Upload
kathleen-hensley
View
50
Download
5
Embed Size (px)
DESCRIPTION
Chapter 19 Chemical Thermodynamics. Entropy, Enthalpy, and Free Energy. Thermodynamic Quantities. H : Enthalpy, heat energy E : Total energy, q + w S : Entropy, disorder G : Free energy, measure of spontaneity, energy available to do work. Relationship between thermodynamic quantities: - PowerPoint PPT Presentation
Citation preview
Chapter 19Chemical Thermodynamics
Entropy, Enthalpy, and Free Energy
Thermodynamic Quantities
• H : Enthalpy, heat energy
• E : Total energy, q + w
• S : Entropy, disorder
• G : Free energy, measure of spontaneity, energy available to do work
• Relationship between thermodynamic quantities:
G = H – TS
• Sign of G
G < 0 Reaction is spontaneous, reaction will proceed in
the forward direction
G < 0 Reaction is not spontaneous, however the reaction
going in the reverse direction is spontaneous
G = 0 The reaction is at equilibrium, nothing will happen
The Meaning of “Spontaneous”
• Spontaneous reaction:– Product favored– Given sufficient time, a combination of
reactants will be converted to products K >> 1
• Non spontaneous reaction:– Reactant favored– Given sufficient time, nothing will happen
K << 1
Problems
a) H2 + I2 < ----- > 2HI Kc = 51
b) N2 + O2 < ----- > 2NO Kc = 1 x 10-30
• Which reaction is spontaneous in the forward direction?
• Which reaction has a negative G0?
What Drives a Reaction to Occur?
• Driving Forces:– The tendencies of concentrated energy and
matter to disperse
• Enthalpy:– Exothermic reactions disperse energy– Energy flows from hot area to cold area
• Entropy:– The tendency of concentrated matter to
disperse
Will a process be spontaneous?
• If energy and matter are both dispersed in a reaction, it is definitely spontaneous
• If only energy or matter is dispersed, then the relative effects of enthalpy and entropy determine spontaneity
• If neither matter nor energy is dispersed, then that process will not be spontaneous and reactants will remain, no matter how long we wait
The Three Laws of Thermodynamics
First Law: The total energy of the universe is a constant
Esystem = -Esurroundings and E = q+w
Second Law: The total entropy of the universe
is always increasing
Third Law: The entropy of a pure perfectly formed crystalline substance at
absolute zero is zero.
Entropy
• Boltzman’s Expression for Entropy: A quantitative measure of matter dispersal or disorder
Ludwig Boltzman (1844 – 1906)S = k log W
S = entropyk = constantW = number of ways atoms or molecules can be arranged
Entropy
• Entropy of a substance is determined by calorimetry
S = q / T
• Absolute entropy of a substance at any temperature can be determined– Absolute entropy at 0 K is 0.– Absolute entropy S0 is the entropy of a pure
substance relative to its entropy at absolute zero
S: Entropy Generalizations
1. Entropies of gases >>> entropy of liquids > entropies of solids
2. Entropies of complex molecules are > than entropies of simpler molecules
3. Entropies of ionic solids become smaller as the attractions between ions become stronger
4. Entropy usually increases when a pure liquid or solid dissolves in a solvent
5. Entropies of liquids comprised of molecules with similar structures are smaller when hydrogen bonding is possible
6. Entropy increases when a dissolved gas escapes from a solution
Problem
• Predict whether S is positive or negative for the following processes
CaCO3(s) --- > CaO(s) + CO2(g)
2CO(g) + O2 --- > 2CO2(g)
Ag+(aq) + Cl-(aq) --- > AgCl(s)
The “naught” Notation and Standard States
S0, H0, G0
Solid: Pure
Liquid: Pure
Gas: 1 atm
Solution: 1 M
Temperature: 25oC
Free Elements
• The absolute entropy S0 of a free element in its standard state is NOT zero
• S0 refers to the entropy increase in warming a pure substance from absolute zero where its entropy is zero, to 25oC
S0 = S0(at 25oC) – S0(at –273oC) = S0 – 0 = S0
Free Elements (continued)
Hf0 of a free element in its standard
state is always equal to zero
Gf0 of a free element in its standard
state is always equal to zero
Second Law of Thermodynamics
• The total entropy of the universe is constantly increasing
• Whenever anything happens, matter, energy or both become more dispersed or disordered
• For any spontaneous process S > 0
Calculating Entropy Changes for a reaction
S0reaction
= n S0products
– n S0reactants
where S0 is the absolute entropy of each compound
S0 is the entropy change that occurs when reactants in their standard states (pure, 1atm or 1M) are converted completely to products in their standard states.
• Calculate S0 for the Haber Process
N2(g) + 3H2(g) --- > 2NH3(g)
S0reaction = 2mol (195.2 J/mol K)
-1 mol (191.5 J/mol K)
-3 (130.6 J/mol K)
= -198.4 J/mol K
Enthalpies and Entropies of Formation
The enthalpy or entropy change associated with forming a compound from its free elements.
Hf0 = Hf
0(compound)
– n Hf0
(elements)
= Hf0
(compound) – 0
Look up Hf0
in a Table
Sf0 = S0
(compound) – n S0
(elements)
Look up S0(compound) and S0
(element) for each element in a Table and substract
S0reaction vs. Sf
0
S0 is the entropy of the reactionthe entropy of formation
S0 is the weighted sum of all the absolute entropies of the product minus the weighted sum of all the absolute entropies of the reactants
Look up all the S0 values for the elements or compounds involved in the reaction and calculate S0 using
S0 = n S0products
– n S0reactants
Sf0 is the entropy change for the reaction
which forms the compound from its elements in their standard states
Look up all the S0 values and calculate Sf0
using
Sf0 = S0
(compound) – n S0
(elements)
Problem
• Calculate the entropy change for the following reaction and calculate DSf0 for CH3OH (l)
• CO(g) + 2H2(g) --- > CH3OH (l)
Solution
S0reaction = S0 [CH3OH(l)] – S0[CO(g)]
-2S0 [H2(g)]
= 1mol 126.8 J/mol K – 1 mol 197.6 J/mol K
-2 mol 130.7 J/mol K = -332.2 J/K
• Formation Reaction
C(s) + 2H2(g) + ½ O2(g) --- > CH3OH (l)
Sf0 = S0 [CH3OH (l)] – S0[C(s)] – 2S0[H2(g)]
- ½ S0 [O2(g)]
= 1 mol (126.8 J/ mol K) – 5.69 J/ mol K
- 2 mol (130.58 J/mol K) – ½ (205 J/mol K)
= -242.6 J/ mol K
Notes:
• Absolute entropies are always positive
• The units for S are Joules / mol K NOT Kilojoules / mol K
• The units for G and H are in kilojoules / mol
Gibbs Free Energy
• A measure of the amount of energy involved in a reaction which is available (free) to do work
G is a quantity which tell us– Whether a reaction is spontaneous or not– Relates enthalpy and entropy– Units: kilojoules / mol
Calculating G
Greaction = Hreaction – TSreaction
Greaction = nGf products – nGf reactants
Greaction = G0reaction + RT ln Q
The sign of G
G < 0Reaction is spontaneous as written
G = 0Reaction is at equilibrium
G > 0 Reverse reaction is spontaneousWork must be done to make the reaction occur
Standard Free Energy G0
G0reaction is the free energy change when
a mixture of only reactants all in their standard states is completely converted to a mixture of all products in their standard states.
The sign of G0 and K
G0 < 0 Reaction is spontaneous as written, product favored at equilibrium K > 1
G0 = 0Very rare, at equilibrium [C]c[D]d = [A]a[B]b
G0 > 0Not spontaneous, reactant favored at equilibrium K < 1
G0 and K
At equilibrium
Greaction = 0Therefore since
Greaction = G0reaction + RT ln Q
At equilibrium
0 = G0reaction + RT ln K
G0reaction = - RT ln K
And
K = e- G0/ RT