Chapter 1: Energy and Chemical Change
Section 5: Reaction Spontaneity
Done by Mr. Bashar Abuhattab
Sukina School - Girls
Grade 12 A
Starter Activity
Spontaneous process: Any physical or chemical change
that once begun, occurs with no outside intervention
Starter (2 min)
Examine given picture.
What chemical Reaction had happened to the boat?
Predict the Reaction?
Something other than ΔH plays a role in determining whether a
chemical process occurs spontaneously under a given set of conditions.
That something is called entropy.
exothermic process
spontaneous
endothermic process
non-spontaneous
What is entropy? ∆S
• Is a measure of the number of possible ways
that the energy of a system can be distributed
and related to the freedom of the system’s
particles to move and number of ways they
can be arranged.
Can you describe the entropy here??
What is entropy?
Entropy (S) is a measure of the number of possible ways that the energy of a system can be
distributed (Randomness of a system)
Gas particles : more
disorder , so they have
the largest entropy
Liquid particles : less
disorder , so they have the
lower entropy than gas
solid particles : less
disorder , so they have the
lower entropy than liquid
Remember : the more distributed the particles the more disorder and the higher
the entropy (number of particles , volume , Energy , freedom of movement )
Less number of arrangements
Less random (-)
more number of arrangements
More random (+)
The tendency towards increased entropy is
summarized in the second Law of thermodynamics
States that, spontaneous process always proceed
in such a way that entropy of the universe
increases.
Predicting changes in entropy
The change in entropy of a system,
ΔSsystem = Sproducts – Sreactants
If the entropy of a system increase
If the entropy of a system decrease
S products >S reactants
S products < S reactants
Δs system is positive.
Δs system is negative.
Predicting changes in entropy
1.
Entropy changes associated with changes in state can be predicted.
Entropy increases as a substance changes from a solid to a liquid and from a liquid to a gas
H2O(l)
H2O(g)
ΔS system > 0
CH3OH(s)
CH3OH(l)
ΔS system > 0
Predicting changes in entropy
2.Assuming no change in the physical state occurs, the entropy of the system usually increases when the
number of gaseous product particles is greater than number of gaseous reactant particles.
Predicting changes in entropy
3. Random motion of particle of a substance- entropy - increases as its temperature increases.
Predicting changes in entropy
4. Dissolving of a gas in solvent always results in a decrease in entropy.
(aq) dissolved in water
O2(g)
O2(aq)
ΔS system < 0
ΔS system is negative for dissolving of oxygen in water
Predicting changes in entropy
5. With some exceptions, entropy increases when a solid or liquid dissolves in a solvent.
(aq) dissolved in water
 Application
• Predict the sign of ΔSsystem for each of the following changes.
• a. ClF(g) + F2(g) → ClF3 (g)
Δssystem negative
• b. NH3 (g) → NH3 (aq)
Δssystem negative
• c. CH3OH(l) → CH3OH(aq)
Δssystem Positive
• d. C10H8 (l) → C10H8 (s)
Δssystem negative
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 Application
What is the sign of ΔSsystem for the following reaction.
• Fe(s) + Zn2+(aq) → Fe2+ (aq) + Zn(s)
A.
B.
C.
D.
Positive
Negative
Equilibrium
Not enough information
The states of the product
and the reactant are
identical, and thus it is not
possible to predict the
ΔSsystem for the reaction,
using only the chemical
equation
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Summarized
• Enthalpy is the sum of the system’s internal energy and the product of
its pressure and volume.
• Entropy: measurable physical property that is most commonly
associated with a state of disorder, randomness, or uncertainty.
• Entropy ∆S is positive = particles is more disorder.
Chapter 1: Energy and Chemical Change
Section 5: Reaction Spontaneity
Done by Mr. Bashar Abuhattab
Sukina School - Girls
Grade 12 A
Entropy, The Universe, And Free Energy
System and
surroundings together
constitute the
universe.
A system in thermodynamics
refers to that part of
universe in which
observation or study
are made.
universe = system
The surroundings
include everything
other than the
system.
+ surroundings
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Entropy, the Universe, and Free Energy
•
The second law of thermodynamics: the entropy of the universe must increase as a result of
a spontaneous reaction.
The following is true for any spontaneous process.
Δ S universe > 0
•
•
Universe = system + surroundings.
Any change in the entropy of the universe is the sum of changes occurring in the system and
surroundings.
Δ S universe = Δ S system + Δ S surroundings
In nature, Δ S universe tends to be positive for reactions and processes
under the following conditions:
1. The reaction or process is exothermic, which means Δ H system is negative the
surroundings. Δ S surroundings is positive.
2. The entropy of the system increases, so Δ S system is positive.
Δ S universe (+)
The entropy of the system increases
The reaction is exothermic
which means Δ H system is
negative
energy related to entropy is useless because it is dispersed and cannot be harnessed to do work
(ΔG °
Gibbs Free Energy (G
system)
system )
is negative, the
reaction
is spontaneous
(ΔG °
system )
is positive, the reaction
is nonspontaneous
Combined enthalpy-entropy function called Gibbs free energy
G system commonly called free energy: energy that is available to do work.
ΔS is usually expressed in J/K
ΔH is expressed in kJ
ΔS must be converted from J to
KJ by dividing the value over
1000.
When a reaction or process occurs under standard conditions (298 K and 1 atm), the standard free energy
change can be expressed as follows.
∆G° system = ∆H° system - T∆ S° system
Calculating free energy change
Calculate ΔG°
4 mol
∆H is negative
N2 (g) + 3H2 (g) → 2NH3 (g)
ΔH° system = -91.8 kJ ΔS° system = -197
J/K
2 mol ///// ∆S decreases
reaction is exothermic
∆S is negative
tends to make the reaction nonspontaneous
tends to make the reaction spontaneous
ΔG will determine whether the
reaction is spontaneous or
nonspontaneous
ΔG° = ΔH° - TΔS°
ΔG° = -91.8 kJ – [(298 K)(-0.197 kJ/K)]
ΔG° = -33.1 kJ
ΔG ° for this reaction is negative, so the reaction is spontaneous
Determine whether each of the following
reactions is spontaneous or non- spontaneous.
• ∆Hsystem=-75.9 kJ, T= 273 K, ∆Ssystem= 138 J/K
Spontaneous
• ∆Hsystem= 365 kJ, T= 388 K, ∆Ssystem= -55.2 J/K
Non-spontaneous
• ∆Hsystem=-27.6 kJ, T = 535 K, ∆Ssystem= -55.2 J/K
Non-spontaneous
• ∆Hsystem= 452 kJ, T= 165 K, ∆Ssystem= 55.7 J/K
Non-spontaneous
T = 3910 K