CHAPTER 14 Thermodynamics: Spontaneous Processes, Entropy, and Free Energy
A tiny fraction of the sun's energy is used to produce complicated, ordered, high- Useful energy is being "degraded" in energy systems such as life the form of unusable heat, light, etc.
• Our observation is that natural processes proceed from ordered, high-energy systems to disordered, lower energy states. • In addition, once the energy has been "degraded", it is no longer available to perform useful work. • It may not appear to be so locally (earth), but globally it is true (sun, universe as a whole).
1 Thermodynamics - quantitative description of the factors that drive chemical reactions, i.e. temperature, enthalpy, entropy, free energy.
Answers questions such as-
• will two or more substances react when they are mixed under specified conditions? • if a reaction occurs, what energy changes are associated with it? • to what extent does a reaction occur to?
Thermodynamics does NOT tell us the RATE of a reaction
Spontaneous Processes A spontaneous process is one that is capable of proceeding in a given direction without an external driving force • A waterfall runs downhill • A lump of sugar dissolves in a cup of coffee • At 1 atm, water freezes below 0 0C and ice melts above 0 0C • Heat flows from a hotter object to a colder object • A gas expands in an evacuated bulb • Iron exposed to oxygen and water forms rust
2 Spontaneous chemical and physical changes are frequently accompanied by a release of heat (exothermic H < 0) -
o C3H8(g) + 5 O2(g) 3 CO2(g) + 4 H2O(l) H = -2200 kJ
Although many spontaneous processes are exothermic (e.g., the previous combustion reaction), not true for all spontaneous reactions:
Endothermic (ΔH > 0), but reaction is spontaneous.
3 Some processes are accompanied by no change in enthalpy at all (Ho = 0.0), as is the case for an ideal gas spontaneously expanding:
spontaneous
nonspontaneous
There's another factor promoting spontaneity in these processes, and that's the increasing randomness or disorder of the system:
1. propane combustion:
o C3H8(g) + 5 O2(g) 3 CO2(g) + 4 H2O(l) H = -2200 kJ
2. water melting: o H2O(s) H2O(l) H = 6.01 kJ
3. gas expansion:
4 Thermodynamics: Entropy
. Second Law of Thermodynamics: • The total entropy of the universe increases in any spontaneous process . Entropy (S): • A measure of the amount of disorder (qualitative), or unusable energy in a system at a specific temperature (quantitative). • Entropy is affected by molecular motion, or disorder from volume changes (e.g. the previous gas expansion example).
Types of Molecular Motion
. Three types of motion: • Translational—movement through space. • Rotational—spinning motion around axis to bond. • Vibrational—movement of atoms toward/away from each other. . As temperature increases, the amount of motion increases.
5 Trends in Entropies
Ssolid < Sliquid < Sgas
Entropy is expected to INCREASE for these types of processes (S > 0) :
6 Changes in Entropy
order S disorder S
S = Sf - Si
S > 0
S < 0
Suniverse
Example - Predict whether the entropy change is greater than or less than zero for each of the following processes:
(a) freezing ethanol (b) evaporating a beaker of liquid bromine at room temperature (c) dissolving sucrose in water (d) cooling nitrogen gas from 80 oC to 20 oC.
7 Entropy Changes in the System (Ssys) 0 The standard entropy of reaction (Srxn ) is the entropy change for a reaction carried out at 1 atm and 250C.
aA + bB cC + dD from Appendix
0 0 0 0 0 Srxn = [c S (C) + dS (D) ] - [a S (A) + bS (B) ]
0 0 0 Srxn = S nS (products) - S mS (reactants)
The Second Law of Thermodynamics: The total entropy of the universe increases in any spontaneous process
Spontaneous process: Suniv = Ssys + Ssurr > 0
sys = system One can be negative but the surr = surroundings other will be even more positive
Equilibrium process: Suniv = Ssys + Ssurr = 0
At equilibrium the forward and reverse rates are equal, e.g vapor pressure
(l) = (g) So there is no net change in entropy
8 Entropy Changes in the Surroundings (Ssurr)
Exothermic Process Endothermic Process
Ssurr > 0 Ssurr < 0
The change in entropy of the surroundings can be calculated:
if Hsys < 0 (exothermic), then Ssurr > 0 (entropy of the surroundings increases) Ssurr -Hsys if Hsys > 0 (endothermic), then Ssurr < 0 (entropy of the surroundings decreases)
If the temperature of the surroundings is already high, Ssurr 1 then pumping heat in or out Tsurr causes less change in disorder than at lower temperatures
9 Combining the two:
Ssurr -Hsys and Ssurr 1 so Ssurr = -Hsys Tsurr T
(Tsurr usually = Tsys)
e.g. N2(g) + 3H2(g) 2NH3(g) Ssys = -198.3 J/K * Hsys = -92.6 kJ The two main driving forces are in opposition to each other - the release of heat favors a spontaneous reaction while the
decrease in entropy does not. Calculating Suniv will decide the issue (next slide). Remember: for a spontaneous reaction the entropy of the universe increases.
0 0 0 *from Ch 5: Hrxn = S nH f (products) - S mHf (reactants)
Is the reaction spontaneous at 25 oC?
Suniv = Ssys + Ssurr
10 The previous example with ammonia illustrated that maybe entropy will decrease in the system, but this will always be accompanied by a greater increase in
the entropy of the surroundings such that Suniv > 0.
Suniv = Ssys + Ssurr > 0
Another way of stating the 2nd Law is that "You Can't Win!"
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