2012 15 3 and 15 4
TRANSCRIPT
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LESSON
Entropy
Spontaneity
IB Chemistry Power Points
Topic 15
Energetics
www.pedagogics.ca
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ENTROPYEntropy, S is a measure of the DISORDER or randomness of a system.
A single coin can have 2 possible configurations.
A system of 4 coins can have 16 (more disorder)
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The greater the number of configurations (or microstates) in a particular system, the greater the entropy (disorder) of the system
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Entropy on the Molecular Scale
Implications to a chemical system (MUST KNOW):
• more particles -> more states -> more
entropy
• higher T-> more energy states -> more
entropy
• less structure (gas vs solid) -> more states -> more
entropy
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Entropy changes (dissolving)
ΔS = Sfinal
– Sinitial
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Entropy changes (heating)
ΔS = Sfinal
– Sinitial
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Entropy changes (change of state)
ΔS = Sfinal
– Sinitial
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Standard entropy values: Sѳ
standard entropy is an absolute value
The standard entropy value for a substance is defined as the entropy increase of the substance when heated from 0 K to 298 K (standard conditions)
ΔSѳ = ΣSѳproducts
–
ΣSѳreactants
The standard entropy change for a reaction can be determined by:
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Consider the following reaction
Describe the change in entropy in this reaction.
Calculate the change in entropy in this reaction.
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Spontaneous Processes
• Spontaneous processes are those that can proceed without any outside intervention.
• The gas in vessel B will spontaneously effuse into vessel A, but once the gas is in both vessels, returning to the original state is not spontaneous
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Spontaneous Processes
Processes that are spontaneous in one direction are non spontaneous in the reverse direction.
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Spontaneous Processes• Processes that are spontaneous at one
temperature may be nonspontaneous at other temperatures.
• Above 0C it is spontaneous for ice to melt.
• Below 0C the reverse process is spontaneous.
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18.4Spontaneous reactions produce substantial amounts of products at equilibrium and release free energy.
Free energy is energy that is available to do work
The Gibbs free energy change, G is the maximum amount of free energy that can be extracted to do work.
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Gibbs Free Energy Change(some things to know)
1. If the G (for a particular reaction under a specific set of conditions) is negative, the forward reaction is spontaneous. (or vice versa)
2. The G for a reaction can be determined from standard free energies of formation, Gf values available on Data Tables. This is analogous to using standard enthalpies of formation, Hf values.
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The key equation to predict spontaneity:
This equation shows how G changes with temperature.
(We assume S & H values are not affected by T.)
G H T S
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Free Energy and Temperature
• There are two parts to the free energy equation: H the enthalpy term TS the entropy term
• The temperature dependence of spontaneity comes from the entropy term.
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By knowing the sign (+ or -) of S and H, we can get the sign of G and determine if a reaction is spontaneous.
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endothermic so ΔH must be positive
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entropy increasing so ΔS must be positive
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3
1
1
176 298 284
9136
0
8
91.4
1
G H T S
Jmol
kJmol
not spontaneous
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3
1
1
176 298 284
9136
0
8
91.4
1
G H T S
Jmol
kJmol
increase in temperature increases spontaneity
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3100 176 284
620
346
o
G H T S
T
T K
C