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Chemical Kinetics and Nuclear Chemistry question

2023 · 24 Jan · Shift 1 · Q19
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Chemical Kinetics and Nuclear Chemistry question

2023 · 24 Jan · Shift 1 · Q19

JEE MainChemistryChemical Kinetics and Nuclear ChemistryNumerical+4 / −1
The number of correct statement/s from the following is ‾\underline{\hspace{2cm}}​ A. Larger the activation energy, smaller is the value of the rate constant. B. The higher is the activation energy, higher is the value of the temperature coefficient. C. At lower temperatures, increase in temperature causes more change in the value of k than at higher temperature D. A plot of ln⁡k\mathrm{\ln k}lnk vs 1T\frac{1}{T}T1​ is a straight line with slope equal to −EaR-\frac{E_a}{R}−REa​​
Numerical answer
View written solutionFree

Correct answer: 4

  1. Use the Arrhenius equation:

k=Ae−Ea/(RT)k = A e^{-E_a/(RT)}k=Ae−Ea​/(RT)

Taking natural log:

ln⁡k=ln⁡A−EaR⋅1T\ln k = \ln A - \frac{E_a}{R}\cdot \frac{1}{T}lnk=lnA−REa​​⋅T1​

This equation will help us test all statements.

  1. Check statement A:

k=Ae−Ea/(RT)k = A e^{-E_a/(RT)}k=Ae−Ea​/(RT)

For fixed AAA and TTT, if EaE_aEa​ increases, then e−Ea/(RT)e^{-E_a/(RT)}e−Ea​/(RT) decreases, so kkk decreases.

So, A is correct.

  1. Check statement B:

Temperature coefficient means the factor by which rate constant increases for a 10∘C10^\circ C10∘C rise in temperature:

temperature coefficient=kT+10kT\text{temperature coefficient} = \frac{k_{T+10}}{k_T}temperature coefficient=kT​kT+10​​

Using Arrhenius form,

ln⁡(kT+10kT)=EaR(1T−1T+10)\ln\left(\frac{k_{T+10}}{k_T}\right) = \frac{E_a}{R}\left(\frac{1}{T} - \frac{1}{T+10}\right)ln(kT​kT+10​​)=REa​​(T1​−T+101​)

This increases with EaE_aEa​. Hence, larger activation energy gives a larger temperature coefficient.

So, B is correct.

  1. Check statement C:

From Arrhenius equation,

ln⁡k=ln⁡A−EaRT\ln k = \ln A - \frac{E_a}{RT}lnk=lnA−RTEa​​

The sensitivity of kkk to temperature is larger at low TTT. More precisely,

d(ln⁡k)dT=EaRT2\frac{d(\ln k)}{dT} = \frac{E_a}{RT^2}dTd(lnk)​=RT2Ea​​

Since this is proportional to 1T2\frac{1}{T^2}T21​, the effect of temperature increase is greater at lower temperature than at higher temperature.

So, C is correct.

  1. Check statement D:

From

ln⁡k=ln⁡A−EaR⋅1T\ln k = \ln A - \frac{E_a}{R}\cdot \frac{1}{T}lnk=lnA−REa​​⋅T1​

A plot of ln⁡k\ln klnk versus 1T\frac{1}{T}T1​ is a straight line with slope

−EaR-\frac{E_a}{R}−REa​​

So, D is correct.

  1. Count correct statements:

A, B, C, and D are all correct.

Therefore, the number of correct statements is

444

  1. Comparison with stored answer:

Stored correct answer = 333

Derived answer = 444

Hence, I disagree with the stored answer. The stored answer likely missed statement C, which is true because the temperature dependence of kkk is stronger at lower temperatures.

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