Download Lecture 7

Outline:
 CAPA TA’s
1/24/07
– Wed & Sunday evening
 Today:
Chapter 14 Thermodynamics:
Enthalpy (Heat)
Entropy
Free Energy
•
•
Thermo = heat
dynamics = movement
Want to test your knowledge?
(and those keypads once again?)
What’s the functional group here?
20% 
20% 
20% 
20% 
20% 
1
2
3
Aldehyde
Alcohol
Ketone
Acid
Ester
4
5
O
OH
What provides the polymeric
backbone to DNA’s double helix?
20%

20%

20%

20%

20%

1
2
3
Ribose sugars
Phosphates
Amino acids
1&2
1&3
4
5
Why are we so interested in heat?
 Chemical
reactions
that produce heat
 Chemical
reactions
that absorb heat
“Exothermic”
“Endothermic”
Heat is related to whether
the reaction will happen!
The first law….
DE = q + w and w = - PDV
DE = q - PDV
q = DE + PDV
For constant pressure:
PDV = D(PV)
q = D(E + PV)
Make a new state variable: E + PV = H
“Constant pressure heat” = Enthalpy (H)
Another definition:
Enthalpy of formation (DHfo) :
The constant-pressure heat required to
form a chemical compound from the
elements (in their most stable form).
DHfo = 0 for elements
 Look up tables of DHfo (Appendix D)
 Define
Examples:
H2(g) + O2 (g)  H20 (l)
0 kJ/mol 0 kJ/mol - 285 kJ/mol
 DH for formation reaction: DHfo
DHrxn = S DHproducts - S DHreactants
Example of DHfo usefulness:
CH4(g) + O2(g)  H2O (g) + CO2 (g)
 How
much heat is generated if you
burn 1.0 mole of methane gas?
Example of DHfo usefulness:
CH4(g) + 2O2(g)  2 H2O (g) + CO2 (g)
1. Balance equation...
2. Info from Appendix D:
DHfo (CH4(g)) = -74.6 kJ/mol
1
DHfo (O2(g)) = 0.0 kJ/mol
2
DHfo (H2O(g)) = -241.8 kJ/mol
2
DHfo (CO2(g)) = -393.5 kJ/mol
1
Example of DHfo usefulness:
CH4(g) + 2O2(g)  2 H2O (g) + CO2 (g)
1. Balance equation...
2. Info from Appendix D…
3. Products – Reactants…
-802.5 kJ/mol
 Practice
!!!
Remember?
A new topic!
Entropy
Start with definition:
Spontaneity: Every chemical process
has a spontaneous direction.
e. g. 2 H2 + O2  2 H2O
apple  brown apple
(fast)
(slow)
What governs spontaneity?
DH (heat of reaction)
?
DE (energy of reaction) ?
Both exothermic and endothermic
reactions can be spontaneous…
T (temperature)
….. Something else
?
?
Answer: Entropy
Definition:
Entropy = “randomness”
= “disorder”
=
S
Second Law of thermodynamics:
Entropy always increases in
spontaneous reactions
(entropy of the universe that is...)
DS = q/T > 0
(spontaneous)

At a molecular level, entropy
(order/disorder) is easy to visualize:
(s)  ()  (g)
Argon example
Entropy is a “state function”:
DS = Change in entropy (randomness)
= related to the flow of heat
(at constant temperature)
TDS = qT
where qT is the heat of
reaction at constant T
Don’t forget  (T in Kelvin)
Examples:
Phase changes ()  (g)
constant temp bath
Macroscopic H2O example:
(1) 36 g water freezes into ice cube
in a freezer at -10°C spontaneously
(2) 36 g ice cube melts to water at
5°C spontaneously
How can both spontaneous
reactions of ice/water have
increasing entropy?
Ice cube example (cont’d):
2 mols H2O(s)  2 mols H2O()
(@ 5oC)
What is DS for this reaction?
q(H2O) = n DHfus = 2 mol (6.0 kJ/mol)
= (heat absorbed by water) = +12 kJ
DS(H2O) = q(H2O) / T = +12 kJ/273K
= +44 J/K
(more entropy / more disordered)
Consider the universe?
q(H2O) = -q(freezer) = -12 kJ
DS(freezer) = q(freezer) / T = -12kJ/278K
= -43.2 J/K
(less entropy / less disordered)
Overall entropy (of universe):
DS(universe) = DS(H2O) + DS(freezer)
= +44.0 + -43.2 = +0.8 J/K
(more entropy / more disordered)
What about freezing water?
2 mols H2O(l)  2 mols H2O(s)
(freezer @ -10oC)
What is DS for this reaction?
q(H2O) = n DHfus = 2 mol (-6.0 kJ/mol)
= (heat lost by water) = -12 kJ
DS(H2O) = q(H2O) / T = -12 kJ/273K
= -44 J/K
(less entropy / less disordered)
But why is it spontaneous?
q(H2O) = -q(freezer) = +12 kJ
DS(freezer) = q(freezer) / T = +12kJ/263K
= +45.6 J/K
(more entropy / more disordered)
Overall entropy (of universe):
DS(universe) = DS(H2O) + DS(freezer)
= -44.0 + 45.6 = +1.6 J/K
(more entropy / more disordered)
On what does entropy depend?
 Temperature
obvious
(s)  ()  (g)
 Molar mass
less obvious
 Concentration
 Phase
Generally, as molar mass ,
intermolecular disorder 
Table 14-2:
H2 = 130.7 J/K mol
(page 580)
F2 = 202.8 J/K mol
Cl2 = 223.1 J/K mol
On what does entropy depend?
 Temperature
obvious
(s)  ()  (g)
 Molar mass
less obvious
 Concentration
 Phase
p.126
Let’s test those minds…
Quiz #2
Please put away all books, papers, etc.
Quiz #2
You may leave when you are done…
hand them into me on your way out.