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Gibbs Free Energy Main Concept:
If a chemical or physical process is not driven by both entropy and enthalpy changes, then the Gibbs free energy change can be used to determine whether the process is thermodynamically favored.
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Thermodynamic Favoritism
Gibbs Free Energy Gibbs Free Energy and Thermodynamic Favoritism Thermodynamic Favoritism Thermodynamic Favored Definition Reverse Reactions and Thermodynamic Favoritism
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- Some exothermic reactions involve decreases in entropy
- When ΔG°>0, process is not thermodynamically favorable - When ΔG°<0, process is thermodynamically favorable.
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- In some reactions, we must consider both enthalpy and entropy to determine if reaction us thermodynamically favorable - freezing of water and dissolution of sodium nitrate in water are examples of need to look at enthalpy and entropy Freezing Water ∆H < 0 ∆S < 0 Dissolution of NaNO3 ∆H > 0 ∆S > 0
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Thermodynamic Favoritism
Main Concept: Some physical or chemical processes involve both a decrease in the internal energy of the components (∆Hº < 0) under consideration and an increase in the entropy of those components (∆Sº > 0). These processes are necessarily “thermodynamically favored” (∆Gº < 0).
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Thermodynamic Favoritism
Thermodynamic Favored Definition Reverse Reactions and Thermodynamic Favoritism
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- enthalpy and internal energy are the same thing
- “thermodynamically favored” means that products are favored at equilibrium (K>1); they will be produced
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- before, “spontaneous” was used to describe processes where ΔG°<0
- “thermodynamically favored” is used to avoid confusion b/c “spontaneous” ≠ “immediately”/“without cause”
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Spontaneous at Lower Temperatures Spontaneous at Higher Temperatures
– + Always Spontaneous Spontaneous at Lower Temperatures Spontaneous at Higher Temperatures Never Spontaneous
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- You should be able to determine the signs of ΔH° and ΔS° for a physical or chemical process
- where ΔH°<0 and ΔS°>0, no need to calculate ΔG° b/c reaction is thermodynamically favored
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- a process that is thermodynamically favored does not mean that it will proceed at a measureable rate - Any process where both ΔH°>0 and ΔS°<0 are not thermodynamically favored, (ΔG°>0) and must favor reactants at equilibrium (K<1) - This should make sense as ΔS° and ΔH° reverse when a chemical or physical process is reversed
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Overcoming Unfavored Reactions
Main Concept: External sources of energy can be used to drive change in cases where the Gibbs free energy change is positive.
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Overcoming Unfavored Reactions
Ways to Overcome Unfavored Reactions Examples Electricity Light
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- Electricity may be used to cause a process to occur that is not thermodynamically favored
Examples: - charging of a battery - electrolysis.
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- Light may be a source of energy for driving a process is not thermodynamically favored.
- Examples: photoionization of an atom (although separation of negatively charged electrons from remaining positively charged ion is highly endothermic, ionization is observed to occur with the absorption of a photon)
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overall conversion of carbon dioxide to glucose through photosynthesis [6CO2 (g) + 6 H2O (l) → C6H12O6 (aq) + 6 O2 (g)] has ΔG°= kJ/molrxn, yet is observed to occur through a multistep process initiated by absorption of several photons in the range of nm
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- thermodynamically unfavorable reactions may be made favorable by coupling it to favorable reactions, such as the conversion of ATP to ADP in biological systems - Here, coupling means the process involves a series of reactions with common intermediates, so the reactions add up to produce an overall reaction with a negative ΔG°
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