Engineering Chemistry S Datta

80 ENGINEERING CHEMISTRY
(iii) ∆G° = – RT ln K
= – 2.303 RT log K.
where R = gas constant, T = temperature in kelvin, K is the equilibrium constant of a
reaction.
(iv) ∆G = 0 (at equilibrium, at constant T and P).
At constant temperature and pressure always system tends to attain a position of
minimum free energy which is the position of equilibrium.
Thus, for a spontaneous process, ∆G must be negative. If ∆G is negative, any change
shall spontaneously occur. If ∆G is zero, the initial and the final states would be at equilibrium
with each other and finally if ∆G is positive, the process will not take place spontaneously, but
it may reverse.
Highlights:
• First law of thermodynamics states—the energy may change from one form to
another, but the sum total of energy of the universe remains constant.
• Mathematical expression of first law:
• Kirchoff’s equation:
dE = dq – dW
∆H2 − ∆H1
( T2 − T1)
= ∆C p
.
• Entropy is a measure of randomness or disorder of a system. It is also defined as
“unavailable energy”. Entropy change, ∆S = q rev
T .
• State function is one which depends only on the initial and final states of the
system but independent of the path followed e.g., ∆G, ∆S, etc.
• Free energy change is a state function and is related to enthalpy change, entropy
change and temperature by Gibb’s-Helmholtz equation as
∆G = ∆H – T∆S.
THERMOCHEMISTRY
All types of chemical reactions are accompanied by heat changes. The reactions in which heat
is evolved are termed exothermic and in which heat is absorbed are endothermic.
The energy from chemical reactions comes from the breaking or formation of bonds. The
thermal changes in chemical processes are governed by two general principles, both of which
are corollaries of the first law.
A. Law of Lavoisier and Laplace. This law states that the heat change accompanying
a chemical process in one direction is equal in magnitude but opposite in sign to that
accompanying the same process taking place in the opposite direction.
1
2 H 2 (g) + 1 2 Cl 2
(g) ⎯⎯→ HCl(g) ∆H = – 92 kJ
HCl(g) ⎯⎯→ 1 2 H 2 (g) + 1 2 Cl 2
(g) ∆H = + 92 kJ
According to the first law, heat content (H) or energy content (E) of the initial reactants
again returns to the same magnitude. As such, the net heat-content change or energy content