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9.9Thermodynamics and Electrochemistry

Cell Potential and Free Energy — AP CHEMISTRY Revision Notes & Topical Questions

5 min read 8 parts Lesson 13 of 15 1 quiz

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Lesson summary

9.9 Cell Potential and Free Energy — AP CHEMISTRY notes: Calculate standard cell potential and relate it to ΔG° and favourability.Standard hydrogen electrode — defined as 0.00 VStandard conditions — 1.00 M, 298 K and 100 kPaMeaning — a more positive E° means the species is more e…

  • What the symbols mean
  • Doubling an equation
  • Minimum voltage
  • Practical voltage
  • Water electrolysis
  • Standard electrode potential

Section 1 of 8Standard electrode potential

Standard electrode potential
Standard hydrogen electrode
  • Standard hydrogen electrode defined as 0.00 V
Standard conditions
  • 1.00 M, 298 K and 100 kPa
Meaning
  • Meaning a more positive E° means the species is more easily reduced

Section 2 of 8Cell potential

Equation
  • E°cell = E°cathode − E°anode = E°red − E°ox
    What the symbols mean: Ecell = standard cell potential (V) · Eright = electrode potential of the positive (reduction) half-cell · Eleft = potential of the negative half-cell (V)
Pattern
  • Cu2+/Cu (+0.34 V) cathode and Zn2+/Zn (−0.76 V) anode give +1.10 V
Doubling an equation
  • E° does not change

Section 3 of 8Favourability

E°cellΔG°Result
PositiveNegativeFavoured (voltaic cell)
NegativePositiveNot favoured; needs applied voltage
ZeroZeroEquilibrium

Section 4 of 8ΔG° from E°

Equation
  • ΔG° = −nFE°cell
F
96 485 C mol-1
Pattern
  • 2Fe3+ + Cu → 2Fe2+ + Cu2+: E° = 0.77 − 0.34 = +0.43 V and ΔG° = −2(96 485)(0.43) = −83 kJ

Section 5 of 8Electrolysis and applied voltage

Minimum voltage
  • Minimum voltage greater than |E°cell|
Practical voltage
  • Practical voltage higher because of overpotential and resistance
Water electrolysis
  • E°cell = −1.23 V, so 1.0 V is not enough

Section 6 of 8More from the specification

1Voltaic cells and spontaneous potential
  • Voltaic cells and spontaneous potential spontaneous redox reactions within voltaic cells drive electric current through external circuits connected between electrolytic half-cells.
2Electrolyte solution ion mobility
  • Electrolyte solution ion mobility aqueous salt solution maintains electrical neutrality within electrochemical cells by conducting migrating ions across porous salt bridges.
3Charge
  • Q = It, measured in coulombs.
    What the symbols mean: Q = charge (C) · I = current (A) · t = time (s)
4Nernst equation
  • Relates cell potential to standard potential and Q.
5Electrolytic cell
  • A cell using electricity to drive a non-spontaneous reaction.
6Reaction quotient
  • The ratio of product to reactant concentrations at any moment.
7Electrochemical cell
  • A device linking redox reactions and electricity.
8Faraday constant
  • The charge of one mole of electrons, 96 500 C mol-1.
9Oxidation
  • Gain of oxygen, loss of electrons or an increase in oxidation number.
10Concentration
  • Moles ÷ volume in dm³, or g/dm³.

Section 7 of 8Key terms

TermMeaning
Standard electrode potentialPotential of a half-cell relative to the standard hydrogen electrode.
E°cellDifference between cathode and anode potentials.
Faraday constant96 485 C mol-1.
Feasible reactionOne with positive E°cell.

Section 8 of 8Exam technique

Avoid these mistakes
Subtracting in the wrong order — cathode minus anode.
Using E° that has been multiplied by the coefficients.
Using ΔG° = −nFE with E in mV.

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