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Section 3: The Second and Third Laws Section 3A Entropy

The document discusses the second law of thermodynamics and the definition of entropy. It states that the second law is concerned with the direction of spontaneous change, such as rivers running downhill and heat not flowing spontaneously from cold to hot. Entropy is then defined as a quantitative measure of this, where the change in entropy for a spontaneous process in the universe is positive. Specifically, the change in entropy of the system plus the surroundings must be greater than zero for a spontaneous process.

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Akib Imtihan
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78 views2 pages

Section 3: The Second and Third Laws Section 3A Entropy

The document discusses the second law of thermodynamics and the definition of entropy. It states that the second law is concerned with the direction of spontaneous change, such as rivers running downhill and heat not flowing spontaneously from cold to hot. Entropy is then defined as a quantitative measure of this, where the change in entropy for a spontaneous process in the universe is positive. Specifically, the change in entropy of the system plus the surroundings must be greater than zero for a spontaneous process.

Uploaded by

Akib Imtihan
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as PDF, TXT or read online on Scribd
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3.

Section 3: The Second and Third Laws

Section 3A Entropy

The 1st law tells us that, within an isolated system, the total energy content is constant:
• Energy can be changed from one form to another

• Energy cannot be created or destroyed

One of the goals of this course is to answer questions such as:
• Will a reaction proceed or not?

• Is a process spontaneous?
It is tempting to use Δr H as an indicator of spontaneity and postulate that exothermic reactions
can occur but not endothermic ones. But this is not true!
Example:

• CaCl2(s) dissolves spontaneously in water and the reaction is exothermic.

• Na2Cr2O7 (s) also dissolves spontaneously in water but the reaction is highly
endothermic.
Δr H is not a good indicator of spontaneity.

The first law tells us about the equivalence of various forms of energy but tells us nothing about
which conversions occur spontaneously. For example, these are consistent with the first law:
• a river runs uphill
• a hand heated by an ice cube
• one end of a metal rod gets hot while the other end cools down
• an ideal gas uniformly distributed in a rigid adiabatic container could undergo a
spontaneous transformation to move to one half of the container 

But these are against common sense. We need a new law! In particular, one that recognizes the
two classes of processes: spontaneous and non-spontaneous

3A.1 The Second Law


The second law is concerned with the direction of spontaneous change. Statements of the 2nd
law:
1. Rivers run downhill
2. “No process is possible in which the sole results is the absorption of heat from a
reservoir and its complete conversion to work” (Kelvin)

3. “Heat does not flow spontaneously from a cool body to a hotter body.” (Clausius)
These statements relate to heat. Recall heat is the energy stored in the kinetic energy of the
random motion of the atoms and molecules in matter. When a substance is heated up, the
magnitude of the kinetic energy is increased (more random motion). This increase is proportional
to the temperature change.

How is work different from heat? When one does mechanical work on a system, one moves all
of the molecules in a strictly coherent fashion.

There is an important difference between:
- raising 11 kg of iron by 1 m where all the Fe atoms are moved in a coherent non-
random fashion.
- Adding the same amount of energy by heating the iron with a burner where the energy
goes into random motion.
3.2

3A.2 The definition of entropy


3A.2(a) The thermodynamics definition of entropy
In order for the second law to be useful, we need a quantitative measure: the entropy, S, which is
a state function.

The differential of S is exact and related to the infinitesimal amount of heat transferred to the
system along a reversible pathway by,
d qrev
dSsystem = (Dimensions of S: J K-1 or molar S: J K-1 mol-1)

T
Therefore, for a change between initial and final states, we have
f
d qrev
∫i
ΔSsystem =
T
For a spontaneous process, the entropy change of the universe is positive: ΔSuniverse > 0
where ΔSuniverse = ΔSsystem + ΔSsurroundings

IMPORTANT: the second law doesn’t say ΔSsystem > 0 for a spontaneous process.



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