Chemistry 100 Chapter 19
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Transcript of Chemistry 100 Chapter 19
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Chemistry 100 Chapter 19
Spontaneity of Chemical and Physical Processes: Thermodynamics
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What Is Thermodynamics?
Study of the energy changes that accompany chemical and physical processes.
Based on a set of laws. In chemistry, a primary application
of thermodynamics is as a tool to predict the spontaneous directions of a chemical reaction.
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What Is Spontaneity?
Spontaneity refers to the ability of a process to occur on its own!
Can the Niagara Falls suddenly reverse?
“Ice will melt, water will boil,” Neil Finn, Tim Finn of Crowded House/Plant ‘It’s Only Natural’.
Water spontaneously freezes on a cold winter day!
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The First Law of Thermodynamics
The First Law deals with the conservation of energy changes.
E = q + w The First Law tells us nothing
about the spontaneous direction of a process.
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Entropy and Spontaneity
Need to examine the entropy change of the process as well
as its enthalpy change (heat flow). Entropy – the degree of randomness
of a system. Solids – highly ordered low entropy. Gases – very disordered high entropy. Liquids – entropy is variable between that
of a solid and a gas.
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Entropy Is a State Variable
Changes in entropy are state functions
S = Sf – Si Sf = the entropy of the final stateSi = the entropy of the initial state
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Entropy Changes for Different Processes
S > 0 entropy increases (melting ice or making steam)
S < 0 entropy decreases (examples freezing water or condensing
steam)
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The Solution Process For the dissolution of NaCl (s) in water
NaCl (s) Na+(aq) + Cl-(aq)
Highly ordered – low entropy
Disordered or random state – high entropy
The formation of a solution is always accompanied by an increase in the
entropy of the system!
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The Entropy Change in a Chemical Reaction
Burning ethane! C2H6 (g) + 7/2O2 (g) 2CO2 (g) + 3H2O (l)
The entropy change rS np S (products) - nr S (reactants)
np and nr represent the number of moles of products and reactants, respectively.
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Finding S Values
Appendix C in your textbook has entropy values for a wide variety of species.
Units for entropy values J / (K mole)
Temperature and pressure for the tabulated values are 298.2 K and 1.00 atm.
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Finding S Values
Note – entropy values are absolute!
Note – the elements have NON-ZERO entropy values!
e.g., for H2 (g) fH = 0 kJ/mole (by def’n)
S = 130.58 J/(K mole)
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Some Generalizations
For any gaseous reaction (or a reaction involving gases).
ng > 0, rS > 0 J/(K mole).ng < 0, rS < 0 J/(K mole).ng = 0, rS 0 J/(K mole).
For reactions involving only solids and liquids – depends on the entropy values of the substances.
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The Second Law of Thermodynamics
The entropy of the universe (univS) increases in a spontaneous process. univS unchanged in an equilibrium
process
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What is univS?
univS = sysS + surrSsysS = the entropy change of the
system.surrS = the entropy change of the
surroundings.
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How Do We Obtain univS?
We need to obtain estimates for both the sysS and the surrS.
Look at the following chemical reaction.
C(s) + 2H2 (g) CH4(g) The entropy change for the systems is
the reaction entropy change, rS. How do we calculate surrS?
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Calculating surrS Note that for an exothermic process,
an amount of thermal energy is released to the surroundings!
Heat
Insulation
surroundings System
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Calculating surrS
Note that for an endothermic process, thermal energy is absorbed from the surroundings!
Heat
surroundings System
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Connecting surrS to sysH
For a constant pressure process qp = H
surrS surrH = -sysH surrS = -sysH / T
For a chemical reactionsysH = rH
surrS = -rH/ T
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The Use of univS to Determine Spontaneity
Calculation of TunivS two system parameters rS rH
Define a system parameter that determines if a given process will be spontaneous?
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The Definition of the Gibbs Energy
The Gibbs energy of the systemG = H – TS
For a spontaneous processsysG = Gf – G i
Gf = the Gibbs energy of the final stateGi = the Gibbs energy of the initial state
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Gibbs Energy and Spontaneity
sysG < 0 - spontaneous processsysG > 0 - non-spontaneous process
(note that this process would be spontaneous in the reverse
direction)sysG = 0 - system is in equilibrium
Note that these are the Gibbs energies of the system under non-
standard conditions
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Standard Gibbs Energy Changes
The Gibbs energy change for a chemical reaction?
Combustion of methane. CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O (l)
Define rG = np fG (products) - nr fG
(reactants) fG = the formation Gibbs energy of the
substance
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Gibbs Energy Changes
fG (elements) = 0 kJ / mole. Use tabulated values of the Gibbs
formation energies to calculate the Gibbs energy changes for chemical reactions.
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The Third Law of Thermodynamics
Entropy is related to the degree of randomness of a substance.
Entropy is directly proportional to the absolute temperature.
Cooling the system decreases the disorder.
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The Third Law of Thermodynamics
The Third Law - the entropy of any perfect crystal is 0 J /(K mole) at 0 K (absolute 0!)
Due to the Third Law, we are able to calculate absolute entropy values.
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At a very low temperature, the disorder decreases to 0 (i.e., 0 J/(K mole) value for S).
The most ordered arrangement of any substance is a perfect crystal!
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Applications of the Gibbs Energy
The Gibbs energy is used to determine the spontaneous direction of a process.
Two contributions to the Gibbs energy change (G) Entropy (S) Enthalpy (H)
G = H - TS
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Spontaneity and Temperature
H S G
+ + < 0 at high temperatures
+ - > 0 at all temperatures
- + < 0 at all temperatures
- - < 0 at low temperatures
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Gibbs Energies and Equilibrium Constants
rG < 0 - spontaneous under standard conditions
rG > 0 - non-spontaneous under standard conditions
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The Reaction Quotient
Relationship between QJ and Keq
Q < Keq
- reaction moves in the forward directionQ > Keq
- reaction moves in the reverse directionQ = Keq
- reaction is at equilibrium
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rG° refers to standard conditions only!
For non-standard conditions - rG rG < 0 - reaction moves in the
forward directionrG > 0 - reaction moves in the
reverse directionrG = 0 - reaction is at equilibrium
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Relating Keq to rG
rG = rG +RT ln QrG = 0 system is at equilibrium
rG = -RT ln Qeq
rG = -RT ln Keq
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Phase Equilibria
At the transition (phase-change) temperature only - trG = 0 kJ
tr = transition type (melting, vapourization, etc.)
trS = trH / Ttr