Transcript chem 102 - Louisiana Tech University

Chemistry 102(001) Fall 2012
CTH 328 10:00-11:15 am
Instructor: Dr. Upali Siriwardane
e-mail: [email protected]
Office: CTH 311 Phone 257-4941
Office Hours: M,W 8:00-9:00 & 11:00-12:00 am;
Tu, Th, F 8:00 - 10:00am..
Exams: 10:00-11:15 am, CTH 328.
September 27, 2012 (Test 1): Chapter 13
October 18, 2012 (Test 2): Chapter 14 &15
November 13, 2012 (Test 3): Chapter 16 &18
Optional Comprehensive Final Exam: November 15, 2012
:
Chapters 13, 14, 15, 16, 17, and 18
CHEM 102, Spring 2012 LA TECH
18-1
Review of Chapter 6. Energy and Chemical
Reactions
6.1 The Nature of Energy
6.2 Conservation of Energy
6.3 Heat Capacity
6.4 Energy and Enthalpy
6.5 Thermochemical Equations
6.6 Enthalpy change for chemical Rections
6.7 Where does the Energy come from?
6.8 Measuring Enthalpy Changes: Calorimetry
6.9 Hess's Law
6.10 Standard Enthalpy of Formation
6.11 Chemical Fuels for Home and Industry
6.12 Food Fuels for Our Bodies
CHEM 102, Spring 2012 LA TECH
18-2
Chapter 18. Thermodynamics: Directionality
of Chemical Reactions
18.1
18.2
18.3
18.4
18.5
18.6
18.7
18.8
18.9
18.10
18.11
Reactant-Favored and Product-Favored Processes
Probability and Chemical Reactions
Measuring Dispersal or Disorder: Entropy
Calculating Entropy Changes
Entropy and the Second Law of Thermodynamics
Gibbs Free Energy
Gibbs Free Energy Changes and Equilibrium
Constants
Gibbs Free Energy, Maximum Work, and Energy
Resources
Gibbs Free Energy and Biological Systems
Conservation of Gibbs Free Energy
Thermodynamic and Kinetic Stability
CHEM 102, Spring 2012 LA TECH
18-3
What forms of energy are found in
the Universe?
mechanical
thermal
electrical
nuclear
mass: E = mc2
others yet to discover
CHEM 102, Spring 2012 LA TECH
18-4
What is 1st Law of Thermodynamics
Eenergy is conserved in the Universe
All forms of energy are inter-convertible
and conserved
Energy is neither created nor destroyed.
CHEM 102, Spring 2012 LA TECH
18-5
What exactly is DH?
Heat measured at constant pressure qp
Chemical reactions exposed to atmosphere
and are held at a constant pressure.
Volume of materials or gases produced can
change.
Volume expansion work = -PDV
DU = qp + w; DU = qp -PDV
qp = DU + PDV; w = -PDV
DH = DU + PDV; qp = DH(enthalpy )
CHEM 102, Spring 2012 LA TECH
18-6
What is the internal energy change (DU)
of a system?
DU is part of energy associated with changes in
atoms, molecules and subatomic particles
Etotal = Eke + E pe + DU
DU = heat (q) + w (work)
DU = q + w
DU = q -P DV; w =- P DV
CHEM 102, Spring 2012 LA TECH
18-7
How is Internal Energy, DU measured?
Heat measured at constant volume qv
Chemical reactions take place inside a
closed chamber like a bomb calorimeter.
Volume of materials or gases produced can
not change. ie: work = -PDV= 0
DU = qv + w
qv = DU + o;
w = 0
DU = qv = DU(internal energy )
CHEM 102, Spring 2012 LA TECH
18-8
Enthalpy
Heat changes at constant pressure
during chemical reactions
Thermochemical equation. eg.
H2 (g) + O2 (g) ---> 2H2O(l) DH =- 256 kJ;
DH is called the enthalpy of reaction.
if DH is + reaction is called endothermic
if DH is - reaction is called exothermic
CHEM 102, Spring 2012 LA TECH
18-9
Entropy, S
The thermodynamic property
related to randomness is
ENTROPY, S.
Product-favored processes:
final state is more
DISORDERED or RANDOM
than the original.
Spontaneity is related to
an increase in
randomness.
CHEM 102, Spring 2012 LA TECH
Reaction of K with
water
18-10
Physical Process”
S[H2O(l)] > S[H2O(s)] at 0 C.
CHEM 102, Spring 2012 LA TECH
18-11
Standard Molar Entropy Values
CHEM 102, Spring 2012 LA TECH
18-12
Chemical Thermodynamics
spontaneous reaction – reaction which
proceed without external assistance once
started
chemical thermodynamics helps predict
which reactions are spontaneous
CHEM 102, Spring 2012 LA TECH
18-13
Thermodynamics
Will the rearrangement of a system decrease its energy?
If yes, system is favored to react — a product-favored
system.
Most product-favored reactions are exothermic.
Often referred to as spontaneous reactions.
“Spontaneous” does not imply anything about time for
reaction to occur. Kinetic factors are more important for
certain reactions.
CHEM 102, Spring 2012 LA TECH
18-14
Thermodynamics Standard States
The thermodynamic standard state of a substance is
its most stable pure form under
standard pressure (1 atm) and at some specific
temperature (25 ºC or 298 K)
superscript circle is used to denote a
thermodynamic quantity that is under standard
state conditions:
ΔH = ΔH°
ΔS = ΔS°
ΔG = ΔG°
CHEM 102, Spring 2012 LA TECH
18-15
1) Give the definitions of the following:
a) Enthalpy (H):
b) Enthalpy change of a thermo-chemical reaction
(DH):
c) Entropy of a substance (S):
d) Entropy change of a chemical reaction(DS):
e) Thermodynamic Standard State(0):
CHEM 102, Spring 2012 LA TECH
18-16
Laws of Thermodynamics
Zeroth: Thermal equilibrium and temperature
First : The total energy of the universe is
constant
Second : The total entropy (S) of the universe is
always increasing
Third : The entropy(S) of a pure, perfectly formed
crystalline substance at absolute zero is zero
CHEM 102, Spring 2012 LA TECH
18-17
2) Give the definitions of the following:
a) Zeroth Law of thermodynamics:
b) First Law of thermodynamics:
c) Second Law of thermodynamics:
d) Third Law of thermodynamics:
CHEM 102, Spring 2012 LA TECH
18-18
Why is it necessary to divide Universe into
System and Surrounding
Universe = System +
Surrounding
universe
system
surroundings
CHEM 102, Spring 2012 LA TECH
18-19
Types of Systems
Isolated system
no mass or energy exchange
Closed system
only energy exchange
Open system
both mass and energy
exchange
CHEM 102, Spring 2012 LA TECH
18-20
Why is it necessary to divide Universe into
System and Surrounding
Universe = System + Surrounding
CHEM 102, Spring 2012 LA TECH
18-21
3) Why we need to divide universe into surroundings
and system for thermodynamic calculations?
Give the signs of the DH (heat) and DS (disorder)
and DG ( free energy) when system lose or gain
them.
Loss
Gain
DH (heat)
DS (disorder)
DG ( free energy)
CHEM 102, Spring 2012 LA TECH
18-22
Second Law of Thermodynamics
In the universe the ENTROPY cannot decrease for
any spontaneous process
The entropy of the universe strives for a
maximum
in any spontaneous process, the entropy of the
universe increases
for product-favored process
DSuniverse = ( Ssys + Ssurr) > 0
DSuniv
= entropy of the Universe
DSsys
= entropy of the System
DSsurr = entropy of the Surrounding
DSuniv =
DSsys +
DSsurr
CHEM 102, Spring 2012 LA TECH
18-23
Entropy of the Universe
DSuniv
=
Dsuniv
DSsys
DSsurr
+
+
+
+
+(DSsys>DSsurr)
-
+
(DSsurr>DSsys)
+
-
CHEM 102, Spring 2012 LA TECH
DSsys +
DSsurr
18-24
4) Explain the ways that DS of the universe, DSuniv
could be +.
DSuniv =
DSsys
+
DSsurr
+
+
+
CHEM 102, Spring 2012 LA TECH
18-25
Entropy and Dissolving
CHEM 102, Spring 2012 LA TECH
18-26
5) Assign a sign to the entropy change for the
following systems.
a) mixing aqueous solutions of NaCl and KNO3
together:
b) spreading grass seed on a lawn:
c) raking and bagging leaves in the fall:
shuffling a deck of cards:
d)
e)
raking and burning leaves in the fall:
CHEM 102, Spring 2012 LA TECH
18-27
Expansion of
a Gas
The positional
probability is
higher when
particles are
dispersed over
a larger volume
Matter tends to
expand unless
it is restricted
CHEM 102, Spring 2012 LA TECH
18-28
Gas Expansion and Probability
CHEM 102, Spring 2012 LA TECH
18-29
Entropies of Solid, Liquid
and Gas Phases
S (gases) >
CHEM 102, Spring 2012 LA TECH
S (liquids)
>
S (solids)
18-30
6) Taking following examples explain how disorder
is related to a measuring positional probability) or
dispersion among the allowed energy states?
a) Expansion of gases: Two gas molecules
trapped in two vessels with a tube with a stop
cock.
CHEM 102, Spring 2012 LA TECH
18-31
6) Taking following examples explain how disorder
is related to a measuring positional probability) or
dispersion among the allowed energy states.
b) Distribution of Kinetic energy at 0, 25 and 100°C
for O2
CHEM 102, Spring 2012 LA TECH
18-32
Entropy and Molecular Structure
CHEM 102, Spring 2012 LA TECH
18-33
Entropy, S
Entropies of ionic solids depend on
coulombic attractions.
o
S (J/K•mol)
CHEM 102, Spring 2012 LA TECH
MgO
26.9
NaF
51.5
18-34
Qualitative Guidelines for Entropy Changes
Entropies of gases higher than liquids higher than
solids
Entropies are higher for more complex structures
than simpler structures
Entropies of ionic solids are inversely related to the
strength of ionic forces
Entropy increases when making solutions of pure
solids or pure liquids in a liquid solvent
Entropy decrease when making solutions of gases
in a liquid
CHEM 102, Spring 2012 LA TECH
18-35
Entropy of a
Solution of a
Gas
CHEM 102, Spring 2012 LA TECH
18-36
7) Arrange following in the order of increasing
entropy?
• a) C(s) (diamond)
• b) C(s) (graphite)
• c) O2(g)
• d) CO2(g)
• e) CO(g)
• f) Hg(l)
CHEM 102, Spring 2012 LA TECH
18-37
Entropy Change
Entropy (DS) normally increase (+) for the following
changes:
i) Solid ---> liquid (melting) +
ii) Liquid ---> gas +
iii) Solid ----> gas most +
iv) Increase in temperature +
v) Increasing in pressure(constant volume, and
temperature) +
vi) Increase in volume +
CHEM 102, Spring 2012 LA TECH
18-38
Qualitative prediction of DS of
Chemical Reactions
Look for (l) or (s) --> (g)
If all are gases: calculate Dn
Dn = Sn (gaseous prod.) - S n(gaseous reac.)
N2 (g) + 3 H2 (g) --------> 2 NH3 (g)
Dn = 2 - 4 = -2
If Dn is - DS is negative (decrease in S)
If Dn is + DS is positive (increase in S)
CHEM 102, Spring 2012 LA TECH
18-39
Predict DS!
2 C2H6(g) + 7 O2(g)--> 4 CO2(g) + 6H2O(g)
2 CO(g) + O2(g)-->2 CO2(g)
HCl(g) + NH3(g)-->NH4Cl(s)
H2(g) + Br2(l) --> 2 HBr(g)
CHEM 102, Spring 2012 LA TECH
18-40
8) Taking following physical and chemical changes
qualitatively predict the sign of DS.
a) 2H2O (g)
------> 2 H2O (l)
b) 2H2O (g)
------> 2 H2 (g) + O2 (g)
c) N2 (g) + 3 H2 (g) ------> 2 NH3 (g)
CHEM 102, Spring 2012 LA TECH
18-41
Entropy Changes for Phase
Changes
For a phase change,
DS = q /T
(q = heat transferred)
Boiling Water
H2O (liq)  H2O(g)
DH = q = +40,700 J/mol
SYS
SYS
q 40,700 J/mol
DS = =
= + 109 J/K • mol
T
373.15 K
CHEM 102, Spring 2012 LA TECH
18-42
9) How is entropy related to the heat and
temperature?
CHEM 102, Spring 2012 LA TECH
18-43
Phase Transitions
Heat of Fusion
energy associated with phase transition solid-toliquid or liquid-to-solid
DGfusion = 0 = DHfusion - T DSfusion
0 = DHfusion - T DSfusion
DHfusion = T DSfusion
Heat of Vaporization
energy associated with phase transition gas-toliquid or liquid-to-gas
DHvaporization = T DSvaporization
CHEM 102, Spring 2012 LA TECH
18-44
10) The normal boiling point of benzene is 80.1°C
and heat of evaporation (∆H°vap)is 30.7 kJ/mol.
Calculate the ∆Ssurr (in J/K mol) for the evaporation
of benzene.
CHEM 102, Spring 2012 LA TECH
18-45
2nd Law of Thermodynamics
2 H2(g) + O2(g)  2 H2O(liq)
DSosys = -326.9 J/K
Entropy Changes in the Surroundings
q surr - DH system
DS surroundings =
=
T
T
o
o
o
Can calc. that DH rxn = DH system = -571.7 kJ
DS o surroundings =
- (-571.7 kJ)(1000 J/kJ)
298.15 K
= +1917 J/K
CHEM 102, Spring 2012 LA TECH
18-46
2nd Law of Thermodynamics
2 H2(g) + O2(g)  2 H2O(liq)
DSosys = -326.9 J/K
DSosurr = +1917 J/K
DSouni = +1590. J/K
The entropy of the universe is increasing, so
the reaction is product-favored.
CHEM 102, Spring 2012 LA TECH
18-47
Gibbs Free Energy, G
DSuniv = DSsurr + DSsys
D S univ
=
-D H sys
T
+
D S sys
Multiply through by (-T)
-TDSuniv = DHsys - TDSsys
-TDSuniv = DGsystem
Under standard conditions —
DGo
= DHo - TDSo
CHEM 102, Spring 2012 LA TECH
18-48
Gibbs Free Energy, G
DGo = DHo - T DSo
Gibbs free energy change =
difference between the enthalpy of a system and
the product of its absolute temperature and
entropy
predictor of spontaneity
Total energy change for system energy lost in disordering the system
CHEM 102, Spring 2012 LA TECH
18-49
11) Define the following:
a) Gibbs Free Energy (G):
b) Gibbs Free Energy change for a reaction
(DG):
c) How is DGsys is related to DSuni and
temperature?
CHEM 102, Spring 2012 LA TECH
18-50
Free energy, DG
The sign of DG indicates whether a reaction
will occur spontaneously.
+
Not spontaneous
0
At equilibrium
Spontaneous
The fact that the effect of DS will vary as a
function of temperature is important. This
can result in changing the sign of DG.
CHEM 102, Spring 2012 LA TECH
18-51
DG and Ecell
The sign of DG indicates whether a reaction
will occur spontaneously. Therefore Ecell
value have to be + (positive) for
spontaneous redox reaction
DG = -nFEcell
n = number of electrons transferred
F = Faraday constant ((96500 C/mol)
Ecell = E½(cathode)- E½(anode)
CHEM 102, Spring 2012 LA TECH
18-52
How do you calculate DG at different T
and P
DG = DGo + RT ln Q
Q = reaction quotient
at equilibrium DG = 0
0 = DGo + RT ln K
DGo = - RT ln K
If you know DGo you could calculate K or
vice versa.
CHEM 102, Spring 2012 LA TECH
18-53
11) Define the following:
d) How you decided from the sign of DG whether and
chemical reaction is?
i) Spontaneous ii) Never take place iii) Equilibrium
e) How is Gibbs Free Energy change (DG°) related to Ecell:
f) How is non standard (DG) related to (DG°) and Q (reaction
quotient)
CHEM 102, Spring 2012 LA TECH
18-54
11) Define the following:
g) How is standard (DG°) related to Keq (equilibrium
constant)?
CHEM 102, Spring 2012 LA TECH
18-55
Gibbs Free Energy, G
o
DG
=
o
DH
o
TDS
-
DHo
DSo
DGo Reaction
exo(-)
increase(+)
-
Prod-favored
endo(+)
decrease(-)
+
React-favored
exo(-)
decrease(-)
?
T dependent
endo(+)
increase(+)
?
T dependent
CHEM 102, Spring 2012 LA TECH
18-56
12) Predict the DGsys changes for different
signs of DHsys and DSsys at low/high
temperatures for the equation:
DGsys =
DHsys
TDSsys
DGsys
DHsys
+
DTDSsys
a)
b)
c)
d)
CHEM 102, Spring 2012 LA TECH
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