Transcript ANALYTICAL SEPARATIONS

Chapter 17
THERMODYNAMICS
What is Thermodynamics?
Thermodynamics is the study of energy changes that
accompany physical and chemical processes.
Word origin:
“Thermo”, from temperature, meaning heat
“Dynamics”, which means motion (under the action of forces)
Chemical thermodynamics answers the questions:
 How much heat is evolved during a chemical reaction
("thermochemistry")?
 What determines the direction of spontaneous chemical
reactions?
Spontaneous Processes
A spontaneous process occurs by itself without any ongoing
outside intervention
Examples: A cube of ice melting in water; a steel pipe rusting;
Spontaneous chemical reactions behave the same way
 Reaction continues until equilibrium is reached
 If a reaction is spontaneous in one direction, it is not
spontaneous in the other
 Spontaneity has nothing to do with rate/speed of reaction
Spontaneity versus Speed/Rate
Spontaneous; 1 atm, 25 0C
C Diamond
C Graphite
Nonspontaneous; 1 atm, 25 0C
Spontaneous = Thermodynamically
favorable; Large Keq)
BUT…
Extremely slow = Kinetically
unfavorable; Small rate)
Spontaneous ≠ Instantaneous (Fast)
Spontaneous Processes
If a reaction is spontaneous in one direction, it is not
spontaneous in the other
 Diamond to graphite: Spontaneous (but extremely slow)
at room T
 Graphite to diamond: Nonspontaneous at room T (otherwise
we’ll all be rich!)
 Melting of ice cube at room T is spontaneous, but freezing of
water at room T is not spontaneous.
In the examples above energy is conserved (1st law). Yet one
process occurs while the other does not.
Spontaneous Processes (Cont.)
Spontaneous processes may need a little “push” to get started
Example: Hydrogen and oxygen gases burn spontaneously
only after being ignited by a spark
2H2 (g) +
O2 (g)
2H2O(l)
 The reverse reaction is nonspontaneous (i.e. Water does
not simply decompose into H2 and O2 gases)
Nonspontaneous reactions
Can we make nonspontaneous reactions happen?
 Yes. It happens everyday. HOW?
 By supplying energy
Examples:
Photosynthesis (CO2 (g) + H2O (l) = Carbohydrates) takes
place upon absorption of solar energy
The nonspontaneous reaction
2H2O(l)
2H2 (g) +
O2 (g)
happens if we pass electricity through water (electrolysis)
Factors Affecting Spontaneity
1. Changes in enthalpy (ΔH)
a) Exothermic reactions ( - ΔH) → tend to be
thermodynamically favorable [ large Keq]
b) Endothermic reactions ( + ΔH) → tend to be unfavorable
Example: All combustion reactions (such as the combustion of
natural gas or methane) are spontaneous and exothermic.
CH4 (g) +
2O2 (g)
Methane
 Spontaneous and
 Exothermic (- ΔH)
CO2 (g)
+
2H2O (g)
ΔHrxn0 = - 802 kJ
Factors Affecting Spontaneity – Cont.
NOTE: Not all exothermic reactions are spontaneous
Examples of endothermic reactions that are spontaneous
 Vaporization of water (+ΔH) at ordinary T and P
 Dissolving of NaCl in water (+ΔH)
Thus, the sign of ΔH does not always predict
spontaneous change
Q. What other factor affects direction of spontaneous
reactions?
Factors Affecting Spontaneity – Cont.
2. Changes in entropy (ΔS) or degree of “disorder”
Recall: Entropy (S) is a measure of the dispersal of energy as
a function of temperature
 Closely associated with randomness or disorder
a) Increase in entropy ( + ΔS) or disorder → tend to be
favorable [ large Keq]
b) Decrease in entropy ( - ΔS) or more order → tend to be
unfavorable
The “Randomness” Factor
In general, nature tends to move spontaneously from a
more ordered state to more random (less ordered) state .
 Randomness also predicts
spontaneous processes
An example of a + ΔS
What? Are you expecting your room to
become more ordered in time?
http://www.lecb.ncifcrf.gov/~toms/
molecularmachines.html
Factors Affecting Spontaneity – Cont.
Change to more disordered (more random) state
= Increase in entropy (+ ΔS)
Q. Which of the two images below has a + ΔS? Is spontaneous
above 0 0C?
Image source:
http://demo.physics.uiuc.edu/Lect
Demo/scripts/demo_descript.idc?
DemoID=1114
MELTING
 More disorder =
higher entropy (+ ΔS)
 Spontaneous when T > 0 0C
Entropy (S) is a State Function
Recall that entropy is a state function:
 Depends only on the initial and final states of the
system (Not on path taken from one state to another)
ΔS = Sfinal - Sinitial
For a chemical reaction, entropy change is expressed as:
ΔSrxn = ∑ n x ΔSproducts - ∑ n x ΔSreactants
In general:
ΔS gas > ΔS liquid > ΔS solid
Most
disordered
Most ordered
(Least disorder)
Entropy (S) and Phase Changes
Exercise: Predict the sign of ΔS in each of the following
processes.
Freezing
- ΔS
Vaporization
+ ΔS (becomes more disordered)
Condensation
- ΔS
Reactions that produce gas (+ ΔS)
tend to be favorable
Entropy and the 2nd Law of Thermo.
Second law of thermodynamics: In a spontaneous process
there is a net increase in the entropy of the universe*
Layman’s term: All real processes occur spontaneously in the
direction that increases disorder.
*
Universe = System + Surrounding
Thus,
ΔSuniverse = ΔSsystem +
ΔSsurrounding
 A process (or reaction) is spontaneous if ∆Suniverse is (+)
Entropy and the 2nd Law of Thermo. – Cont.
ΔSuniverse = ΔSsystem +
ΔSsurrounding
Q. When is ∆Suniverse positive?
 (+) ∆Suniverse when: ΔSsystem + ΔSsurrounding > 0
If this decreases, ∆Ssurr. must increase even more to offset the
system’s decrease in entropy
Example: During growth of a child, food molecules (carbs,
proteins, etc) are broken down into CO2, water and energy
 Increases ∆Ssurrounding
Exothermic ( -∆H)
But formation of body mass decreases ∆Ssystem
Thus, ↓ ∆Ssystem is offset by ↑ ∆Ssurrounding = Still obeys 2nd law
Entropy and the 3rd Law of Thermo.
Q. What happens to a system if we decrease the temperature
continuously?
 Entropy will decrease (more order)
 Recall: ∆S (g) > ∆S (l) > ∆S (s)
Third law of thermodynamics: The entropy of a pure
crystalline substance at 0 K is zero.
= System
All motion/vibration at absolute zero has stopped
 Particles in highest order
 Ssystem = 0 at T = 0 K
Entropy and 3rd Law - Cont.
It follows that as the temperature of a system increases, its
entropy also increases.
 Thus, T (related to ∆H) and entropy (∆S) both influence
spontaneous reactions
 ∆H can be measured using a calorimeter
 How is ∆S measured?
Standard Molar Entropies, S0
Standard Molar Entropy, S0
 Entropy (S) at standard conditions of 1 atm pressure
and 25 0C temperature for one mole of a substance
 Unit for S0: J/mol•K
 NOTE: Be able to calculate ΔS0rxn from given S0 values
ΔS0rxn = ∑ n x ΔS0products - ∑ n x ΔS0reactants
Table 17.1: Standard molar entropies of substances
 Elements and compounds have (+) S0
 Aqueous ions may have (+) or (-) S0
Gibbs Free Energy, G
 So far you’ve learned that both ∆H and ∆S affect reaction
spontaneity
 Is there an equation that relates these two thermodynamic
quantities?
At a constant temperature, the free energy change, ΔG, of a
system is given by the Gibbs-Helmholtz equation:
ΔG
Free energy
change
=
ΔH - TΔS
Enthalpy
change
Kelvin T * Entropy
change
Standard Free Energy Change, ΔG0
The standard free energy change, ΔG0, applies to standards
states as follows:
(1) 1 atm pressure for pure liquids and solids
(2) 1 atm partial pressure for gases
(3) 1 M concentration for solutions
Mathematically:
ΔG0 = ΔH0 - TΔS0
Importance of Free Energy Change:
 The sign of ΔG0 determines reaction spontaneity
Importance of ΔG0
The sign of ΔG0 determines reaction spontaneity
(1) - ΔG0 = spontaneous reaction*
(2) + ΔG0 = nonspontaneous reaction
(3) ΔG0 = 0 for reactions at equilibrium (occurs in either
direction)
*At
a constant T and P, reactions go in the direction that
lowers the free energy of the system.
ΔG0 and Spontaneous Reactions
Q. Just when is a reaction spontaneous by looking at ΔG0?
ΔG0 = ΔH0 - TΔS0
Source: Principles of General Chemistry by Silberberg, © 2007.
ΔG0 versus ΔG
ΔG0 vs. ΔG
ΔG0 = free energy of the system at standard conditions
[Pgas = 1 atm; [ ] = 1 M for species in solution (aq)]
 Fixed during a reaction under std. conditions
ΔG = free energy of the system at any given condition
[i.e. at any point in the reaction]
 Changes during a reaction
Importance of ΔG0
For a given chemical reaction aA + bB → cC + dD
the free energy of the reaction is given by the equation
∆G = ∆G0 + RT ln Q
Reaction quotient
Std. free
energy
Gas
constant
Kelvin
temp.
Q=
[C]c [D]d
[A]a [B]b
Importance of ΔG0 – Cont.
At equilibrium, reaction seem to have stopped (ratefwd = raterev.)
and:
At equilibrium ∆G = 0 ; Q = Keq
∆G = ∆G0 + RT ln Q
= Keq
=0
Thus,
or
0 = ∆G0 + RT ln Keq
at equilibrium ∆G0 = - RT ln Keq
or Keq
(∆G0/RT)
=e
Importance of this equation:
 Allows us to calculate ∆G0 given Keq or Keq given ∆G0