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Chemical Thermodynamics

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Title: Chemical Thermodynamics


1
Chemical Thermodynamics Part II (Chapter 19)
2
Key Concepts of Chapter
Identifying Spontaneous Processes.
Identifying reversible and irreversible
processes. Entropy and its relation to
randomness. Second Law of Thermodynamics.
Predicting Entropy Changes of a Process. Third
Law of Thermodynamics. Relate temperature
change to entropy change. Calculating change in
standard entropy.
3
Free energy in terms of enthalpy and
entropy. Relating free energy change to
spontaneity. Calculating standard free energy
change. Relationship between free energy and
work. Calculating free energy ? under
nonstandard conditions.
4
From Chapter 5
Chemical thermodynamics is the study of energy
relationships in chemistry.
The First law of Thermodynamics
- energy cannot be created or destroyed only
converted from one form to another.
5
  • Enthalpy
  • heat transfer between the system and its
    surroundings under const. press.
  • Enthalpy is a guide to whether a reaction is
    likely to proceed.
  • It is not the only factor that determines whether
    a reaction proceeds.

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Spontaneous Processes
  • Occur without outside intervention
  • Have a definite direction.
  • The reverse process is not spontaneous.
  • Temperature has an impact on spontaneity.
  • Ex Ice melting or forming
  • Ex Hot metal cooling at room temp.

9
KI (aq) Pb(NO3)2 (aq) ? PbI2(s) KNO3 (aq)
When mixed ? Precipitate forms spontaneously. It
does not reverse itself and become two clear
solutions.
10
Reversible Irreversible
  • Reversible
  • System changes state and can be restored by
    reversing original process.
  • Ex Water (s) Water (l)
  • Irreversible
  • System changes state and must take a different
    path to restore to original state.
  • Ex CH4 O2 ? CO2 H2O

11
  • Whenever a system is in equilibrium, the reaction
    can go reversibly to reactants or products (water
    ? water vapor at 100 º C).
  • In a Spontaneous process, the path between
    reactants and products is irreversible. (Reverse
    of spontaneous process is not spontaneous).
  • Scrambled eggs dont unscramble

12
The Second Law of Thermodynamics
- The entropy of the universe always increases
in a spontaneous process and remains unchanged in
an equilibrium process.
13
But ma, its not my fault the universe wants my
room like this!gt
14
Entropy (S)
  • A measure of randomness or disorder
  • S entropy in J/Kmole
  • Increasing disorder or increasing randomness is
    increasing entropy.
  • Three types of movement can lead to an increase
    in randomness.

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Entropy is a state function
  • Change in entropy of a system
  • ?S Sfinal- Sinitial
  • Depends only on initial and final states, and not
    the pathway.
  • -?S indicates a more ordered state(think lt
    disorder or - disorder)
  • Positive () ?S less ordered state
  • (think gt disorder or disorder)

19
Entropy, S - a measure of disorder
Ssolid
Sliquid
Sgas
?
?
20
Increasing Entropy
21
Increasing Entropy
22
Increasing Entropy
23
If entropy always increases, how can we account
for the fact that water spontaneously freezes
when placed in the freezer?
Movement of compressor Evaporation and
condensation of refrigerant Warming of air
around container Net increase in the entropy of
the universe
24
  • On the AP exam, you will likely be asked to
  • predict whether a process leads to an increase in
    entropy or a decrease in entropy.
  • Determine if ?S is or
  • Determine substances or reactions that have the
    highest entropy.

25
  • Processes that lead to an Increase in Entropy
  • When a solid melts.
  • When a solid dissolves in solution.
  • When a solid or liquid becomes a gas.
  • When the temperature of a substance increases.
  • When a gaseous reaction produces more molecules.
  • If no net change in of gas molecules, can be
    or -, but small.

26
Predict whether the entropy change is greater
than or less than zero for each of the following
processes
  • Freezing liquid bromine
  • Evaporating a beaker of ethanol at room
    temperature
  • Dissolving sucrose in water
  • Cooling N2 from 80ºC to 20ºC

?Slt0
?Sgt0
?Sgt0
?Slt0
27
Predict whether the entropy change of the system
in each of the following reactions is positive or
negative
1)?S 2)?S 3)?S?
1.) Ag(aq) Cl-(aq)?AgCl(s) 2.) NH4Cl(s)?
NH3(g) HCl(g) 3.) H2(g) Br2(g)?2HBr(g)
28
According to the 2nd law of thermodynamics the
entropy of the universe always increases. ? What
if the entire senior class assembles in the
auditorium? Arent we decreasing disorder, and
therefore decreasing entropy? If so, how can
the second law of thermodynamics be true?
29
  • If we consider the senior class as the system,
    the
  • ?S of the system would indeed decrease.
  • ?S of the system is
  • In order for the students to gather, they would
  • Metabolize food (entropy increase of
    surroundings)
  • Generate heat (entropy increase of surroundings)
  • The magnitude of the entropy increase of the
    surroundings will
  • always be greater than the entropy decrease of
    the system.

30
Theoretical values ?Suniverse ?Ssystem
?Ssurroundings ?Suniverse (-10)
(20) ?Suniverse 10 means
entropy increases
31
The same can be considered in a chemical
process. When a piece of metal rusts 4Fe(s)
O2(g) ? 2Fe2O3(s) The entropy of the solid
slowly decreases.
Although this is a slow process, it is
exothermic, and heat is released into the
surroundings causing an overall increase in
entropy of the universe!
32
The Second Law of Thermodynamics
- The entropy of the universe increases in a
spontaneous process and remains unchanged in an
equilibrium process.
?Suniverse ?Ssystem ?Ssurroundings
?Suniverse gt 0 for spontaneous rxn
?Suniverse 0 at equilibrium
33
In terms of temperature, how would you describe
an object that has an entropy value of 0?
0 K Perfect solid crystal with no motion
Only Theoretical It is not possible to reach
absolute 0! Entropy of universe is always
increasing!
34
3rd Law of Thermodynamics the entropy of a
perfect crystalline substance is zero at absolute
zero Handout3 Laws
Based on 0 entropy as a reference point, and
calculations involving calculus beyond the scope
of this course, data has been tabulated
for Standard Molar Entropies ?Sº Pure substances,
1 atm pressure, 298 K
35
  • Standard Molar Entropies
  • ?Sº
  • Standard molar entropies of elements are not 0
    (unlike ?Hºf).
  • (0 entropy is only theoretical not really
    possible)
  • S.M.E of gases gt S.M.E of liquids and solids.
  • (gases move faster than liquids)
  • 3) S.M.E. increase with increasing molar mass.
  • (more potential vibrational freedom with more
    mass)
  • 4) S.M.E. increase as the number of atoms in a
    formula increase.
  • (same as above)

36
Calculating the Entropy Change
?Sorxn ?n So(products) - ?m So(reactants)
Units for ?S ?SJ/molK Since we are considering
?S J/K are often used because moles are assumed
and cancel in the calculations when considering
standard states.
37
Calculate the standard entropy change (?Sº) for
the following reaction at 298K
Al2O3(s) 3H2(g)?2Al(s) 3H2O(g)
38
?Sorxn ?n So(products) - ?m So(reactants)
Al2O3(s) 3H2(g)?2Al(s) 3H2O(g)
  • ?So 2Sº(Al) 3Sº(H2O) - Sº(Al2O3)
    3Sº(H2)

180.4 J/K
39
Predict the sign of ?Sº of the following
reaction. 2SO2(g) O2(g)? 2SO3(g) Entropy
decreases, - Lets Calculate
40
Calculate the standard entropy change (?Sº) for
the following reaction at 298K
2SO2(g) O2(g)? 2SO3(g)
?Sº -187.8 J K-1
41
Predicting spontaneous reactions
  • Spontaneous reactions result in an increase in
    entropy in the universe.
  • Rxs that have a large and negative ?? tend to
    occur spontaneously.
  • Spontaneity depends on enthalpy, entropy, and
    temperature.

42
Gibbs Free Energy (G)
  • Provides a way to predict the spontaneity of a
    reaction using a combination of enthalpy and
    entropy of a reaction.

43
If Both T and P are constant, the relationship
between ?G and spontaneity is
  • ?G is (-), rx is spon. forward
  • ?G is 0, rx is at equilibrium
  • ?G is () forward rx is not spontaneous(requires
    work) reverse rx is spontaneous.

44
(sum of standard free energies of formation of
products) minus (sum of standard free energies of
formation of reactants)
Coefficients from equation
the sum of
45
The values of ?Gºf of elements in there most
stable form is 0, just as with enthalpy of
formations.
46
Calculate the ?Grxn for the combustion of
methane at 298K and determine if the reaction is
spontaneous.
-818.0 KJ Spontaneous
47
Calculate the (?Gº) for the thermite reaction
(aluminum with iron(III) oxide).
-835.5 KJ spontaneous
48
Gibbs Free Energy Equation 2
  • This equation allows us to determine if a process
    provides energy to do work. A spontaneous
    reaction in the forward direction provides energy
    for work.
  • If not spontaneous, ?G equals the amount of
    energy needed to initiate the reaction.
  • Allows us to calculate the value of ?G as
    temperature changes.
  • Gibbs free energy (G) is a state function
    defined as
  • ?G ?H T?S (Given on AP Exam)
  • T is the absolute temperature
  • ?G ?H T?S G ?H T?S (when
    nonstandard)
  • If given a temperature change and asked to
    determine spontaneity or value of G, this is the
    equation you would use.
  • Value of ?G tells us if a reaction is spontaneous.

49
?G ?H T(?S) If we know the conditions of
?H and ?S, we can predict the sign of ?G. We
will see that Two conditions always produce the
same result, and two conditions depend on
temperature.
50
Predicting Sign of ?G in Relation to Enthalpy and
Entropy
?H
?S
?G
-
Always negative (spontaneous)
-
Always positive (nonspontaneous)
- -
Neg. (spontaneous) at low temp Pos.
(nonspontaneous) at high temp.

Pos. (nonspontaneous) at low temp Neg.
(spontaneous) at high temp.
51
?H
?S
?G
-
Different sign, not temperature dependent.
-
- -
Freezing
Same sign, temperature dependent.

Melting
52
?G ?H T(?S)
Some reactions are spontaneous because they give
off energy in the form of heat (?H lt 0). Others
are spontaneous because they lead to an increase
in the disorder of the system (?S gt 0).
Calculations of ?H and ?S can be used to probe
the driving force behind a particular reaction.
53
Example 19.2 The entropy change of the system
is negative for the precipitation
reaction Ag(aq) Cl-(aq) ?
AgCl(s) ?Ho -65 kJ Since ?S decreases rather
than increases in this reaction, why is this
reaction spontaneous?
54
Yes, entropy increases, which goes against the
second law. However, in this case, the entropy
decrease is minimal compared to the magnitude of
change in enthalpy. Therefore, the release of
heat drives the reaction to stability, which is
why it is spontaneous.Theoretical Values
?G ?H T?S
?G -65 298(-.030) -73.94
55
  • Problem
  • For a certain reaction, ?Hº -13.65 KJ and ?Sº
    -75.8 J K-1.
  • What is ?Gº at 298 K?
  • B) Will increasing or decreasing the temperature
    make the reaction become spontaneous? If so, at
    what temperature will it become spontaneous?

56
Given ?Hº -13.65 KJ ?Sº -75.8
J K-1 T 298 K
A) What is ?Gº at 298 K?
At 298 K the free energy is ?G ?H
T?S ?G -13.65 KJ 298(-.0758 KJ K-1)
8.94 KJ (Reaction is not spontaneous
at 298 K)
57
B) Will increasing or decreasing the temperature
make the reaction become spontaneous? If so, at
what temperature will it become spontaneous?
Because enthalpy and entropy have the same
signs, spontaneity is indeed temperature
dependent. Since going from not spontaneous to
spontaneous crosses the point of equilibrium, and
?G 0 at equilibrium, we can make ?G to 0 to
find the temperature at which equilibrium is
crossed.
0 ?H T?S 0 -13.65 KJ T(-.0758 KJ
K-1) T 13.65 KJ
0.0758 KJ K-1
T 180 K
Reaction is spontaneous below 180 K, not
spontaneous above 180 K
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