Question at position 25 A reaction has a Gibbs free energy change of –55 kJ/mol at 80 °C. What must be true? The reaction is at equilibrium.The reaction is non-spontaneous at all temperatures.The reaction is spontaneous at 80 °C.The reaction occurs rapidly.

Question

BlackTom AI · Accepted Answer

1) Relevant knowledge points and concepts
A negative Gibbs free energy change (ΔG < 0) at a given temperature means the process is thermodynamically spontaneous under those conditions. ΔG = 0 corresponds to equilibrium. ΔG alone does not give information about reaction rate (kinetics). Temperature dependence of ΔG can be understood from ΔG = ΔH − TΔS, so sign can change with temperature, but the provided ΔG value applies at the stated temperature.

2) Complete reasoning process step by step
Given: ΔG = −55 kJ·mol−1 at 80 °C. Convert temperature if needed: 80 °C = 353.15 K (not required to assess sign but shows the temperature reference). The numerical value is negative, so thermodynamic spontaneity criterion is satisfied at that temperature: ΔG < 0 → process is spontaneous under the stated conditions.

3) Calculations and intermediate steps
No detailed algebra required beyond recognizing the sign. For clarity:
- ΔG = −55 kJ·mol−1 (given)
- Compare to zero: −55 kJ·mol−1 < 0 → spontaneous at 80 °C.

If one wanted to consider how temperature might change ΔG, the relation ΔG = ΔH − TΔS shows T can alter sign; however, the given ΔG already reflects the effect of temperature (80 °C).

4) Why the correct answer is correct
“The reaction is spontaneous at 80 °C” is correct because spontaneity is defined by ΔG < 0 at the specified conditions. The negative value (−55 kJ·mol−1) directly indicates the reaction is thermodynamically favorable at 80 °C.

5) Why the other options are incorrect
- “The reaction is at equilibrium.” Incorrect because equilibrium requires ΔG = 0, not a negative value.  
- “The reaction is non-spontaneous at all temperatures.” Incorrect because ΔG is negative at 80 °C (so it is spontaneous there); also the statement “at all temperatures” is unjustified since ΔG depends on T.  
- “The reaction occurs rapidly.” Incorrect because ΔG concerns thermodynamic favorability, not the reaction rate; kinetics (activation energy, catalysts) determine how fast a spontaneous reaction proceeds.

6) Conclusion
Because ΔG = −55 kJ·mol−1 at 80 °C (ΔG < 0), the reaction is spontaneous at 80 °C. This confirms the provided correct answer: “The reaction is spontaneous at 80 °C.”

Question at position 25 A reaction has a Gibbs free energy change of –55 kJ/mol at 80 °C. What must be true? The reaction is at equilibrium.The reaction is non-spontaneous at all temperatures.The reaction is spontaneous at 80 °C.The reaction occurs rapidly.单项选择题

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