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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

⁠

more particles

less energy

⁠

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

⁠

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

⁠

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)

⁠

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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