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9.8Thermodynamics and Electrochemistry

Galvanic (Voltaic) and Electrolytic Cells — AP CHEMISTRY Revision Notes & Topical Questions

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9.8 Galvanic (Voltaic) and Electrolytic Cells — AP CHEMISTRY notes: Compare voltaic and electrolytic cells and describe their parts and reactions.Oxidation — occurs at the anode in every cell typeReduction — occurs at the cathode in every cell typeMemory aid — RED CAT, AN OXFeatu…

  • Half-cells
  • Salt bridge
  • Daniell cell
  • Single line
  • Double line
  • Arrangement

Section 1 of 8Redox in cells

Oxidation
  • Oxidation occurs at the anode in every cell type
Reduction
  • Reduction occurs at the cathode in every cell type
Memory aid
  • RED CAT, AN OX

Section 2 of 8Comparison

FeatureVoltaicElectrolytic
ReactionSpontaneous (E° > 0)Non-spontaneous (needs applied voltage)
Anode signNegativePositive
Cathode signPositiveNegative
EnergyProduces electrical energyConsumes electrical energy

Section 3 of 8Voltaic cell

Half-cells
  • Half-cells each has an electrode in a solution of its ions, separated so electrons flow through the external wire
Salt bridge
  • Salt bridge allows ions to flow and completes the circuit
EMF
  • EMF the potential difference between the electrodes, read on a high-resistance voltmeter
Daniell cell
  • Zn | Zn2+ || Cu2+ | Cu with E°cell = +1.10 V

Section 4 of 8Cell notation

Single line
  • Single line a phase boundary
Double line
  • Double line salt bridge
Arrangement
  • Arrangement anode (oxidation) on the left and cathode (reduction) on the right

Section 5 of 8Electrolytic cell: molten salt

Ion movement
  • Ion movement cations move to the cathode and anions to the anode
Cathode
  • Pb2+ + 2e− → Pb
Anode
  • 2Br− → Br2 + 2e−
Rule
  • Rule metals form at the cathode and non-metals at the anode

Section 6 of 8More from the specification

1Charge
  • Q = It, measured in coulombs.
    What the symbols mean: Q = charge (C) · I = current (A) · t = time (s)
2Nernst equation
  • Relates cell potential to standard potential and Q.
3Reaction quotient
  • The ratio of product to reactant concentrations at any moment.
4Electrochemical cell
  • A device linking redox reactions and electricity.
5Standard conditions
  • 298 K and 101 kPa.
6Faraday constant
  • The charge of one mole of electrons, 96 500 C mol-1.
7Equilibrium
  • The state where forward and reverse transfer rates are equal.
8Concentration
  • Moles ÷ volume in dm³, or g/dm³.

Section 7 of 8Key terms

TermMeaning
AnodeElectrode where oxidation occurs.
CathodeElectrode where reduction occurs.
Salt bridgeConnection completing the circuit with mobile ions.
EMFElectromotive force; cell potential.
ElectrolyteLiquid containing ions that conducts electricity.

Section 8 of 8Exam technique

Avoid these mistakes
Thinking oxidation occurs at the cathode in electrolytic cells.
Thinking electrons flow through the salt bridge.
Assuming the anode is always negative.

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