Spontaneous Processes
Chapter 19 – Section 1

Introduction
 The first law of thermodynamics –
 energy is conserved.
 The total energy remains the same.
 We can transfer it from the system to the
surroundings
 We can change its form.
 Potential energy to kinetic energy
 Kinetic energy to potential energy
 The total energy remains the same.
Spontaneity
 We already know that chemical and physical
processes have a directional character.
 Sodium metal and chlorine gas easily combine to
form sodium chloride.
 However, sodium chloride does not easily
decompose into sodium metal and chlorine gas.
 The first law holds for both processes.
 But, one process occurs and the other does not.
Spontaneity
 A process that occurs of its own accord, without any
ongoing, outside intervention, is said to be
spontaneous.
Spontaneity
 A spontaneous process occurs with a definite
direction in time.
 Placing a single drop of dye in water produces the
dye distributed throughout the water.
➙
⇠
spontaneous
nonspontaneous
Spontaneity
 A spontaneous process occur with a definite
direction in time.
 A gas will expand into a vacuum.
➙
⇠
spontaneous
nonspontaneous
Spontaneity
 Processes that are spontaneous in one direction,
are nonspontaneous in the opposite direction.
Spontaneity
 Experimental conditions are often important in
determining whether a process is spontaneous.
 Temperature
 Pressure
 For example, the freezing and melting of water is
dependent on the temperature.
spontaneous at T < 273 K
⇨
⇦
spontaneous at T > 273 K
Spontaneity
 Experimental conditions are often important in
determining whether a process is spontaneous.
 Temperature
 Pressure
 At t = 273 K, there is an interconversion between
liquid water and ice with no preferred direction.
spontaneous at T < 273 K
⇨
⇦
spontaneous at T > 273 K
Spontaneity
 It is important to realize that knowing that a
reaction is spontaneous does not mean that it will
occur at an observable rate.
 A reaction is spontaneous if it occurs on its own,
regardless of its speed.
 Thermodynamics can tell us about the direction
and extent of the reaction but not the speed of the
reaction.
Sample Exercise 19.1 | Identifying Spontaneous Processes
Predict whether the following processes are spontaneous as
described, spontaneous in the reverse direction, or in equilibrium:
a) When a piece of metal is heated to 150°C is added to water at
40°C, the water heats up.
This process is spontaneous. Whenever two objects at
different
temperatures
are brought
into contact,
heat
b) Water
at room
temperature
decomposes
into H2(g)
andisO2(g).
transferred
fromus
the
hotter
one. Thus,
Experience tells
that
this object
processtoisthe
notcolder
spontaneous
– we
heat
is transferred
the
hot metal
the
water.to
certainly
have never
seen
hydrogen
and
gases
c) Benzene
vapor,
C6Hfrom
at
a pressure
ofto1oxygen
atmcooler
condenses
6(g),
The
final temperature,
after
theof
metal
andRather,
water the
attain
the
spontaneously
bubbling
up out
water!
reverse
liquid
benzene
at
the
normal
boiling
point
of
benzene,
80.1°C.
same
temperature
(thermal
equilibrium),
will
process
– the reaction
of H2 and
O2 to form
H2be
O – is
By
definition,
the
normal
boiling
point
is
the
temperature
somewhere
spontaneous.between the initial temperatures of the metal at
which
vapor at 1 atm is in equilibrium with its liquid. Thus,
and thea water.
this is an equilibrium situation. If the temperature were
below 80.1°C, condensation would be spotaneous.
Seeking a Criterion for Spontaneity
 Loss of energy is a common feature of spontaneous
change in mechanical systems.
 A marble rolling down hill
 A brick falling from a ledge
Seeking a Criterion for Spontaneity
 In the 1870’s, Marcellin Bertholet suggested that
spontaneous chemical change were determined by
a loss of energy.
 He suggested that all chemical and physical changes
were exothermic.
 But, it doesn’t take long to come up with exceptions
to this generalization.
 Melting ice at room temperature is spontaneous even
though it is endothermic.
Seeking a Criterion for Spontaneity
 Clearly, there must be some factor other than a
decrease in energy to account for spontaneity.
 We need to look closely at the ways in which the
state of a system can change.
 Temperature, internal energy, and enthalpy are state
functions.
 The values of q and w are very dependent on the
path take – these are not state functions.
 We need to look at reversible and irreversible paths
between states.
Reversible and Irreversible Processes
 These processes were first studied by Sadi Carnot.
 He was a French engineer.
 He found that not all of the energy content of a fuel
could be converted into work.
 There was a significant amount of heat lost to the
surroundings.
 His studies eventually became the discipline of
thermodynamics.
Reversible and Irreversible Processes
 Carnot’s work was extended by Rudolph Clausius.
 He was a German Physicist.
 He determined that there was a special significance
to the ratio of the heat delivered to an ideal engine
and the temperature at which it is delivered, q/T.
 He called this ratio entropy.
 In an ideal system, maximum efficiency is achieved
when all of the processes are reversible.
 Heat is lost to the surroundings when there are
irreversible steps in the process.
Reversible and Irreversible Processes
 Let’s look at some samples of reversible and
irreversible processes.
 When we put two objects at different temperatures
together, heat will spontaneously flow from the
hotter one to the cooler one.
heat
hot
 We cannot make the heat flow from the cooler object
cool
to the hotter object.
 The flow of heat is irreversible.