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Spontaneity
Recap of Enthalpy
• Describes chemical potential energy
stored in matter.
• Can only measure changes in enthalpy.
• Enthalpy is arithmetical.
– Reverse equation, reverse sign of H
– Multiply equation by some #, multiply H by
that number.
– Add two equations, add the H’s.
Spontaneous Processes
• A physical or chemical change that occurs
with no outside intervention.
• Some energy may be supplied to get the
process started – activation energy
Examples of Exothermic
Spontaneous Processes
• Burning methane gas, CH4, in the bunsen
burner.
CH4(g) + 2O2(g)  CO2(g) + 2H2O(l) H = -891 kJ
• Iron rusting
4Fe(s) + 3O2(g)  2Fe2O3(s) H = -1625 kJ
Some Spontaneous Processes are
Endothermic
• Dissolving NH4NO3(s) in water is an
example of a spontaneous endothermic
process.
• NH4NO3(s)  NH4+(aq) + NO3-(aq)
• Ice melting is another spontaneous,
endothermic process.
• H2O(s)  H2O(l)
Nature is lazy & disorganized
• 2 driving forces in nature
– Get to the lowest energy state
– Get to the most chaotic state
• For a system to get lower in energy, it must
release energy.
• So nature favors exothermic processes
with -H.
• But spontaneous endothermic processes
occur, so something else is important, too.
Entropy, S
• A measure of the disorder or randomness
of the particles of a system.
• Law of Disorder: spontaneous processes
always occur in the direction that
increases the chaos of the universe.
Unmelt.mov
Unmix.mov
Unshatr.mov
Chaos
• The more degrees of freedom a system
has, the more chaotic it can be.
• Degrees of freedom = ways you can move
& places you can be.
• Gases have much more entropy than
liquids & liquids have more than solids.
Entropy = 0?
• A pure crystal with no imperfections at 0K.
• Every atom is where it’s supposed to be.
• Nothing is moving.
Changes in Entropy, S
• S = Sfinal – Sinitial or Sproducts – Sreactants
• Nature wants to increase S, so Sfinal > Sinitial
• Nature wants S to be positive.
Predicting Changes in Entropy
• Sgas > Sliquid > Ssolid
• SMixture > SPure Substance
• SDissolved Solid > SSolid
• Sgas > SDissolved Gas
but
Predicting Entropy
• Which has more entropy, 1 mole of dry ice
or 1 mole of CO2 gas?
• 1 mole of CO2 gas
Predicting Changes in Entropy
• 2SO3(g)  2SO2(g) + O2(g)
S > 0
• 2 moles of gas on the reactant side.
• 3 moles of gas on the product side.
• Plus, the product side is a mixture.
Temperature & Entropy
• An increase in temperature increases the
random motion of the particles, so entropy
increases with temperature.
Predicting Spontaneity
• Use Gibbs free energy expression
• G = H - TS
• If G is negative, rxn is spontaneous.
• If G is positive, rxn is not spontaneous.
G = H + (-TS)
H
S
-TS
G
+
+
-
?
+
_
+
-
+
-
-
-
+
G = f(T)
Never
NOT
negative spont.
Always SPONT.
negative
?
G = f(T)
G = H - TS
• H is negative; S is positive.
Always spontaneous.
• H is negative; S is negative.
Spontaneity depends on temperature.
• H is positive; S is positive.
Spontaneity depends on temperature.
• H is positive; S is negative.
Never spontaneous.
G = H - TS
G is negative for spontaneous processes.
S is neg.
 in chaos.
S is pos.
 In chaos.
H is neg.
 in energy.
Can’t tell.
G = f(T).
H is pos.
 In energy.
G > 0.
Never
spontaneous
G < 0.
Always
spontaneous
Can’t tell.
G = f(T).
Summary
• Nature has 2 driving forces
• Tends to minimize enthalpy (potential
energy). Wants H to be negative.
• Tends to maximize entropy (chaos).
Wants S to be positive.
Summary
• Spontaneity is determined by the
combination of the enthalpy change and
the entropy change.
Predict the sign of G
• LiBr(s)  Li+(aq) + Br-(aq) + 48.83 kJ
• The reaction is exothermic so H is
negative. The chaos increases so S is
positive. Both enthalpy and entropy are
going in the direction preferred by nature.
This reaction is always SPONTANEOUS.
Predict the sign of G
• N2(g) + 2 O2(g) + 66.4 kJ  2 NO2(g)
• The reaction is endothermic so H is
positive. The chaos decreases (3 moles
of a gas to 2 moles of a gas) so S is
negative. Neither the enthalpy nor the
entropy goes in the direction that nature
prefers, so this reaction is NEVER
spontaneous.
Predict the sign of G
• 2 H2(g) + O2(g)  2 H2O(l) + 571.6 kJ
•
•
•
•
•
The reaction is exothermic: H is negative.
Chaos decreases: S is negative.
One driving force, enthalpy, is with nature.
Entropy is going against nature.
CANNOT predict the spontaneity of this rxn.