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

Question

In a study of the decomposition of ammonia on a platinum surface at 856 degree C NH_3 rightarrow 1/2 N_2 + 3/2 H_2 the following data were obtained: The observed half life for this reaction when the starting concentration is 2.62 times 10^-3 M is and when the starting concentration is 1.31 times 10^-3 M is The average rate of disappearance of NH_3 from t = 0 s to t = 1000 s is M s^-1. The average rate of disappearance of NH_3 from t = 1000 s to t = 1.50 times 10^3 s is M s^-1. Based on these data, the rate constant for this order reaction is M s^-1.

Explanation / Answer

Q1.

HL =half life

so

from

2.62*10^-3 --> half value is 1.31*10^-3

then

time required = 1000-0 = 1000 seconds

2)

Avg. disspareance = (Cfinal - Cinitial)/(Tfinal - Tinitial) = (1.31*10^-3 - 2.62*10^-3)/(1000-0) = -0.00000131 M/s

3)rate from:

1000s to 1500 -->

dC = (6.55*10^-4) - 1.31*10^-3 = -0.000655 C

dt = 1500-1000 = 500 s

avg rate = 0.000655/500 = -0.00000131 M/s

Q4.

the rate cosnatn for this is FIRST ORDER

since the rate won't change

so

HL = ln(2)/k

k = ln(2)/(1000) = 0.00069314 1/s

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