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Thermodynamics question

2018 · Shift 2 · Q18
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Thermodynamics question

2018 · Shift 2 · Q18

JEE AdvancedChemistryThermodynamicsMultiple correct+3 / −0.75
For a reaction, A  ⇌  P,A\,\,\rightleftharpoons\,\,P,A⇌P, the plots of [A]\left[ A \right][A] and [P]\left[ P \right][P] with time at temperature T1{T_1}T1​ and T2{T_2}T2​ are given below. JEE Advanced 2018 Paper 2 Offline Chemistry - Thermodynamics Question 30 English If T2>T1,{T_2} \gt {T_1},T2​>T1​, the correct statement(s) is (are) (Assume ΔH∘\Delta {H^ \circ }ΔH∘ and ΔS∘\Delta {S^ \circ }ΔS∘ are independent of temperature and ratio of lnKlnKlnK at T1{T_1}T1​ to lnKlnKlnK at T2{T_2}T2​ is greater than T2T1.{{{T_2}} \over {{T_1}}}.T1​T2​​. Here H,S,GH,S,GH,S,G and KKK are enthalpy, entropy, Gibbs energy and equilibrium constant, respectively.)
  1. A
    ΔH∘<0,ΔS∘<O\Delta {H^ \circ } \lt 0,\Delta {S^ \circ } \lt OΔH∘<0,ΔS∘<O
  2. B
    ΔG∘<0,ΔH∘>0\Delta {G^ \circ } \lt 0,\Delta {H^ \circ } \gt 0ΔG∘<0,ΔH∘>0
  3. C
    ΔG∘<0,ΔS∘<0\Delta {G^ \circ } \lt 0,\Delta {S^ \circ } \lt 0ΔG∘<0,ΔS∘<0
  4. D
    ΔG∘<0,ΔS∘>0\Delta {G^ \circ } \lt 0,\Delta {S^ \circ } \gt 0ΔG∘<0,ΔS∘>0
View written solutionFree

Correct answer: A, C

Step-by-step Solution

  1. Analyze the equilibrium plots to determine the equilibrium constants.

    • At temperature T₁: The first plot shows that at equilibrium, the concentration of the product P is greater than the concentration of the reactant A, i.e., [P]eq,T1>[A]eq,T1[P]_{eq, T_1} > [A]_{eq, T_1}[P]eq,T1​​>[A]eq,T1​​. The equilibrium constant is given by K1=[P]eq,T1[A]eq,T1K_1 = \frac{[P]_{eq, T_1}}{[A]_{eq, T_1}}K1​=[A]eq,T1​​[P]eq,T1​​​. Therefore, K1>1K_1 > 1K1​>1.
    • At temperature T₂: The second plot shows that at equilibrium, the concentration of the reactant A is greater than the concentration of the product P, i.e., [A]eq,T2>[P]eq,T2[A]_{eq, T_2} > [P]_{eq, T_2}[A]eq,T2​​>[P]eq,T2​​. The equilibrium constant is given by K2=[P]eq,T2[A]eq,T2K_2 = \frac{[P]_{eq, T_2}}{[A]_{eq, T_2}}K2​=[A]eq,T2​​[P]eq,T2​​​. Therefore, K2<1K_2 < 1K2​<1.
  2. Determine the sign of the standard enthalpy change, ΔH°. We are given that T2>T1T_2 > T_1T2​>T1​. We observe that the equilibrium constant KKK decreases as the temperature increases (since K1>1K_1 > 1K1​>1 and K2<1K_2 < 1K2​<1, we have K1>K2K_1 > K_2K1​>K2​). According to Le Chatelier's principle, if an increase in temperature shifts the equilibrium to the left (favoring reactants), the forward reaction must be exothermic. Therefore, ΔH∘<0\Delta H^\circ < 0ΔH∘<0. This is also described by the van't Hoff equation: ln⁡(K2K1)=ΔH∘R(1T1−1T2)\ln \left( \frac{K_2}{K_1} \right) = \frac{\Delta H^\circ}{R} \left( \frac{1}{T_1} - \frac{1}{T_2} \right)ln(K1​K2​​)=RΔH∘​(T1​1​−T2​1​) Since T2>T1T_2 > T_1T2​>T1​, the term (1T1−1T2)\left( \frac{1}{T_1} - \frac{1}{T_2} \right)(T1​1​−T2​1​) is positive. Since K2<K1K_2 < K_1K2​<K1​, the term ln⁡(K2K1)\ln \left( \frac{K_2}{K_1} \right)ln(K1​K2​​) is negative. For the equation to hold, ΔH∘\Delta H^\circΔH∘ must be negative.

  3. Determine the sign of the standard entropy change, ΔS°. The standard Gibbs free energy change, ΔG∘\Delta G^\circΔG∘, is related to the equilibrium constant by ΔG∘=−RTln⁡K\Delta G^\circ = -RT \ln KΔG∘=−RTlnK. It is also given by ΔG∘=ΔH∘−TΔS∘\Delta G^\circ = \Delta H^\circ - T\Delta S^\circΔG∘=ΔH∘−TΔS∘.

    • At T1T_1T1​, since K1>1K_1 > 1K1​>1, ln⁡K1>0\ln K_1 > 0lnK1​>0. Thus, ΔGT1∘=−RT1ln⁡K1<0\Delta G^\circ_{T_1} = -RT_1 \ln K_1 < 0ΔGT1​∘​=−RT1​lnK1​<0. The reaction is spontaneous.
    • At T2T_2T2​, since K2<1K_2 < 1K2​<1, ln⁡K2<0\ln K_2 < 0lnK2​<0. Thus, ΔGT2∘=−RT2ln⁡K2>0\Delta G^\circ_{T_2} = -RT_2 \ln K_2 > 0ΔGT2​∘​=−RT2​lnK2​>0. The reaction is non-spontaneous.

    The reaction changes from spontaneous at a lower temperature (T1T_1T1​) to non-spontaneous at a higher temperature (T2T_2T2​). This happens when both ΔH∘\Delta H^\circΔH∘ and ΔS∘\Delta S^\circΔS∘ are negative. The reaction is enthalpy-driven, but opposed by entropy. At low temperatures, the favorable enthalpy term dominates, making ΔG∘\Delta G^\circΔG∘ negative. At high temperatures, the unfavorable entropy term (−TΔS∘-T\Delta S^\circ−TΔS∘, which is positive if ΔS∘<0\Delta S^\circ < 0ΔS∘<0) becomes larger and makes ΔG∘\Delta G^\circΔG∘ positive. Mathematically:

    • ΔGT1∘=ΔH∘−T1ΔS∘<0  ⟹  ΔH∘<T1ΔS∘\Delta G^\circ_{T_1} = \Delta H^\circ - T_1\Delta S^\circ < 0 \implies \Delta H^\circ < T_1\Delta S^\circΔGT1​∘​=ΔH∘−T1​ΔS∘<0⟹ΔH∘<T1​ΔS∘
    • ΔGT2∘=ΔH∘−T2ΔS∘>0  ⟹  ΔH∘>T2ΔS∘\Delta G^\circ_{T_2} = \Delta H^\circ - T_2\Delta S^\circ > 0 \implies \Delta H^\circ > T_2\Delta S^\circΔGT2​∘​=ΔH∘−T2​ΔS∘>0⟹ΔH∘>T2​ΔS∘ Combining these gives T2ΔS∘<ΔH∘<T1ΔS∘T_2\Delta S^\circ < \Delta H^\circ < T_1\Delta S^\circT2​ΔS∘<ΔH∘<T1​ΔS∘. This implies T2ΔS∘<T1ΔS∘T_2\Delta S^\circ < T_1\Delta S^\circT2​ΔS∘<T1​ΔS∘. Since T2>T1T_2 > T_1T2​>T1​, this inequality can only be true if ΔS∘<0\Delta S^\circ < 0ΔS∘<0.
  4. Analyze the textual condition. The problem states that the "ratio of lnKlnKlnK at T1{T_1}T1​ to lnKlnKlnK at T2{T_2}T2​ is greater than T2T1{{{T_2}} \over {{T_1}}}T1​T2​​", which means ln⁡K1ln⁡K2>T2T1\frac{\ln K_1}{\ln K_2} > \frac{T_2}{T_1}lnK2​lnK1​​>T1​T2​​. From our analysis of the plots, we found ln⁡K1>0\ln K_1 > 0lnK1​>0 and ln⁡K2<0\ln K_2 < 0lnK2​<0. This makes the left side of the inequality, ln⁡K1ln⁡K2\frac{\ln K_1}{\ln K_2}lnK2​lnK1​​, negative. The right side, T2T1\frac{T_2}{T_1}T1​T2​​, is positive since temperatures are positive. A negative number cannot be greater than a positive number. Therefore, this condition is mathematically impossible and likely a typo in the question. The problem should be solved based on the unambiguous information from the plots.

  5. Evaluate the options based on the findings from the plots. Our analysis based on the plots concluded that ΔH∘<0\Delta H^\circ < 0ΔH∘<0 and ΔS∘<0\Delta S^\circ < 0ΔS∘<0. Also, the reaction is spontaneous at T1T_1T1​, so ΔG∘<0\Delta G^\circ < 0ΔG∘<0 is a valid statement for the conditions at T1T_1T1​.

    • A: ΔH∘<0,ΔS∘<O\Delta {H^ \circ } < 0, \Delta {S^ \circ } < OΔH∘<0,ΔS∘<O - This is consistent with our findings. Correct.
    • B: ΔG∘<0,ΔH∘>0\Delta {G^ \circ } < 0, \Delta {H^ \circ } > 0ΔG∘<0,ΔH∘>0 - This is incorrect because we found ΔH∘<0\Delta H^\circ < 0ΔH∘<0.
    • C: ΔG∘<0,ΔS∘<0\Delta {G^ \circ } < 0, \Delta {S^ \circ } < 0ΔG∘<0,ΔS∘<0 - This is consistent. We found ΔS∘<0\Delta S^\circ < 0ΔS∘<0, and ΔG∘<0\Delta G^\circ < 0ΔG∘<0 is true at temperature T1T_1T1​. Correct.
    • D: ΔG∘<0,ΔS∘>0\Delta {G^ \circ } < 0, \Delta {S^ \circ } > 0ΔG∘<0,ΔS∘>0 - This is incorrect because we found ΔS∘<0\Delta S^\circ < 0ΔS∘<0.

Final Answer

Based on the analysis of the provided plots, both statements A and C are correct. The additional textual condition is inconsistent and must be disregarded.

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