Electrochemistry

Thermochemistry : 9.3 Born-Haber Cycle

First Ionization Energy (IE₁)
- Energy required for 1 mol of gaseous atom to lose 1 mol of electrons.

Affinity Electron (EA)

Energy change that occurs when 1 mol of gaseous atom gains 1 mol of electrons.

Lattice Energy

Energy change when 1 mol of solid ionic compound formed from its gaseous ions

Magnitude Lattice Energy

Indicates the strength of ionic bonding
The more negative lattice energy, the stronger ionic bonding
It influences :
- Melting point
- Hardness
- Solubility

Determining Lattice Energy

Lattice energy cannot be measured directly.
- Can be calculated by Born-Haber cycle

Born-Haber Cycle

A series of chosen steps from elements to ionic compound for which all the enthalpies are known , except lattice energy.
ΔH_f= sum of ΔH^o for multistep path

Calculation Step

Example : LiF

Overall : Formation of LiF compound

Li(s) + ½ F₂(g) LiF(s) ΔH^o
_overall = 617 kJ

ΔH^o_overall = ΔH^o_f

Step 1 :- atomization of lithium

Li(s) Li(g) ΔH_f = 161 kJ

ΔH₁^o = ∆H^o_atom or ∆H^o_sublim

Step 2 :- atomization of fluorine

½ F₂(g) F(g) ∆H₂^o = 79.5 kJ

∆H₂^o= ∆H^o_atom

Step 3 :- ionization of lithium atom

Li(g) Li⁺(g) + e^- ∆H₃^o = 520 kJ

∆H₃^o = IE₁

Step 4 :- Electron affinity of F atom

F(g) + e^- F^-(g) ∆H₄^o = -328 kJ

∆H₄^o = EA

Step 5 :- Formation of LiF(s) from its gaseous ions

Li⁺(g) + F^-(g) LiF(s) ∆H₅^o= ?

∆H₅^o = ∆H^o_latticeof LiF

Factors Affecting Lattice Energy
- From Coulomb's Law

ΔE α (n⁺Q⁺)(n^-Q^-)

---------------------------

∆H : electrostatic energy

n⁺ : numberof positive charge

n^-: number of negative charge

Q⁺: electrostatic charge (+)

Q^-: electrostatic charge (-)

d : distance between the ions = r⁺+ r^-(radius of ions)

Ionic bond strength increase when
- Q, charge (Q) increase

Hydration Process of Ionic Crystal in Water
- 2 main process
  - Breaking lattice energy of ionic crystal

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Electrochemistry