CHEM 1312 Chapter Notes - Chapter 14: Thermodynamic Equilibrium, Boltzmann Constant, Reaction Quotient
Chapter 14: Entropy, Free Energy,
and Equilibrium
Spontaneous Processes:
• Spontaneous: process that does occur under a specific set
of conditions
• Nonspontaneous: process that does not occur under a
specific set of conditions
• previously (first semester):
-- used enthalpy (ΔH) to predict if reaction spontaneous
ΔH < 0 exothermic = spontaneous
ΔH > 0 endothermic = not spontaneous
• NOT ALWAYS THE CASE:
-- dissolving NaI(s) in water is endothermic, but
spontaneous
-- ice melting above 0 °C ΔH > 0 but spontaneous
-- H2O evaporating ΔH > 0 but spontaneous
• something else must be important in determining
spontaneity
• what else is changing?
-- dissolving NaI(s) more “random”, more disorder
-- ice melting more “random”, more disorder
-- H2O evaporating more “random”, more disorder
Entropy (S): a measure of how spread out or dispersed the
system’s energy is [often correlated with the disorder, or
randomness, of a system
-- increase disorder increase entropy (ΔS > 0)
• one way to visualize order and disorder probabilities
-- Deck of cards:
One way to have cards in MANY ways to have cards out
perfect order of order (after tossing in air)
-- probable event something that can happen in many
ways
= e.g. random (disordered) pile of cards
-- improbable event something that happens in a
limited number of ways
= e.g. perfectly ordered pile of cards
-- nature moves towards more probable (disordered)
states (i.e. increase in entropy)
Microstates: specific arrangement of particles
-- distribution with largest number of (energetically
equivalent) microstates most probable (because
most random, or energy most dispersed)
-- arrangements of 4 gas particles into 2 bulbs
• all 4 in left side:
• all 4 in right side:
• one on left side:
• one on right side:
• two on each side: MOST PROBABLE
• 1868 – Ludwig Boltzmann – quantitative definition of entropy
S = k ln W
-- k = Boltzmann constant (1.38 x 10−23J K–1)
-- W = number of energetically equivalent microstates
Document Summary
Spontaneous processes: spontaneous: process that does occur under a specific set of conditions, nonspontaneous: process that does not occur under a specific set of conditions, previously (first semester): - used enthalpy ( h) to predict if reaction spontaneous. H > 0 endothermic = not spontaneous: not always the case: - dissolving nai(s) in water is endothermic, but spontaneous. - ice melting above 0 c h > 0 but spontaneous. - h2o evaporating h > 0 but spontaneous: something else must be important in determining spontaneity. - dissolving nai(s) more random , more disorder. - ice melting more random , more disorder. Entropy (s): a measure of how spread out or dispersed the system"s energy is [often correlated with the disorder, or randomness, of a system. - increase disorder increase entropy ( s > 0) - deck of cards: one way to visualize order and disorder probabilities. One way to have cards in perfect order.
