In a study of the decomposition of ammonia on a platinum surface at 856 °C the f

Question

In a study of the decomposition of ammonia on a platinum surface at 856 °C the following data were obtained NH3], M 7.98x10-3 00x10-3 1.50x103 3.99x10-3 1.00x10-3 1.75x103 seconds 1000 Hint: It is not necessary to graph these data. The observed half life for this reaction when the starting concentration is 7.98x10*3 M is 3.99x103M is s and when the starting concentration is The average rate of disappearance of NH3 from t-0 s tot 1000 s is M sl The average rate of disappearance of NH3 from t 1000 s to t 1.50x10' s is Based on these data, the rate constant for this order reaction is M sl

Explanation / Answer

Theoretical half life is defined as the time required for the concentration to drop to 50% of the initial value. Here the initial concentration is 7.98*10-3 and it has dropped to 7.98*10-3/2= 3.99*10-3 in 1000 seconds

the average rate= drop in concentration/ time = (7.98*10-3- 3.98*10-3)/1000 = 3.98*10-6 M/s

for the case of initial concentration of 3.99*10-3, t= 1000 second and when the concentration dropped to 50% of this value, i.e 3.99*10-3/2= 2*10-3, the time is 1500 second. So the time for half the concentration to drop to this 50% = 1500-1000=500 sec

Averate rate= (3.99*10-3- 2.*10-3)/500 = 3.98*10-6 M/s

The data suggests the rate of reaction is independent of concentration. So this suggests zero order reaction. For zero order reaction -d[NH3]/dt= K[NH3]n, n is order of reaction , where K is rate constant. for ero order, n=0 and -d[NH3]/dt= K, so the rate is consant and indepdent of concentratino for zero order reaction.

K= 3.9*10-6 M/s

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