Electrochemistry

ELECTROCHEMISTRY NOTES

INTRODUCTION TO ELECTROCHEMICAL CELLS

• Galvanic (Voltaic) Cells:

  • Thermodynamically favorable chemical reactions.

  • Generates useful electrical energy.

  • Often referred to as batteries.

• Electrolytic Cells:

  • Driven by thermodynamically unfavorable reactions that require an input of electrical energy.

MNEMONIC DEVICES FOR GALVANIC CELLS

• Mnemonics to remember key concepts:

  • AN OX RED CAT: Oxidation occurs at Anode, Reduction occurs at Cathode.

  • FAT CAT: Electrons flow from the Anode to the Cathode.

  • CA+HODE: Cathode is positive in galvanic cells.

VOCABULARY FOR GALVANIC CELLS

• Anode:

  • Site of oxidation, mass decreases over time.

  • Example Reaction: $Y <br>ightarrow Y^+ + e^-$

• Cathode:

  • Site of reduction, gains mass over time.

  • Example Reaction: $Z^+ + e^- <br>ightarrow Z$

• Inert Electrodes:

  • Conduct electricity without participating in the reaction (e.g., Platinum (Pt) and Graphite).

• Salt Bridge:

  • Maintains electrical neutrality by preventing charge buildup in a galvanic cell.

  • Usually filled with a salt solution (e.g., KNO3).

• Electron Flow: Always flows from the Anode to the Cathode.

• Voltmeter: Measures the cell potential (emf) in volts.

GALVANIC CELL EXAMPLES

1. Example Reaction:

   • Oxidation Half-Reaction:
     $ext{Zn(s) } <br>ightarrow ext{Zn}^{2+}(aq) + 2e^-$

   • Reduction Half-Reaction:
     $ext{Cu}^{2+}(aq) + 2e^- <br>ightarrow ext{Cu(s)}$

   • Overall Reaction:
     $ext{Zn(s)} + ext{Cu}^{2+}(aq) <br>ightarrow ext{Zn}^{2+}(aq) + ext{Cu(s)}$

   • Cell Notation for Zn/Cu Cell:
     $ext{Zn | Zn(NO}<em>3 ext{)}</em>2 || ext{Cu(NO}<em>3 ext{)}</em>2 | ext{Cu}$

2. Balance Redox Reaction:

   • For reaction: $ext{MnO}_4 + ext{Fe}^{2+} <br>ightarrow ext{Mn}^{2+} + ext{Fe}^{3+}$ (in acidic solution).

   • Determine Oxidizing Agent (OA) and Reducing Agent (RA).

CELL POTENTIAL

• Potential (emf) indicates the ability of the cell to push electrons through a circuit:

  • $E_{ ext{cell}}$ or $ext{Emf}$

  • Measured in Volts (1 V = 1 J/C).

• Standard Reduction Potentials:

  • Hydrogen electrode assigned 0.00 V, similar to C-12 atomic mass standardization.

  • Standard conditions: 1 atm for gases, 1.0 M for solutions, at 25°C.

  • Results in an activity series where:

  • More positive values are easily reduced (usually nonmetals).

  • Less positive values are easily oxidized (usually metals).

GIBB'S FREE ENERGY

• Relates to thermodynamic favorability:

  • $ext{ΔG} = -nF E^{ ext{o}}$

  • Where n = moles of electrons and F = 96,485 J/V·mol.

• Negative $ext{ΔG}$ indicates a thermodynamically favorable reaction.

THE NERNST EQUATION

• Captures effect of concentration on cell potential:

• General Form:

  • $E<em>{ ext{cell}} = E^ ext{o}</em>{ ext{cell}} - rac{RT}{nF} ext{ln}(Q)$

• @25°C Simplifies to

  • $E<em>{ ext{cell}} = E^ ext{o}</em>{ ext{cell}} - 0.0592/n ext{log}(Q)$

• Q is the reaction quotient calculating the ratio of products to reactants.

PREDICTING REACTION FAVORABILITY

• Changes in concentration affect $E_{ ext{cell}}$ following Le Chatelier's principle.

• For example, increasing reactant concentration enhances forward reaction compatibility and will increase $E_{ ext{cell}}$.

ELECTROLYTIC CELLS

• Characteristics:

  • Thermodynamically unfavorable: E_{ ext{cell}} < 0 and ΔG > 0.

  • Requires an external energy input for operation.

  • Electron Flow: Flows from anode (positive) to cathode (negative).

• Electrolysis of Water:

  • $ext{2H}<em>2 ext{O} + 2e^- ightarrow ext{H}</em>2 + 2 ext{OH}^-$ and oxidation: $ext{2H}<em>2 ext{O} ightarrow ext{O}</em>2 + 4H^+ + 4e^-$.

• Applications:

  • Electroplating, extraction of metal from ores, industrial processes, etc.

EXAMPLES OF ELECTROLYTIC PROCESSES

1. Calculate Mass Deposition:

   • Example: If current = 5.00 A and time = 2 hours, calculate grams of deposited Ag.

2. Example Reaction:

   • Electrolysis of CuSO4 producing Cu(s).

FUEL CELLS

• Convert chemical potential energy (energy in molecular bonds) into electrical energy.

• Main fuel: Hydrogen

  • Mechanism involves proton exchange mechanisms and efficiency in water or air cooling.

CONCLUSIONS AND PRACTICE

• Practice problems for understanding concepts concurrently through various electrochemical reactions and predictions of thermodynamic favorability through both galvanic and electrolytic cells.