16-4-2011
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Thermodynamics
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Introduction
Thermodynamics examines the heat and work
as well as spontaneity of a reaction.
Spontaneity is the notion of whether or not a
process can take place unassisted.
Entropy is a mathematical concept describing
disorder and randomness in a system.
Free energy is a thermodynamic function that
relates enthalpy and entropy to spontaneity,
and can also be related to equilibrium
constants.
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Why Study Thermodynamics?
• With a knowledge of thermodynamics
and by making a few calculations before
embarking on a new venture, scientists
and engineers can save themselves a
great deal of time, money, and
frustration.
• Thermodynamics tells us what
processes are possible.
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– (Kinetics tells us whether the process is
practical.)
Law of Conservation of Energy
• energy cannot be created or
destroyed
– First Law of Thermodynamics
• energy can be transferred between
objects
• energy can be transformed from one
form to another
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Spontaneous processes and
entropy
As far as we are concerned, we would like to
be able to predict whether a chemical
reaction proceeds spontaneously under a
specified set of conditions or not.
A chemical reaction is said to be spontaneous
if it occurs under a given set of conditions. If
it does not occur under this set of conditions
it is said to be non-spontaneous.
If a reaction is spontaneous in one direction, it
is non-spontaneous in the other, and vice
versa.
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Examples of spontaneous
processes
• A waterfall runs downhill, but never uphill.
• A lump of sugar spontaneously dissolves in
a cup of tea, but never dissolved sugar
lumps back spontaneously.
• Heat transfers from a hot object to colder
surroundings, but never heat from a cold
surrounding transfers to heat a body of
higher temperature
• Iron exposed to water and oxygen rusts, but
never the opposite.
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Spontaneous Processes
• Spontaneous
processes are those
that can proceed
without any outside
intervention.
• The gas in vessel B will
spontaneously effuse
into vessel A, but once
the gas is in both
vessels, it will not
accumulate in one only.
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Many exothermic processes
are Spontaneous
In general, product-favored reactions are
exothermic and
spontaneous
E.g. thermite reaction
Fe2O3(s) + 2 Al(s)
 2 Fe(s) + Al2O3(s)
H = - 848 kJ
H+ + OH- g H2O
H = - 56.2 kJ
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Many endothermic processes
are also spontaneous
NH4NO3(s) + heat  NH4+ (aq) + NO3- (aq)
Hsol = +25.7 kJ/mol
H2O (s) g H2O (l)
Ho = +6.01 kJ/mol
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Water falling (higher to
lower potential energy) is
a spontaneous process.
H2 and O2 combine
spontaneously to form water
(exothermic)
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Conclusion: enthalpy alone is
not a sufficient criterion for
prediction of spontaneity.
… liquid water vaporizes
spontaneously at room
temperature; an
endothermic process.
In some cases, endothermic reactions that are
non-spontaneous at room temperature may
become spontaneous when the temperature
is raised. Example:
2HgO(s) g 2 Hg(l) + O2(g)
Ho = +90.7 kJ/mol
Conclusion: Exothermicity favors spontaneity
of a reaction but does not guarantee it. In
addition, endothermic reactions can be
spontaneous. Therefore, we can not judge
spontaneity considering H only.
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Entropy
• Entropy, S, is a measure of the
disorder of a system.
• Spontaneous reactions proceed to
lower energy or increase entropy.
• In ice, the molecules are very well
ordered because of the H-bonds,
therefore, ice has a low entropy.
• Melting ice will increase entropy, a
spontaneous process although
endothermic.
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Entropy




Entropy is a state function.
For a system, S = Sfinal - Sinitial
If S > 0 the randomness increases
if S < 0 the order increases
 Ssolid < Sliquid << Sgas
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Entropy
 There is a balance between energy and
entropy considerations.
 When an ionic solid is placed in water
two things happen:
 the water organizes into hydrates
about the ions (so the entropy
decreases), and
 the ions in the crystal dissociate (the
hydrated ions are less ordered than
the crystal, so the entropy increases).
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The Molecular Interpretation of
Entropy
 A gas is less ordered than a liquid which
is less ordered than a solid
 Any process that increases the number
of gas molecules leads to an increase in
entropy
 When NO(g) reacts with O2(g) to form
NO2(g), the total number of gas
molecules decreases, and the entropy
decreases
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2NO(g) + O2(g) g 2NO2(g)
 Boiling corresponds to a much greater
change in entropy than melting.
 Entropy will increase when:
 Liquids or solutions are formed from
solids
 Gases are formed from solids or liquids
 The number of gas molecules increases
 The temperature is increased
 The number of atoms in a molecule
increases without change in number of
moles of gas molecules
 The molar mass increases
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Which of the following processes
produces a decrease in the entropy
of the system?
A. Boiling water to form steam
B. Dissolution of solid KCl in water
C. Mixing of two gases into one
container
D. Freezing water to form ice
E. Melting ice to form water
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Consider a pure crystalline solid that is
heated from absolute zero to a temperature
above the boiling point of the liquid. Which
of the following processes produces the
greatest increase in the entropy of the
substance?
A. melting the solid
B. heating the liquid
C. heating the gas
D. heating the solid
E. vaporizing the liquid
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Predict whether each of the following leads to an
increase or decrease in the entropy of a system.
(a) The synthesis of ammonia:
N2(g) + 3 H2(g)  2 NH3(g)
(b) Preparation of a sucrose solution:
C12H22O11(s) g C12H22O11(aq)
(c) Evaporation to dryness of a solution of urea,
CO(NH2)2, in water:
CO(NH2)2(aq)  CO(NH2)2(s)
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