In a study of the decomposition of ammonia on a tungsten surface at 1100 Degree

Question

In a study of the decomposition of ammonia on a tungsten surface at 1100 Degree C NH_3 Right arrow 1/2 N_2 + 3/2 H_2 the following data were obtained: it is not necessary to graph these data. The observed half life for this reaction when the starting concentration is 4.59 x 10^-3 M is s and when the starting concentration is 2.30 x 10^-3 M is s. The average rate of disappearance of NH_3 from t = 0 s to t = 440 s is M s^-1. The average rate of disappearance of NH_3 from t = 440 s to t = 661 s is M S^-1. Based on these data, the rate constant for this order reaction is M s^-1.

Explanation / Answer

1)

As it is happening on the surface of tungsten and generally the reaction that occurs on the surface of other atom is zero order reaction

so half life = [NH3]0 / 2 * K

I found from google that the rate constant fort the decomposition of NH3 is =  2.08·10-4 mol·L-1·s-1.

=> Half life at initial Conc. 4.59 * 10-3 =  4.59 * 10-3 /2* 5.204 * 10-6 = 441.006 Sec

Half life at the initial conc. 2.3 * 10-3 = 2.3 * 10-3 / 2* 5.204 * 10-6 =220.9 Sec

2 ) Average rate of disappearence of NH3 from 0 sec to 440 sec = change in Conc. / Change in time

= 4.59 * 10-3 - 2.3 * 10-3 / 440 = 5.204 * 10-6 mol L-1 Sec-1

3)

Average rate of disappearence of NH3 from 440 sec to 661 sec = change in Conc. / Change in time

= 2.3 * 10-3 - 1.15 * 10-3 / 661-440 = 5.204 * 10-6 mol L-1 Sec-1

4 )Based on these two datas we can conclude that it is a zero order reaction as the reaction does not depend upon the Conc. of the reactant . and the rate constant = 5.204 * 10-6 mol L-1 Sec-1

Than you

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