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Question #44051

Difficulty: 8/10 | 🟢 Free

The Question

Consider the gas-phase synthesis reaction at 300 K300\text{ K}:

\ceA(g)+2B(g)−>C(g)\ce{A(g) + 2B(g) -> C(g)}

The standard thermodynamic parameters at 300 K300\text{ K} are provided below:

SpeciesΔHf∘ (kJ mol−1)Sm∘ (J K−1mol−1)Cp,m∘ (J K−1mol−1)\ceA(g)2025050\ceB(g)1015040\ceC(g)035030\begin{array}{|c|c|c|c|} \hline \text{Species} & \Delta H^\circ_f \text{ (kJ mol}^{-1}\text{)} & S^\circ_m \text{ (J K}^{-1}\text{mol}^{-1}\text{)} & C^\circ_{p,m} \text{ (J K}^{-1}\text{mol}^{-1}\text{)} \\ \hline \ce{A(g)} & 20 & 250 & 50 \\ \ce{B(g)} & 10 & 150 & 40 \\ \ce{C(g)} & 0 & 350 & 30 \\ \hline \end{array}

Assuming the molar heat capacities at constant pressure (Cp,m∘C^\circ_{p,m}) are independent of temperature, determine the standard Gibbs free energy change (ΔG∘\Delta G^\circ) for the reaction at 500 K500\text{ K}.

(Given: ln⁡(5/3)=0.51\ln(5/3) = 0.51)

Options

  • A.

    +65.5 kJ mol−1+65.5\text{ kJ mol}^{-1}

  • B.

    +60.0 kJ mol−1+60.0\text{ kJ mol}^{-1}

  • C.

    +40.0 kJ mol−1+40.0\text{ kJ mol}^{-1}

  • D.

    +85.5 kJ mol−1+85.5\text{ kJ mol}^{-1}

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Numerical_Precision

Improve calculation stamina and avoid rounding errors in high-stakes competitive examinations.

Concepts Behind This Question

Gibbs-Helmholtz Equation|Enthalpy Definition|Hess's Law Application

Why This Question Is Tricky

Common Mistakes

This problem tests the rigorous application of the Gibbs-Helmholtz equation coupled with Kirchhoff's Laws.

The 'Hidden Variable' trap is the temperature dependence of Enthalpy and Entropy.

Students frequently default to the linear approximation (ΔGT2=ΔHT1−T2ΔST1\Delta G_{T_2} = \Delta H_{T_1} - T_2\Delta S_{T_1}), ignoring the ΔCp\Delta C_p term over large temperature ranges.

Doing so yields the primary distractor (+60.0 kJ mol−1+60.0\text{ kJ mol}^{-1}).

Partially updating only Enthalpy or only Entropy yields the other distractors, strictly penalizing incomplete thermodynamic integration.

Quick Facts About This Question

Question Information

SubjectChemistry
ChapterThermodynamics - Class 11 Part 1
ConceptGibbs-Helmholtz Equation
Difficulty8
Cognitive Load7
Target Solve Time160 sec
AccessFree