2 of 19

Unit 9.1

Enduring Understanding:

Some chemical or physical processes cannot occur without intervention

Learning Objective:

Identify the sign and relative magnitude of the entropy change associated with chemical or physical processes

3 of 19

Entropy

Entropy can be described in several ways:

Measure of the thermal energy of a system divided by a unit of temperature (in K)

This energy is unavailable to use to do work

Measure of the energy lost to molecular disorder or randomness in a sample

Units: J/K (Joules per Kelvin)
Symbol: ΔS
Positive values = increased entropy
Negative values = decreased entropy
Third Law of Thermodynamics: entropy of a perfect crystal at 0K is 0

4 of 19

Entropy

Microstates: possible configurations
Take two particles in this small container for example:

5 of 19

Entropy

Microstates: possible configurations
Take two particles in this small container for example:

6 of 19

Entropy

Microstates: possible configurations
Take two particles in this small container for example:

There is 1 way (microstate) in which both particles are on the left

7 of 19

Entropy

Microstates: possible configurations
Take two particles in this small container for example:

There is 1 way (microstate) in which both particles are on the left
There is 1 way (microstate) in which both particles are on the right

8 of 19

Entropy

Microstates: possible configurations
Take two particles in this small container for example:

There is 1 way (microstate) in which both particles are on the left
There is 1 way (microstate) in which both particles are on the right
There are 2 ways (microstates) in which there is one particle on each side

9 of 19

Entropy

Microstates: possible configurations
Take two particles in this small container for example:

There is 1 way (microstate) in which both particles are on the left
There is 1 way (microstate) in which both particles are on the right
There are 2 ways (microstates) in which there is one particle on each side

This is the more probably configuration because it has the most microstates

10 of 19

Entropy

Real systems have HUGE numbers of microstates
When the particles or energy in a system are more spread out into the microstates we say that the system has more disorder; more entropy
When the particles or energy in a system are less spread out among the microstates we say that the system has less disorder; less entropy

11 of 19

States of Matter & Entropy

Solids	Liquids	Gases
Regular arrangement of particles Fixed positions with only vibrational KE Fewer microstates	Particles able to move within their volume Greater freedom of motion and can move throughout the system More microstates	Greatest freedom and molecular motion of particles Expand to fill volume thus increase in disorder Many microstates
Least amount of entropy	Greater amount of entropy	Greatest amount of entropy

12 of 19

States of Matter & Entropy

If you have equal amounts of gas in two containers with different volumes, the gas in the larger container will have greater entropy

More microstates
Greater disorder and dispersal

In an equation, a decrease or increase in entropy can be determined by the number of moles of gas

If there are more moles of gas on the product side, the entropy of the reaction has increased

ΔS is positive

13 of 19

Energy & Entropy

Entropy also increases when energy is more dispersed within a system
Let’s look at two identical systems at different temperatures

KMT states that the system at the higher temperature will have particles with greater average kinetic energy
The energy distributed into the particles will be greater
This leads to greater entropy of a system as temperature increases

14 of 19

Entropy Increase & Decrease--Summary

ENTROPY INCREASES System becomes more disorganized	ENTROPY DECREASES System becomes less disorganized
Decomposition reactions AB → A + B	Synthesis reactions C + D → CD
Increase in temperature	Decrease in temperature
Dissociation of ions or dissolution of a solute to form a solution (s) → (aq)	Precipitation or formation of a solid from aqueous solutions (aq) → (s)

15 of 19

Entropy Increase & Decrease--Summary

ENTROPY INCREASES System becomes more disorganized	ENTROPY DECREASES System becomes less disorganized
Endothermic phase change (s) → (l) → (g)	Exothermic phase change (g) → (l) → (s)
A reaction which results in an increase in the number of moles of gas	A reaction which results in a decrease in the number of moles of gas
Gas expansion (decreasing pressure, increasing volume)	Gas compression (increasing pressure, decreasing volume)

16 of 19

Practice: I Do

For each of the following pairs, choose the substance with the higher entropy at a given temperature. Explain your reasoning.

CO₂ (g) → CO₂ (s)
He (g) at 2.0 atm and He (g) at 0.020 atm
CaCO₃ (s) and CaO (s) + CO₂ (g)

17 of 19

Practice: I Do

For each of the following pairs, choose the substance with the higher entropy at a given temperature. Explain your reasoning.

CO₂ (g) → CO₂ (s)

CO₂ (g) has the higher entropy because, in a gaseous state, particles are in a more random arrangement and have more microstates compared to the solid state

18 of 19

Practice: I Do

For each of the following pairs, choose the substance with the higher entropy at a given temperature. Explain your reasoning.

He (g) at 2.0 atm and He (g) at 0.020 atm

He (g) at 0.020 atm has the higher entropy because gas will expand (volume increase) at lower pressures with increases random particle arrangement and microstates

19 of 19

Practice: I Do

For each of the following pairs, choose the substance with the higher entropy at a given temperature. Explain your reasoning.

CaCO₃ (s) and CaO (s) + CO₂ (g)

CaO (s) + CO₂ (g) because it has a gaseous species which has more entropy than a solid