Chemistry_Today_April_2017_vk_com_stopthepress

Isothermal reversible expansion :
∆S sys > 0, ∆S sur < 0
∆S total = 0
Adiabatic reversible expansion :
∆S sys = 0, ∆S sur = 0
∆S total = 0
Adiabatic irreversible expansion :
∆S sys > 0, ∆S sur = 0
∆S total = 0
Isothermal irreversible compression :
∆S sys < 0, ∆S sur > 0
∆S total < 0
And we conclude that since all natural processes are
irreversible the entropy of the universe increases.
This is another statement of second law.
Notes :
• The entropies of all perfectly crystalline material
approaches zero as temperature approaches zero
kelvin, this is third law.
• Few substances have residual entropies even at zero
kelvin like CO, NO, N 2 O, even H 2 .
• For bigger molecules, standard entropy value is
higher .
+
• For H (aq) , standard entropy is zero.
• The increase in temperature results in increase in
entropy.
Free energy
At constant T and P the equilibrium condition is
the minimisation of Gibb’s free energy (G).
The greatest advantage of (at constant T and P)
Gibb’s free energy is that it
can predict the spontaneity
of the process by looking
into the system only (unlike
entropy which considers
G
equilibrium
Time
both system and surroundings).
∆G sys (const. T and P) < 0
is the criteria of spontaneity.
Also, –∆G = w net
For a reversible process at constant T and P, the decrease
in Gibb’s energy corresponds to maximum work done
by the system excluding P-V work.
60 CHEMISTRY TODAY | APRIL ‘17