Page 2 - LN

+
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Na + Cl → NaCl(s); ∆ lattice H
According to Hess’s law, ∆ H = ∆ H + ∆H + 1/2∆ bond H + ∆ H + ∆ lattice H
sub
eg
f
i
 Spontaneity: It means having the potential to proceed without assistance
of external energy or force.
 Spontaneous process: The process which takes place without assistance of
any external agency or force. It is an irreversible process which can be
reversed by some external agent.

 Is decrease in enthalpy a criterion for spontaneity:
(i)For spontaneous process, decrease in enthalpy takes place.
-1
Example: C(s) + O (g) → CO (g); ∆ H = -393.5 KJ mol
2
r
2
(ii)Some endothermic reactions which require less amount of energy are
also spontaneous.
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Example: 1/2N + O → NO ; ∆ H = 33.2 KJ mol
r
2
2
2
So, decrease in enthalpy may be a contributory factor for spontaneity but
not true for all cases.
 Entropy(S):It is the degree of randomness or disorderness of a system.
∆S = q / T = ∆H / T
rev
∆S Total = ∆S System + ∆S Surrounding
(i)Decrease in regularity in structure means increase in entropy.

(ii)Crystalline solid state is the state of lowest entropy.
(iii)Gaseous state is the state of highest entropy.
(iv)Heat has randomising influence on the system and temperature is the
measure of average random motion of particles in the system.

(v)Heat added to the system at low temperature causes greater
randomness than when the same quantity of heat is added at higher
temperature.

(vi)At equilibrium, ∆S = 0 and entropy is maximum.
(vii)For irreversible process, ∆S System + ∆S Surrounding ≠ 0
(vii)If ∆S = +ve, process is spontaneous.

If ∆S = -ve, process is non-spontaneous.
If ∆S = 0, process is at equilibrium state.
Second law of Thermodynamics:It has the following statements.

(i)All spontaneous processes are thermodynamically irreversible.
(ii)Without the help of an external agency, a spontaneous process cannot
be reversed.