ChemSphere

Unit 9 · 7–9% of the exam

Applications of Thermodynamics

Unit 9 asks whether a process is thermodynamically favorable, not just how much heat it releases. Entropy and Gibbs free energy connect to the equilibrium constant, and the same ideas power electrochemistry: galvanic cells that make electricity and electrolytic cells that use it.

Galvanic cell builder

Pick two half-cells, read E°cell, ΔG° and K, and watch electrons flow through the wire.

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Topics in this unit

  1. 9.1

    Introduction to Entropy

    Entropy measures how dispersed matter and energy are; it increases from solid to liquid to gas, with more moles of gas, and with higher temperature.

  2. 9.2

    Absolute Entropy and Entropy Change

    ΔS°rxn = ΣS°(products) − ΣS°(reactants), using tabulated absolute entropies.

  3. 9.3

    Gibbs Free Energy and Thermodynamic Favorability

    ΔG° = ΔH° − TΔS°; a process is thermodynamically favorable when ΔG° < 0, and the signs of ΔH and ΔS decide how temperature affects that.

  4. 9.4

    Thermodynamic and Kinetic Control

    A thermodynamically favorable reaction may not happen at an observable rate if its activation energy is high (kinetic control).

  5. 9.5

    Free Energy and Equilibrium

    ΔG° = −RT ln K, so ΔG° < 0 corresponds to K > 1 and ΔG° > 0 to K < 1.

  6. 9.6

    Coupled Reactions

    An unfavorable reaction can be driven by coupling it to a favorable one, so the sum has ΔG < 0 (e.g., ATP hydrolysis in cells).

  7. 9.7

    Galvanic (Voltaic) and Electrolytic Cells

    Galvanic cells use a favorable redox reaction to produce electrical energy; electrolytic cells use external energy to drive an unfavorable one. Oxidation always happens at the anode.

  8. 9.8

    Cell Potential and Free Energy

    E°cell = E°(cathode) − E°(anode), and ΔG° = −nFE°, so a positive E°cell means a thermodynamically favorable reaction.

  9. 9.9

    Cell Potential Under Nonstandard Conditions

    When Q < 1, Ecell is greater than E°cell; when Q > 1, Ecell is smaller. Ecell reaches zero when the cell is at equilibrium (the Nernst equation describes this).

  10. 9.10

    Electrolysis and Faraday's Law

    Charge q = It, and moles of electrons = q/F, which links current and time to the amount of substance deposited or produced.

Must know

  • ΔS°rxn = ΣS°(products) − ΣS°(reactants)
  • ΔG° = ΔH° − TΔS° (convert ΔS° from J/K to kJ/K)
  • ΔG° = −RT ln K (R = 8.314 J·mol−1·K−1)
  • ΔG° = −nFE°, with F = 96,485 C/mol e−
  • E°cell = E°(cathode) − E°(anode); do not multiply E° by coefficients
  • Nernst equation: E = E° − (RT/nF) ln Q
  • q = It (coulombs = amps × seconds); mol e− = q/F
  • Anode = oxidation, cathode = reduction ('An Ox, Red Cat'); electrons flow anode → cathode through the wire

Common mix-ups

  • A thermodynamically favorable reaction happens quickly.

    ΔG says nothing about rate. Diamond turning into graphite is favorable but immeasurably slow because the activation energy is huge.

  • Doubling the coefficients in a half-reaction doubles its E°.

    E° is an intensive property and stays the same; ΔG° doubles because n doubles.

  • Exothermic reactions are always favorable.

    If ΔS < 0, an exothermic reaction becomes unfavorable at high enough temperature, because the −TΔS term outweighs ΔH.

Practice questions

AP-style multiple choice. Every answer comes with the reasoning, including why the tempting wrong answer is wrong.

1 / 6

Which process has the largest positive entropy change?

Free-response practice

A galvanic cell is built from a Cu electrode in 1.0 M Cu2+ and an Ag electrode in 1.0 M Ag+. Standard reduction potentials: Ag+ + e− → Ag, E° = +0.80 V; Cu2+ + 2e− → Cu, E° = +0.34 V. (a) Write the balanced overall reaction and calculate E°cell. (b) Calculate ΔG° for the reaction. (c) Identify the anode and the direction of electron flow. (d) If [Ag+] is decreased to 0.10 M, will Ecell increase, decrease or stay the same? Justify.

Flashcards

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See it in real life

Everyday reactions that use Unit 9 ideas, animated atom by atom.