Transcript chem 102

Chemistry 102(01) Spring 2014
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 9:30 - 11:30 am. or by appointment.
Test Dates: 9:30-10:45 am., CTH 328
March 31,
April 23,
May 19,
May 21,
2014 (Test 1): Chapter 13
2014 (Test 2): Chapter 14 &15
2014 (Test 3) Chapter 16 &17
2014 (Make-up test) comprehensive:
Chapters 13-17
CHEM 102, Fall 2014 LA TECH
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Chapter 6. Thermochemistry
6.1 Chemical Hand Warmers 231
6.2 The Nature of Energy: Key Definitions
232
6.3 The First Law of Thermodynamics: There Is No Free Lunch
234
6.4 Quantifying Heat and Work
240
6.5 Measuring for Chemical Reactions: Constant-Volume Calorimetry246
6.6 Enthalpy: The Heat Evolved in a Chemical Reaction at Constant
Pressure
249
6.7 Constant-Pressure Calorimetry: Measuring
253
6.8 Relationships Involving
255
6.9 Determining Enthalpies of Reaction from Standard Enthalpies
of Formation
257
6.1 0 Energy Use and the Environment
263
CHEM 102, Fall 2014 LA TECH
17-2
Chapter 17. Free Energy and Thermodynamics
17.1 Nature’s Heat Tax: You Can’t Win and You Can’t Break Even
769
17.2 Spontaneous and Nonspontaneous Processes
771
17.3 Entropy and the Second Law of Thermodynamics
773
17.4 Heat Transfer and Changes in the Entropy of the Surroundings 780
17.5 Gibbs Free Energy
784
17.6 Entropy Changes in Chemical Reactions: Calculating
788
17.7 Free Energy Changes in Chemical Reactions: Calculating
792
17.8 Free Energy Changes for Nonstandard States: The Relationship
between and
798
17.9 Free Energy and Equilibrium: Relating to the Equilibrium Constant
(K)
CHEM 102, Fall 2014 LA TECH
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What forms of energy are found in
the Universe?
mechanical
thermal
electrical
nuclear
mass: E = mc2
others yet to discover
CHEM 102, Fall 2014 LA TECH
17-4
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, Fall 2014 LA TECH
17-5
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.
CHEM 102, Fall 2014 LA TECH
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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, Fall 2014 LA TECH
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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, Fall 2014 LA TECH
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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, Fall 2014 LA TECH
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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, Fall 2014 LA TECH
Reaction of K with
water
17-10
Physical Process”
S[H2O(l)] > S[H2O(s)] at 0 C.
CHEM 102, Fall 2014 LA TECH
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Gibbs Free Energy, G
o
DG
=
o
DH
- T
o
DS
1. If DH is negative it helps product to be favored
2. If DS is positive it helps product to be favored
3. If DG is negative reaction is product favored
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 of the system energy lost in disordering the system
CHEM 102, Fall 2014 LA TECH
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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, Fall 2014 LA TECH
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Standard Thermodynamic Data
o
ΔH f
o
ΔG f
S
o
- Standard Enthalpy of Formation
- Standard Free Energy of Formation
- Standard Free Energy of Formation
Hydrogen
ΔHof
(kJ/mol)
ΔGof
(kJ/mol)
So (J/mol K)
H2 (g)
H (g)
H2O (l)
H2O (g)
0
218.0
-285.8
-241.8
0
203.2
-237.1
-228.6
130.7
114.7
69.9
188.8
H2O2 (l)
-187.8
-120.4
109.6
CHEM 102, Fall 2014 LA TECH
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Standard Molar Entropy Values
CHEM 102, Fall 2014 LA TECH
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Chemical Thermodynamics
spontaneous reaction – reaction which
proceed without external assistance once
started
chemical thermodynamics helps predict
which reactions are spontaneous
CHEM 102, Fall 2014 LA TECH
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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, Fall 2014 LA TECH
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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, Fall 2014 LA TECH
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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, Fall 2014 LA TECH
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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, Fall 2014 LA TECH
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Why is it necessary to divide Universe into
System and Surrounding
Universe = System + Surrounding
universe
system
surroundings
Boundary?
CHEM 102, Fall 2014 LA TECH
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Types of Systems
Isolated system
no mass or energy exchange
Closed system
only energy exchange
Open system
both mass and energy
exchange
CHEM 102, Fall 2014 LA TECH
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Why is it necessary to divide Universe into
System and Surrounding
Universe = System + Surrounding
CHEM 102, Fall 2014 LA TECH
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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, Fall 2014 LA TECH
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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, Fall 2014 LA TECH
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Entropy of the Universe
DSuniv
=
Dsuniv
DSsys
DSsurr
+
+
+
+
+(DSsys>DSsurr)
-
+
+
(DSsurr>DSsys)
CHEM 102, Fall 2014 LA TECH
DSsys +
DSsurr
17-26
4) Explain the ways that DS of the universe, DSuniv
could be +.
DSuniv =
DSsys
+
DSsurr
+
+
+
CHEM 102, Fall 2014 LA TECH
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Entropy and Dissolving
CHEM 102, Fall 2014 LA TECH
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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, Fall 2014 LA TECH
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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, Fall 2014 LA TECH
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Gas Expansion and Probability
CHEM 102, Fall 2014 LA TECH
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Entropies of Solid, Liquid
and Gas Phases
S (gases) >
CHEM 102, Fall 2014 LA TECH
S (liquids)
>
S (solids)
17-32
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, Fall 2014 LA TECH
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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, Fall 2014 LA TECH
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Entropy and Molecular Structure
CHEM 102, Fall 2014 LA TECH
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Entropy, S
Entropies of ionic solids depend on
coulombic attractions.
o
S (J/K•mol)
CHEM 102, Fall 2014 LA TECH
MgO
26.9
NaF
51.5
17-36
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, Fall 2014 LA TECH
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Entropy of a
Solution of a
Gas
CHEM 102, Fall 2014 LA TECH
17-38
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, Fall 2014 LA TECH
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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, Fall 2014 LA TECH
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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, Fall 2014 LA TECH
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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, Fall 2014 LA TECH
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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, Fall 2014 LA TECH
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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
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9) How is entropy related to the heat and
temperature?
CHEM 102, Fall 2014 LA TECH
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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, Fall 2014 LA TECH
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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, Fall 2014 LA TECH
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2nd Law of Thermodynamics
2 H2(g) + O2(g)  2 H2O(liq)
DSosys = -326.9 J/K
Entropy Changes in the Surroundings
DS
o
surroundin gs
q surr - DH system
=
=
T
T
o
o
Can calc. that DH rxn = DH system = -571.7 kJ
DS o surroundin gs =
- (-571.7 kJ)(1000 J/kJ)
298.15 K
= +1917 J/K
CHEM 102, Fall 2014 LA TECH
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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, Fall 2014 LA TECH
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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, Fall 2014 LA TECH
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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, Fall 2014 LA TECH
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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, Fall 2014 LA TECH
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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, Fall 2014 LA TECH
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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, Fall 2014 LA TECH
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How do you calculate DG at different T
and P
Nerst Equation, since DG = -nFE ell
c
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.
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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, Fall 2014 LA TECH
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11) Define the following:
g) How is standard (DG°) related to Keq (equilibrium
constant)?
CHEM 102, Fall 2014 LA TECH
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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, Fall 2014 LA TECH
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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, Fall 2014 LA TECH
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