Thermodynamics
Thermo- heat
Dynamics- movement
Thermodynamics is the study of energy changes and the flow of energy form
one substance to another. The energy flow accompanied by physical and
chemical changes.
Zeroth Law of Thermodynamics
“When two bodies are in thermal equilibrium with a third body, then they
must be in thermal equilibrium with each other.”
First Law of Thermodynamics
“Energy is neither crated more destroyed. It can only be transformed form
one form into another”
During the energy transformations, the total amount of energy does not change.
When one form disappears, an equal quantity in another form must appear. It
can be said that the energy of the universe is constant.
Second Law of Thermodynamics
“Efficiency of Energy Transformation”
How much work do you get from an energy source?
Can all of the chemical energy of a fuel such as gasoline be converted to
mechanical energy?
During combustion, some of the chemical energy of gasoline and oxygen is not
transformed to other forms of energy- it remains as chemical energy of carbon
dioxide and water.
Efficiency is measure in relation to the quantity of chemical energy that is
made available.
Free Energy. The quantity of energy that is available to do mechanical work
The poor efficiency of the car is due to transformation of some of the chemical
energy of fuels into other form of energy, mainly as heat, that are not used to
propel the car.
Waste of energy occurs at several places:
*Some heat form combustion is radiated from the engine to the surrounding air
*Some heat is transferred to the cooling water that passes around the engine. In
the radiator, this heat energy is transferred from the water to the surrounding
air.
*Some heat energy is passed into the air in the exhaust gases.
* Some of the mechanical energy is transformed to heat energy as a result of
friction between the tires and the road
*Some of the mechanical energy is transformed into heat energy because of
friction between moving parts in the power transmission
*Some of the chemical energy is converted during combustion into sound energy
Only 25% of the heat produced by gasoline combustion is used to propel the car.
The rest is “lost” as follows;
35% to the cooling system
35% to the atmosphere in the exhaust gases and by radiation
5% as friction of the moving parts
Hence,
Efficiency is obtained by the ratio of the amount of energy converted to useful
work and the total energy available.
These limitations on the conversion of energy from one form into another leads
to the second law of thermodynamics
Example:
Estimate the efficiency of a motor when it produced only 2.2x10 6 joules of
mechanical energy while consuming 1 kwhr of electricity.
Third Law of Thermodynamics: Law of Entropy
All real changes have a direction which we consider as natural. The
transformation in the opposite sense would be unnatural; it would be unreal.
The system to undergo an “unnatural change”, there exist such a property of s
system called ENTROPY
In nature, rivers run form the mountains to the sea, never in the opposite way. A
tree blossoms, bears fruit, and later sheds its leaves.
The dry leaves won’t rise, attach themselves to the tree, and later shrink into
buds.
Entropy ( S ) – A property relates to the degree of randomness of a system. It
is a measure of the degree of randomness or disorderliness of a system.
As a system goes from a more orderly to a less orderly state, there is an
increase in its randomness, and hence, it is an increase in entropy. Conversely,
if the change is one in which there is an increase in the orderliness, there is a
decrease in entropy.
Example
The process of vaporization produces an increase in randomness in the
distribution of molecules with a resulting increase in entropy. A heating process,
with the accompanying increase in kinetic energy, rotational and vibrational
energies, is accompanied by an increase in entropy of the system. And again,
conversely, cooling results in a decrease in entropy.
The change in entropy, ΔS, for any process is given by the equation
ΔS = S final - S initial
Thus, if S final > S initial, the randomness of the system increases, the entropy of
the system increases.
Hence,
“The energy of the universe is constant but the entropy approaches a
maximum”
At absolute zero of temperature, 00K, a substance will be in a state of perfect
order and its entropy of a pure crystal will be zero
S = 0
at T = 00K
As we cool a liquid substance and turns it into solid, the molecules become
highly ordered and the entropy of the system is low
ENTROPY CHANGE IN CHEMICAL REACTIONS
If the entropy of one mole of a substance is determined at a temperature of
298 0K (25 0C) and a standard entropy pressure of 1 atm, we call it the
standard entropy (Units of entropy – calories per Kelvin or joules per Kelvin)
The standard entropy change, ΔS0 for a chemical reaction by,
ΔS = (sum of S0 of products) - (sum of S0 of reactants)
Examples:
1. Urea (from urine) hydrolyses slowly in the presence of water to produce
ammonia and carbon dioxide.
CO(NH2)2 (aq) + H2O (l)
CO2 (g) + 2 NH3 (g)
Standard entropies (250C, 1 atm)
CO(NH2)2 (aq)
= 41.55 cal/mol-K
H2O (l)
= 16.72 cal/mol-K
CO2 (g)
= 51.06 cal/mol-K
NH3 (g)
= 46.01 cal/mol-K
What is the standard entropy change, in cal/0K, for this reaction?
Exercise
Calculate the standard entropy change S0 for the following reactions
1. The oxidation of C2H6 (g) to CO2 (g) and H2O (g)
2. Nitrogen dioxide dissolves in rainwater to form a dilute solution of nitric
acid by the reaction,
NO2 (g) +
H2O (l)
HNO3 (l) +
NO (g)
ENTROPY CHANGE IN PHASE CHANGES
The general equation for the computation of entropy change in phase is,
ΔS = ΔH transformation / ΔT transformation
Where, T is expressed in degree Kelvin
Example
Calculate the entropy change associated with the vaporization of water at
1000C
FREE ENERGY
The Gibbs Free energy and Spontaneous Change
What is free energy?
The Gibbs free energy change for a spontaneous process is the maximum
amount of energy produced by a reaction that can be theoretically harnessed.
It is that energy that need not be lost as heat and, therefore, free to be used for
doing work.
What does the quantity free energy give us?
Gibbs Free Energy is defined as
G = H - TS
For a change at constant temperature and pressure, the equation becomes
ΔG = ΔH - TΔS
This means that,
ΔG = Gfinal - Ginitial
What is of particular importance to us is the fact that a change can only be
spontaneous if it is accompanied by a decrease in free energy. In other words,
Gfinal < Ginitial ; G must be negative
What does this mean in terms of signs of ΔH and ΔS?
When a change is exothermic and is also accompanied by an increase in
entropy, both factors favor spontaneity
ΔH is negative
ΔS is positive
ΔG = ΔH – TΔS
= (-) - T (+)
In such change, ΔG will be negative regardless of the value of the absolute
temperature, T (which can only have positive value). Therefore, the change
will occur spontaneously at all temperatures. A good example is the oxidation
of fuels. The oxidation process is exothermic and the products have higher
entropy than the reactant.
Case 2
If a change is endothermic and is accompanied by a decrease in entropy, both
factors work against spontaneity
ΔG = ΔH – TΔS
= (+) - T (-)
In this case, ΔG will be positive at all temperatures and the change will always
be non- spontaneous
An example is a situation wherein hollow blocks rise by themselves (+ΔH) and
assemble to make a wall ( - ΔS ). This, of course, is fantastically untrue.
Case 3
When ΔH and ΔS have the same sign, the temperature becomes critical in
determining whether or not an event is spontaneous. If both ΔH and ΔS are
positive in the equation
ΔG = ΔH – TΔS
= (+) - T (+)
Only at relatively high T will the value T s be larger than H so that their
difference, G is negative. A familiar example is the melting of ice.
H2O (s)
H2O(l)
Here is a change that is endothermic and which occurs with an increase in
entropy. At temperatures above 00C (when pressure is 1 atm), ice melts
because the quantity TΔS is bigger that the ΔH
Example:
Calculate the ΔS and ΔG for the melting of ice at 00C
At lower temperatures, ice doesn’t melt because the smaller value of T gives a
smaller of TΔS and ΔG
Example:
Compute the lowest value of T at which ice will melt spontaneously
Case 4:
When both ΔH and ΔS are negative, ΔG will be negative only at relatively low
T, for similar reasons as in Case 3. This is the reason why the freezing of
water occurs spontaneously at low temperature, below 00C.
STANDARD FREE ENERGY
When ΔG is determined at 250C and 1 atm, we call standard free energy
change, ΔG0.
The formula for ΔG0 formation is
ΔG0 = (sum of the ΔG0 of products) - (sum of ΔG0 of reactants)
Example:
Calculate the G0 for the combustion of octane, a component of gasoline, at
250C and 1 atm
FREE ENERGY AND MAXIMUM WORK
Free energy change, ΔG, for a spontaneous reaction is a measure of the
maximum work that can be obtained from a reaction.
One of the most important uses to which put spontaneous chemical reactions
is the production of useful work. Fuels are burned in gasoline or diesel engines
to power automobiles and machineries. Chemical reactions in batteries start
our autos and run all sort of electronic gadgets.
When chemical reactions occur, however, their energy is not always harnessed
to do work. For instance, if we burn gasoline in an open dish, the energy
produced is lost entirely as heat and no useful work is accomplished.
In the design of machines, the primary goal is to maximize the efficiency with
which chemical energy is converted to work and to minimize the amount of
energy transferred unproductively to the environment and lost entirely as
heat
Example:
Calculate the maximum work available, expressed in kilojoules, from the
oxidation of 1 mole of octane, C8H8 (l), to give CO2 (g) and H2O (l) at 250C and
1 atm pressure.
FREE ENERGY AND EQUILIBRIUM
The algebraic sign of free energy tells us if the transformation can occur I the
direction in which we imagine it. There are three possibilities:
1. ΔG = - ; the transformation can occur spontaneously, or naturally
2. ΔG = + ; the transformation is non spontaneous, the natural direction is
opposite to the direction we have imagined for the transformation
Transformations with positive values for ΔG include such fantastic things as
water flowing uphill and the automobile converting water and carbon dioxide
to gasoline as it is pushed down the street.
3. ΔG = 0 ; the system is at equilibrium with respect to the transformation;
What is the implication of a zero value for ΔG as far as work is concerned?
Since ΔG is zero, the amount of work available is zero also. Therefore, no
work can be obtained from equilibrium it. Such a system is said to be in
equilibrium.
Let us examine the common lead storage battery.
When the battery is fully charged, there are virtually no products of the
reaction present. The reactants are present in large amount. Therefore, the
total free energy of the reactant far exceeds the total free energy of the
products, and since ΔG = Gproducts - Greactants, the ΔG of the system has a
large negative value. This means that a lot of work is available.
As the battery discharges, the reactant are converted to products and the
Gproducts get larger while Greactants get smaller. Thus, ΔG becomes lesser and
lesser negative and lesser work is available. Finally the battery reaches
equilibrium. The total free energies of the products and that of the reactants
become equal and ΔG = 0. No further work can be extracted; the battery is
dead.