Question

Calculating the Change in Energy for the formation of NaCl (Born-Haber Cycle) ?​

Answer 1

Answer:

Explanation:

The Born-Haber cycle takes advantage of the state function property of the change in enthalpy to indirectly determine the lattice energy of ionic compounds through processes that utilize known thermodynamic quantities like ionization energy and electron affinity.

Let's take NaCl as an example. We begin by writing the formation reaction, which is by definition from the elemental states at 25°C and 1 atm:

Na(s)+1/2 Cl2 (g)→NaCl(s)

Answer 2

∆Hf = heat of formation of sodium chloride = 411.3 kJ mol-1
∆H1 = heat of sublimation of Na(s) = 108.7 kJ mol-1
∆H2 = ionisation energy of Na(s) = 495 kJ mol-1
∆H3 = dissociation energy of Cl2(s) = 244 kJ mol-1
∆H4 = Electron affinity of Cl(s) = – 349 kJ mol-1 U = lattice energy of NaCl
∆Hf = ∆H1 + ∆H2 + 1/2 ∆H3 + ∆H4 + U
∴ U = (∆Hf) – (∆H1 +∆H2 + 1/2 ∆H3 + ∆H4)
U = (-411.3) – (108.7 + 495 + 122 – 349) U = (-41 1.3) – (376.7)
∴ U = -788 kJ mol-1

This negative sign in lattice energy indicates that the energy is released when sodium is tormea from its constituent gaseous ions Na+ and Cl-


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