Answer to Question #86351 in Organic Chemistry for Chanchal

Question

Write down the born Huber cycle for cacl2 formation

Answer 1

The Born Haber cycle for the formation of calcium chloride (CaCl₂) form its constituent elements involves following steps:

Atomization enthalpy of Calcium: This step involves the conversion of solid calcium to gaseous state.

Ca(s) = Ca(g), ΔHa°(Ca) = 178 kJ/mol

Ionization enthalpy of Calcium: Calcium forms di-positive ion (Ca²⁺), therefore the energy involve in the removal of first electron is called as first ionization energy and valued 590 kJ/mol. The removal of second electron requires more energy as it’s difficult to remove electron from a cation. Hence the second ionization energy for calcium ion is 1145 kJ/mol.

Ca(g) = Ca⁺(g) + e-, ΔH_IE°= 590 kJ/mol

Ca⁺(g) = Ca²⁺(g) + e-, ΔH_IE°= 1145 kJ/mol

Atomization enthalpy of Chlorine: This step involves dissociation of Cl₂(g) into Cl(g) atoms. The reaction enthalpy is half of the bond dissociation enthalpy of chlorine.

½ Cl₂(g) = Cl(g), ΔHa°= ½ΔH_Cl-Cl°= 121 kJ/mol

For the formation of calcium chloride, two Cl(g) requires, therefore total atomization enthalpy will be double that is 242 kJ/mol.

Electron affinity of Chlorine: This is the amount of energy released during the addition of electron in an isolated neutral gaseous chlorine atom.

Cl(g) + e^- = Cl^-(g), ΔH_EA°= -364 kJ/mol

Lattice enthalpy: The combination of one Ca²⁺ ion and two chloride ions (Cl^-)to form one mole of calcium chloride release lattice energy (ΔH_LE).

Ca²⁺ (g) + 2Cl^-(g) = CaCl₂(s), ΔH_LE°= -364 kJ/mol

Ca(s) + Cl₂(g) = CaCl₂(s), ΔH_f°

The heat of formation of calcium chloride can be calculated by using the Born Haber cycle in which the sum of enthalpy in a cycle is zero. By applying the Hess’s Law,

 

Heat of formation (ΔH_f^o) = Heat of atomization (ΔHa°) + Dissociation energy (ΔHd°) + (sum of Ionization energies) + (sum of Electron affinities) + Lattice energy (ΔH_LE°)

 

ΔH_f^o (CaCl₂) = 178 kJ/mol + 2*121 kJ/mol + 2*(-364 kJ/mol) + 590 kJ/mol + 1145 kJ/mol + (-2223 kJ/mol) = 796 kJ / mol.

ΔH_f^o (CaCl₂) = 796 kJ / mol.

 

 

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https://chemistry.tutorvista.com/inorganic-chemistry/born-haber-cycle.html