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SUBJECT – CHEMISTRY

CHAPTER – THEMODYNAMICS

Module - 3

SUBTOPIC: Different enthalpies and Spontaneity

KEYNOTES:

 Bond enthalpy or Bond dissociation enthalpy(∆ bond H): It is the amount of
energy required to break 1 mol of bonds of a type between two atoms in
the gaseous state of a substance.
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Example: H (g) → 2H(g); ∆ bond H = 433 KJ mol
2
 Mean bond enthalpy: It is the average of the bond dissociation enthalpy of
the same type in a gaseous substance.

 Atomisation enthalpy(∆ H): It is the enthalpy change on breaking 1 mol of
a
bonds of diatomic molecule to obtain atoms in gas phase.
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Example: O (g) → 2O(g); ∆ H = 498KJ mol
2
a
 Lattice enthalpy(∆ lattice H): It is the amount of energy required to break 1
mol of ionic compound into its constituent ions in gas phase.
-
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+
Example: NaCl(s) → Na (g) + Cl (g); ∆ lattice H = +788 KJ mol
 Enthalpy of solution(∆ H):It is the enthalpy change when 1 mol of a
sol
substance is dissolved in a solvent at a given temperature.
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Example: NaCl(s) + aq→ NaCl(aq); ∆ H = + 5.35 KJ mol
sol
 Enthalpy of hydration(∆ H):It is the enthalpy change when 1 mol of the
hyd
anhydrous salt changes to a hydrated salt by combining with specified no.
of moles of water.
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Example: CuSO (s) + 5 H O → CuSO .5H O(s); ∆ H = -78.2 KJ mol
2
4
2
4
hyd
∆ H= ∆ lattice H + ∆ H
hyd
sol
 Born-Haber cycle: For the formation of NaCl
NaCl can be formed by the following methods.
(i)Direct method:
Na(s) + ½ CL → NaCl(s); ∆ H
2
f
(ii)Indirect method:
+
Na(s) → Na(g) → Na (g); ∆ H + ∆H
i
sub
-
1/2Cl (g) → Cl(g) → Cl (g); 1/2∆ bond H + ∆ H
eg
2