Electrode Potentials and Oxidizing Reactions

Standard Electrode Potential Trends

• The general trend of less negative E values is related to the increasing sum of first and second ionization enthalpies.
• Manganese, nickel, and zinc have more negative E values than expected, deviating from the trend.
• Chromium(II) is reducing because its configuration changes from d⁴ to d³, achieving a half-filled t₂g level.
• Manganese(III) is oxidizing because its configuration changes from d⁴ to d⁵, resulting in a half-filled d shell, which is more stable.

Thermochemical Data and Electrode Potentials

• The table shows relationships between thermodynamic properties and E values.
• A higher negative enthalpy of hydration (∆hydH) for nickel contributes to its negative E value.
• Stability of the half-filled d⁵ subshell in Mn and the completely filled d¹⁰ configuration in Zn contribute to their respective E values.

Important Oxidising Reactions of KMnO₄ in Acid Solutions

• Potassium permanganate (KMnO₄) is a strong oxidant capable of oxidizing several substances in acidic solutions.
• KMnO₄ oxidizes iodide to iodine, Fe²⁺ to Fe³⁺, oxalate ion or oxalic acid to carbon dioxide, hydrogen sulfide to sulfur, sulfurous acid or sulphite to sulfate or sulfuric acid, and nitrite to nitrate.

Important Oxidising Reactions of KMnO₄ in Neutral or Faintly Alkaline Solutions

• KMnO₄ oxidizes iodide to iodate, thiosulphate to sulphate, and manganous salt to MnO₂ in neutral or faintly alkaline solutions.

Uses of KMnO₄

• Potassium permanganate (KMnO₄) is used in analytical chemistry, preparative organic chemistry, as a bleach for textiles, and for decolorization in oils.

Inner Transition Elements: Lanthanoids and Actinoids

• Lanthanoids (Ln) and Actinoids are the two series of f-block elements, following Lanthanum and Actinium, respectively.
• Lanthanoids are more similar to each other than other transition metals due to their similar oxidation states.
• Actinoid chemistry is more complex than that of lanthanoids.

Characteristics of Lanthanoids

• Hardness increases with atomic number, with samarium being steel hard.
• Melting points range between 1000 to 1200 K, except for samarium which melts at 1623 K.
• Lanthanoids have metallic structures, are good conductors of heat and electricity, and exhibit smooth changes in density and properties, except for Eu and Yb.
• Trivalent lanthanoid ions are often colored, attributed to the presence of f electrons.
• Lanthanoid ions are paramagnetic, except for f⁰ (La and Ce) and f¹⁴ (Yb and Lu) type ions.

Electronic Configuration and Ionization Enthalpies of Lanthanoids

• First ionization enthalpies of lanthanoids are around 600 kJ mol⁻¹, and the second is about 1200 kJ mol⁻¹, comparable to calcium.
• The third ionization enthalpy shows variation due to the exchange enthalpy considerations, leading to stability for empty, half-filled and completely filled f orbitals.
• This is indicated by abnormally low third ionization enthalpy values for lanthanum, gadolinium, and lutetium.

Chemical Reactivity of Lanthanoids

• The earlier members of the lanthanoid series are highly reactive, similar to calcium, but reactivity decreases as atomic number increases.
• With increasing atomic number, lanthanoids exhibit more similarities to aluminum.