Engineering Chemistry S Datta

136 ENGINEERING CHEMISTRY
18. The half-life periods for the decomposition of ammonia (catalytically) are as follows :
Pressure in mm 50 100 200
Half-life period in hrs. 3.52 1.92 1
Find the reaction is of which order.
19. The decomposition of Cl 2
O 7
at 400 K in the gas-phase to Cl 2
and O 2
is a 1st order reaction.
(i) After 55 seconds at 400 K the pressure of Cl 2
O 7
falls from 0.062 to 0.044 atm. Calculate the
rate constant.
(ii) Calculate the pressure of Cl 2
O 7
after 100 sec. of decomposition at this temperature.
[Ans. 6.236 × 10 –3 sec –1; 0.0332 atm]
20. A 2nd order reaction, where a = b, is 20%, is completed in 500 seconds. How long will it take to go
to 60% completion?
[Ans. 3000 seconds]
21. The following data were obtained in the hydrolysis of ethyl acetate, using equal concentration of
ester and NaOH:
Time (minutes): 0 5 15 25 35
Vol. of HCl (c.c.) 16.0 10.24 6.13 4.32 3.41
Show that reaction is of 2nd order.
22. A 1st order reaction has rate constant equal to 1.25 × 10 –4 sec –1 at 298 K and 8.5 × 10 –4 sec –1 at
318 K. Calculate the activation energy of the reaction. [Ans. 18, 050 Cal mol –1 ]
23. Two reactions:
(i) A ⎯→ Products, (ii) B ⎯→ Products, follow first order kinetics. The rate of the reaction (i) is
denoted when the temperature is raised from 300 K to 310 K. The half-life for this reaction at
310 K is 30 minutes. At the same temperature B decomposes twice as fast as A. If the energy
of activation for the reaction (ii) is half that of reaction (i), calculate the rate of the reaction
(ii) at 300 K. [Ans. 0.0327 min –1 ]
24. The activation energy for the reaction
2HI (g) H 2
(g) + I 2
(g)
is 209.5 kJ mol –1 at 581 K. Calculate the fraction of molecules of reactants having energy equal
to or greater than activation energy. [Ans. 1.462 × 10 –19 ]
25. When log K (velocity constant for a reaction) was plotted against 1/t, the slope of the line was
found to be – 5400 K. Calculate the energy of activation for this reaction (R = 8.314 JK –1 mol –1 )
[Ans. 103.4 kJ mol –1 ]