What’s wrong with these events?. Entropy. Introduction. We’ve all spent enough time in this universe to know that things don’t work this way… but how do we explain it?. ©1965, James Frankfort & The Curtis Publishing Co. Entropy. Here are some things we observe:
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What’s wrong with these events?
Entropy
Introduction
We’ve all spent enough time in this universe to know that things don’t work this way… but how do we explain it?
©1965, James Frankfort & The Curtis Publishing Co.
Entropy is a measure of the amount of dispersal or scattering of matter and energy.
Clausius chose the word "entropy" because the meaning, from Greek, en+tropein, translated is “transformed content“.
Introduced in 1865 by Rudolph Clausius
Clausius observed that heat cannot flow spontaneously from a material at lower temperature to a material at higher temperature. He also noted that any differences in temperature, pressure, and chemical potential tend to even out in a physical system that is isolated from the outside world.
Entropy is a measure of how much this eveningout process has progressed.
Additionally, Clausius observed that whenever energy is moved or converted, a small amount of energy is incrementally dissipated (Example: frictional losses in mechanical systems).
Entropy (S)
Historically, the concept of entropy evolved in order to explain why some processes are spontaneous and others are not; systems tend to progress in the direction of increasing entropy.
For isolated systems, entropy never decreases. This fact has several important consequences in science:
Lesser Entropy
Greater Entropy
Organized
Separated
Orderly
Disorganized
Mixed
Random
Increasing
Increasing
Increasing
Increasing
?
How does the entropy of a system change for each of the following processes?
(a) Condensing water vapor
Randomness decreases increases
Entropy decreases increases
(b) Forming sucrose crystals from a supersaturated solution
Randomness decreases increases
Entropy decreases increases
(c) Heating hydrogen gas from 600C to 800C
Randomness decreases increases
Entropy decreases increases
(d) Subliming dry ice
Randomness decreases increases
Entropy decreases increases

S
nS0(reactants)
S
nS0(products)
=
Entropy Changes in the System (DSsys)
The standard entropy of reaction ( ) is the entropy change for a reaction carried out at 1 atm and 25°C.
For our purposes in ChemB, entropy is determined this way:
Where q is the flow of heat in or out of a reaction, T is the constant Kelvin temperature, and mol is the amount of matter gaining or losing the heat.
Third Law of Thermodynamics
The entropy of a perfect crystalline substance is zero at the absolute zero of temperature.

S
nS0(reactants)
S
nS0(products)
=
What is the standard entropy change for the following reaction at 25°C?
197.9
205
213.6
DS = () means that entropy is DECREASING… matter and energy are more organized by this reaction.
The negative DS IS predictable in this reaction.
This reaction is going from 3 moles of gas to 2 moles of gas. When any homogeneous reaction reduces the number of particles, entropy will decrease.
Example Problem 2
What is the sign of the entropy change for the following reaction?
2 Zn (s) + O2 (g) 2 ZnO (s)
Entropy Changes in the System (DSsys)
When gases are produced (or consumed)
The total number of gas molecules goes down, DS is negative.
Entropy Changes in the Surroundings (DSsurr)
Endothermic Process
DSsurr < 0
Exothermic Process
DSsurr > 0
In a system, a process that occurs will tend to increase the total entropy of the universe.
Thus, while a system can go through some physical process that decreases its own entropy, the entropy of the universe (which includes the system and its surroundings) must increase overall.
The second law of thermodynamics is an axiom of thermodynamics concerning heat, entropy, and the direction in which thermodynamic processes can occur.
Spontaneous process:
DSuniv = DSsys + DSsurr > 0
Equilibrium process:
DSuniv = DSsys + DSsurr = 0