Ethylene (CH2=CH2) represents one of the most valuable industrial chemicals. It is commonly used to synthesis styrene (C6H5C=CH2), the building block of polystyrene, a versatile plastic used to make a wide variety of consumer products. The overall chemical synthesis of styrene is given below: C6H6(l) + CH2=CH2(g) → C6H5C=CH2(l) + H2 (g) [table]
Compound | ∆Hf0 (298.15K), kJ mol-1 | S0 (298.15K), J K-1 mol-1 C6H6(l) | 48.7 | 173.26 CH2=CH2(g) | 52.47 | 219.32 C6H5C=CH2(l) | 108.0 | 240.5 H2 (g) | 0.0 | 114.7
[/table] The standard Gibbs free energy of the reaction, ∆Gr0, is 17.97 kJ mol-1 at 25.00 °C. Which of the following expressions best describes the spontaneity of the reaction?

Question

BlackTom AI · Accepted Answer

To assess spontaneity, we use the standard Gibbs free energy change ∆G° for the reaction. A negative ∆G° indicates a spontaneous process under standard conditions, while a positive ∆G° indicates non-spontaneity, and ∆G° = 0 would correspond to equilibrium.

Option a: Spontaneous. This would require ∆G° < 0. In the problem, ∆G° is given as +17.97 kJ/mol at 25.0 °C, which is positive. Therefore, this statement is incorrect because the reaction is not spontaneous under standard conditions.

Option b: Cannot be determined. The provided data include the standard Gibbs free energy change ∆G° for the reaction, explicitly 17.97 kJ/mol, which is sufficient to determine spontaneity in standard conditions. Thus, claiming it cannot be determined is inaccurate.

Option c: Non-spontaneous. Since ∆G° > 0, the reaction tends to proceed in the reverse direction under standard conditions, and it is non-spontaneous as written. This aligns with the thermodynamic criterion for spontaneity and the given positive ∆G° value.

Option d: At equilibrium. A reaction at standard conditions is at equilibrium only when ∆G° = 0. Here ∆G° is not zero; it is +17.97 kJ/mol, so the system is not at equilibrium under the stated standard conditions.

In summary, the sign and magnitude of ∆G° directly determine spontaneity: a positive value indicates non-spontaneity, a negative value indicates spontaneity, and zero indicates equilibrium. The provided data clearly show a positive ∆G°, making the reaction non-spontaneous under standard conditions.

CHM1051 MUM S2 2025 CHM1051 Practice Exam 2

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