9701 · 23.4
Gibbs free energy change, ΔG flashcards
Revision flashcards for Cambridge 9701 Gibbs free energy change, ΔG (syllabus 23.4). Flip, recall, then mark a real past-paper question.
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What is Gibbs free energy change, ΔG?
The overall change in energy during a chemical reaction that is available to do useful work. It determines the spontaneity of a reaction.
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What is the Gibbs free energy equation?
ΔG = ΔH - TΔS, where ΔH is the enthalpy change, T is the absolute temperature in Kelvin, and ΔS is the entropy change.
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What does a negative ΔG value indicate?
A negative ΔG indicates that a reaction is spontaneous (feasible) under the given conditions.
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What does a positive ΔG value indicate?
A positive ΔG indicates that a reaction is non-spontaneous (not feasible) under the given conditions. The reverse reaction will be spontaneous.
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What does it mean if ΔG = 0?
The system is at equilibrium. The rates of the forward and reverse reactions are equal.
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If ΔH is negative and ΔS is positive, is the reaction spontaneous?
Yes, always. ΔG will be negative at all temperatures.
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If ΔH is positive and ΔS is negative, is the reaction spontaneous?
No, never. ΔG will be positive at all temperatures.
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What is the most common mistake when calculating ΔG?
A units mismatch. ΔH is usually in kJ mol⁻¹, while ΔS is in J K⁻¹ mol⁻¹. You must convert one to match the other, typically by dividing ΔS by 1000.
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How is standard Gibbs free energy change (ΔG°) related to the equilibrium constant (K)?
ΔG° = -RT ln K, where R is the gas constant (8.31 J K⁻¹ mol⁻¹).
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How is standard Gibbs free energy change (ΔG°) related to standard cell potential (E°)?
ΔG° = -nFE°, where n is the number of moles of electrons transferred, and F is the Faraday constant (96500 C mol⁻¹).
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For a reaction with ΔH > 0 and ΔS > 0, how does temperature affect spontaneity?
The reaction becomes spontaneous only at high temperatures, when the TΔS term becomes larger than the ΔH term, making ΔG negative.
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What does the superscript '°' (plimsoll) signify in ΔG°, ΔH°, and S°?
It indicates standard conditions: 100 kPa pressure, 298 K (25 °C) temperature, and 1.0 mol dm⁻³ concentration for solutions.