Engineering Chemistry S Datta

84 ENGINEERING CHEMISTRY
Variation of bond energy
(i) Bond energy varies with the size of the atoms.
Example: bond energy for H—F and H—Cl are 56.5 and 43.1 kJ/mol, respectively.
(ii) Bond energy varies with the difference between the electronegativity of the two atoms.
Bond energy decreases as H—F > H—H > C—C > N—N > F—F.
(iii) Bond energy varies with the total environment of the molecule.
Application of bond energies
(i) The heat of reaction can be calculated from the bond energies since in a reaction
certain bonds are formed and certain bonds are broken. So, the total enthalpy change involved
in these bond breaking and making gives the heat of reaction.
i.e., ∆H reaction = Σ ∆H (bonds broken) + Σ ∆H (bonds formed)
= Σ Bond energy (reactants) – Σ Bond energy (products)
(ii) Heat of formation can also be calculated from the average bond energy summation.
Example: Heat of formation of ethane is,
H
⏐
H
⏐
2C(g) + 6H(g) ⎯⎯→ H— C—
C—H(g)
⏐ ⏐
H H
∆H f
for ethane is = 1 × (bond energy for C—C) + 6 × (bond energy for C—H)
= (1 × 79.3 + 6 × 98.5) Kcal.
= 670.3 Kcal mol –1 .
SECOND LAW OF THERMODYNAMICS
Heat is readily available and man is interested to convert this heat energy to useful work.
Second law of thermodynamics guides the conditions under which heat and work are
interconvertible. For the conversion of heat into work two conditions must be fulfilled.
1. Without the help of a “thermodynamic engine” the conversion of heat into work is
impossible and the engine must work in a reversible cyclic process.
2. The engine must take up heat at higher temperature, convert a portion of it into work
and give up the rest of heat to a body at lower temperature.
From the above experience, second law of thermodynamics can be stated in some different
important forms,
(a) It is impossible to convert heat completely into an equivalent amount of work, without
producing some other effects in the system.
(b) It is impossible for a self-acting machine unaided by any external agency, to convey
heat from a body at a low to one at a high temperature.
(c) All spontaneous processes tend to equilibrium.
(d) The energy of the universe is constant, but the entropy approaches a maximum.
Effect of temperature on free energy-(Gibbs-Helmholtz equation)
We know,
H = E + PV and G = H – TS
= E + PV – TS