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Thermodynamics question

2019 · 9 Jan · Shift 2 · Q23
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Thermodynamics question

2019 · 9 Jan · Shift 2 · Q23

JEE MainChemistryThermodynamicsMCQ+4 / −1
The entropy change associated with the conversion of 1 kg of ice at 273 K to water vapours at 383 K is : (Specific heat of water liquid and water vapour are 4.2 kJ K −-− 1 kg −-− 1 and 2.0 kJ K −-− 1 kg −-− 1; heat of liquid fusion and vapourisation of water are 334 kJ −-− 1 and 2491 kJ kg −-− 1, respectively). (log 273 = 2.436, log 373 = 2.572, log 383 = 2.583)
  1. A
    7.90 kJ kg −-− 1 K −-− 1
  2. B
    2.64 kJ kg −-− 1 K −-− 1
  3. C
    8.49 kJ kg −-− 1 K −-− 1
  4. D
    9.26 kJ kg −-− 1 K −-− 1
View written solutionFree

Correct answer: D

  1. Break the process into reversible steps for 1 kg1\,\text{kg}1kg of substance:

    1. Ice at 273 K273\,\text{K}273K →\to→ water at 273 K273\,\text{K}273K
    2. Water at 273 K273\,\text{K}273K →\to→ water at 373 K373\,\text{K}373K
    3. Water at 373 K373\,\text{K}373K →\to→ vapour at 373 K373\,\text{K}373K
    4. Vapour at 373 K373\,\text{K}373K →\to→ vapour at 383 K383\,\text{K}383K

    Total entropy change: ΔS=ΔS1+ΔS2+ΔS3+ΔS4\Delta S = \Delta S_1+\Delta S_2+\Delta S_3+\Delta S_4ΔS=ΔS1​+ΔS2​+ΔS3​+ΔS4​

  2. Step 1: Melting of ice at 273 K273\,\text{K}273K

    ΔS1=LfT=334273\Delta S_1 = \frac{L_f}{T} = \frac{334}{273}ΔS1​=TLf​​=273334​

    ΔS1≈1.223 kJ kg−1K−1\Delta S_1 \approx 1.223\,\text{kJ kg}^{-1}\text{K}^{-1}ΔS1​≈1.223kJ kg−1K−1

  3. Step 2: Heating liquid water from 273 K273\,\text{K}273K to 373 K373\,\text{K}373K

    For heating at constant pressure: ΔS2=mcℓln⁡T2T1\Delta S_2 = m c_{\ell} \ln\frac{T_2}{T_1}ΔS2​=mcℓ​lnT1​T2​​

    Here m=1m=1m=1, cℓ=4.2 kJ kg−1K−1c_{\ell}=4.2\,\text{kJ kg}^{-1}\text{K}^{-1}cℓ​=4.2kJ kg−1K−1.

    ΔS2=4.2ln⁡373273\Delta S_2 = 4.2\ln\frac{373}{273}ΔS2​=4.2ln273373​

    Using ln⁡x=2.303log⁡x\ln x = 2.303\log xlnx=2.303logx, ln⁡373273=2.303(log⁡373−log⁡273)\ln\frac{373}{273}=2.303(\log 373-\log 273)ln273373​=2.303(log373−log273)

    =2.303(2.572−2.436)=2.303(0.136)=0.313=2.303(2.572-2.436)=2.303(0.136)=0.313=2.303(2.572−2.436)=2.303(0.136)=0.313

    Therefore, ΔS2=4.2×0.313≈1.315 kJ kg−1K−1\Delta S_2 = 4.2\times 0.313 \approx 1.315\,\text{kJ kg}^{-1}\text{K}^{-1}ΔS2​=4.2×0.313≈1.315kJ kg−1K−1

  4. Step 3: Vaporisation at 373 K373\,\text{K}373K

    ΔS3=LvT=2491373\Delta S_3 = \frac{L_v}{T} = \frac{2491}{373}ΔS3​=TLv​​=3732491​

    ΔS3≈6.678 kJ kg−1K−1\Delta S_3 \approx 6.678\,\text{kJ kg}^{-1}\text{K}^{-1}ΔS3​≈6.678kJ kg−1K−1

  5. Step 4: Heating steam from 373 K373\,\text{K}373K to 383 K383\,\text{K}383K

    ΔS4=mcvln⁡383373=2.0ln⁡383373\Delta S_4 = m c_v \ln\frac{383}{373} = 2.0\ln\frac{383}{373}ΔS4​=mcv​ln373383​=2.0ln373383​

    ln⁡383373=2.303(log⁡383−log⁡373)\ln\frac{383}{373}=2.303(\log 383-\log 373)ln373383​=2.303(log383−log373)

    =2.303(2.583−2.572)=2.303(0.011)=0.0253=2.303(2.583-2.572)=2.303(0.011)=0.0253=2.303(2.583−2.572)=2.303(0.011)=0.0253

    Hence, ΔS4=2.0×0.0253≈0.0506 kJ kg−1K−1\Delta S_4 = 2.0\times 0.0253 \approx 0.0506\,\text{kJ kg}^{-1}\text{K}^{-1}ΔS4​=2.0×0.0253≈0.0506kJ kg−1K−1

  6. Add all contributions

    ΔS=1.223+1.315+6.678+0.0506\Delta S = 1.223 + 1.315 + 6.678 + 0.0506ΔS=1.223+1.315+6.678+0.0506

    ΔS≈9.27 kJ kg−1K−1\Delta S \approx 9.27\,\text{kJ kg}^{-1}\text{K}^{-1}ΔS≈9.27kJ kg−1K−1

    This matches closest with: 9.26 kJ kg−1K−1\boxed{9.26\,\text{kJ kg}^{-1}\text{K}^{-1}}9.26kJ kg−1K−1​

  7. Option check

    • A: 7.907.907.90 ❌
    • B: 2.642.642.64 ❌
    • C: 8.498.498.49 ❌
    • D: 9.269.269.26 ✅

Therefore, the correct option is D.

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