Entropy
Mr. Shields
Regents Chemistry
U13 L08
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Entropy
One thing you need to understand is that
the Universe tends toward disorder.
For example …
If I drop a box of crayons organized by color in a box
They don’t stay organized as they fall and hit the floor.
New born babies grow old as time passes as cellular defects
Continue to pile up. Old people do not grow young.
And chemical reactions don’t suddenly
Undo what has already taken place
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Entropy
-Here’s some more examples…
Your room !
- If I drop a glass on the floor neat one day and messy the
it shatters!
next!
- And what if I gathered up all those pieces
and re-dropped them on the floor again? Do
they recombine to form the original glass?
Of course not! These are examples of the universe tending
Towards disorder
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Entropy
Entropy (S) is the second thermodynamic term we’ll discuss.
This term is a measure of the DISORDER of a system
Greater disorder
This will happen
Less order
But this won’t
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Entropy of states
When matter changes states Entropy either increases
Or Decreases
Consider for example the following changes of states:
ICE

Water

STEAM
Has Entropy increased or decreases?
Sure, entropy has increased (i.e ∆S increases, and when
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Entropy increases the change in entropy is positive, +∆S).
Entropy of States
So we now know that when we change phase Entropy increases
in this order:
Solids < Liquids < Gas
∆S
What about the entropy of a dissolved solid ??? What has
Greater entropy, the solid
or the dissolved solid?
The solid exists as a
crystalline substance. The
solvated ions have less
order and thus have more
entropy
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Entropy Values
The larger the entropy value (i.e. the more positive) the
greater the disorder
Just as the change in Enthalpy is DH = Hproduct – H reactant
The change in Entropy is
DS = Sproduct – Sreactant
We can however measure DH directly. For instance, the heat
Lost or gained can be measured
in a CALORIMETER
But …
DS can not be measured directly.
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Entropy Values
We will not focus on how DS can be measured
Instead we will simply focus on attributes that will enable
Us to determine whether DS is + or –
If DS is +
- Entropy has increased
- Products have more disorder
- And Sproducts > Sreactants
For example: Propane (l)  Propane (g)
The entropy change would be positive (+DS) because
liquid Propane is more ordered than Propane gas.
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Entropy Values
If DS is (–)
- Entropy has decreased
- Products have less disorder
- And Sproducts < Sreactants
For example: Water(l)
 water(s)
Entropy is decreasing for this phase change since a solid
is more ordered than a liquid (∆S is negative)
Another example could be …
I2(g)  I2 (s)
why?
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Increasing Entropy
Summary of Positive changes in ENTROPY (+DS):
- Solid 
Liquid 
Solid 
Liquid 
liquid
Gas
Solution
Solution
- Entropy also increases
if the Temp. of the
system increases
Example: Hg (l, 20 deg C)  Hg (l, 40 deg C)
Higher ∆S … Why?
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Positive Entropy
Entropy also increases if there are more molecules of
product than reactant (+∆S)
examples: 2NH3(g)  N2(g) + 3H2(g)
(+∆S)
HgO  Hg + O2
Problem: What’s the sign of DS in the following reaction
NH3(g) + HCl(g)  NH4CL (s)
Negative (-DS) since order is increasing because we are
Going from more particles to fewer particles
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Spontaneity
Since the Universe favors disorder…
In any SPONTANEOUS process, the overall
ENTROPY of the Universe will always increases (+DS)
Therefore, if the DS of the rxn is +
Then the process will tend to be spontaneous
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Spontaneity
- We’ve discussed in this unit two thermodynamic terms
- Enthalpy and Entropy; Keep in mind…
- A change in Enthalpy (DHrxn) can be (+) or (-)
- If it is (-) the reaction is exothermic
- Exothermic reactions tend to be spontaneous
- A change in Entropy (DSrxn) can be (+) or (-)
- If DSrxn is (+) the rxn tends towards greater disorder
- Increasing disorder favors rxn spontaneity
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Predicting Spontaneity
So how can we predict if a reaction is spontaneous or not?
Know case 1 & 2
Case 1:
Spontaneous reaction
Case
DH
Rxn
DS
Rxn
Spontaneous
Rxn?
1
(-)
(+)
YES
favorable
favorable
Case 2:
Non-spontaneous reaction
(+)
(-)
NO
unfavorable
unfavorable
(-)
(-)
Case 3: ?
Depends upon the value
of DS (is it smaller than DH)
favorable
unfavorable
(+)
(+)
unfavorable
favorable
2
3
4
?
?
Case 4: ?
Depends upon the value
of DH (is it smaller than DS)
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PROBLEM: What is the sign of ∆H and ∆S for each
reaction and Predict if the rxn is Spontaneous, not
spontaneous or not predictable:
a) C6H12O6(s) + 6O2(g)  6CO2(g)+ 6H2O(g) + Heat
b) 2CO(g) + O2(g)  CO2(g) + Heat
c) 2C(s) + 2H2(g) + heat  C2H4(g)
a) -∆H, +∆S, Spontaneous
b) -∆H, -∆S, Not Predictable
c) +∆H, -∆S, Not Spontaneous
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