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Chemical Kinetics and Nuclear Chemistry question

2017 · 8 Apr · Shift 1 · Q19
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Chemical Kinetics and Nuclear Chemistry question

2017 · 8 Apr · Shift 1 · Q19

JEE MainChemistryChemical Kinetics and Nuclear ChemistryMCQ+4 / −1
The rate of a reaction A doubles on increasing the temperature from 300 to 310 K. By how much, the temperature of reaction B should be Increased from 300 K so that rate doubles if activation energy of the reaction B is twice to that of reaction A.
  1. A
    9.84 K
  2. B
    4.92 K
  3. C
    2.45 K
  4. D
    19.67 K
View written solutionFree

Correct answer: B

  1. Use Arrhenius equation

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

So for two temperatures, the ratio of rate constants is ln⁡(k2k1)=EaR(1T1−1T2)\ln\left(\frac{k_2}{k_1}\right)=\frac{E_a}{R}\left(\frac{1}{T_1}-\frac{1}{T_2}\right)ln(k1​k2​​)=REa​​(T1​1​−T2​1​)

Since rate doubles, we take k2k1=2\frac{k_2}{k_1}=2k1​k2​​=2


  1. For reaction A

Given reaction A doubles when temperature increases from 300300300 K to 310310310 K.

Thus, ln⁡2=EAR(1300−1310)\ln 2 = \frac{E_A}{R}\left(\frac{1}{300}-\frac{1}{310}\right)ln2=REA​​(3001​−3101​)


  1. For reaction B

Activation energy of B is twice that of A: EB=2EAE_B = 2E_AEB​=2EA​

Let the required temperature rise for B from 300300300 K be ΔT\Delta TΔT. Then final temperature is 300+ΔT300+\Delta T300+ΔT

Again rate doubles, so ln⁡2=EBR(1300−1300+ΔT)\ln 2 = \frac{E_B}{R}\left(\frac{1}{300}-\frac{1}{300+\Delta T}\right)ln2=REB​​(3001​−300+ΔT1​)

Substitute EB=2EAE_B=2E_AEB​=2EA​: ln⁡2=2EAR(1300−1300+ΔT)\ln 2 = \frac{2E_A}{R}\left(\frac{1}{300}-\frac{1}{300+\Delta T}\right)ln2=R2EA​​(3001​−300+ΔT1​)


  1. Compare the two equations

From reaction A: ln⁡2=EAR(1300−1310)\ln 2 = \frac{E_A}{R}\left(\frac{1}{300}-\frac{1}{310}\right)ln2=REA​​(3001​−3101​)

From reaction B: ln⁡2=2EAR(1300−1300+ΔT)\ln 2 = \frac{2E_A}{R}\left(\frac{1}{300}-\frac{1}{300+\Delta T}\right)ln2=R2EA​​(3001​−300+ΔT1​)

Equating, EAR(1300−1310)=2EAR(1300−1300+ΔT)\frac{E_A}{R}\left(\frac{1}{300}-\frac{1}{310}\right)=\frac{2E_A}{R}\left(\frac{1}{300}-\frac{1}{300+\Delta T}\right)REA​​(3001​−3101​)=R2EA​​(3001​−300+ΔT1​)

Cancel EAR\frac{E_A}{R}REA​​: (1300−1310)=2(1300−1300+ΔT)\left(\frac{1}{300}-\frac{1}{310}\right)=2\left(\frac{1}{300}-\frac{1}{300+\Delta T}\right)(3001​−3101​)=2(3001​−300+ΔT1​)

Now, 1300−1310=10300⋅310=19300\frac{1}{300}-\frac{1}{310}=\frac{10}{300\cdot 310}=\frac{1}{9300}3001​−3101​=300⋅31010​=93001​

So, 19300=2(1300−1300+ΔT)\frac{1}{9300}=2\left(\frac{1}{300}-\frac{1}{300+\Delta T}\right)93001​=2(3001​−300+ΔT1​)

Hence, 1300−1300+ΔT=118600\frac{1}{300}-\frac{1}{300+\Delta T}=\frac{1}{18600}3001​−300+ΔT1​=186001​


  1. Solve for ΔT\Delta TΔT

1300−1300+ΔT=ΔT300(300+ΔT)=118600\frac{1}{300}-\frac{1}{300+\Delta T} = \frac{\Delta T}{300(300+\Delta T)} = \frac{1}{18600}3001​−300+ΔT1​=300(300+ΔT)ΔT​=186001​

So, ΔT300(300+ΔT)=118600\frac{\Delta T}{300(300+\Delta T)}=\frac{1}{18600}300(300+ΔT)ΔT​=186001​

Cross-multiply: 18600ΔT=300(300+ΔT)18600\Delta T = 300(300+\Delta T)18600ΔT=300(300+ΔT)

18600ΔT=90000+300ΔT18600\Delta T = 90000 + 300\Delta T18600ΔT=90000+300ΔT

18300ΔT=9000018300\Delta T = 9000018300ΔT=90000

ΔT=9000018300≈4.92 K\Delta T = \frac{90000}{18300} \approx 4.92\text{ K}ΔT=1830090000​≈4.92 K


  1. Check options

The required increase in temperature is 4.92 K4.92\text{ K}4.92 K

So the correct option is B.

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