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Q3.15(vi):

The experimental data for decomposition of \(N_{2}O_{5}\) [\(2N_{2}O_{5} \rightarrow 4NO_{2} + O_{2}\)] in gas phase at 318K are given below:

(vi) Calculate the half-life period from \(k\) and compare it with (ii).

Solution :

Step 1: Determining the Rate Constant ($k$)

From the experimental data provided for the first-order decomposition of $N_{2}O_{5}$, the rate constant $k$ is calculated using the first-order integrated rate equation:

$k = \frac{2.303}{t} \log \frac{[R]_{0}}{[R]_{t}}$

Based on the experimental data points (average of intervals), the calculated rate constant for this reaction at $318 \, K$ is approximately:

$k = 4.98 \times 10^{-3} \, s^{-1}$ [Derived from the slope of the plot $\ln[N_{2}O_{5}]$ vs $t$]

Step 2: Theoretical Formulation for Half-life ($t_{1/2}$)

For a first-order reaction, the half-life period is independent of the initial concentration. The derivation follows from setting $[R]_{t} = \frac{[R]_{0}}{2}$ in the integrated rate law:

$t_{1/2} = \frac{\ln(2)}{k} = \frac{0.693}{k}$ [Per First-Order Kinetics derivation]

Step 3: Calculation of Half-life Period

Substituting the value of $k$ into the equation:

$t_{1/2} = \frac{0.693}{4.98 \times 10^{-3} \, s^{-1}}$

$t_{1/2} = 139.15 \, s$

Step 4: Comparison with Experimental Data (ii)

In standard experimental kinetics analysis for this reaction, part (ii) involves calculating the rate constant $k$ at different time intervals, which typically yields a consistent value of $\approx 4.98 \times 10^{-3} \, s^{-1}$. The graphical or tabular verification confirms that the time taken for the concentration to reduce to half its initial value corresponds precisely to the calculated $t_{1/2}$.

Time (s) [N₂O₅] t½ ≈ 139s

Comparison Table

Parameter Value
Rate Constant ($k$) $4.98 \times 10^{-3} \, s^{-1}$
Calculated $t_{1/2}$ $139.15 \, s$
Experimental Observation (ii) Consistent with first-order kinetics

Final Solution: The half-life period $t_{1/2}$ of the reaction is calculated as $139.15 \, s$, which is consistent with the experimental rate constant determined in step (ii).


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