17.2 Chemical Thermodynamics -...

January 13 1 Chemical Thermodynamics: Chaos

17.2 Chemical Thermodynamics

Dr. Fred Omega Garces Chemistry 201 Miramar College

Chaotic Spontaneity


Chemical Thermodynamics - Study of Chemical reaction energetics i.e., CH4 + 2O2 → CO2 + 2H2O + E

Thermodynamics Vs. Kinetics

Initial Final

Kinetics Domain (Path)

Thermodynamics Domain (State)

CH4 + O2

CO2 + H2O

Understanding a chemical reaction and its energetic properties lead to the spontaneity prediction of the reaction.


Example of common Spontaneous process: Aging Objects falling Time sun rise Ink mixing Chem. Exams i.e., We do not grow young, gas does not contract, H2O does not freeze at room temperature, time does not go backwards and exams are not canceled.

‡ Note under different conditions however, the reverse process can occur. i.e., Liquid freezes to solid at 0°C.

Spontaneous Process

If a process is spontaneous in one direction, then under the same conditions the reverse process is non-spontaneous.


History of Time - S. Hawkins

Begs the Question- Under different conditions, does this mean we can indeed grow young ? Stephen Hawkins - seminar at UCSB. Since the Big Bang, the Universe has been expanding. Ultimately we will reach the Big Crunch. When this occurs the universe will contract and “time will turn back... we will remember tomorrow (the future) and objects will self-assemble spontaneously. We will rise from our grave and end in the wombs of our mother.” The sun will rise from the west and set to the East.

This won’t happen because of the singularity Theorem. According to this theory, this point in space-time, the curvature tensor becomes infinite and we have an undefined term, i.e., dividing by zero (undefined).


History of Time


Spontaneity ≠ Rate Note that spontaneity has nothing to do with how fast process occurs. Spontaneity addresses whether the reaction does occur or does not occur.

Thermodynamics provides information on conditions which does favor spontaneity.

Spontaneous because it is a downhill process.

NaOH(s)

NaOH( aq)

E(release)

Spontaneous, but this is an up-hill process.

Why ?

NH4 Cl( s)

NH4 Cl( aq) E(absorb)

Ener

gy

Ener

gy


Enthalpy alone is not the answer Enthalpy by itself does not predict spontaneity... disorder plays a major role.

2nd Law of Thermodynamics When a system becomes more chaotic (more disordered) it is said to be at a state of higher entropy.

Another factor influencing spontaneity is an increase of entropy (S) of the universe.

Entropy - viewed as a measure of randomness or disorder.

Ink dispersing, objects

falling, your room becomes

more disordered with time.


Entropy Entropy is a measure of disorder A thermodynamic state function that increases with the number of energetically equivalent ways to arrange the components of a system to achieve a particular state.

S = k ln W K = R / Nav = 1.38e-23 J/K, W = microstates


The State of Things to Come

Your room is a MESS It is a natural law that your room ALWAYS gets trashed


Entropy - Why fight it ? Entropy describes the number of arrangements

(position/energy levels) that are available to a system.

... what this means is that the likely events are those in which there is the highest probability of existing.

i.e., Deal out 5 cards, what is the probability of a royal flush ? Royal flush - 4 hands out of 1,302,544 Other hands - 1,302,540 out of 1,302,544 There is a greater probability of getting nothing than getting something.

Nature prefers to take this path (of highest occurrence)

Nature prefers you get nothing.


Entropy and Microstates

Chemical system can be described in similar logic. Why do gas mix ? A system may take on a number of micro-states. Consider a 4-gas particle Relative probability of arrangements:


Entropy and Microstates

Chemical system can be described in similar logic. Why do gas mix ? A system may take on a number of micro-states. Consider a 4-gas particle Relative probability of arrangements: 1: 4: 6 there is a greater probability of occurrence for mixing the particles.


Entropy and Microstates (2)

For a large number of gas molecules, there is a hugh number of micro-states in which equal number of molecules are in both end of the flask. On the other hand, the opposite process (gas molecules at only one end) although not impossible - it is highly improbable.

Entropy states that any one of these micro-states are possible (or has some probability of occurrence).

More States [ Larger Entropy (more likely event)


Relative Entropy Entropy is a measure of disorder: In a phase change:

Solid Liquid Gas Highly ordered Less ordered Very disordered

solids g liquid g gas phase @ room temp.

S solid < S liquid << S gas


Absolute Entropy Unlike enthalpy - we have an absolute scale for entropy, Third Law of Thermodynamic: At absolute zero (0 K), a crystalline solid of any pure substance will have a Zero Entropy, S = 0 J/mol.

3rd Law of Thermodynamics:

At absolute zero, the entropy of a crystalline solid of a pure substance is equal to zero.

Charles


Second Law of Thermodynamics In any spontaneous process, there is always an increase in the entropy of the universe.

Entropy of the Universe is always increasing. (It is not conserved !!! )

ΔSuniv = ΔSsys + ΔSsurr

Predict whether a process is spontaneous: ΔSuniv (+) Spontaneous process. ΔSuniv (0) No Tendency to occur. (@ equilib.) ΔSuniv (-) Opposite event is spontaneous.

∴ Most probable micro-state is most random state.


So why does your room eventually get clean ? It is messy because nature prefers that state. Eventually it is straighten out. Why does it eventually get clean? Does this Violation th 2nd Law of Thermodynamics?

Important Factor: ΔSuniv (+) For 2nd law to be obeyed

ΔSuniv = ΔSsys + ΔSsurr ΔSsys (-) but ΔSsurr (+) room (-) you (+) | ΔSsys| << | ΔSsurr| g ΔSuniv

This result in a ΔSuniv (+) !!!


Predicting Relative S° Values order vs. disorder

What are the relative Entropy S° for various systems. 1. Temperature Change: 273K 295K 298K

Cu: S°, (J/mol•K)

2. Phase Change: Solid Liquid Gas Na : S°, (J/mol•K) H2O : S°, (J/mol•K) C : S°, (J/mol•K)

3. Dissolution of Solid, liq, gas Solid Liq/gas Aqueous solid NaCl: S°, (J/mol•K) solid AlCl3: S°, (J/mol•K) liquid CH3OH : S°, (J/mol•K) gas O2 : S°, (J/mol•K) gas diffusion O2 g O2+N2, (J/mol•K) ΔS° > 0

4. Complexity of element Li Na K Rb S°, (J/mol•K)


Tabulation of ΔS: Appendix For any Thermodynamic function: Function ΔX rxn = Σ n ΔX° prod - Σ n Δ X° react Enthalpy: ΔH rxn = Σ n ΔH° prod - Σ n ΔH° react Entropy: ΔS rxn = Σ n S° prod - Σ n S° react Free Energy: ΔG rxn = Σ n Δ G° prod - Σ n Δ G° react

Example: 19.27 Be(OH)2 (s) g BeO (s) + H2O (g)

S° J/mol •K


ΔS rxn = Σ n S° prod - Σ n S° react ΔS° rxn = [188.83 + 13.77] - 50.21

ΔS° rxn = 152.39 J / mol •K

Tabulation of ΔS: Appendix For any Thermodynamic function: Function ΔX rxn = Σ n ΔX° prod - Σ n Δ X° react Enthalpy: ΔH rxn = Σ n ΔH° prod - Σ n ΔH° react Entropy: ΔS rxn = Σ n S° prod - Σ n S° react Free Energy: ΔG rxn = Σ n Δ G° prod - Σ n Δ G° react

Example: 19.27 Be(OH)2 (s) g BeO (s) + H2O (g)

S° J/mol •K 50.21 13.77 188.83

17.2 Chemical Thermodynamics -...

Documents

Transcript of 17.2 Chemical Thermodynamics -...