Download Chapter 17: Thermodynamics

CHAPTER 18:
THERMODYNAMICS
Vanessa N. Prasad-Permaul
Valencia College
CHM 1046
1
Thermodynamics
Thermodynamics: The study of interconversion of
heat and other forms of energy.
Internal energy (U): the sum of the kinetic and
potential energies of the particles making up a
system.
State Function: a property of a system that depends
only on its present state which is determined by
variables such as temperature and pressure.
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Thermodynamics
Heat (q): energy that moves into or out of the system
because of a temperature difference between the system and
it’s surroundings
Work (w): energy exchange that results when a force
(F)moves an object through distance (d) w = F x d
1st Law of Thermodynamics:
DU = q + w
The change in internal energy of a system is equal to heat
plus work.
if heat evolves from system (-)
if heat is absorbed by the system (+)
work done by the system (-)
work done on a system (+)
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Thermodynamics
Spontaneous process: Process that proceeds on its
own without external influence
Non-spontaneous: Needs continuous external
influence to take place
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Thermodynamics
Entropy (S): a thermodynamic quantity that is a measure
of how dispersed the energy of a system is among the
different possible ways that system can contain energy.
2nd Law of Thermodynamics:
DS = q
T
For a spontaneous process at a given temperature, the
change in entropy of a system is greater than the heat
divided by the absolute temperature.
The total entropy of a system and its surroundings always
increases for a spontaneous process
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Entropy and temperature
3rd Law of Thermodynamics
a) The entropy of a perfectly ordered crystalline
substance at 0 K is zero
b) As the temperature increases, the KE
increases, Molecular motion increases,
entropy increases
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Thermodynamics
Entropy (S): Disorder, molecular randomness
DS = Sfinal - Sinitial
When disorder increases +DS
When disorder decreases -DS
Enthalpy (H): Heat flow
In to the system +DH
Out of the system -DH
Enthalpy change:
DH = TDS
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Thermodynamics
The Entropy of a system usually increases in the
following situations:
1. A reaction in which a molecule is broken down
into 2 or more smaller molecules
2. A reaction in which there is an increase in moles
of gas
3. A process on which a solid changes to liquid or
gas or a liquid changes to a gas
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Example 1:
Predict the sign of DS in the system for each of the
following
a)
H2O(g)  H2O(l)
b)
I2(g)  2I-(g)
c)
CaCO3(s)  CaO(s) + CO2(g)
d)
Ag+(aq) + Br-(aq)  AgBr(s)
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Example 2:
Which of the following reactions has an increase
in entropy?
1. H2O(g)  H2O(l)
2. H2O(l)  H2O(g)
3. H2O(g)  H2O(s)
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Standard Molar Entropies and Standard Entropies of Reaction
Standard Molar Entropy, S
The entropy of one mole of the pure substance at 1
atm pressure and a specific temperature usually 25C
Standard Entropy of Reaction, DS
Entropy change for a chemical reaction
DS = Sproducts - Sreactants
Based on 1 mole of substance so you have to multiply S by the
number of moles present
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Standard Entropy of Reaction, DS
aA + bB  cC + dD
DS = [c S(C) + d S(D)] – [a S(A) + b S(B)]
Units: coefficients are moles
S = J/K mol
DS = J/K
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Example 3:
Calculate the standard entropy of reaction at 25C
for the decomposition of calcium carbonate
CaCO3(s)  CaO(s) + CO2(g)
Substance
CaCO3(s)
CaO(s)
CO2(g)
S (J/K mol)
92.9
39.7
213.6
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Entropy and the Second Law of Thermodynamics
1st Law of Thermodynamics
In any process, spontaneous or nonspontaneous,
the total energy of a system and its surroundings
is constant
2nd Law of Thermodynamics
In any spontaneous process, the total entropy of
a system and its surroundings always increases
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Entropy and the Second Law of Thermodynamics
DStotal = DSsystem + DSsurroundings
if DS > 0 spontaneous
if DS< 0 non spontaneous
if DS = 0 equilibrium
DSsurr = -DH / T
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Entropy and the Second Law of Thermodynamics
a) Exothermic reaction: DH<0, because the surroundings
gain heat (entropy increases), heat is lost from the system
b) Endothermic reaction: DH>0, surroundings lose heat
(entropy decreases), and system gains the heat
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Example 4:
Which of the following reactions is endothermic?
1.
N2O4(g)  2 NO2(g)
DH = +57.1 kJ
2.
2 NO2(g)  N2O4(g)
DH = -57.1 kJ
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Free-Energy
Free energy (G): A thermodynamic quantity defined
by the equation:
G = H – TS
DG = DH - TDS
if DG < 0 spontaneous
if DG > 0 nonspontaneous
if DG = 0 equilibrium
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Example 5:
Consider the decomposition of gaseous N2O4
N2O4(g)  2 NO2(g)
a)
DH = +57.1 kJ
DS = +175.8 J/K
Is this reaction spontaneous under standardstate conditions at 25C?
b) Estimate the temperature at which the reaction
becomes spontaneous
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Standard Free-Energy Changes for Reactions
1.
Standard State Conditions: Solids, liquids, and
gases in pure form at 1 atm pressure, Solutes at
1M concentration, specified temperature, usually
25 celsius
2.
Standard Free Energy Change, DG: The change
in free energy that occurs when reactants in their
Std. States are converted to products in their Std.
States.
3.
DG = DH - TDS
DG = DH - TDS
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Example 6:
Consider the thermal decomposition of calcium
carbonate
CaCO3(s)  CaO(s) + CO2(g)
DH = 178.3 kJ
DS = 160.4 J/K
a) Calculate the standard free energy change for this
reaction at 25C
b) Will a mixture of solid CaCO3, CaO, and gaseous CO2 at 1
atm pressure react spontaneously at 25C?
c) Assuming that DH and DS are independent of
temperature, estimate the temperature at which the
reaction becomes spontaneous
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Standard Free Energies of Formation
1. Standard Free Energy of Formation, DGf
The free energy change for formation of one mole of the
substance in its standard state from the most stable form of
its constituent elements in their standard states
2.
DGf measures the substances thermodynamic stability with
respect to its constituent elements
3.
-DGf are stable and do not decompose to their constituent
elements under standard state conditions
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Standard Free Energies of Formation
4.
+DGf are thermodynamically unstable with respect to their
constituents elements
a) There is no point in trying to synthesize a substance that
has a +DGf because it would degrade into it’s constituents
b) You would need to synthesize it at different temperatures
and or pressures or start with different starting materials
that has a reaction with a -DGf
5.
DG = DGf(products) - DGf(reactants)
6.
General reaction: aA +bB  cC + dD
DG = [cDGf(C) + dDGf(D)] – [aDGf(A) + bDGf(B)]
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Standard Free Energies of Formation
To judge the spontaneity of a reaction:
1.
When DGo is a large negative number (more than -10kJ), the
reaction is spontaneous as written and reactants transform
almost entirely to products when equilibrium is reached.
2.
When DGo is a large positive number (more than 10kJ), the
reaction is nonspontaneous as written and reactants do not
give significant amounts of products at equilibrium.
3.
When DGo has a small negative or positive value, the
reaction gives an equilibrium mixture with significant
amounts of both reactants and products.
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Example 7:
Calculate the standard free energy change for the
reaction of calcium carbide with water. Might this
reaction be used for the synthesis of acetylene (C2H2)?
CaC2(s) + 2 H2O(l)  C2H2(g) + Ca(OH)2(s)
DGf (CaC2) = -64.8 kJ/mol
DGf (H2O(l)) = -237.2 kJ/mol
DGf (C2H2) = 209.2 kJ/mol
DGf (Ca(OH)2) = -898.6 kJ/mol
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Free Energy Changes and Composition of the Reaction Mixture
Standard state conditions are unrealistic, the
reaction itself will change the temperature and
pressure, so what can we use to calculate the
free energy change under non-standard state
conditions?
DG = DG + RT ln Q
R = gas constant
T = temperature in Kelvins
Q = reaction quotient (Qc or Qp)
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Free Energy Changes and Composition of the Reaction Mixture
Thermodynamic Equilibrium Constant (K):
the equilibrium constant in which gases are
expressed in partial pressures (atm) whereas the
concentrations of solutes in liquid solutions are
expressed in molarities.
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Example 8:
Calculate the Free energy change for the formation of
ethylene (C2H4) from carbon and hydrogen at 25C
when the partial pressures are 100 atm H2 and 0.10
atm C2H4
2 C(s) + 2 H2(g)  C2H4(g)
DG = 68.1 kJ/mol
Is the reaction spontaneous in the forward or reverse
direction?
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Free Energy and Chemical Equilibrium
1. When the Reaction Mixture is mostly reactants
Q<<1 RT lnQ <<0
DG<0
2. When the Reaction Mixture is mostly products
Q>>1 RT lnQ >>0
DG>0
3. DG= -RT ln K
K = equilibrium constant Kc or Kp
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Example 9:
Calculate the Kp at 25C for the reaction
CaCO3(s)  CaO(s) + CO2(g)
DG= 130.5 kJ/mol
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Example 10:
Which of the following can always predict the
spontaneity of a reaction?
1. DH
2. DS
3. DG
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Spontaneity and Temperature Change
Effect of Temperature on the Spontaneity of Reactions
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