Dayap National Integrated High School
Chemical
Thermodynamics
GENERAL CHEMISTRY 2
QUARTER 4_M1
Martinez, Joven A.
Special Science Teacher I
Q4W1 _ Module 1
Spontaneous Change, Entropy,
and Free Energy
Most Essential Learning Competencies
Predict the spontaneity of a process based on entropy. STEM_GC11CTIVa-b-140
Explain the second law of thermodynamics and its significance. STEM_GC11CTIVa-b-142
Use Gibbs’ free energy to determine the direction of the reaction. STEM_GC11CTIVa-b-143
Thermodynamics
A branch of Physics which deals with the energy
and work of a system.
Thermodynamics is, in some ways, the science
that most influences our daily lives, because we use
its concepts and information in the ways we design
and operate so many of the devices we take for
granted in our daily lives.
Thermodynamics
CHEMISTRY
Thermodynamics tells chemists whether a particular
reaction is energetically possible in the direction in
which it is written, and it gives the composition of
the reaction system at equilibrium.
It does not, however, say anything about whether an
energetically feasible reaction will actually occur as
written, and it tells us nothing about the reaction rate
or the pathway by which it will occur (described by
chemical kinetics).
Chemical thermodynamics provides a bridge between
the macroscopic properties of a substance and the
individual properties of its constituent molecules and
atoms.
As you can see, thermodynamics explains why
graphite can be converted to diamond; how chemical
energy stored in molecules can be used to perform
work; and why certain processes, such as iron rusting
and organisms aging and dying, proceed
spontaneously in only one direction, requiring no net
input of energy to occur.
Spontaneous
vs
Non-Spontaneous
Spontaneous
Non-Spontaneous
Spontaneous
Spontaneous
Non-Spontaneous
Spontaneous
Non-Spontaneous
Spontaneous
SPONTANEOUS PROCESSES
Chemical and physical processes have a natural
tendency to occur in one direction under certain
conditions.
A spontaneous process occurs without the need for
a continual input of energy from some external
source, while a nonspontaneous process requires
such.
SPONTANEOUS PROCESSES
Systems undergoing a spontaneous process
may or may not experience a gain or loss of energy,
but they will experience a change in the way matter
and/or energy is distributed within the system.
NON-SPONTANEOUS PROCESSES
They cannot proceed unless there is a driving force
or outside help that acts on a system. For example,
a ball cannot be brought uphill unless someone
pushes it. Another non-spontaneous process is the
conversion of water to hydrogen gas and oxygen
gas, which cannot take place unless electric current
is applied on the system.
Chemical processes can be spontaneous as well.
These are chemical reactions that occur without any
intervention in a given set of conditions.
An example is spontaneous combustion, wherein a
flammable substance burns by itself even without
direct application of spark or flame.
SPONTANEOUS COMBUSTION
Spontaneous reactions such as burning of coal or
gasoline go to completion even without further
input of energy once triggered. Such a process is
said to be irreversible.
Generally, any spontaneous
process is irreversible. A system
is irreversible if it cannot be
restored to its original state
using the same path.
A reaction that does occur under the given set
of conditions is called a spontaneous reaction.
If a reaction does not occur under specified
conditions, it is said to be nonspontaneous.
A waterfall runs downhill.
A. Spontaneous
B. Nonspontaneous
A
Water freezes spontaneously below 0°C.
Ice melts spontaneously above 0°C (at 1 atm).
Heat flows from colder
object to hotter one.
A. Spontaneous
B. Nonspontaneous
B
A piece of sodium metal reacts violently with
water to form sodium hydroxide and hydrogen
gas.
Hydrogen gas does not react with sodium
hydroxide to form water and sodium.
2Na + 2H2O
2NaOH + H2
Iron exposed to water and
oxygen forms rust.
A. Spontaneous
B. Nonspontaneous
A
Iron exposed to water and oxygen forms
rust, but rust does not spontaneously
change back to iron.
HEAT OF REACTION
Also known as the Enthalpy of Reaction, is the
change in the enthalpy of a chemical reaction that
occurs at a constant pressure.
The notation ΔH° or ΔH°rxn then arises to explain
the precise temperature and pressure of the heat of
reaction ΔH. The standard enthalpy of reaction is
symbolized by ΔH° or ΔH°rxn and can take on both
positive and negative values.
The units for ΔH° are kilojoules per mole, or kj/mol.
ΔH° or ΔH°rxn
Δ = represents the change in the enthalpy; (ΔH products – ΔH
reactants)
- a positive value indicates the products have greater enthalpy,
or that it is an endothermic reaction (heat is required)
- a negative value indicates the reactants have greater enthalpy,
or that it is an exothermic reaction (heat is produced)
° = signifies that the reaction is a standard enthalpy change, and
occurs at a preset pressure/temperature
rxn = denotes that this change is the enthalpy of reaction
wherein:
vp = stoichiometric coefficient of the product from the balanced
reaction
vr = stoichiometric coefficient of the reactants from the balanced
reaction
ΔHºf = standard enthalpy of formation for the reactants or the
products
Example 1: The Combustion of Acetylene
Calculate the enthalpy change for the combustion
of acetylene (C2H2).
ΔH° = -2512.4 kJ
Example 2
Calculate the enthalpy of reaction for:
H2(g) + O3(g)
H2O(g)
ΔH° = -868.1 kJ
Example 3:
The combustion of methane, CH4 releases 890.4 kJ/mol of
heat. That is when one mole of methane is burned, 890.4 kJ are
given off to the surroundings. This means that the products have
890.4 kJ less energy stored in the bonds than the reactants. Thus,
ΔH for the reaction is equal to -890.4 kJ. A negative symbol for ΔH indicates
an exothermic reaction.
A. How much energy is given off when 2.00 mol of methane are
burned?
B. How much energy is released when 22.4 g of methane are
burned?
C. If you were to attempt to make 45.0 g of methane from carbon
dioxide and water (with oxygen also being produced), how much
heat would be absorbed during the reaction?
Example 4
Given a simple chemical equation with the
variables A, B and C representing diff. compounds.
A+B
C
and the standard enthalpy of formation values:
Example 5
NO2(g) is formed from the combination of NO(g) and
O2(g) in the following reaction:
High entropy
or
Low entropy
High entropy
Low entropy
Low entropy
High entropy
High entropy
Low entropy
Low entropy
High entropy
Low entropy
High entropy
ENTROPY
Entropy, denoted S, is a measure of
the randomness or disorder of a system.
This thermodynamic property is also a
measure of how much energy is
unavailable for conversion into work.
ENTROPY
The Second Law of Thermodynamics states that the state
of entropy of the entire universe, as an isolated system,
will always increase over time.
The second law also states that the changes in the
entropy in the universe can never be negative.
ENTROPY
The more random or disordered a system is, the greater the
entropy. Since entropy, like enthalpy, is a state function, it is
independent of the path or the route taken in attaining the final
state. Therefore, the change in entropy, ΔS, depends only on the
entropies of the final and initial states of the system.
ΔS = S final – S initial
ENTROPY
A positive (+) value of ΔS (ΔS > 0), indicates that the
final state is more random or disordered than the initial
state. A negative (-) ΔS (ΔS < 0), indicates that the final
state is more ordered than the initial state.
FACTORS AFFECTING ENTROPY
Qualitatively, it is possible to determine whether a
change in the system would result in a decrease or
increase in entropy. There are several factors that
influence the amount of entropy present in a system at a
particular state.
These factors include change in phase or physical state,
change in temperature, and change in concentration or
number of particles.
Processes that lead to an increase
in entropy of the system:
A.
B.
C.
D.
Melting
Vaporization
Dissolving
Heating
ENTROPY
• S, is a thermodynamic quantity.
• It is used to measure how spread out or dispersed the
energy of a system is.
• It is used to describe if the process is spontaneous and
can occur in a defined direction or nonspontaneous
and will occur in the reverse direction.
Predicting positive or negative entropy
CHANGE IN PHASE
In general, a liquid has higher entropy than the solid
from which it is formed, while a gas has a higher entropy
than the liquid counterpart. In changing from solid to
liquid, and from liquid to gas, the arrangement of
molecules become more disorderly.
CHANGE IN PHASE
A
B
C
Increasing entropy from solid to liquid to gas. (A) particles are confined to fixed
positions (B) particles are slightly far from each other (C) particles are very far apart
CHANGE IN TEMPERATURE
Raising the temperature increases the average kinetic
energy of molecules. Greater translational, vibrational, and
rotational motion lead to a more disorderly state.
With an increase in temperature, solid particles vibrate
more energetically, while liquid and gas particles move about
more rapidly. Hence, increasing the temperature increases the
entropy of a system. Conversely, a decrease in temperature
results in lowering of entropy.
NUMBER OF PARTICLES
The phrase “the more, the merrier” can certainly be applied to
entropy. When a lot of people are present, there will be more
movement, more noise, more wastes, and definitely, greater
disorder. This means greater entropy.
A2 B
2A + B
Which part of the reaction has the higher entropy?
Sample Exercise
Directions: Determine the entropy of a system for each of the folllwing
processes.
Positive
_________1.
A solid melts.
Negative
_________2.
A liquid freezes.
Negative
_________3.
A vapor is converted to liquid.
Negative
_________4.
A vapor condenses to liquid.
Positive
_________5.
A solid sublimes.
Sample Exercise
Directions: Determine the entropy of a system for each of the folllwing
processes.
Negative
_________6.
2H2(g) + O2(g)
2H2O(l)
Positive
_________7.
NH4Cl(s)
Positive
_________8.
H2(g) + Br2(g)
Positive
_________9.
CaCO3(g)
Positive
_________10.
H2(g) + Cl2(g)
NH3(g) + HCl(g)
2HBr(g)
CaO(s) + CO2(g)
2HCl(g)
Sample Exercise
Directions: Predict whether the entropy increases or decreases for each
of the following processes. Consider the degree of disorder, from initial
to final state or from reactants to products.
a. Entropy increases
b. Entropy decreases
c. Entropy increases
d. Entropy decreases
e. Entropy increases
ENTROPY CHANGE DETERMINATION
A numerical value for entropy can possibly be determined
for any substance under a given set of conditions. Entropy
change can be measured by a calorimeter, the same instrument
used in determining enthalpy change.
However, measurement of ΔS for reactions in the laboratory
is not that easy. The change in entropy is related to the heat
transferred during the process. The relationship between ΔS and
the heat transferred is similar to that of ΔH and heat transferred
under constant pressure.
ENTROPY CHANGE DETERMINATION
It is also possible to calculate entropy change for various
processes that occurs under standard conditions using standard
entropy values of substances.
ΔS° = ΣyS°(products) - ΣzS°(reactants)
Where:
° = in the symbol S° indicates standard conditions (25°C and 1 atm pressure)
y = number of moles (products)
z = number of moles (reactants)
ENTROPY CHANGE DETERMINATION
Sample Problem:
Calculate the entropy change when graphite burns in sufficient
supply of oxygen as shown in the following equation:
Directions: Using the data of S°, calculate the standard entropy change
for each of the following reactions at 25°C. For each reaction, interpret the
sign and magnitude of the reaction entropy.
1. CaCO3(s)
CO2(g) + CaO(s)
2. KClO3(s)
KClO4(s) + KCl(s)