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CHEM112 LRSVDS Thermodynamics part 1 !
reactants
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products
CHEM112 LRSVDS Thermodynamics part 1 !
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THERMOCHEMISTRY
LAWS OF THERMODYNAMICS
1st Law:
The total energy in the universe is constant.
!Euniverse = 0
!Euniverse = ! Esystem + ! Esurroundings
!Esystem = - ! Esurroundings
Heat and work:
!Esystem = q + w
and for PV work at const. pressure,
!Esystem = q – P!V
CHEM112 LRSVDS Thermodynamics part 1 !
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CHEM112 LRSVDS Thermodynamics part 1 !
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Example Problem: What is the Heat of Formation of gaseous mercury?
CHEM112 LRSVDS Thermodynamics part 1 !
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What are the Standard States of the
Elements? (at 298K and 1 atm)
1. Metals: all are ___________ (except ____)
2. Metalloids: all are _________
3. Nonmetals
A: Noble gases (Group 8): _________!
B: Diatomics: (there are 7)_____________
which are gases? _____________
which is a liquid? ____
which is a solid? ____
C: Other non-metals are __________
C (graphite), S, P, Se
CHEM112 LRSVDS Thermodynamics part 1 !
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SPONTANEOUS REACTIONS
Define spontaneous reaction:
Note: spontaneity has nothing to do with the rate at which a
reaction will occur. (A spontaneous reaction may be fast or slow.)
Bunsen Burner: Do I have to keep lighting it?
CH4(g) + 2 O2(g) " CO2(g) + 2 H2O(g)
Spontaneous or Non-Spontaneous?
Reverse of Rusting?
2 Fe2O3(s) " 4 Fe(s) + 3 O2(g)
Spontaneous or Non-Spontaneous?
NaCl(s) " Na(s) + " Cl2(g)
Spontaneous or Non-Spontaneous?
CHEM112 LRSVDS Thermodynamics part 1 !
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SPONTANEITY AND WORK
Is this process spontaneous?
Can this process do work?
What is the reverse of this process?
Is the reverse process spontaneous?
Which process can do work?
2 H2O " 2 H2 + O2
2 H2 + O2 " 2 H2O
CHEM112 LRSVDS Thermodynamics part 1 !
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Spontaneity and Equilibrium
At -10°C
At + 10°C
H2O(l) " H2O(s)
Is it Spontaneous?
H2O(l) " H2O(s)
At 0°C?
Is it Spontaneous?
H2O(s)
H2O(l)
When a chemical system is at equilibrium, reactants and
products can interconvert reversibly.
In a spontaneous process, the path between reactants and
products is irreversible.
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CHEM112 LRSVDS Thermodynamics part 1 !
EXAMPLES
H2O(l) " H2O(s)
At -10°C
!H = ?
2NaCl(s) " 2Na(s) + Cl2(g)
!H=?
4Fe(s) + 3O2(g) " 2 Fe2O3(s)
!H=?
N2(g) " 2N(g)
!H=?
2H2(g) + O2(g) " 2H2O(g)
!H=?
CHEM112 LRSVDS Thermodynamics part 1 !
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If !H is negative, does that mean the
reaction is spontaneous?
Examples:
H2O(s) " H2O(l)
At +10°C !H = ?
Ba(OH)2•8H2O(s) + 2NH4SCN(s) "Ba(SCN)2(aq) + 2NH3(aq) + 10H2O(l)
!!H = ?
NH4Cl(s) + H2O(l) " NH4Cl(aq)
!H=?
CHEM112 LRSVDS Thermodynamics part 1 !
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SPONTANEITY
Nature tends to move spontaneously from a state of
lower probability to one of higher probability
»!G.N. Lewis (Nobel Laureate)
!H alone doesn’t determine spontaneity; what also
affects spontaneity?
CHEM112 LRSVDS Thermodynamics part 1 !
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ENTROPY!
A thermodynamic parameter (S) that is a measure of the
disorder or randomness in a system.
The more disordered a system, the greater its entropy.
Entropy is a state function -- its value depends only upon the state
of the system (not how it got there).
We are usually concerned with the change in entropy (!S ) during a
process such as a chemical reaction.
!S = S
final
- S initial
The Second Law of Thermodynamics
For spontaneous processes !S >0
!Suniv = (!Ssystem + !Ssurroundings) > 0
CHEM112 LRSVDS Thermodynamics part 1 !
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ENTROPY!
Which processes have !S > 0?
S "S
Unopened deck of cards " Cards spread out on table
Unassembled car parts " Assembled car
Seed + CO2 + H2O + Minerals " Tree
1 mole gas in 1 liter flask " 1 mole gas in 2 liter flask
NH4Cl(s) " NH4+(aq) + Cl-(aq)
CHEM112 LRSVDS Thermodynamics part 1 !
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Probability and Locations of Gas Molecules
Before Stopcock is
opened
Possibilities After
Stopcock is opened
Increase in number of microstates #
CHEM112 LRSVDS Thermodynamics part 1 !
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Molecular Basis of Entropy
!!As # of atoms in a molecule increases, S increases.
!S > 0
!SAr < !SHCl < !SCH4
CHEM112 LRSVDS Thermodynamics part 1 !
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Molecular Basis of Entropy
!! It is an extensive property (more moles, more entropy)
As # of molecules increases
!S > 0
Al2O3(s) + 3H2(g) " 2 Al(s) + 3 H2O(g)
!S =
Example: Is !S for this reaction positive or negative?
2NO(g) + O2(g)
2NO2(g)
CHEM112 LRSVDS Thermodynamics part 1 !
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Molecular Basis of Entropy
Dissolution of KCl(s): is !S for this process positive or negative?
CHEM112 LRSVDS Thermodynamics part 1 !
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Molecular Basis of Entropy
!! Which has more Entropy: Gas, Liquid or Solid State?
!S for gases
liquids
solids
Trends:
sol " gas; sol " liq; liq " gas
!S = ?
gas " liq; gas " sol; liq " sol
!S = ?
Structure of ice
What happens when T increases within the SAME phase?
CHEM112 LRSVDS Thermodynamics part 1 !
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Molecular Basis of Entropy
!! Adding heat increases entropy
CHEM112 LRSVDS Thermodynamics part 1 !
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ENTROPY
Examples: Which has the greater entropy?
S(1 mol HCl(g))
S(1 mole NaCl(s))
S(2 mol HCl(g))
S(1 mole HCl(g))
S(1 mol HCl(g))
S(1 mole Ar(g))
S(1 mol N2(g))
@ 300K
S(1 mole N2(g))
@ 200K
More Examples
!S (+ or - ?)
Effect
H2O (l, 25oC) " H2O(g)
CaCO3 (s) " CaO(s) + CO2(g)
N2(g) + 3 H2(g) " 2 NH3(g)
Ag+(aq) + Cl-(aq) " AgCl(s)
Au(s) at 298K " Au(s) at 1000K
CHEM112 LRSVDS Thermodynamics part 1 !
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Phase Changes
!! During phase changes what happens to
temperature?
!! What happens to the amount of disorder
during phase changes?
!!!S is always _________ for melting or boiling.
Entropy and heat at a fixed temperature:
!Stransition = qrev = !Htransition
T
T
For melting ice, !Sfus =
Example: Calculate entropy change when 1 mole of liquid water
evaporates at a constant temperature of 100oC. !Hvap = 40.6 kJ/mol
CHEM112 LRSVDS Thermodynamics part 1 !
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LAWS OF THERMODYNAMICS
1st Law:
The total energy in the universe is constant.
!Euniverse = 0
!Euniverse = !Esystem + !Esurroundings
!Esystem = - !Esurroundings
2nd Law:
The total entropy in the universe is increasing.
! Suniverse > 0
! Suniverse = !Ssystem + !Ssurroundings > 0
3rd Law:
The entropy of every pure substance at 0K (absolute zero
temperature) is zero.
S = 0 at 0K.
CHEM112 LRSVDS Thermodynamics part 1 !
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3rd Law of Thermodynamics!
S = 0 at T = 0 K
This means we can measure absolute entropy S (not just !S)
Define Absolute Entropy: S° (Standard Molar Entropy)
S° is always positive (>0) for pure substances
CHEM112 LRSVDS Thermodynamics part 1 !
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Entropy Changes for REACTIONS
!S°(rxn) = $nS°(products) - $ mS° (reactants)
What is !S° for the following reaction?
N2(g) + 3 H2(g) " 2 NH3(g)
!S > 0 when:
"!Gases form from either liquids or solids
"!Liquids or solutions form from solids
"!The number of molecules of gas increase during a chemical
reaction.
CHEM112 LRSVDS Thermodynamics part 1 !
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