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Chemical Equilibrium question

2008 · Shift 1 · Q14
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Chemical Equilibrium question

2008 · Shift 1 · Q14

JEE AdvancedChemistryChemical EquilibriumMCQ+3 / −1
Statement 1 : For every chemical reaction at equilibrium, standard Gibbs energy of reaction is zero. and Statement 2 : At constant temperature and pressure, chemical reactions are spontaneous in the direction of decreasing Gibbs energy.
  1. A
    Statement 1 is True, Statement 2 is True; Statement 2 is correct explanation for Statement 1.
  2. B
    Statement 1 is True, Statement 2 is True; Statement 2 is NOT correct explanation for Statement 1.
  3. C
    Statement 1 is True, Statement 2 is False.
  4. D
    Statement 1 is False, Statement 2 is True.
View written solutionFree

Correct answer: D

Analysis of Statement 1

  1. Statement 1: For every chemical reaction at equilibrium, standard Gibbs energy of reaction is zero.

  2. Let's consider the relationship between the Gibbs energy of reaction (ΔrG\\\Delta_r GΔr​G) and the standard Gibbs energy of reaction (\\\Delta_r G^\\circ). The equation is given by: ΔrG=ΔrG∘+RTln⁡Q\Delta_r G = \Delta_r G^\circ + RT \ln QΔr​G=Δr​G∘+RTlnQ where RRR is the gas constant, TTT is the absolute temperature, and QQQ is the reaction quotient.

  3. At equilibrium, two conditions are met:

    • The Gibbs energy change for the reaction is zero: ΔrG=0\\\Delta_r G = 0Δr​G=0.
    • The reaction quotient QQQ is equal to the equilibrium constant KKK: Q=KQ = KQ=K.
  4. Substituting these equilibrium conditions into the equation, we get: 0=ΔrG∘+RTln⁡K0 = \Delta_r G^\circ + RT \ln K0=Δr​G∘+RTlnK Rearranging this gives the relationship between the standard Gibbs energy and the equilibrium constant: ΔrG∘=−RTln⁡K\Delta_r G^\circ = -RT \ln KΔr​G∘=−RTlnK

  5. From this equation, we can see that the standard Gibbs energy of reaction, \\\Delta_r G^\\circ, is zero only if ln⁡K=0\\\ln K = 0lnK=0, which implies that the equilibrium constant K=1K = 1K=1.

  6. However, a chemical reaction can be at equilibrium with any value of KKK (i.e., K>1K > 1K>1, K<1K < 1K<1, or K=1K = 1K=1). It is not necessary for KKK to be 1 for every reaction at equilibrium.

  7. Therefore, the statement that \\\Delta_r G^\\circ is zero for every chemical reaction at equilibrium is false. It is ΔrG\\\Delta_r GΔr​G (the Gibbs energy change under the specific equilibrium conditions) that is zero, not necessarily \\\Delta_r G^\\circ (the standard Gibbs energy change).

Analysis of Statement 2

  1. Statement 2: At constant temperature and pressure, chemical reactions are spontaneous in the direction of decreasing Gibbs energy.

  2. This statement describes the fundamental criterion for spontaneity of a process under conditions of constant temperature and pressure. The change in Gibbs energy, ΔG\\\Delta GΔG, determines the direction of a spontaneous process.

    • If ΔG<0\\\Delta G < 0ΔG<0 (negative), the process is spontaneous in the forward direction. A negative ΔG\\\Delta GΔG signifies a decrease in the system's Gibbs energy.
    • If ΔG>0\\\Delta G > 0ΔG>0 (positive), the process is non-spontaneous in the forward direction, but the reverse process is spontaneous.
    • If ΔG=0\\\Delta G = 0ΔG=0, the system is at equilibrium, and there is no net change.
  3. Thus, a chemical reaction will proceed spontaneously in the direction that leads to a decrease in the Gibbs energy of the system.

  4. Therefore, Statement 2 is true.

Conclusion

  • Statement 1 is False.
  • Statement 2 is True.

Based on this analysis, the correct option is the one that identifies Statement 1 as false and Statement 2 as true.

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