Engineering Chemistry S Datta

98 ENGINEERING CHEMISTRY
Sol. From Van’t Hoff equation, we have
ln
K
K
P2
P1
= ∆H R
L
N
M
T − T
TT
2 1
1 2
Here, T 1
= 27°C = 300 K; T 2
= 37°C = 310 K.
∴
log
log
K
P2
K
K
3.49 × 10
O
Q
P
K P1 = 3.49 × 10 –2 atm –1 ; K P2 = ?
∆H = – 11.2 Kcal = – 11200 cal; R = 1.987 cal deg –1 mol –1 .
P2
P1
=
− 2 =
∆H
2.303R
L
N
M
T − T
TT
2 1
1 2
− 11200
2.303 × 1.987
L
NM
O
Q
P
310 − 300
300 × 310
or, K P2 = 1.9038 × 10 –2 atm –1 .
O
QP
SHORT QUESTIONS AND ANSWERS
Q. 1. State First law with mathematical expressions.
Ans. See text page 68. Alternatively, it can be stated as the net energy of the universe
remains constant.
Q. 2. State intensive property of a system.
Ans. See text page 66.
Q. 3. Why is enthalpy more useful than internal energy?
Ans. Internal energy and ethalpy of a system are measured by keeping the volume and
the pressure constant, respectively. Since, in most of the reactions, they occur at atmospheric
pressure, so enthalpy is more useful than internal energy.
Q. 4. The molar heat of sublimation of a solid is always greater than the molar
heat of vaporization of the corresponding liquid. Why?
Ans. Since the molar heat of sublimation is the summation of heat of melting and heat
of vaporization so it is always greater than the heat of vaporization of the corresponding liquid.
Q. 5. When does the entropy of a substance become zero?
Ans. The entropy of a substance becomes zero when the substance is perfectly crystalline
i.e., ordered state and at the temperature of 0 K.
Q. 6. Derive Clapeyron-Clausius equation and mention its applications.
Ans. See text page 84.
Q. 7. Derive Gibbs-Helmholtz equation and mention its significance.
Ans. See text page 85.
Q. 8. What is enthalpy?
Ans. See text page 71.