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AP Chemistry
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      • Notes
        • 1. Chemical foundations
        • 2. Atoms, molecules, and ions
        • 3. Stoichiometry
        • 4. Types of chemical reactions and solution stoichiometry
        • 5. Gases
        • 6. Thermochemistry
        • 7. Atomic structure and periodicity
        • 8. Bonding: general concepts
        • 9. Covalent bonding: orbitals
        • 10. Liquids and solids
        • 11. Properties of solutions
        • 12. Chemical kinetics
        • 13. Chemical equilibrium
        • 14. Acids and bases
        • 15. Acid-base equilibria
        • 16. Solubility and complex ion equilibria
        • 17. Spontaneity, entropy, free energy
        • 18. Electrochemistry
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    • Textbook (incomplete)
      • 1. Chemical foundations
      • 2. Atoms, molecules, and ions
      • 3. Stoichiometry
      • 4. Types of chemical reactions and solution stoichiometry
      • 5. Gases
      • 6. Thermochemistry
      • 7. Atomic structure and periodicity
      • 8. Bonding: general concepts
    • CED
      • 1. Atomic structure and properties
      • 2. Compound structure and properties
      • 3. Properties of substances and mixtures
      • 4. Chemical reactions
      • 5. Kinetics
      • 6. Thermochemistry
      • 7. Equilibrium
      • 8. Acids and bases
      • 9. Thermodynamics and electrochemistry
AP Chemistry
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17. Spontaneity, entropy, free energy
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17. Spontaneity, entropy, free energy

Advanced thermodynamics

enthalpy (H): measures heat
ΔH: measures flow of heat
entropy (S): measures how “chaotic” it is
ΔS: change in entropy (positive: more entropic/chaotic)
second law of thermodynamics: the entropy is the universe is always increasing
spontaneous: when a reaction occurs without outside influence
examples
more entropy
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more particles
less energy
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fewer particles

Entropy

entropy: probability of an atom/molecule having energy (usually J/K)
example: gas A and gas B in two glass balls connected by glass tube
four variations
A and B on left
A on left, B on right
B on left, A on right
A and B on right
probability is 25% for each
solid/liquid: lower ΔS because they move less
third law of thermodynamics: a perfect crystal at 0 K has an entropy of 0 J/K
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thermodynamically favorable: when a reaction occurs without outside influence (spontaneous)
Gibb’s free energy (G): measurement of spontaneity
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ΔG: Gibb’s free energy
ΔH: enthalpy
T: temperature (K)
ΔS: entropy
ΔG is negative: thermodynamically favorable
ΔG is positive: thermodynamically unfavorable
ΔH
ΔS
favorable temperature
-
+
any
+
+
high
-
-
low
+
-
none (never favorable)
There are no rows in this table
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ΔG

controlled by kinetics: reaction is thermodynamically favorable but slow (high activation energy)
equilibrium
reactants → products
K < 1: favors reactants (ΔG > 0)
K > 1: favors products (ΔG < 0)
K = 1: equilibrium (ΔG = 0)
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ΔG: Gibb’s free energy
R: gas constant (8.3145 J/K×mol)
T: temperature (K)
K: equilibrium constant

Reactions when ΔG > 0

electrolysis: adding energy to reverse the reaction
e.g. charging a phone
when it stops working (V = 0): equilibrium
couple it with a reaction that has a very large and negative ΔG
first step must have a product that is a reactant in the second step
e.g. cellular respiration

Advanced thermodynamics
Entropy
ΔG
Reactions when ΔG > 0
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