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UNIT 5
How do we
predict
chemical
change?
Our ability to synthesize new chemical compounds or to
control chemical processes depends on how well we can predict the extent and rate of chemical reactions based on the
analysis of the composition and structure of the substances
involved. Our predictions may be enhanced by making use
of experimental information about key properties of the
reactants and products.
To predict the likelihood of a chemical reaction we need to
study the directionality, extent, mechanism, and rate of the
process. Of these four factors, directionality and extent are determined by thermodynamic properties of the substances involved,
while mechanism and rate are associated with kinetic properties.
The central goal of this Unit is to help you identify and apply the
four different factors that help predict the likelihood of chemical
reactions. To illustrate these ideas, we will analyze processes that
may have played a central role in the origin of life on Earth.
282
Chemical Thinking
UNIT 5 MODULES
M1. Analyzing Structure
Comparing the thermodynamic stability of
reactants and products.
M2. Comparing Free Energies
Quantifying the directionality and extent of
chemical reactions.
M3. Understanding Mechanism
Analyzing the changes that lead from reactants
to products.
M4. Measuring Rates
Exploring changes in the concentration of reactants and products as a function of time.
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There are multiple reasons to be interested in why and how chemical reactions occur. For example, we may want to predict whether a new polymer used to m a k e
bottles will react with the liquids it contains to produce toxic
substances. We may be interested in predicting whether
the combustion products of a new type of gasoline will
react with compounds in the atmosphere. We may
want to identify the types of chemical reactions that
led to the formation of the basic components of life
on Earth. In all these cases, we want to determine the
extent to which a given chemical process occurs as well
as the rate at which the reaction proceeds. Some chemiRusting is a slow
cal processes may occur to a great extent (i.e., reactants are almost
process but occurs to
a great extent
completely converted into products) but at such a slow rate that the
probability of observing the process will be very small. In other cases,
the extent of the reaction may be limited but the process could be so fast that it is
cost-effective for producing the substances that we want.
Chemists use a variety of approaches to predict the extent and rate of chemical reactions. Here in Module 1, we will focus on the analysis of the composition
and structure of reactants and products to make qualitative predictions about the
direction in which chemical processes are likely to occur.
By Laitr Keiows (Own work)
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U5: MODULE 1
Analyzing
Structure
THE CHALLENGE
Primitive Combinations
Nitrogen gas, N2(g), and hydrogen gas, H2(g), are suspected to be two important components of the primitive Earth. These two substances could have
reacted to form ammonia NH3(g) in the atmosphere.
•
•
How would you go about predicting the extent to which N2(g) and H2(g)
would combine to form NH3(g)?
What factors must be considered when making this prediction?
Share and discuss your ideas with one of your classmates.
This module will help you develop the type of chemical thinking that is used
to answer questions similar to those posed in the challenge. In particular, the central goal of Module 1 is to learn to predict reaction directionality based on the
relative thermodynamic stability of reactants and products.
Chemical Thinking
U5
How do we predict chemical change?
285
Chemical Stability
By Einar Helland Berger (Own work)
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Not every combination of substances will lead to the formation of new compounds
via a chemical reaction. How can we predict when a chemical process takes place?
One approach could be to compare the relative stability of reactants and products.
We might expect that chemical reactions will proceed in the direction in which
more stable substances are formed. However, for this strategy to work we need to
clearly define what we mean by “stability” and we need to identify the compositional and structural features that can be used to evaluate the relative stability of
different substances.
In general, chemists distinguish between two types of chemical stability, called
thermodynamic stability and kinetic stability. Thermodynamic stability is a measure of how stable a system is with respect to changes in the surroundings, such as
changes in temperature and pressure, or to the addition of new components. The
greater the thermodynamic stability of a substance, the less likely it is to change
to a different state or to react with other substances. On the other hand, kinetic
stability is a measure of how long it would take for a system to reach a more stable
state without external intervention. Although the reaction between two substances
may be favored because it would lead to the formation of more thermodynamically stable products, the process may take a long time to occur without external
intervention because the reactants are kinetically stable (Figure 5.1).
Kinetic stability is affected by the value of the activation energy Ea needed to
initiate a process; this quantity determines the rate of the reaction but not its directionality. On the other hand, thermodynamic stability is affected by energetic
and entropic factors:
• Energetic Factors influence the internal potential energy of a substance due to
interactions between its submicroscopic components (i.e., electrons, atoms, ions,
molecules);
Figure 5.1 Mixtures of
common fuels (e.g., methane, wood, glucose) with
oxygen tend to be thermodynamically unstable but
kinetically stable.
Entropic Factors influence the number of different configurations in which the
submicroscopic components of the substance may exist.
•
These two types of factors determine the directionality and extent of the chemical
process. In this module, we will focus our attention on the analysis of compositional and structural features of chemical substances that can be used to judge
their relative thermodynamic stability. This type of analysis is useful in making
predictions about the direction in which a chemical reaction may occur.
LET’S THINK
Directionality?
Consider the following chemical processes:
2 H2(g) + O2(g)
•
2 H2O(l)
2 H2O(l)
2 H2(g) + O2(g)
Which compositional and structural features of reactants and products may affect
which of these two processes is more likely to take place? Identify features that
can affect the potential energy of the molecules involved (energetic factors) as
well as the number of configurations they can adopt (entropic factors).
Share and discuss your ideas with a classmate, and clearly justify your reasoning.
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MODULE 1
Analyzing Structure
Energetic Factors
Chemical substances in which the submicroscopic components interact strongly
tend to have low internal potential energies and to be more thermodynamically
stable. Thus, paying attention to compositional and structural features such as
bond strength (i.e., the strength of atom–atom interactions in molecular compounds and ion–ion interactions in ionic compounds) and the type of intermolecular forces (i.e., the strength of interactions between independent particles) may
help us predict relative thermodynamic stabilities To begin our analysis, let us
identify some useful patterns in the bond energies of different types of covalent
bonds.
LET’S THINK
Bond Energies
Consider the bond energy (BE), expressed in kJ/mol, of the following types of bonds:
Bond
BE (kJ/mol)
Bond
BE (kJ/mol)
Bond
BE (kJ/mol)
C–C
N–N
O–O
F–F
347
163
142
159
C–H
N–H
O–H
F–H
414
389
464
565
C–N
C–O
C–F
C–Cl
305
360
485
339
•
How does the heterogeneity of a bond (i.e., whether the bond involves atoms of the
same or different types) affect bond strength?
•
How would you use bond properties, such as bond length and bond polarity, to explain this effect?
•
How could the analysis of the types of bonds in the molecules of different molecular
substances be used to predict relative stabilities?
Share and discuss your ideas with a classmate, and clearly justify your reasoning.
Molecular compounds with stronger covalent bonds tend to be more thermodynamically stable than substances with weaker bonds because more energy
is needed to induce atomic rearrangements. In general, chemical bonds involving
atoms of the same type (A–A bonds) are weaker than bonds between atoms of
different types (A–B bonds). A–B bonds tend to be shorter and more polar than
A–A bonds, factors that increase the strength of atom–atom interactions and decrease the internal potential energy of the system. Thus, from an energetic point
of view, chemical reactions are more likely to proceed in the direction that leads to
the formation of molecules with the larger proportion of A–B bonds. Apply this
predictive rule cautiously, though, because some A–A bonds are strong (e.g., BE =
–436 kJ/mol for H–H) and there may be instances in which reactions that involve
breaking these types of bonds will not be energetically favored. We also need to
consider the strength of the intermolecular forces between particles in the system
and the number of configurations that different types of particles may adopt.
Chemical Thinking
U5
LET’S THINK
Initial Predictions
It is likely that Earth’s atmosphere contained a considerable amount of
H2(g) when the first life forms appeared. Given the information listed in
the table to the right:
•
Analyze how thermodynamically favored it would have been for
H2(g) to react with other elementary substances such as O2(g),
F2(g), Cl2(g), Br2(l), and I2(s) to form the associated binary compounds [i.e., H2O(l), HF(g), etc.]
HINT: Write the balanced chemical equations for each potential reaction
and compare the bond energies of reactants and products.
Share and discuss your ideas with a classmate, and clearly justify your
reasoning.
Bond
BE (kJ/mol)
H–H
F–F
Cl–Cl
Br–Br
I–I
O=O
O–H
F–H
Cl–H
Br–H
I–H
436
159
243
193
151
498
464
565
431
364
297
Recall from Unit 4 that differences in the bond energy of reactants and products are responsible for the absorption or release of energy during a chemical reaction. If the products of a chemical process are made up of molecules with stronger
bonds than those present in the molecules of the reactants, the internal potential
energy of the products will be lower than that of the reactants and energy will be
released during the reaction (it will be an exothermic process). Thus, reactions that
lead to the formation of chemical compounds with a larger proportion of A-B
bonds than A-A bonds will likely be exothermic (DHrxn< 0) (Figure 5.2). From
the energetic point of view, chemical processes will have a higher probability of
occurring in the direction in which less net energy (or no net energy) needs to be
invested for the reaction to happen. As a result, exothermic processes tend to be
more energetically favored than endothermic processes.
The strength of the intermolecular interactions between the molecules that
make up a compound also has an effect on the internal potential energy of the
substance. Molecules are closer to each other in the solid and liquid states than
in the gaseous state, so the internal potential energy of a substance is lower in the
condensed phases. Consequently, the formation of the liquid or the solid phase of
a substance is more energetically favored than the formation of the gaseous phase.
For example, the formation of liquid water H2O(l) from the reaction between
H2(g) and O2(g) at 25 oC is more exothermic (DHrxn= –286 kJ/mol) than the formation of water vapor H2O(g) (DHrxn= –242 kJ/mol). As we will see in the next
section, this effect competes with configurational factors that facilitate the formation of gases over liquids and solids, particularly at high temperatures.
Ep
A2 + B2
Energy released
Reactants
2 CO2(g) + 2 H2O(g)
|
2 CO2(g) + 2 H2O(l)
Products
Figure 5.2 Generic ener-
gy diagram for a reaction
leading to the formation of
compounds with a larger
proportion of A–B bonds.
Which of these two processes is more likely to occur? Clearly justify your reasoning:
2 HCOOH(l) + O2(g)
2 AB
Reaction Path
LET’S THINK
Directionality?
•
287
How do we predict chemical change?
2 HCOOH(l) + O2(g)
288
MODULE 1
Analyzing Structure
r
-+
Figure 5.3 The distance
(r) between charged particles is measured from
center to center.
The likelihood of chemical reactions involving ionic compounds depends on
factors that influence the strength of ion–ion interactions. Ionic compounds consists of positive ions (cations) and negative ions (anions) arranged in a crystalline
network, not of independent molecules. The strength of the interaction between
anions and cations in the network is determined by Coulomb’s law (F= q1q2/r2),
and therefore it depends on the electric charge (q) of the ions and on their size
(Figure 5.3). Based on Coulomb’s law, the strength of ion–ion interactions should
increase when the charge of the ions is larger and their sizes are smaller. Stronger
attractive interactions between neighboring anions and cations in an ionic lattice
lead to lower internal potential energies, thus increasing the thermodynamic stability of the substance.
LET’S THINK
Ionic Compounds
Ionic compounds, either in solid form or dissolved in water, are thought to have played several
important roles in the origin and evolution of life on Earth. Given their crystalline structure, the
surfaces of common rock-forming oxides and carbonates select and concentrate specific amino
acids and sugars, and they also increase the rate of a variety of chemical reactions.
•
Consider the following ionic oxides present on Earth: Na2O, K2O, Al2O3, CaO, MgO. Arrange them in order of decreasing internal potential energy (or increasing thermodynamic
stability).
Share and discuss your ideas with a classmate, and clearly justify your reasoning.
The energy released during the formation of a solid ionic network starting
from the cations (C+) and anions (A-) in the gas phase can be used to compare the
relative internal potential energy of different ionic compounds. This process can
be represented using the following generic chemical equation:
C+(g) + A-(g)
Lattice Energy (kJ/mol)
-500
-600
-700
-800
-900
-1000
-1100
F-
Cl-
Br-
I-
CA(s)
The energy released during this process (DHrxn) is
known as the lattice energy of the compound CA(s).
The more negative the lattice energy of an ionic comK+ pound, the more energy will be required to separate
the ions that make up the system and the more ther+
Na modynamically stable the compound can be expected
to be. Figure 5.4 illustrates the variation in the lattice
Li+
energy for a set of binary ionic compounds involving
ions with different sizes. The experimental results included in this figure and its caption confirm our hypothesis that relative ion sizes and charges can be used
to predict the relative thermodynamic stability of ionic
compounds.
Figure 5.4 Lattice energy for different sets of ionic compounds. The lattice
energy for compounds involving ions with a larger charge is much more
negative. For example, it is –2522 kJ/mol for MgCl2 and –2253 kJ/mol for
CaCl2.
Chemical Thinking
U5
How do we predict chemical change?
289
LET’S THINK
Metallic Oxides
Earth’s original atmosphere did not contain large amounts of oxygen. The action of UV radiation
on water molecules, combined with the production of oxygen by photosynthetic organisms, increased dramatically the proportion of O2(g) in the atmosphere. Oxygen gas reacted with different
metals on Earth’s surface to produce a variety of ionic compounds, as in the following processes:
DHrxn (kJ/mol)
Chemical Reaction
2 Fe(s) + 3/2 O2(g)
Ti(s) + O2(g)
•
–824
Fe2O3(s)
TiO2(s)
–944
Ca(s) + 1/2 O2(g)
CaO(s)
–635
Ba(s) + 1/2 O2(g)
BaO(s)
–548
Apply the ideas discussed in this module to explain the observed differences in the heats
of reaction for these chemical processes. Discuss why some of these processes may be more
energetically favored than others.
Share and discuss your ideas with a classmate, and clearly justify your reasoning.
Entropic Factors
Number of Configurations
LET’S THINK
The simulations that can be opened by clicking on the image illustrate the behavior of the submicroscopic components of four different types of systems.
•
Arrange these systems, from fewest to most, according to the number of
configurations that the particles in the system can adopt. The different
configurations represent different ways in which the particles may arrange in space or different manners in which energy may be distributed
among them.
Share and discuss your ideas with a classmate, and justify your reasoning.
CLICK TO PLAY
http://www.chem.arizona.edu/tpp/chemthink/resources/U5_M1/conf.html
The thermodynamic stability of chemical compounds does not depend only on
the potential energy of their submicroscopic components. We also have to take
into account the number of configurations in which these components may be
found. The larger the number of configurations that the atoms, ions, or molecules
that make up a substance can adopt in a given state, the smaller the probability
that random movements will lead them to a completely different state or to form
new compounds when interacting with other systems. Thus, the larger the number
of configurations that the particles that make up a substance can adopt, the more
thermodynamically stable the substance should be expected to be.
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MODULE 1
Analyzing Structure
In general, the larger the number of particles in a system, the more diverse
these particles are, and the greater the number of different types of interactions
among such particles, the larger the number of configurations in which the system
can exist. Particles in a system may adopt different configurations not only by
changing their positions in space, but also by adopting different configurations
in which the total energy is distributed in different ways among its components.
For example, a system may adopt two different sets of configurations in which the
total kinetic energy is the same but the energy distribution is rather different. In
some configurations all of the particles may be moving at similar medium speeds,
while in other configurations some particles may have high speeds while others
may have low speeds.
Physical scientists have been able to identify a measurable property of chemical substances that can be used to quantify the number of configurations that their
submicroscopic components can adopt at a certain temperature. This property is
known as the entropy of the substance and is represented by the letter S. The entropy S of a system is related to the number of available configurations W through
the Boltzmann’s entropy equation:
(5.1)
S = kB ln W
where kB is Boltzmann’s constant (kB = 1.380 x 10-23 J/K) and (ln) is the natural
logarithm. The entropy of a substance can be determined experimentally from its
heat capacity at different temperatures. Of particular interest is the value of the
standard molar entropy of formation Sfo , measured in J/(mol K) for the substance of
interest. This quantity is a measure of the different configurations that matter and
energy can take in one mole of the substance at 25 oC and 1 atm. The reported values of Sfo assume that the standard entropy of the perfect crystalline substance at T
= 0 K is equal to 0. According to Equation (5.1), this implies that at 0 K particles
can exist only in one configuration (the perfectly ordered solid).
The standard molar entropy of formation Sfo of chemical substances is affected
by a variety of factors, some of which depend on the composition and structure of
its individual molecules or ions (single particle level) while others are related to the
distribution and interactions of the many particles present in a mole of substance
(multiparticle level). These factors are summarized in the following table and their
effects will be explored and analyzed in the following activities.
Molecular Compounds
Single Particle Level
Multiparticle Level
Molecular Mass
State of Matter
Molecular Complexity
Ionic Compounds
Single Particle Level
Multiparticle Level
Ion Mass
State of Matter
Ion Charge
Ion Size
Chemical Thinking
U5
How do we predict chemical change?
LET’S THINK
291
State of Matter
The tables list the standard molar entropy of formation Sfo of a group of substances that played a
key role in the development of life on Earth.
•
Substance
Sfo (J mol-1 K-1)
Substance
Sfo (J mol-1 K-1)
Substance
Sfo (J mol-1 K-1)
H2O(s)
41.0
C(s, diamond)
2.4
CO2(s)
51.1
H2O(l)
70.0
C(s, graphite)
5.7
CO2(aq)
117.6
H2O(g)
188.8
C(g)
158.1
CO2(g)
213.8
According to these data, how does the state of matter affect the entropy of molecular compounds? How would you explain these patterns using the ideas discussed in this module?
The following tables lists values of Sfo for important ionic compounds in our planet in different
states:
Substance Sfo (J mol-1 K-1)
Substance Sfo (J mol-1 K-1)
•
NaCl(s)
72.1
NaNO3(s)
116.5
NaCl(aq)
115.5
NaNO3(aq)
205.4
How would you explain the different effects on the entropy of dissolving a gaseous substance, such as CO2(g), in water and dissolving an ionic compound in this liquid?
Share and discuss your ideas with a classmate, and clearly justify your reasoning.
As the data in the previous activity illustrate, the state of matter has a major
effect on the entropy of all types of chemical substances. In general, the more
configurations particles can adopt in any given state, the larger the entropy of the
substance in that phase. Given that temperature and pressure affect the state of
matter of chemical substances, we can expect these two variables to influence the
entropy values.
LET’S THINK
Temperature Effects
•
How would you explain the different effects that
changing temperature has on the value of S at
different stages during the heating process?
Share and discuss your ideas with a classmate, and
clearly justify your reasoning.
Entropy
The graph illustrates the effect of changing temperature on the entropy S of water.
Temperature
292
MODULE 1
Analyzing Structure
LET’S THINK
Molecular Mass
The table lists values of Sfo for substances in the same state of matter,
with similar molecular complexity, but different molecular mass.
•
How would you explain the differences in standard molar entropy values?
Share your ideas with a classmate and justify your reasoning.
LET’S THINK
H2(g)
130.7
N2(g)
191.6
O2(g)
205.2
Cl2(g)
223.1
Molecular Complexity
The table lists values of Sfo for substances in the same state of matter,
with similar molecular mass, but different molecular complexity
•
Substance Sfo (J mol-1 K-1)
How would you explain the differences in standard molar entropy values?
Substance
Sfo (J mol-1 K-1)
N2(g)
191.6
CO(g)
197.7
C2H4(g)
219.3
Share your ideas with a classmate and justify your reasoning.
LET’S THINK
The tables to the right lists values of Sfo for a set of common ionic
compounds:
•
Explain the differences in standard molar entropy values within
the fluorides, within the oxides, and between the fluorides and
the oxides? What factors seem to have a dominant effect on the
value of the entropy of ionic compounds?
Share your ideas with a classmate, and clearly justify
your reasoning.
Ionic Compounds
Substance
Sfo (J mol-1 K-1)
LiF(s)
35.6
NaF(s)
51.5
KF(s)
66.6
Substance
Sfo (J mol-1 K-1)
BeO(s)
13.8
MgO(s)
27.0
CaO(s)
38.1
Given that substances with higher values of entropy have a larger number of accessible configurations
at any given temperature, they should be more thermodynamically stable. Chemical reactions should then
be more likely to proceed in the direction that produces substances with higher entropy. If we were to
measure the difference in standard molar entropy between products and reactants DSorxn, we should expect
DSorxn > 0 for processes that are entropically favored. However, the same factors that result in high entropy
values often result in high internal potential energy values and thus DHrxn > 0. For example, the formation
of gases is favored entropically but disfavored energetically. The frequent competition between energetic
and entropic contributions to the thermodynamic stability of chemical substances means that we need to
devise reliable ways to quantify these two types of effects. This will be our central goal in Module 2.
Chemical Thinking
FACING THE CHALLENGE
Primordial Atmosphere
The origin of life on Earth seems to have occurred
in a relatively short period of time in our planet’s history. Experimental evidence suggests that
the solar system is close to 4.65 billion years old.
Life on Earth seems to have appeared between 3.9
and 3.8 billion years ago. The formation of life on
Earth required the accumulation and organization of chemical compounds needed to sustain it.
The synthesis and accumulation of these types of
substances would have depended on the prevailing
environmental conditions on the primitive Earth.
Unfortunately, little is
known with certainty
about the primordial atmospheric composition
of our planet.
H2, H2O, CH4, CO,
CO2, and NH3 are the
most abundant molecular gases in our solar system, and this was probably the case during the
By NASA/Jenny Mottar
formation of our planet.
It is unlikely, however, that Earth kept much of
its atmosphere during the formation process. It
has thus been suggested that the primordial atmosphere derived from outgassing (gases from volcanic emissions), at temperatures between 300 and
1500 oC. The chemical composition of the early
mantle should have then determined the chemical nature of the gases released during outgassing.
As a result, the primordial atmosphere was likely
composed of mainly nitrogen, N2, and smaller
amounts of other gases such as CO, H2O, and H2.
The amount of O2 is speculated to have been negligible.
The main components in the primordial atmosphere could have reacted with each other to form
other chemical compounds. For example, the following processes proceed to a large extent due to
the thermodynamic stability of the products:
U5
How do we predict chemical change?
CO2(g) + 4 H2(g)
CH4(g) + 2 H2O(g)
CO(g) + 3 H2(g)
CH4(g) + H2O(g)
N2(g) + 3 H2(g)
2 NH3(g)
The first forms of life may have relied on these
types of chemical reactions as a source of energy.
Methanogens, for example, are modern microbes
that depend on the formation of methane, CH4,
in the first reaction above for their survival. These
organisms are strictly anaerobic and are believed to
be evolutionarily ancient.
Geological evidence suggests that only low
levels of O2 existed in the atmosphere before 2.4
billion years ago. The rise of oxygen in the atmosphere is somewhat linked
to the development of
organisms that produced
organic matter (represented generically as CH2O)
through photosynthesis:
CO2 + H2O
CH2O + O2
Respiration and decay
processes reverse this
chemical reaction on a
time scale of 100 years, consuming over 99% of
the oxygen produced by photosynthesis. However,
around 0.1-0.2% of the organic matter escapes the
reconversion into CO2 and H2O through burial
in sediments. This process can eventually lead to
a net accumulation of O2 in the atmosphere. This
accumulation is possible as long as other thermodynamically favored processes in which oxygen is
consumed occur at a lower rate:
4 FeO(s) + O2(g)
C(s) + O2(g)
H2(g) + O2(g)
2 Fe2O3(s)
CO2(g)
2 H2O(g)
Existing evidence suggests that there were two distinct periods in the history of Earth in which O2
amounts rose considerably. In each case, accumulation was due to the burial of chemical substances
that otherwise may have reacted with O2 to form
more thermodynamically stable products.
293
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MODULE 1
Analyzing Structure
Let’s Apply
Chemistry on the Primitive Earth
ASSESS WHAT YOU KNOW
Analysis of the thermodynamic stability of potential reactants and products is critical for building models and theories about the extent of the chemical reactions and the nature of the chemical
substances that led to and sustained the development of life on Earth.
Methane Formation
The following chemical processes were likely involved in the formation of methane,
CH4(g), in Earth’s primordial atmosphere:
CO2(g) + 4 H2(g)
CO(g) + 3 H2(g)
C(s) + 2 H2(g)
•
CH4(g) + 2 H2O(g)
CH4(g) + H2O(g)
CH4(g)
Discuss whether each of these reactions would be product-favored (
) or
reactant-favored (
)based on whether the processes are a) energetically
favored (if necessary, compare the bond energies of the molecules involved as
reactants and products), and b) entropically favored.
Share and discuss your ideas with a classmate, and clearly justify your reasoning.
Oxygen Sinks
The following chemical reactions represent processes that may have played an important role in hindering the accumulation of oxygen, O2(g), in Earth’s atmosphere:
4 FeO(s) + O2(g)
4 FeS2(s) + 15 O2(g) + 8 H2O(l)
C(s) + O2(g)
•
2 Fe2O3(s)
2 Fe2O3(s) + 8 H2SO4(aq)
CO2(g)
Discuss whether each of these reactions would be product-favored (
) or
reactant-favored (
) based on whether the processes are a) energetically
favored (if necessary, compare the bond energies of the molecules involved as
reactants and products), and b) entropically favored.
Share and discuss your ideas with a classmate, and clearly justify your reasoning.
Chemical Thinking
U5
How do we predict chemical change?
295
Hydrogen Sources
Some models propose that H2(g) was a major component of Earth’s primordial atmosphere. It is such a light gas, though, that most of it is assumed to have escaped from
Earth’s gravitational pull. The following table lists the heat of reaction (DHrxn) and the
difference in entropy DSorxn (difference in the standard molar entropy of products versus
reactants) at 25 oC for reactions that may have led to the formation of H2(g) on Earth:
DHrxn (kJ mol-1) DSorxn (J mol-1 K-1)
Chemical Reaction
S(s) + 4 H2O(l)
2 FeO(s) + H2O(l)
•
2 H2(g) + CO2(g)
90.1
91.9
3 H2(g) + H2SO4(aq)
233.9
98.5
5.6
26.5
H2(g) + Fe2O3(s)
Analyze each of the chemical reactions and justify the signs of DHrxn and DSorxn for
each of them. Discuss why some processes are more thermodynamically favored
than others.
Share and discuss your ideas with a classmate.
Amino acids
Amino acids were the first chemical compounds of biological interest produced in an
experiment simulating conditions on the primitive Earth. These substances have been
produced using an electrical discharge through different types of simple mixtures, such as
H2O/CH4/NH3/H2, H2O/CH4/N2/NH3, and CO/N2/H2. Ball-and-stick models of six
different amino acids are as follows:
Leucine
Serine
Alanine
Glycine
•
Proline
Valine
Based on your analysis of their composition and structure, arrange these amino
acids in order of increasing standard molar entropy.
Share and discuss your ideas with a classmate, and clearly justify your reasoning.
ASSESS WHAT YOU KNOW
C(s) + 2 H2O(g)
296
MODULE 1
Analyzing Structure
Let’s Apply
ASSESS WHAT YOU KNOW
Smelting
Chemical interactions between metals in the Earth’s crust and gases in our atmosphere led to the
formation of the wide variety of chemical compounds that make up the minerals that we routinely
mine today. Smelting is the industrial process used to extract metals such as silver, iron, copper, and
lead from these minerals using a combination of chemical reactions.
Common Minerals
The following chemical compounds are the major components of minerals from which the
widely used metals lead, iron, and silver are commonly extracted:
•
PbS (in galena)
Fe2O3 (in Hematite)
Ag2S (in argentite)
Arrange these substances in order of a) decreasing internal potential energy (or more
negative lattice energy), and b) increasing standard molar entropy.
Share and discuss your ideas with a classmate, and clearly justify your reasoning.
Iron Smelting
Iron is extracted from minerals containing Fe2O3(s) using coal (made mostly of carbon) and air
as source of oxygen. The process is carried out at high temperatures. The table lists the heat of
reaction (DHrxn) and the difference in entropy DSorxn (difference in the standard molar entropy
of products versus reactants) at 25 oC for the sequence of reactions that lead to the formation
of metallic iron:
Chemical Reaction
2 C(s) + O2(g)
3 Fe2O3(s) + CO(g)
Fe3O4(s) + CO(g)
FeO(s) + CO(g)
•
2 CO(g)
DHrxn (kJ mol-1)
DSorxn (J mol-1 K-1)
–221.0
178.8
2 Fe3O4(s) + CO2(g)
–47.2
45.9
3 FeO(s) + CO2(g)
441.9
52.5
Fe(s) + CO2(g)
–11.0
–17.41
Analyze each of the chemical reactions and justify the signs of DHrxn and DSorxn for each
of them. Discuss why some processes are more thermodynamically favored than others.
Share and discuss your ideas with a classmate, and clearly justify your reasoning.
Chemical Thinking
U5
How do we predict chemical change?
297
Lead Smelting
Metallic lead is produced from galena (PbS) via the sequential processes of roasting and
smelting:
a) In roasting the mineral is heated in the presence of air to convert the metallic sulfide
into the metallic oxide:
2 PbS(s) + 3 O2(g)
2 PbO(s) + 2 SO2(g)
•
Analyze the composition and structure of reactants and products in these two reactions and then predict the signs of DHrxn and DSorxn in each case.
Share and discuss your ideas with a classmate, and clearly justify your reasoning.
Metals and their Oxides
The following table lists the standard molar entropy of formation of several metals and of common metallic oxides found in minerals.
Substance
Fe(s)
Ni(s)
Zn(s)
Substance
Fe3O4(s)
NiO(s)
ZnO(s)
Sfo (J mol-1 K-1)
Substance
Ag(s)
Hg(l)
42.6
75.9
Al(s)
Sn(s)
Pb(s)
Sfo (J mol-1 K-1)
Substance
Sfo (J mol-1 K-1)
Substance
CuO(s)
Ag2O(s)
HgO(s)
33.2
Substance
27.3
29.9
41.6
146.4
38.0
43.7
Cu(s)
Sfo (J mol-1 K-1)
42.6
121.3
70.3
Al2O3(s)
SnO(s)
PbO(s)
Sfo (J mol-1 K-1)
28.3
51.2
64.8
Sfo (J mol-1 K-1)
50.9
57.2
68.7
•
What main compositional and structural factors seem to determine the standard molar
entropy of formation of metals?
•
What main compositional and structural factors seem to determine the standard molar
entropy of formation of metallic oxides?
•
What accounts for the difference between the standard molar entropy of formation of a
metal and that of its associated oxide?
Share and discuss your ideas with a classmate, and clearly justify your reasoning.
ASSESS WHAT YOU KNOW
b) In smelting the metal oxide is reacted with coal (carbon) to fomr the metal:
2 PbO(s) + C(s)
2 Pb(s) + CO2(g)