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LESSON
Entropy
Spontaneity
IB Chemistry Power Points
Topic 15
Energetics
www.pedagogics.ca
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)
The greater the number of configurations (or microstates) in a particular system, the greater the entropy (disorder) of the system
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
Entropy changes (dissolving)
ΔS = Sfinal
– Sinitial
Entropy changes (heating)
ΔS = Sfinal
– Sinitial
Entropy changes (change of state)
ΔS = Sfinal
– Sinitial
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:
Consider the following reaction
Describe the change in entropy in this reaction.
Calculate the change in entropy in this reaction.
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
Spontaneous Processes
Processes that are spontaneous in one direction are non spontaneous in the reverse direction.
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.
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.
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.
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
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.
By knowing the sign (+ or -) of S and H, we can get the sign of G and determine if a reaction is spontaneous.
endothermic so ΔH must be positive
entropy increasing so ΔS must be positive
3
1
1
176 298 284
9136
0
8
91.4
1
G H T S
Jmol
kJmol
not spontaneous
3
1
1
176 298 284
9136
0
8
91.4
1
G H T S
Jmol
kJmol
increase in temperature increases spontaneity
3100 176 284
620
346
o
G H T S
T
T K
C