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Chemical Kinetics question

2024 · Q141
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Chemical Kinetics question

2024 · Q141

NEETChemistryChemical KineticsMCQ+4 / −1

The rate of a reaction quadruples when temperature changes from 27∘C27^{\circ} \mathrm{C}27∘C to 57∘C57^{\circ} \mathrm{C}57∘C. Calculate the energy of activation.

Given R=8.314 J K−1 mol−1,log⁡4=0.6021\mathrm{R}=8.314 \mathrm{~J} \mathrm{~K}^{-1} \mathrm{~mol}^{-1}, \log 4=0.6021R=8.314 J K−1 mol−1,log4=0.6021

  1. A
    38.04 kJ/mol38.04 \mathrm{~kJ} / \mathrm{mol}38.04 kJ/mol
  2. B
    380.4 kJ/mol380.4 \mathrm{~kJ} / \mathrm{mol}380.4 kJ/mol
  3. C
    3.80 kJ/mol3.80 \mathrm{~kJ} / \mathrm{mol}3.80 kJ/mol
  4. D
    3804 kJ/mol3804 \mathrm{~kJ} / \mathrm{mol}3804 kJ/mol
View written solutionFree

Correct answer: A

To solve this problem, we will use the Arrhenius equation, which relates the rate constant ($k$) of a chemical reaction to the temperature ($T$) and the activation energy ($E_a$). The equation in its logarithmic form is:

log⁡k=log⁡A−Ea2.303RT\log k = \log A - \frac{E_a}{2.303RT}logk=logA−2.303RTEa​​

where:

  • $k$ is the rate constant,
  • $A$ is the pre-exponential factor (frequency factor),
  • $E_a$ is the activation energy,
  • $R$ is the universal gas constant ($$8.314 \, \text{J mol}^{-1} \text{K}^{-1}$$),
  • $T$ is the temperature in Kelvin,
  • $2.303$ is the factor to convert from natural log to common log.

According to the problem, the rate of the reaction quadruples ($k_2 = 4k_1$) when the temperature increases from $27^{\circ}C$ (which equals $$27 + 273.15 = 300.15 \, K$$) to $57^{\circ}C$ (which equals $$57 + 273.15 = 330.15 \, K$$). Using the logarithmic form of the Arrhenius equation, for two different temperatures we have:


log⁡k1=log⁡A−Ea2.303R×300.15\log k_1 = \log A - \frac{E_a}{2.303R \times 300.15}logk1​=logA−2.303R×300.15Ea​​



log⁡k2=log⁡A−Ea2.303R×330.15\log k_2 = \log A - \frac{E_a}{2.303R \times 330.15}logk2​=logA−2.303R×330.15Ea​​

With $k_2 = 4k_1$, substituting in the values and taking their difference:

log⁡4k1−log⁡k1=(log⁡A−Ea2.303×8.314×330.15)−(log⁡A−Ea2.303×8.314×300.15)\log 4k_1 - \log k_1 = \left(\log A - \frac{E_a}{2.303 \times 8.314 \times 330.15}\right) - \left(\log A - \frac{E_a}{2.303 \times 8.314 \times 300.15}\right)log4k1​−logk1​=(logA−2.303×8.314×330.15Ea​​)−(logA−2.303×8.314×300.15Ea​​)



log⁡4=Ea2.303×8.314(1300.15−1330.15)\log 4 = \frac{E_a}{2.303 \times 8.314} \left(\frac{1}{300.15} - \frac{1}{330.15}\right)log4=2.303×8.314Ea​​(300.151​−330.151​)

Given that $\log 4 = 0.6021$, we now solve for $E_a$:


0.6021=Ea2.303×8.314(1300.15−1330.15)0.6021 = \frac{E_a}{2.303 \times 8.314} \left(\frac{1}{300.15} - \frac{1}{330.15}\right)0.6021=2.303×8.314Ea​​(300.151​−330.151​)

First, calculate $$\frac{1}{300.15} - \frac{1}{330.15}$$:

1300.15≈0.003332and1330.15≈0.003029\frac{1}{300.15} \approx 0.003332 \quad \text{and} \quad \frac{1}{330.15} \approx 0.003029300.151​≈0.003332and330.151​≈0.003029



1300.15−1330.15=0.003332−0.003029=0.000303\frac{1}{300.15} - \frac{1}{330.15} = 0.003332 - 0.003029 = 0.000303300.151​−330.151​=0.003332−0.003029=0.000303

So,

0.6021=Ea19.141872×0.0003030.6021 = \frac{E_a}{19.141872} \times 0.0003030.6021=19.141872Ea​​×0.000303



Ea19.141872=0.60210.000303=1987.05\frac{E_a}{19.141872} = \frac{0.6021}{0.000303} = 1987.0519.141872Ea​​=0.0003030.6021​=1987.05

Solve for $E_a$:

Ea=1987.05×19.141872=38067 J/mol≈38.067 kJ/molE_a = 1987.05 \times 19.141872 = 38067 \, \text{J/mol} \approx 38.067 \, \text{kJ/mol}Ea​=1987.05×19.141872=38067J/mol≈38.067kJ/mol

Therefore, the activation energy $E_a$ is $$38.07 \, \text{kJ/mol}$$, which best matches Option A:

38.04 kJ/mol38.04 \, \text{kJ/mol}38.04kJ/mol

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