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ChemistryMediumClass 11

Ionic Bond ā€” Lattice energy, Born-Haber cycle

Chemical Bonding

15

JEE Qs

8%

Hard

75

min

Master the correct application of Hess's Law with precise sign conventions and stoichiometry for each step of the Born-Haber cycle to accurately calculate unknown enthalpy values.

šŸ§® Key Formulas

Ī”H_f = Ī”H_sub + 1/2 * Ī”H_dissociation + I.E. + E.A. + U_lattice (for MX type compound, adjust coefficients accordingly)
U_lattice āˆ (Z+ * Z-) / (r+ + r-)

āœ… Key Points for JEE

  • 1Lattice energy (U_lattice) is the energy released when one mole of a crystalline ionic solid is formed from its constituent gaseous ions (it is an exothermic process, U_lattice < 0).
  • 2The magnitude of lattice energy determines several properties of ionic compounds: higher U_lattice generally means higher melting point, greater hardness, and lower solubility in polar solvents.
  • 3Lattice energy is directly proportional to the product of the charges on the ions (Z+ * Z-) and inversely proportional to the sum of their ionic radii (r+ + r-). Therefore, smaller ions and higher charges lead to higher lattice energy.
  • 4The Born-Haber cycle is an application of Hess's Law, allowing the calculation of one unknown enthalpy change (usually lattice energy) by summing the known enthalpy changes for a series of steps that transform elements into an ionic compound.
  • 5Each step in the Born-Haber cycle (sublimation, dissociation, ionization, electron gain, lattice formation) must be assigned the correct enthalpy change sign (endothermic +ve, exothermic -ve) and stoichiometric coefficients.

āš ļø Common Mistakes

  • āœ•Incorrectly assigning signs to enthalpy changes in the Born-Haber cycle (e.g., treating ionization energy or sublimation as exothermic).
  • āœ•Failing to account for stoichiometric coefficients, especially for dissociation of diatomic molecules (e.g., 1/2 * Ī”H_diss) or when forming compounds like MgCl2.
  • āœ•Confusing electron gain enthalpy (Ī”H_eg) with electron affinity (E.A.), leading to sign errors, especially for subsequent electron additions which are endothermic.
  • āœ•Misinterpreting the magnitude of lattice energy when comparing compounds with different charges and sizes simultaneously.

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NCERT Chapters

  • Class 11 Chemistry Ch 4: Chemical Bonding and Molecular Structure