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Exercises · 3.15
Q.

The experimental data for decomposition of N2O5N_2O_5

[2N2O5→4NO2+O2][2N_2O_5 \rightarrow 4NO_2 + O_2]

in gas phase at 318 K are given below:

tt/s0400800120016002000240028003200
102×[N2O5]10^2\times[N_2O_5]/mol L−1\text{mol L}^{-1}1.631.361.140.930.780.640.530.430.35
  1. Plot [N2O5][N_2O_5] against tt.
  2. Find the half-life period for the reaction.
  3. Draw a graph between log⁡[N2O5]\log[N_2O_5] and tt.
  4. What is the rate law?
  5. Calculate the rate constant.
  6. Calculate the half-life period from kk and compare it with (ii).
Telangana TsbieTextbookSubjective· 5mImportance★★★★★
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The plot of log⁡[N2O5]\log[N_2O_5] against tt is a straight line, so the reaction is first order: Rate=k[N2O5]\text{Rate} = k[N_2O_5]. The rate constant is k≈4.81×10−4 s−1k \approx 4.81 \times 10^{-4}\ \text{s}^{-1}, giving t1/2≈1441 st_{1/2} \approx 1441\ \text{s} from kk, in good agreement with ≈1450 s\approx 1450\ \text{s} read from the graph.

(i) Plot [N2O5][N_2O_5] vs tt. Plotting the concentration against time gives a smooth downward curve (steep at first, then flattening). It is not a straight line, so the reaction is not zero order.

(ii) Half-life from the graph. The initial concentration is [N2O5]0=1.63×10−2 mol L−1[N_2O_5]_0 = 1.63 \times 10^{-2}\ \text{mol L}^{-1}, so half of it is 0.815×10−2 mol L−10.815 \times 10^{-2}\ \text{mol L}^{-1}. Reading the curve, this value is reached at about t1/2≈1450 st_{1/2} \approx 1450\ \text{s}.

(iii) Plot log⁡[N2O5]\log[N_2O_5] vs tt. Computing log⁡[N2O5]\log[N_2O_5] (with [N2O5][N_2O_5] in mol L−1\text{mol L}^{-1}):

tt/s[N2O5][N_2O_5]/mol L−1\text{mol L}^{-1}log⁡[N2O5]\log[N_2O_5]
01.63×10−21.63 \times 10^{-2}−1.788-1.788
4001.36×10−21.36 \times 10^{-2}−1.866-1.866
8001.14×10−21.14 \times 10^{-2}−1.943-1.943
12000.93×10−20.93 \times 10^{-2}−2.031-2.031
16000.78×10−20.78 \times 10^{-2}−2.108-2.108
20000.64×10−20.64 \times 10^{-2}−2.194-2.194
24000.53×10−20.53 \times 10^{-2}−2.276-2.276
28000.43×10−20.43 \times 10^{-2}−2.367-2.367
32000.35×10−20.35 \times 10^{-2}−2.456-2.456

These points fall on a straight line, which confirms first-order kinetics.

(iv) Rate law. A linear log⁡[N2O5]\log[N_2O_5] vs tt plot means the reaction is first order in N2O5N_2O_5:

Rate=k[N2O5]\text{Rate} = k[N_2O_5]

(v) Rate constant. For a first-order reaction, log⁡[N2O5]=log⁡[N2O5]0−k2.303 t\log[N_2O_5] = \log[N_2O_5]_0 - \dfrac{k}{2.303}\,t, so the slope is −k2.303-\dfrac{k}{2.303}. Using the endpoints t=0t = 0 and t=3200 st = 3200\ \text{s}: …

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