Title: Thermodynamics
1Thermodynamics
22 H2 O2 ? 2 H2O
- Given a chemical reaction
- Does it happen?
- How fast does it happen?
- Is it an equilibrium reaction?
- How does it compare with a competing reaction?
(If I mix H2, O2 and N2, what do I get?)
3Joes Rule of the Possible
- If it can happen, it will happen.
- But, that doesnt tell you how much, how fast,
how often, how easily - Thermodynamics picks up where Joes Rule leaves
off.
4Thermodynamics
- Thermodynamics deals with energy, as the name
implies, but not just energy. It includes the
study of all the different possible states of a
system and how the system moves between different
states.
5States
- I mix 1 molecule of O2 and 1 molecule of H2 in an
evacuated 1 L flask. How many different states
of this system are there? - A nearly infinite number of them!
6States of a system
H2
O2
O2
H2
7States of a system
O2
O2
H2
H2
8What the can we do?
- Thermodynamics deals with statistical analysis of
ensembles of states. - In our case, we are usually looking at a single
representative state of the system that is the
most probable state.
9Putting the thermo in thermodynamics
- As the name implies, thermo-dynamics is about
energy (thermoheat). - What does this mean for a reaction?
10Reaction Energies
- The energy change associated with a chemical
reaction is called the enthalpy of reaction and
abbreviated ?H. - ? H Hfinal - Hinitial
11Enthalpy of Reactions
- There are actually a number of different types of
enthalpies because enthalpy depends on
conditions. THEY ARE ALL JUST SPECIFIC TYPES OF
A GENERAL CONCEPT CALLED ENTHALPY. - ? H Hfinal - Hinitial
12Types of ? H
- ? H generic version
- ? Hrxn generic version
- ? Hº - enthalpy change under Standard Temperature
and Pressure (298 K, 1 atm) - ? Hf enthalpy of formation, refers to a
specific reaction type
13General Reaction Scheme hot pack
14Endothermic Reaction cold pack
Ea
Energy
Products
Reactants
?H
Reaction Coordinate
15Where does the Energy go?
- In the case of a chemical reaction, you need to
keep the different types of energy separate in
your mind - Bond energy energy INSIDE the molecules
- Thermal energy (heat) kinetic energy of the
molecules - Energy of the bath kinetic energy of solvent
or other molecules in the system
16Energy changes
- ? H represents the change in INTERNAL MOLECULAR
ENERGY. - ? H Hfinal - Hinitial
17Exothermic Reaction hot pack
18Exothermic energy changes
- ? H Hfinal Hinitial lt 0
- HinitialgtHfinal
- This energy is internal to the molecule.
- The excess gets absorbed by the rest of the
system as heat causing the molecules to move
faster (more kinetic energy) and the temperature
to increase.
19Endothermic Reaction cold pack
Ea
Energy
Products
Reactants
?H
Reaction Coordinate
20Endothermic energy changes
- ? H Hfinal Hinitial gt 0
- HinitialltHfinal
- This energy is internal to the molecule and must
come from somewhere. - The additional energy required by the system gets
absorbed from the rest of the system as heat
causing the molecules to move slower (less
kinetic energy) and the temperature to decrease.
21The hard part is getting over the hump.
22Ea Activation Energy
- The tale of a reaction is not limited strictly to
the identity and energetics of the products and
reactants, there is a path (reaction coordinate)
that must get followed. - The hump represents a hurdle that must be
overcome to go from reactants to products.
23How do you get over the hump?
- If you are at the top, it is easy to fall down
into the valley (on either side), but how do you
get to the top?
24How do you get over the hump?
- The molecules acquire or lose energy the same
way by colliding with each other! - The energy comes from the bath, the rest of the
system.
25Types of ? H
- ? H generic version
- ? Hrxn generic version
- ? Hº - enthalpy change under Standard Temperature
and Pressure (298 K, 1 atm) - ? Hf enthalpy of formation, refers to a
specific reaction type
26Enthalpy is a State Function
- Whats a state function?
- A state function is a value that is a function
only of the initial and final states of the
system, not the path you take to get there!
27Climbing Mt. Everest
- Suppose you start at Himalayan Base Camp 1,
climb to the summit of Everest over the course of
3 weeks, then return to Himalayan Base Camp 1.
28Climbing Mt. Everest
- Back at base camp, I figure out my altitude
change. What is it? - ZERO Im back where I started
29Climbing Mt. Everest
- I did a lot of work along the way, but all that
matters is Im back where Im started. The net
change in altitude is NADA, ZERO, ZILCH!
30Enthalpy as a State Function
- Enthalpy is like that. It doesnt care how you
got where you are going, it simply looks at the
difference from where you started.
31Path doesnt matter!
Actual path
Products
? H
Reactants
32Energy Considerations
- Energy is an important consideration in any
physical or chemical process. - You need to climb the hill!
33Have you ever seen a ball roll uphill?
- The universe is a lazy place!
- Everything seeks its lowest energy state.
- Given the chance, systems always seek the lowest
energy possible.
34Am I lying?
- How did the ball get to the top of the hill in
the first place?
35Little nit-picking definitions
- There are always two (at least) regions to
consider - The system the object under study
- The surroundings the rest of the universe
36You can go upbut someone goes down.
- You can add energy to the system, but you must
take it from the surroundings. - The issue with my lazy mans definition of the
universe is a thing called spontaneity.
37Spontaneity is
- Spontaneity means that the observed change
happens without a push. It naturally occurs
without being forced. - The ball spontaneously rolls down the hill. We
can force it back up the hill, but we have to put
in energy.
38Still seems like Im lying
- If what I say is true, then ALL observed changes
in the world around us that happen without being
forced would have to be downhill in energy. - Is that true?
39If energy were the whole story
- Why would water evaporate?
- It is an endothermic process with an activation
barrier, so it requires energy to be put into the
system. Yet, water spontaneously evaporates even
at near freezing temperatures. (And actually
sublimes when frozen!)
40- BUT
- ENERGY CHANGES ARENT THE WHOLE STORY!
41The rest of the story
- The energy of the molecules and their motions are
one part of the story the thermo part. - There is also the distribution of atoms within
the allowed states. It not only matters what the
average energy of the system is, but which
molecules have what energies and what positions!
42The rest of the story
- is entropy (S) - is a measure of the
distribution of states. - Entropy is sometimes defined as disorder or
randomness. It is really more complicated than
that and represents the total number of different
micro-states available to the system.
43States of a system
O2
O2
O2
O2
44States of a system
O2
O2
O2
O2
45States of a system
O2
O2
O2
O2
46Imagine 3 molecules!
O2
O2
O2
O2
O2
O2
47Imagine 3 molecules!
O2
O2
O2
O2
O2
O2
48Imagine a MOLE of MOLECULES!!!
O2
O2
O2
BIG EFFING MESS!!!!
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
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O2
O2
O2
O2
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O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
THANK GOD FOR STATISTICS
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
49Entropy is
- a state function.
- Entropy gets handled much the same as enthalpy.
- There are tables of entropy values, and it is
usually the change (? S) that matters more than
the absolute amount.
50Some examples
- What has more entropy 1 mole of water or 1 mole
of steam? Why? - 1 mole of steam the molecules in steam are not
associated with each other and are, therefore,
free to explore more positions and energy states!
51Some examples
- What has more entropy 1 mole of water or ½ mole
of water mixed with ½ mole of methanol? Why? - The mixture there are the same number of
molecules in both systems, but the mixture allows
for more possible distributions of the molecules!
52Clicker question
- In the following process what is the sign of ?S
- 2 H2 (g) O2 (g) ?2 H2O (g)
- Positive
- Negative
- Zero
- I dont know, Im just glad to be here.
53Clicker
- If delta S is negative, and that is the only
consideration, should the reaction - Happen
- Not Happen
- I dont know
- You are a beautiful animal
- Your mother
54Suppose I want an exact number?
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56The Laws of Thermodynamics
- 1st Law Conservation of Energy
- 2nd Law The Entropy of the universe is always
increasing for spontaneous changes. - 3rd Law A perfect crystal at 0 K has no entropy.
573rd Law
- The third law is interesting (and important).
Unlike enthalpy, we have an absolute zero for
entropy. Thats why Appendix II shows S values
not ?S values.
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59Suppose I want an exact number?
- 2 H2 (g) O2 (g) ?2 H2O (g)
- ?S0 SS0products SS0reactants
- ?S0 2S0(H2O(g))
- 2S0(H2(g)) S0(O2(g))
- ?S0 2mol188.8 J/molK
- 2mol130.7 J/molK 1 mol205.2 J/molK
- ?S0 - 89 J/K
60From a thermodynamic standpoint
- Delta S - 89 J/K is
- Good
- Bad
- Indifferent
- You are still a beautiful animal.
612 H2(g) O2 (g) ?2 H2O (g)
- ?S0rxn -89.0 J/K
- Again, this assumes stoichiometric quantities of
everything. - Does this number make sense?
- What does a NEGATIVE change in entropy mean?
622 H2(g) O2 (g) ?2 H2O (g)
- ?S0rxn -89.0 J/K
- What does a NEGATIVE change in entropy mean?
- ?S0rxn S (products) S (reactants)lt0
- S(products) lt S(reactants)
- of product states lt of reactant states
- Does this make sense?
- ABSOLUTELY there are fewer molecules so there
are fewer states!
63Does this number make sense?
- 2 H2 (g) O2 (g) ?2 H2O (g)
- ?S0 - 89 J/K
- ?S is negative meaning
- The products have LESS entropy than the
reactants. - 3 moles of gas vs. 2 moles of gas
- 2 different gases vs. 1 gas
642 particles vs. 3 particles!
H2
H2O
H2O
O2
H2
65- 2 H2 (g) O2 (g) ?2 H2O (g)
- So, why does it happen if ?S is negative?
- ?H is also negative (?H0 -241.8 kJ/mol) the
lazy man wins this one!
66Thermodynamics is
- All about balancing H and S to determine
spontaneity.
67Spontaneous change
- A spontaneous change is one that happens
naturally, without being forced by an outside
agent. - Spontaneous change
- Water evaporating at room temperature.
- A rock rolling down hill.
- Non-spontaneous change
- Freezing water at room temperature.
- Rolling a rock uphill.
68Spontaneous change
- A spontaneous change is thermodynamically
favorable.
69Spontaneous change
- Thermodynamics is all about balancing enthalpy
and entropy. - Some processes are enthalpically and entropically
favorable. - Some process are enthalpically and entropically
unfavorable. - What about when one property is favorable and the
other is unfavorable?
70Balancing entropy and enthalpy
- Gibbs Free Energy
- ? G ? H -T ? S
- If ? G gt0 then reaction is NOT spontaneous.
- If ? G lt0 then reaction IS spontaneous
- If ? G 0 thenthe reaction is at equilibrium
more later!
71Balancing entropy and enthalpy
- Gibbs Free Energy
- ? G ? H -T ? S
- 4 possibilities
- ? H is -, ? S is -
- ? H is -, ? S is
- ? H is , ? S is
- ? H is , ? S is -
72Balancing entropy and enthalpy
- ? G ? H -T ? S
- ? H is -, ? S is -
- ? H is -, ? S is this is the best!!!
- ? H is , ? S is
- ? H is , ? S is - this is the worst!!!
73Balancing entropy and enthalpy
- ? G ? H -T ? S
- ? H is -, ? S is this is the best!!!
- ?G will ALWAYS be negative
- ? (-) ()()
74Balancing entropy and enthalpy
- ? G ? H -T ? S
- ? H is , ? S is - this is the worst!!!
- ?G will ALWAYS be positive
- ? () ()(-) ()()
75Balancing entropy and enthalpy
- ? G ? H -T ? S
- ? H is , ? S is
- Its gonna depend on the temperature.
- ? G ( ?H) (T)( ?S) ( ?H)-(T ?S)
- Is T ?S bigger than or smaller than ?H?
76Balancing entropy and enthalpy
- ? G ? H -T ? S
- ? H is -, ? S is -
- Its gonna depend on the temperature.
- ? G (- ?H) (T)(- ?S) (- ?H)(T ?S)
- Is T ?S bigger than or smaller than ?H?
772 H2 (g) O2 (g) ?2 H2O (g)
- ?S0 - 89 J/K
- ?H0 - 241.8 kJ
- This reaction is spontaneous at some
temperatures, not all!
782 H2 (g) O2 (g) ?2 H2O (g)
- ?S0 - 89 J/K
- ?H0 - 241.8 kJ
- G ? H -T ? S
- G (-241.8 kJ) T(-0.089 kJ/K)
- 0 (-241.8 kJ) T(0.089 kJ/K)
- 241.8 kJ T(0.089 kJ/K)
- T 2717 K
792 H2 (g) O2 (g) ?2 H2O (g)
- G ? H -T ? S
- G (-241.8 kJ) T(-0.089 kJ/K)
- T 2717 K
- When Tlt2717, the reaction is spontaneous.
- When Tgt2717, the T ? S term is now bigger than ?
H and the reaction is no longer spontaneous. - This reaction is better at lower temperatures!
80Clicker Question
- Who is the most powerful superhero of all?
- Superman
- Batman
- Santa Claus
- Joe
- Anyone but Joe
81What Holiday are you celebrating?
- Hanukkah
- Christmas
- Kwanzaa
- Christmukkah
- None of the above
82Question
- Is the following reaction spontaneous at 298 K?
- 8 H2(g) S8(s) ? 8 H2S (g)
- If I care about spontaneous, I care about ?G!
83? G0 ? H0 - 298 ? S0
- But if you look in Appendix II, youll find the
magic 3rd column - ? Gf0
848 H2(g) S8(s) ? 8 H2S (g)
- From Appendix II
- H2(g) S8(s) H2S (g)
- ? Gf0 0 kJ/mol 49.7 kJ/mol -33.4
kJ/mol - ? Hf0 0 kJ/mol 102.3 kJ/mol -20.6
kJ/mol - S0 130.7 J/mol K 430.9 J/mol K 205.8 J/mol K
858 H2(g) S8(s) ? 8 H2S (g)
- ?Grxn0 S? Gf0(products) - S? Gf0 (reactants)
- 8 mol -33.4 kJ/mol 8 mol 0 kJ/mol1 mol
49.7 kJ/mol - -267.2 kJ 49.7 kJ
- ?Grxn0 -316.9 kJ
- ?Grxn0 lt0, reaction is spontaneous.
86? G0 ? H0 - 298 ? S0
- 8 H2(g) S8(s) ? 8 H2S (g)
- ?Hrxn0 S ? Hf0(products) - S ? Hf0
(reactants) - ?Hrxn0 8-20.6kJ/mol 80102.3kJ/mol
- ?Hrxn0 -267.1 kJ
- ?Srxn0 S S0(products) - S S0 (reactants)
- ?Srxn0 8205.8 J/molK-8130.7 J/molK430.9
J/molK - ?Srxn0 169.9 J/K
87? G0 ? H0 - 298 ? S0
- ?Hrxn0 -267.1 kJ
- ?Srxn0 169.9 J/K
- ?Srxn0 0.1699 kJ/K
- G0 -267.1 kJ - 298 (0.1699 kJ/K)
- G0 -317.73 kJ
- Compare to ? G0 calc using ? G0f -316.9 kJ
88Why the difference?
- Remember ?H0f and ?G0f are relative. S0 is
absolute. - By definition ?G0f is 0 for an element as is ?H0f
but not S0
89Question
- Is the following reaction spontaneous at 500 C?
- 8 H2(g) S8(s) ? 8 H2S (g)
- If I care about spontaneous, I care about ?G!
- But now theres no naught!
908 H2(g) S8(s) ? 8 H2S (g)
- Can we take the short way?
- ?Grxn0 S? Gf0(products) - S? Gf0 (reactants)
- Wont work no naught!
- Only ? G0 ? H0 - T ? S0 works if T is not 298.
91? G0 ? H0 - 298 ? S0
- ?Hrxn0 -267.1 kJ
- ?Srxn0 169.9 J/K
- ?Srxn0 0.1699 kJ/K
- Same as beforeI only have one Appendix II!!!
- Can this be right?
- Yes and no.
92ASSUMING
- ? H0 and ? S0 are temperature independent.
- Is this a good assumption? Not perfect. We
already said that heating a material usually
increases its entropy. But it is a better
assumption for H and the S has a T term soits
the best we can do!
93? G0 ? H0 - T ? S0
- ?Hrxn0 -267.1 kJ
- ?Srxn0 169.9 J/K
- ?Srxn0 0.1699 kJ/K
- Same as beforeI only have one Appendix II!!!
- G0 -267.1 kJ (500C273.15K) (0.1699 kJ/K)
- G0 -398 kJ
- Even more spontaneous at 500 K.