Lecture Notes: Key Chemistry Concepts for Exam Preparation

Introduction

• Video duration: 10-15 minutes.
• Coverage of topics likely to earn 40 marks.
• Topics include:
  • Fajan's rules
  • Spectral lines
  • Quantum numbers
  • Valence electrons calculation
  • Period and group identification
  • Spin magnetic moment
  • Boiling point from Van’t Hoff factor
  • Reaction order from half-life

Fajan's Rules

• Covalent Character: Proportional to polarizing power of cation and polarizability of anion.
  • Polarizing Power: Depends on charge and size of cation.
  • Polarizability: Depends on charge and size of anion.
• Examples:
  • Compare Al+ with different anions (F-, Cl-, Br-, I-).
  • Cation examples: La+, Na+, K+, Rb+, Cs+.
• Conclusion: Larger anion size = higher covalent character; Smaller cation size = higher covalent character.

Calculation of Spectral Lines

• Formula: Total lines = n(n-1)/2 where n is the shell transition.
  • Example: Transition from 5th to 1st shell = 10 lines total.
  • Distribution into series (Lyman, Balmer, etc.) using specific formulas.

Quantum Numbers

• Principal (n), Azimuthal (l), Magnetic (m), and Spin (s) Quantum Numbers.
  • Valid ranges: l = 0 to n-1, m = -l to +l, s = ±1/2.
• Example Evaluation:
  • n=4, l=3, m=-3, s=+1/2 is correct.
  • n=4, l=4 is incorrect (l cannot equal n).

Valence Electrons Calculation

• Method: Analyze ionization energy jumps to determine valence electron count.
  • Large increase in energy = transition from valence to inner shell.
  • Example: Ionization energies (5, 10, 20, 2000 eV) suggest 3 valence electrons.

Spin Magnetic Moment

• Formula: ( \sqrt{n(n+2)} )
  • n = number of unpaired electrons.
  • Example: Fe2+ in complex with strong field ligands results in zero unpaired electrons.

Boiling Point and Van’t Hoff Factor

• Dissociation affects boiling point: Proportional to Van’t Hoff factor.
  • More ions = higher boiling point.
  • Example order: NaCl < BaCl2 < AlCl3.

Reaction Order from Half-Life

• Formula: ( \frac{t_{1/2}^{1}}{t_{1/2}^{2}} = \left(\frac{[A]_1}{[A]_2}\right)^{1-n} )
  • Example: Reaction with concentration change from 50M to 100M to 200M, half-life 4s, 2s, 1s yields order n=2.

Period and Group Identification

• Period: Determined by highest principal quantum number.
• Group:
  • S block: Number of electrons in ns shell.
  • P block: 10 + sum of electrons in ns and np shells.
  • D block: Sum of electrons in (n-1)d and ns shells.

Conclusion

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