calculated using the formula ∆S = k ln 1 − k ln 2.

    <p>W1, W2</p> Signup and view all the answers

    Match the thermodynamic terms with their descriptions:

    <p>Entropy = Measure of disorder of a system Microstate = Specific detailed configuration of a system Macroscopic state = Large-scale properties of a system Second Law of Thermodynamics = Entropy of the universe tends to increase</p> Signup and view all the answers

    Which of the following statements regarding entropy is accurate?

    <p>Entropy increases with vaporizing of liquids.</p> Signup and view all the answers

    Engineers work to increase the disorder in the universe.

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

    What happens to the entropy of a system when the complexity of molecules increases?

    <p>Entropy increases</p> Signup and view all the answers

    What happens to the degrees of freedom and microstates as the complexity of a molecule increases?

    <p>Both increase</p> Signup and view all the answers

    A pure crystalline solid at absolute zero has infinite microstates.

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

    What does the 3rd Law of Thermodynamics state about a pure crystalline solid at T = 0 K?

    <p>S = 0</p> Signup and view all the answers

    According to the 3rd Law of Thermodynamics, as we cool any system, it has less energy, less degrees of freedom, and less __________.

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

    Match the terms related to chemical reactions with their descriptions:

    <p>∆S° = Change in standard entropy Ri = Reactants in a chemical reaction Pj = Products in a chemical reaction ai = Stoichiometric coefficients for reactants</p> Signup and view all the answers

    What does W represent in the context of the 3rd Law of Thermodynamics?

    <p>Number of microstates</p> Signup and view all the answers

    The formula for determining ∆H for chemical reactions includes only reactants.

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

    What are standard molar entropies represented as in the given content?

    <p>S°</p> Signup and view all the answers

    What is the key factor in determining if a reaction is spontaneous?

    <p>Change in free energy</p> Signup and view all the answers

    A process is considered spontaneous if the total entropy change, Suniv, is less than zero.

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

    What is the standard symbol for Gibbs Free Energy?

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

    A spontaneous process requires that the total change in entropy, ______, is greater than or equal to zero.

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

    Match the concepts with their definitions:

    <p>G = Gibbs Free Energy used to determine spontaneity Suniv = Total change in entropy of the universe ΔG° = Standard free energy change Spontaneous process = A process that occurs without external intervention</p> Signup and view all the answers

    Which of the following represents a spontaneous reaction?

    <p>ΔG &lt; 0</p> Signup and view all the answers

    The standard free energy of formation is always zero.

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

    What is assumed when calculating G for a system?

    <p>Constant temperature and pressure</p> Signup and view all the answers

    What is the value of ∆S° for the reaction N2 (g) + 3 H2 (g) → 2 NH3 (g)?

    <p>−199 J/mol⋅K</p> Signup and view all the answers

    A negative value of ∆S° indicates that the reaction is spontaneous.

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

    What does a positive value for ∆G° indicate about the spontaneity of a reaction?

    <p>Non-spontaneous</p> Signup and view all the answers

    The criterion for spontaneity requires that ∆G° be _____ than 0.

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

    Match the following terms with their definitions:

    <p>∆G° = Gibbs free energy change ∆H° = Enthalpy change ∆S° = Entropy change Spontaneity = Natural tendency of a process to occur</p> Signup and view all the answers

    How does the reaction N2 (g) + 3 H2 (g) → 2 NH3 (g) affect the number of gas species?

    <p>Decreases the number of gas species</p> Signup and view all the answers

    The reaction where ∆G° is calculated as ∆S° + ∆H° indicates spontaneous behavior when ∆G° is negative.

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

    What is the combined change in Gibbs free energy (∆G°) for the reaction N2 (g) + 3 H2 (g) → 2 NH3 (g) if ∆S° is −199 J/mol⋅K and ∆H° is +310 kJ/mol⋅K?

    <p>+111 J/mol⋅K</p> Signup and view all the answers

    What does a negative $ΔG$ value indicate about a reaction?

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

    A process with $ΔG = 0$ denotes that the process is irreversible.

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

    What are the standard conditions for measuring free energy change?

    <p>1 atm and 25 °C</p> Signup and view all the answers

    The maximum amount of work that can be extracted from a process is given by $ΔG = ___$.

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

    Which condition leads to a reaction being classified as reversible?

    <p>$ΔG = 0$</p> Signup and view all the answers

    All reactions with positive enthalpy change ($ΔH$) will have positive free energy change ($ΔG$).

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

    How can we use $ΔG°$ values in chemical reactions?

    <p>To determine the spontaneity of reactions</p> Signup and view all the answers

    Study Notes

    Lecture 5 Announcements

    • Lecture topics include spontaneous processes, the second law of thermodynamics, the molecular interpretation of entropy and the third law, entropy changes in chemical reactions, Gibbs free energy, and free energy and temperature.
    • Problem Set 4 is due tomorrow before exercise 5 and uploaded on Moodle.
    • Problem Set 5 is posted on Moodle; due before exercise 6 next week.
    • Study center hours are Wednesdays, 6:00 PM - 8:00 PM in ETA F 5.
    • Professor office hours are Thursdays, 5:00 PM - 6:00 PM in LEE P 210.

    Lecture 6 Next Week

    • Lecture topics will be the wave nature of light, quantized energy and photons, line spectra and the Bohr model, the wave behavior of matter, quantum mechanics and atomic orbitals, representation of orbitals, multi-electron atoms, electron configurations, electron configurations and the periodic table, effective nuclear charge, sizes of atoms and ions, ionization energy, and electron affinity.

    Review

    • Lecture 4 covered the first law of thermodynamics.
    • The first law of thermodynamics includes the system and surroundings, internal energy (E), state functions (P, V, E), energy diagrams, endothermic and exothermic processes, enthalpy (H = E + PV), pressure-volume work, calorimetry, heat capacity, Hess’s law, heats of reaction, and heats of formation.

    Heat and Work: Sign Convention

    • Heat gained (q > 0), heat lost (q < 0).
    • Work done on system (w > 0), work done by system (w < 0).
    • Energy deposited into system (ΔE > 0)
    • Energy withdrawn from system (ΔE < 0).

    Enthalpy Diagram for Propane Combustion

    • Diagram shows the enthalpy change for the combustion of propane.
    • The reaction involves decomposition into elements, formation of 3 CO2, and formation of 4 H2O.

    Thermodynamics II

    • A spontaneous process occurs without any external assistance.
    • Spontaneous processes seem to favor lowering their energy.
    • At constant P, a process is spontaneous if ΔH < 0 (exothermic).
    • Many spontaneous processes have ΔH > 0 (endothermic).
    • Ice melting is an example of a spontaneous endothermic process.

    Spontaneous versus Nonspontaneous Processes

    • If a process is spontaneous, its reverse process is nonspontaneous.
    • The experimental conditions, like temperature (T) and pressure (P), influence spontaneity.
    • Spontaneity does not guarantee speed.

    Reversible versus Irreversible Processes

    • In a reversible process, no change occurs in surroundings when the process is reversed.
    • In an irreversible process, the surroundings change when the process is reversed
    • Hot objects transfer heat to cold objects is a spontaneous process
    • Cold-to-hot heat transfer is nonspontaneous

    Example: Heat Transfer

    • Heat flows spontaneously from hot system to cold surroundings.
    • For a reversible heat transfer, the system and surroundings' temperatures must differ infinitesimally.
    • To reverse a heat flow, the difference in temperatures must change from positive to negative infinitesimally.

    Another Example: Isothermal Expansion

    • Expansion of ideal gas into a vacuum at constant temperature.
    • To reverse the process, the surroundings must do work on the gas to compress it.

    Real Processes

    • All real processes are irreversible.
    • All spontaneous processes are real processes.
    • All spontaneous processes are irreversible.
    • To return a system to its initial state after a spontaneous process, work is required which will lead to an increase of energy in system.
    • During a spontaneous process, energy spreads out and becomes less useful for work.

    Entropy, S

    • Entropy (S) is a measure of the tendency for energy to spread which reduces a system's ability to do work
    • Entropy is related to randomness or disorder in a system.
    • Entropy is a state function
    • For an isothermal process, ΔS = qrev/T, where qrev is the heat flow for a reversible process and T is absolute temperature.

    ΔS for Phase Changes

    • Melting and boiling are isothermal processes.
    • For ice melting, the change in entropy of the system (ΔSsys) is calculated.

    Conservation of S

    • When an ice cube melts, the total entropy of the universe increases.

    2nd Law of Thermodynamics

    • For a reversible process, ΔSuniv = 0.
    • For an irreversible process, ΔSuniv > 0.
    • The entropy of the universe increases during any spontaneous process.

    Entropy: Molecular Theory of Gases

    • Molecules move with a statistical distribution.
    • Statistical thermodynamics connects macroscopic states to microscopic arrangements of molecules.
    • A microstate is a particular arrangement of molecules.

    Boltzmann's Equation

    • S = kB ln W.
    • W = Number of microstates consistent with a specific macroscopic state
    • kB = Boltzmann constant.
    • Entropy measures the number of microstates consistent with the macroscopic state of a system.
    • The entropy of a system increases with the number of possible microstates.

    Implications

    • Entropy (S) and the number of possible molecular arrangements (W) increase with increases in volume(V), temperature(T), and the number of molecules present.
    • Spontaneity occurs with changes like melting of solids and vaporization of liquids

    Engineers: Entropy Warriors

    • Engineers combat the 2nd Law of Thermodynamics by creating highly ordered materials.

    3rd Law of Thermodynamics

    • As a system cools, energy, degrees of freedom, and the number of microstates decrease.
    • A pure, perfect crystalline solid substance at absolute zero (0 K) has zero entropy (S = 0).

    Determining ΔS for Chemical Reactions

    • ΔS for a chemical reaction is calculated using standard molar entropies (S°) for substances in their standard states.

    Determining ΔS for Chemical Reactions?

    • For the reaction N2(g) + 3H2(g) --> 2NH3(g), the value of ΔS is negative.

    What Does This Mean for Spontaneity of a Reaction?

    • To determine reaction spontaneity, analyses of both the system and surroundings are needed, because the entropy of the universe change is determined by two factors, the change in entropy of the reaction system and the change in entropy of the surroundings.

    Gibbs Free Energy

    • The Gibbs Free Energy (G) is related to enthalpy (H) and entropy (S) by the equation G = H − TS where T is temperature.
    • The change in Gibbs free energy (ΔG) for a process is a criterion for spontaneity.
    • A process is spontaneous if ΔG < 0; , process is non-spontaneous if ΔG > 0; and process is at equilibrium if ΔG = 0.

    Standard Free Energy of Formation

    • The standard free energy of formation (ΔGf°) is the change in free energy during the formation of one mole of a substance from its elements under standard conditions (1 atm, 25 °C).

    Summary of ΔHrxn, ΔS°, and ΔGrxn

    • Formulas to calculate enthalpy(ΔHorxn), entropy (ΔSo), and Gibbs free energy (ΔGorxn) for chemical reactions given standard molar enthalpy (ΔHoi) , standard molar entropy (Soi), and standard Gibbs free energy of formation (ΔGoi) from tabulated values

    Importance of Temperature

    • The spontaneity criteria ΔG < 0 depends on temperature.
    • The spontaneity of a reaction can change depending on temperature based on factors like enthalpy and entropy changes.

    What We Learned

    • Summary of key topics from the lecture, Spontaneous/ Nonspontaneous processes, Reversible/Irreversible processes, Isothermal Processes, Entropy, 2nd Law of Thermodynamics, Boltzmann's equation, microstates, 3rd Law of Thermodynamics, Gibbs free energy, and standard free energies.

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    Description

    Test your knowledge on the concepts covered in Lecture 5 of Thermodynamics. This quiz includes questions about entropy, thermodynamic laws, and key definitions. Dive deeper into the principles that govern energy and its transformations.

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