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Credit Sprint / Study guides / Chemistry Unit 9

Credit Sprint's Guide to Chemistry Unit 9: Applications of Thermodynamics

AP® weight 7–9%CLEP® weight 2–4%

Entropy and free energy tell you whether a process is spontaneous and how far it goes. Electrochemistry connects free energy to cell potential and lets you predict batteries and electrolysis.

Galvanic cell e⁻ flow → salt bridge Anode (Zn)oxidationCathode (Cu)reduction Zn²⁺Cu²⁺

What you need to know

  • Entropy S measures dispersal of matter and energy: gas > liquid > solid, more moles of gas, dissolving and mixing raise it.
  • Second law: a spontaneous process increases the entropy of the universe.
  • Gibbs: ΔG = ΔH − TΔS. Spontaneous when ΔG < 0. (−,+) always spontaneous; (+,−) never; (−,−) at low T; (+,+) at high T.
  • ΔG° = −RT ln K. K > 1 means ΔG° < 0. ΔG = ΔG° + RT ln Q; at equilibrium ΔG = 0.
  • Thermodynamically favorable doesn't mean fast.
  • Galvanic cells: oxidation at the anode, reduction at the cathode; electrons flow anode → cathode; the salt bridge balances charge.
  • E°cell = E°cathode − E°anode. Positive E°cell means spontaneous. ΔG° = −nFE°.
  • Nernst: E = E° − (0.0592/n) log Q at 25 °C. A cell at equilibrium has E = 0.
  • Electrolysis forces a nonspontaneous reaction: moles of electrons = I·t ÷ 96,485.
  • Concentration cells have E° = 0; the dilute half-cell is the anode.

Equations and rules

ΔG = ΔH − TΔS
ΔG° = −RT ln K
ΔG° = −nFE°
E = E° − (0.0592/n) log Q
moles e⁻ = It/F, F = 96,485 C/mol

Exam traps

Watch forConvert ΔS from J/K to kJ/K before using ΔG = ΔH − TΔS.
Watch forDon't multiply E° by the coefficients when balancing half-reactions.
Watch forIn every cell, oxidation is at the anode and reduction is at the cathode.
Watch forA dead battery has Q = K and E = 0.

Key terms

  • Entropy (S): A measure of the dispersal of matter and energy; gases have the most.
  • Gibbs free energy: ΔG = ΔH − TΔS; negative means spontaneous.
  • Spontaneous process: One that occurs without continuous outside input; says nothing about speed.
  • ΔG° and K: ΔG° = −RT ln K.
  • Galvanic cell: A cell that uses a spontaneous redox reaction to produce electricity.
  • Anode: The electrode where oxidation occurs.
  • Cathode: The electrode where reduction occurs.
  • Standard cell potential: E°cell = E°cathode − E°anode.
  • Nernst equation: E = E° − (0.0592/n) log Q at 25 °C.
  • Faraday constant: 96,485 C per mole of electrons.
  • Electrolysis: Using electrical energy to drive a nonspontaneous redox reaction.
  • Sacrificial anode: A more active metal that corrodes in place of a protected metal.

Practice questions

Five of the 51 Unit 9 questions in Credit Sprint. Every choice is explained in the app.

1. For which process is ΔS most likely positive?
  1. H₂O(l) → H₂O(g)
  2. H₂O(g) → H₂O(l)
  3. N₂(g) + 3 H₂(g) → 2 NH₃(g)
  4. CO₂(g) → CO₂(s)
Show answer

H₂O(l) → H₂O(g). A gas has far more disorder than a liquid.

2. For a reaction, ΔH = −92 kJ and ΔS = −199 J/K. What is ΔG at 298 K?
  1. −32.7 kJ
  2. +32.7 kJ
  3. −151 kJ
  4. −92 kJ
Show answer

−32.7 kJ. ΔG = −92 − (298)(−0.199) = −92 + 59.3 = −32.7 kJ.

3. For the vaporization of a liquid, ΔH = +40.7 kJ/mol and ΔS = +109 J/mol·K. What is the normal boiling point?
  1. 4.40 K
  2. 273 K
  3. 298 K
  4. 373 K
Show answer

373 K. At equilibrium ΔG = 0, so T = ΔH/ΔS = 40,700 ÷ 109.

4. A reaction has K = 1.0 × 10⁵ at 298 K. Which is the value of ΔG° (J/mol)?
  1. 0
  2. −2480
  3. −28,500
  4. +28,500
Show answer

−28,500. ΔG° = −(8.314)(298) ln(1.0 × 10⁵) = −28,500 J/mol.

5. In a galvanic cell, electrons flow
  1. through the salt bridge
  2. from the anode to the cathode through the external wire
  3. from the cathode to the anode
  4. from the cathode through the solution
Show answer

from the anode to the cathode through the external wire. Electrons leave the oxidation site.