Topic 16: Redox & Equilibria

Standard Electrode Potential definiton

The EMF of a half-cell compared with a standard hydrogen half cell, measured under standard conditions.

Standard Conditions:

• 298 K temperature

• 100 kPa pressure

  • higher pressure causes a more negative emf cause e- are produced

• 1.00 mol dm⁻³ ion concentration

  • higher concentration causes a more positive emf cause fewer e- are produced

Absolute Potential Difference definition

the potential difference between a metal and a solution of its ions

Standard Electrode Potential Values

more positive Eº value = more likely to get reduced equilibrium is to the right

The Standard Hydrogen Electrode (SHE)

• Platinum (unreactive) electrode coated with platinum black (inert, conducts electrons)

• platinum foil on the bottom, covered in porous platinum (for a large surface area)

• equilibrium between H+ ions and H gas

how to Measure the Standard Electrode Potentials

by conducting the standard hydrogen electrode (negative pole) , metal electrode and solution; via a circuit containing a high resistance voltmeter and a salt bridge.

Writing Cell Diagrams

Anode (oxidation) || Cathode (reduction).

Reaction Feasibility

• thermodynamically feasible if Eº cell > 0

• a + Eº cell means the overall reaction is energetically favorable

• but in practice, the value has to be more than 0.4

Electrochemical Series

more + Eº are stronger oxidizing agents

Relationship to Entropy & ln k

Eº cell ∝ΔSº

Eº cell ∝ln k
ΔG=-RT ln k

Acidic Hydrogen-Oxygen Fuel Cells Electrolyte:

• anode: H2 → 2H+ + 2e-

• cathode: O2 + 4H+ + 4e- → 4OH-

Alkaline Hydrogen-Oxygen Fuel Cells Electrolyte:

• anode: 2H2 + 4OH- → 4H2O + 4e-

• cathode: O2 + 2H2O + 4e- → 4OH-

advantages of Hydrogen-Oxygen Fuel Cells

• no harmful products

• efficient

• rechargeable

disadvantages of Hydrogen-Oxygen Fuel Cells

• hight costs

• Hydrogen manufactured from fossil fuels

• don’t work well in low temperatures

Potassium Manganate (VII) Titrations Equation

MnO4−​ + 8H+ + 5Fe2+ →. Mn2+ + 5Fe3+ +. 4H2​O

reagent, condition and color change of KMn reaction

Mn (VII) gets reduced to 2+

Fe gets oxidized to 3+

purple → colorless

Iodine-Thiosulfate Titration Equation

2S2O3(aq) 2- + I2 (aq) → 2I- (aq) + S4O6 (aq) 2-