16.3 The Effect of Temperature on Spontaneity
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Direction of heat flow
- Entropy changes in the surroundings are primarily determined by heat flow
- Exothermic reactions in a system at constant temperature increase the entropy of the surroundings
- Endothermic reactions in a system at constant temperature decrease the entropy of the surroundings
- The impact of the transfer of a given quantity of energy as heat to or from the surroundings will be greater at lower temperatures
- ∆Ssurr = - ∆H/T
- Where ∆Ssurr is the entropy of the surrounding ∆H is the enthalpy and T is the temperature in Kelvin
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Energy (G), also called “Gibbs Free Energy”
- Calculating Free Energy change at constant temperature and pressure
- ∆G = ∆H - T∆S
- where H is enthalpy, T is temperature in Kelvin, and S is entropy
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Free Energy and Spontaneity
- Reactions proceed in the direction that lowers their free energy (-∆G)
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|
∆H |
T∆S |
∆G |
Spontaneity |
|
negative |
positive |
negative |
spontaneous |
|
positive |
negative |
positive |
Non-spontaneous |
|
negative |
negative |
??? |
Spontaneous at low temperatures |
|
positive |
positive |
??? |
Spontaneous at high temperatures |
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