A.P. Chapter 18 Outline
Thermodynamics: Directionality of Chemical Reactions
I.
II.
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Reactant-Favored and Product-Favored Reactions: these designations indicate
the direction in which a chemical reaction will take place; either forward or
reverse
Chemical Reactions and the Dispersal of Energy
Energy will spread out (disperse) unless it is hindered from doing so
Most exothermic reactions are product favored at room temperature
Dispersal of energy occurs because the probability is much higher that energy
will be spread out over many particles than it will be concentrated in a few
If energy can be dispersed over a larger number of particles, it will
Energy becomes more dispersed when a system consisting of atoms or
molecules expands to occupy a larger volume
III.
Measuring Dispersal of Energy: Entropy, S
 ΔS = Sfinal - Sinitial = qrev/T
 The standard molar entropy of a substance at temperature T is a measure
of the quantity of energy that must be dispersed in that substance
for it to exist at T, it is ΔS form 0K to T
 Entropies of gases are usually much larger than those of liquids, which are
larger than those of solids
 Entropies of more complex molecules are larger than those of simpler
molecules
 Entropies of ionic solids that have similar formulas are larger when the
attraction among the ions are weaker
 Entropies decrease when a gas dissolves in a liquid
IV.
Calculating Entropy Changes
 ΔSº=Σ(moles product) x Sº(product) – Σ(moles reactant) x Sº(reactant)
V.
Entropy and the Second Law of Thermodynamics
 Whenever a product favored chemical reaction or physical process occurs,
energy becomes more dispersed
 The second law of thermodynamics states that the total entropy of the
universe (a system plus its surroundings) is continually increasing
 ΔSuniverse = ΔSsystem + ΔSsurroundings
 ΔSºsurroundings = -ΔH/T
 A reaction is certain to be product favored if it is exothermic and the
entropy of products is greater than the reactants.
Sign of ΔHsystem
Sign of ΔSsystem
Product-favored
negative
positive
yes
negative
negative
only at low T
positive
positive
only at high T
positive
netative
no
VI.
Gibbs Free Energy, G
 ΔGsystem = -TΔS
 If the entropy of the universe increases, the Gibbs free energy must
decrease
 A decrease in Gibbs free energy is characteristic of a process that ais
product-favored at constant temperature and pressure
 ΔG°system = ΔHºsystem - TΔSºsystem
 A negative value of ΔG°system indicates that the reaction is product favored
 Two conditions make ΔG°system more negative: if the reaction is
exothermic and if the products have greater entropy than the reactants
 Because ΔG°system is multiplied by T, the entropy of the system is more
important at higher temperatures
 ΔG° = Σ(moles of product) x ΔG°f (product) – Σ(moles of reactant) x ΔG°f
(reactant)
 Most reactions are product favored at some temperatures and ractant
favored at other temperatures
 T (at which ΔG° changes sign) = ΔH°/ΔSº
VII.
Gibbs Free Energy Changes and Equilibrium Constants
 ΔGº = -RT + ln Kº
 A simple correction can be made to ΔG° for nonstandard conditions
ΔG = ΔG° + RT ln Q
VIII.
Gibbs Free Energy, Maximum Work, and Energy Resources
 ΔG represents the maximum useful work that can be done by a product
favored system on its surroundings. It also represents the minimum work
that must be done to force a reactant favored reaction to occur
 Coupling a product favored reaction with a reactant favored reaction can
be done to cause the latter reaction to occur