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Chemical Kinetics and Nuclear Chemistry question

2023 · 10 Apr · Shift 1 · Q20
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Chemical Kinetics and Nuclear Chemistry question

2023 · 10 Apr · Shift 1 · Q20

JEE MainChemistryChemical Kinetics and Nuclear ChemistryNumerical+4 / −1
A molecule undergoes two independent first order reactions whose respective half lives are 12 min and 3 min. If both the reactions are occurring then the time taken for the 50% consumption of the reactant is ‾\underline{\hspace{2cm}}​ min. (Nearest integer)
Numerical answer
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Correct answer: 2

  1. For a first-order reaction, the rate constant is related to half-life by k=ln⁡2t1/2k=\frac{\ln 2}{t_{1/2}}k=t1/2​ln2​

  2. The molecule undergoes two independent first-order reactions simultaneously, so the overall rate constant is the sum: ktotal=k1+k2k_{\text{total}}=k_1+k_2ktotal​=k1​+k2​

  3. Compute the individual rate constants: k1=ln⁡212k_1=\frac{\ln 2}{12}k1​=12ln2​ k2=ln⁡23k_2=\frac{\ln 2}{3}k2​=3ln2​

Thus, ktotal=ln⁡2(112+13)k_{\text{total}}=\ln 2\left(\frac{1}{12}+\frac{1}{3}\right)ktotal​=ln2(121​+31​) =ln⁡2(1+412)=ln⁡2⋅512=\ln 2\left(\frac{1+4}{12}\right)=\ln 2\cdot \frac{5}{12}=ln2(121+4​)=ln2⋅125​

  1. If both reactions are occurring, the reactant disappears with effective first-order constant ktotalk_{\text{total}}ktotal​. For 50% consumption, NN0=12\frac{N}{N_0}=\frac{1}{2}N0​N​=21​ For a first-order process, t=ln⁡2ktotalt=\frac{\ln 2}{k_{\text{total}}}t=ktotal​ln2​

Substitute: t=ln⁡2(ln⁡2)(5/12)=125=2.4 mint=\frac{\ln 2}{(\ln 2)(5/12)}=\frac{12}{5}=2.4\text{ min}t=(ln2)(5/12)ln2​=512​=2.4 min

  1. Nearest integer: 2.4≈22.4 \approx 22.4≈2

Therefore, the required time is 2 min\boxed{2\text{ min}}2 min​

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