GENERAL CHEMISTRY 2
Quarter 4_Week 1
SECOND AND THIRD LAW OF
THERMODYNAMICS
Teacher: Ms. Joreen S. Dael
LEARNING COMPETENCIES
- Predict the spontaneity of process based on entropy (STEM_GC11CTIVa-b-140)
- Explain the second law of thermodynamics and its significance (STEM_GC11CTIVa-b-142)
- Use Gibb’s free energy to determine the direction of a reaction (STEM_GC11CTIVa-b-143)
References:
• Negros Oriental, General Chemistry 2 module.
• Silberberg, M. (2015). Chemistry: The molecular nature of matter and change, 7th
ed. McGraw-Hill Companies, Inc.: New York.
Introduction to Spontaneity
In the previous module, you have learned that energy cannot be created nor destroyed it
can only be transferred from one form to another. In addition, a discussion of energy as a form of
heat and work and in relation to internal energy. For a chemical reaction, energy in the form of
heat (measured in enthalpy) can either be exothermic or endothermic process.
In this module we will be discussing spontaneity or spontaneous process. A spontaneous
process is one that occurs without the addition of external energy. The process or action may take
place quickly or slowly as long as it occurs and it is not directly related to rate. Example of a
spontaneous process is heat transfer. Heat always transfer from a hotter body to a colder body until
such a thermal equilibrium is reach between them. Another is corrosion of iron nails exposed in the
atmosphere. The iron in the nail spontaneously reacts with oxygen in the air to form rust or iron oxide.
Another is ice melting at room temperature.
The Degree of Disorder or Entropy
The second law of thermodynamics describes the entropy (ΔS), the unit is in terms of J/K or
J/mol●K for 1 mol of a substance. Entropy is defined as the measure of a system's thermal energy
per unit temperature that is unavailable for doing useful work. Because work is obtained from
ordered molecular motion, the amount of entropy is also a measure of the molecular disorder, or
randomness of a system. In simplest meaning, the system favors disorder than order. Entropy is
different than enthalpy, entropy is the degree of disorder and enthalpy is the total heat at constant
temperature (see Figure 1). The more disorder the system is the higher the entropy value.
Figure 1. Enthalpy is the total heat content in a
thermodynamic system whereas entropy is the degree
of disorder in the same.
Source: https://byjus.com/physics/differencesbetween-enthalpy-and-entropy/
As solid water is place left outside at room
temperature, it spontaneously converted into liquid water
at room temperature. This is also consistent with the
experimental data of H2O(s) 41= J/mol●K and H2O(l) =
69.95 J/mol●K (note that molecules in liquid is more
disordered than solid water and at room temperature so
the process is spontaneous). In figure 2, it shows the
differences in the entropy of solid, liquid and gas.
Figure 2. The comparison of the different values of
entropy of water (not to scale) notice that the ΔS(solid) <
ΔS(liquid) < ΔS(gas).
Another way to explain entropy, through statistical thermodynamics, which was first
proposed by Ludwig Boltzmann, an Austrian physicist. He was the first person who used statistics in
calculating entropy using the equation;
S = k ln Ω
Where: S = entropy, k = Boltzmann Constant (1.38 x 10-23 J/K), Ω is the microstates. Microstates
is a term used to describe the number of different possible arrangements of molecular position
and kinetic energy at a particular thermodynamic state. A process that gives an increase in the
number of microstates therefore increases the entropy.
The Gibbs Free Energy.
The Gibbs free energy (denoted by ΔG) combines the first and second law of
thermodynamics, enthalpy and entropy into a single value. The change in free energy, ΔG, is
equal to the sum of the enthalpy plus the product of the temperature and entropy of the system.
ΔG can predict the direction of the chemical reaction under two conditions; a.) constant
temperature and b.) constant pressure. If ΔG is positive, then the reaction is nonspontaneous (i.e.,
the input of external energy is necessary for the reaction to occur) and if it is negative, then it is
spontaneous (occurs without external energy input). Gibbs energy was developed in the 1870’s
by Josiah Willard Gibbs. He originally termed this energy as the “available energy” in a system.
his quantity is the energy associated with a chemical reaction that can be used to do work, and
is the sum of its enthalpy (H) and the product of the temperature and the entropy (S) of the
system. This quantity is defined as follows:
ΔG = ΔH – TΔS
where ΔG = Gibbs free energy, ΔH = enthalpy, T = temperature, and ΔS is entropy. Since the
changes of entropy of chemical reaction are not measured readily, thus, entropy is not typically
used as a criterion. To obviate this difficulty, we can use ΔG. The sign of ΔG indicates the direction
of a chemical reaction and determine if a reaction is spontaneous or not.
Below are the possible values of Gibbs free energy.
• ΔG < 0: (negative value) reaction is spontaneous in the direction written (i.e., the reaction is
exergonic)
• ΔG = 0: the system is at equilibrium and there is no net change either in forward or reverse
direction.
• ΔG > 0: (positive value) reaction is not spontaneous and the process proceeds spontaneously in
the reserve direction. To drive such a reaction, we need to have input of free energy (i.e., the
reaction is endergonic)
The factors affect ΔG of a reaction (assume ΔH and ΔS are independent of temperature) is
summarized below:
Note:
• ΔG depends only on the difference in free energy of products and reactants (or final state and
initial state). ΔG is independent of the path of the transformation and is unaffected by the
mechanism of a reaction.
• ΔG cannot tell us anything about the rate of a reaction.
For The standard Gibbs energy change (the standard-state free energy of reaction (ΔG°) is defined
as the free energy of reaction at standard state conditions) for a reaction in the form of aA + bB →
cC + dD, where A, B, C, and D are substances and a, b, c, and d are stoichiometric ratio or
coefficients; the formula is;
ΔrG° = cΔfG°(C)+ dΔfG°(D) − aΔfG°(A) − bΔfG°(B)
or summation of the total standard Gibbs free energy of the products minus the total Gibbs free
energy of the reactants;
ΔfG° = ΣΔfG°(products)−ΣΔfG°(reactants)
NAME: _____________________________
SECTION: ____________________
SCORE: ______ (Written)
_______(Performance)
PREPARED BY: Joreen S. Dael
General Chemistry I – Activity Sheets
Quarter 4 – Week 1
Written Task
A. True or False. Write the statement True if the statement is correct and False if
the statement is incorrect.
_________ 1. There is an increase in entropy if the state changes from gas to solid.
_________ 2. A gas from a container decreases the entropy as it undergoes
diffusion or effusion.
_________ 3. For a chemical reaction if the value of Gibbs free energy if zero, it
means that the reaction does not react.
_________ 4. According the second law of thermodynamics, if the system is
randomly arranged it has much higher entropy than that of the properly arrange
system.
_________ 5. Gibbs free energy can be used to determine how fast the energy
reaction is.
Identification
____________ 1. It is the amount of heat per unit mass required to raise the
temperature by one degree Celsius.
____________ 2. It is the change in the enthalpy of a chemical reaction that occurs
at a constant pressure.
____________ 3. A type of system where no heat and matter exchange.
____________ 4. A reactions or processes that release energy.
____________ 5. A thermodynamic system which is the energy contained within it.
B. Multiple Choice. Choose the correct letter of the answer for the following
questions.
1. Comparing reaction a and b, below;
a. a
b. b
c. both are equal
Which has entropy
value?
d. both have low value
2. Which of the following statements will always apply when a reversible
chemical reaction has attained equilibrium?
a. All reactants will convert to
products.
b. The reaction proceeds alternately
in the forward and reverse directions.
c. The Gibbs free energy of the
system reaches a minimum.
d. The forward reaction will
dominate over the reverse reaction.
3. Which of the following statements best describes the second law of
thermodynamics?
a. The internal energy of the universe
is constant.
b. Energy can be neither created
nor destroyed.
4. The entropy of an isolated system
can never ____?
a. increases
b. decreases
c. zero d. none of the mentioned
the entropy of the system will
increase.
c. When an isolated system
undergoes a spontaneous change,
d. At absolute zero, the entropy of a
perfect crystal is zero.
5. Which of the following can be considered as an application of entropy
principle?
a. transfer of heat through a finite
c. maximum temperature
temperature difference
obtainable from two finite bodies
b. mixing of two fluids
d. all of the mentioned
6. The entropy of an isolated system always ____ and becomes a ____ at the
state of equilibrium.
a. decreases, minimum b. increases, maximum
c. increases, minimum d. decreases, maximum
7. A system with _________ enthalpy and _________ entropy will never be
spontaneous.
a. positive/positive b. negative/positive
c. positive/negative d. negative/negative
8. Of the following reactions, which of the following is only spontaneous at high
enough temperatures?
a. ΔH +, ΔS + b. ΔH +, ΔS – c. ΔH –, ΔS – d. ΔH –, ΔS +
9. Which of the following transformations most likely involves a decrease in
entropy?
a. 2NaHCO3 (s) → Na2CO3 (s) + H2O
c. N2(g) + 3 H2(g) → 2NH3(g)
(g) + CO2 (g)
d. CO2 (s) → CO2 (g)
b. NH4NO3 (s) → NH4+(aq) + NO3(aq)
10. Which of the phase changes illustrated here illustrates an increase in the
entropy of the substance?
Performance Task: Problem Select the substance with the higher entropy in each
pair, and explain your choice [assume constant temperature, except in part
(e)]:
(a) 1 mol of SO2(g) or 1 mol of SO3(g)
(b) 1 mol of CO2(s) or 1 mol of CO2(g)
(c) 3 mol of O2(g) or 2 mol of O3(g)
(d) 1 mol of KBr(s) or 1 mol of KBr(aq)
(e) Seawater at 28C or at 238C
(f) 1 mol of CF4(g) or 1 mol of CCl4(g)
Select the substance with the lower entropy in each pair, and explain your
choice (assume 1 mol of each at the same T ):
(a) LiBr(aq) or NaBr(aq) (b) quartz or glass