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Questions and Answers
What is the primary purpose of Gibbs free energy in thermodynamics?
What is the relationship between the entropy of the universe and the ΔG of the system?
What is the unit of Gibbs free energy (G)?
What is the condition for a reaction to be spontaneous according to the Gibbs free energy change (ΔG)?
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What is the equation for Gibbs free energy (G)?
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What is the primary condition for a reaction to be spontaneous?
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What is the relationship between ΔG and ΔSuniv for a spontaneous reaction?
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What is the formula for calculating ΔG at standard conditions?
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What is the unit of entropy (S°) in the given sample problem?
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What is the purpose of calculating ΔG for a reaction?
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Study Notes
Gibbs Free Energy Overview
- Gibbs Free Energy (G) predicts the spontaneity of a chemical reaction by focusing on system changes.
- Defined as G = H – TS, where H is enthalpy, T is temperature, and S is entropy.
- All components (H, S, G) are state functions with energy units.
Spontaneity and ΔG
- A change is spontaneous when ΔG is negative.
- If the entropy of the universe increases, the ΔG for the system will decrease.
- Positive ΔSuniv correlates with negative ΔG for the system, indicating a spontaneous process.
Calculating ΔG
- For constant temperature, ΔG = ΔH – TΔS.
- At standard conditions: ΔG⁰ = ΔH⁰ – TΔS⁰.
- Free energy (ΔG) represents the energy available to perform work in the system.
- Standard measurements typically reference a temperature of 25°C (298 K).
Example Calculation: Mg Reaction
- Reaction: Mg + ½ O2 → MgO
- Standard entropy values:
- S°(Mg) = 32.7 J/K·mol
- S°(O2) = 205.0 J/K·mol
- S°(MgO) = 26.9 J/K·mol
- Standard enthalpy values:
- ΔH°f(Mg) = 0
- ΔH°f(O2) = 0
- ΔH°f(MgO) = -601.2 kJ/mol
Steps for Determining ΔG
-
Step 1: Calculate ΔS°
- ΔS° = S°(MgO) – [S°(Mg) + ½ S°(O2)]
- ΔS° = 26.9 - (32.7 + 0.5 × 205.0) = -108.3 J/K·mol
-
Step 2: Calculate ΔH°
- ΔH° = Σ nΔHf° (products) – Σ mΔHf° (reactants)
- ΔH° = [-601.2] – [0 + 0] = -601.2 kJ
Understanding ΔSuniv and ΔG Relationship
- The direction of spontaneous change is indicated by negative ΔG.
- Conditions for spontaneity:
- ΔSuniv increases → spontaneous change (ΔG < 0)
- No change at equilibrium: ΔSuniv = 0, ΔG = 0
- ΔSuniv decreases → not spontaneous (ΔG > 0)
Implications of Gibbs Free Energy
- ΔG is essential for predicting whether a chemical reaction can occur under specified conditions.
- The spontaneity is directly tied to the free energy change, aiding in analyses of various chemical processes and reactions.
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Description
Learn about Gibbs free energy and its relation to the spontaneity of a reaction. Understand the definition of Gibbs free energy, its units, and how it is used to predict changes in a system.
