11) At elevated temperatures, dinitrogen pentoxide decomposes to nitrogen dioxid

Question

11) At elevated temperatures, dinitrogen pentoxide decomposes to nitrogen dioxide and oxygen: 2 N2O5(g) 4 NO2 (g) + O2 (g) When the rate of formation of NO2 is 5.5 * 10-4 M/s, the rate of formation of O2 is______ M/s.

A) 1.4 * 10-4 B) 2.8 *10-4 C) 5.5 * 10-4 D) 10.1 * 10-4 E) 2.2 * 10-3

12) 2 NOBr (g) 2 NO (g) + Br2 (g) is a 2nd-order reaction with a rate constant (k) of 0.80 M-1s-1 at 11°C. If the initial concentration ([A]o) of NOBr is 0.0440 M, the concentration of NOBr after 17.0 seconds (t) is _____.

A) 0.0400 M B) 0.0350 M C) 0.0325 M D) 0.0300 M E) 0.0275 M

Explanation / Answer

11.

2 N2O5 (g) ----------------> 4 NO2 (g) + O2 (g)

General expression for rate of reaction,

Rate = - (1/2) d[N2O5]/dt = +(1/4)d[NO2]/dt = + d[O2]/dt

Now,

(1/4)d[NO2]/dt = d[O2]/dt

d[O2]/dt = (1/4) * (5.5 * 10-4)

d[O2] / dt = 1.375 * 10-4 = 1.4 * 10-4 M/s

So, (A)

12.

For a second order reaction,

1 / [NOBr]t = kt + 1 / [NOBr]0

1 / [NOBr]17.0 = (0.80 * 17.0) + 1 / 0.0440)

1 / [NOBr]17.0 = 36.3

[NOBr]17.0 = 0.0275 M

(E)

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