Engineering Chemistry S Datta

THERMODYNAMICS 103
8. Find heat of formation of caustic soda from the following data:
Na + H 2
O = NaOH (aq) + 1 2 H 2 + 98,000
H 2
+ 1 2 O 2 = H 2 O + 68,380
NaOH + water = NaOH (aq) + 13,300.
[Ans. 153080 cals]
9. Calculate the heat of formation of benzene given that the heats of combustion of benzene, carbon
and hydrogen are 754300 cals, 94380 cals and 68380 cals, respectively. [Ans. – 17120 cals]
10. Isothermally at 27°C one mole of a van der Waal’s gas expands reversibly from 2 litres to 20
litres. Calculate the work done if a = 1.42 × 10 12 dynes cm 4 mol –1 and b = 30 c.c mol –1 .
V
[Hint. W = RT ln 2 V1
−
−
b
b
L
N
M
O
Q
P
1 1
+ a − .
V2 V1
20 − 0.03
W = 2.303 × 8.31 × 10 7 × 300 log + 1.42 × 10
2 − 0.03
12
L
NM
O
QP
2000 − 20000
20000 × 2000 ergs
= 5677 × 10 7 ergs.]
11. 70 g of nitrogen gas was initially at 50 atm and 25°C. It was allowed to expand isothermally
against a constant external pressure of one atmosphere. Calculate ∆Q, ∆W, ∆U, ∆H.
[Ans. 1.5 Kcal; 1.5 Kcal; 0; 0]
12. From the following bond enthalpies calculate ∆H for the reaction, H 2 + Br 2 → 2 HBr. Bond
energy H—H = 435.24 kJ mol –1 ; Br—Br bond energy = 192.5 kJ mol –1 ; H—Br bond energy =
368.3 kJ mol –1 . [Ans. – 108.86 kJ]
13. The heat of combustion of methyl alcohol is – 173.65 Kcal. Given, ∆H (H
2 O, l ) = – 68320 cal;
∆H f (CO , g ) = – 94050 cal. Evaluate the heat of formation of methyl alcohol. [Ans. – 57040 cal]
2
14. Calculate ∆S, ∆A and ∆G for vaporisation of 2 moles of benzene (b.p. = 353.2 K and G = 101 cal g –1 )
[Ans. ∆S = 44.5 cal/°K, ∆G = 0, ∆A = – 1.4 Kcal]
15. The van der Waal’s constant for CO 2
, a = 3.59 litre 2 atm/mol –2 and b = 0.043 litre mol –1 . What is
the inversion temperature of the gas? [Ans. 2036 K]
16. The sp. heat of benzene is 0.39. Find the change in entropy when 1 kg of benzene at 10°C is
mixed with 4.0 kg of benzene at 30°C.
[Ans. 0.9879 eV]
17. Calculate the entropy change in vaporisation of ether. Given, boiling point = 35°C, L = 6500 cal
mol –1 .
[Ans. 21.1 eV]
18. Calculate the free energy change of g mole of SO 2
at 25°C and 10 atm pressure. Given, ∆G° 298
= – 71.8 Kcal mol –1 . (Ans. – 70.43 Kcal/mol –1 ]
19. ∆G and ∆H for a particular reaction at 127°C are – 12 Kcal and 17.5 Kcal. Find out ∆S.
[Ans. – 13.75 eV]
20. The vapour pressure of water at 90°C is 530 mm Hg. The average latent heat of vaporisation of
water is 546 cal g –1 within the temperature range 90°C – 100°C. Calculate the vapour pressure
at 100°C.
[Ans. 760 mm Hg]