how do we predict chemical change? -...

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UNIT 5How do we predict chemical change?Our ability to synthesize new chemical compounds or to control chemical processes depends on how well we can pre-dict 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 de-termined 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.

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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 re-actants and products as a function of time.

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1 There are multiple reasons to be interested in why and how chemical reactions oc-cur. 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 chemi-cal processes may occur to a great extent (i.e., reactants are almost 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 chemi-cal 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.

THE CHALLENGE Primitive Combinations

Nitrogen gas, N2(g), and hydrogen gas, H2(g), are suspected to be two im-portant 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 cen-tral goal of Module 1 is to learn to predict reaction directionality based on the relative thermodynamic stability of reactants and products.

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285U5 How do we predict chemical change?Chemical Thinking

Chemical Stability

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 composi-tional 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 mea-sure 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 thermodynami-cally 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 di-rectionality. 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);

• 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 composi-tional 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.

Figure 5.1 Mixtures of common fuels (e.g., meth-ane, wood, glucose) with oxygen tend to be thermo-dynamically unstable but kinetically stable.

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Directionality? LET’S THINKConsider 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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286 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 com-pounds and ion–ion interactions in ionic compounds) and the type of intermo-lecular 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:

• 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 ex-plain this effect?

• How could the analysis of the types of bonds in the molecules of different molecular substances be used to predict relative stabilities?


Bond BE (kJ/mol)

C–C 347N–N 163O–O 142F–F 159

Bond BE (kJ/mol)

C–H 414N–H 389O–H 464F–H 565

Bond BE (kJ/mol)

C–N 305C–O 360C–F 485C–Cl 339

Molecular compounds with stronger covalent bonds tend to be more ther-modynamically 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 de-crease 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.


Recall from Unit 4 that differences in the bond energy of reactants and prod-ucts are responsible for the absorption or release of energy during a chemical reac-tion. 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 for-mation 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 forma-tion of gases over liquids and solids, particularly at high temperatures.

Initial Predictions LET’S THINKIt 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 com-pounds [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.


Bond BE (kJ/mol)

H–H 436F–F 159

Cl–Cl 243Br–Br 193

I–I 151O=O 498O–H 464F–H 565Cl–H 431Br–H 364I–H 297

Figure 5.2 Generic ener-gy diagram for a reaction leading to the formation of compounds with a larger proportion of A–B bonds.

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Reaction Path

Reactants Products

A2 + B2

2 ABEnergy released

Directionality? LET’S THINK• Which of these two processes is more likely to occur? Clearly justify your reasoning:

2 HCOOH(l) + O2(g) 2 CO2(g) + 2 H2O(g) | 2 CO2(g) + 2 H2O(l) 2 HCOOH(l) + O2(g)


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. Ar-range them in order of decreasing internal potential energy (or increasing thermodynamic stability).


The likelihood of chemical reactions involving ionic compounds depends on factors that influence the strength of ion–ion interactions. Ionic compounds con-sists 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/r

2), 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 sta-bility of the substance.

+-r

Figure 5.3 The distance (r) between charged par-ticles is measured from center to center.

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) 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 com-pound, the more energy will be required to separate the ions that make up the system and the more ther-modynamically stable the compound can be expected to be. Figure 5.4 illustrates the variation in the lattice energy for a set of binary ionic compounds involving ions with different sizes. The experimental results in-cluded in this figure and its caption confirm our hy-pothesis 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.

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Metallic Oxides LET’S THINKEarth’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, in-creased 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:

• 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.


Entropic Factors

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.

Chemical Reaction DHrxn (kJ/mol)

2 Fe(s) + 3/2 O2(g) Fe2O3(s) –824

Ti(s) + O2(g) TiO2(s) –944

Ca(s) + 1/2 O2(g) CaO(s) –635

Ba(s) + 1/2 O2(g) BaO(s) –548

Number of Configurations LET’S THINKThe simulations that can be opened by clicking on the image illustrate the be-havior 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 ar-range 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.

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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 chemi-cal 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 en-tropy 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 Sf

o , 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 val-ues of Sf

o 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 CompoundsSingle Particle Level Multiparticle Level

Molecular MassMolecular Complexity

State of Matter

Ionic CompoundsSingle Particle Level Multiparticle Level

Ion MassIon Charge

Ion Size

State of Matter


LET’S THINK State of Matter The tables list the standard molar entropy of formation Sf

o of a group of substances that played a key role in the development of life on Earth.

• According to these data, how does the state of matter affect the entropy of molecular com-pounds? 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:

• How would you explain the different effects on the entropy of dissolving a gaseous sub-stance, such as CO2(g), in water and dissolving an ionic compound in this liquid?


Substance Sfo (J mol-1 K-1)

H2O(s) 41.0

H2O(l) 70.0

H2O(g) 188.8


C(s, diamond) 2.4

C(s, graphite) 5.7

C(g) 158.1


CO2(s) 51.1

CO2(aq) 117.6

CO2(g) 213.8


NaCl(s) 72.1

NaCl(aq) 115.5


NaNO3(s) 116.5

NaNO3(aq) 205.4

LET’S THINK Temperature Effects The graph illustrates the effect of changing tempera-ture on the entropy S of water.

• How would you explain the different effects that changing temperature has on the value of S at different stages during the heating process?


Entr

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Temperature

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 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 en-tropy values?

Share your ideas with a classmate and justify your reasoning.

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 DSo

rxn, we should expect DSo

rxn > 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.


H2(g) 130.7

N2(g) 191.6

O2(g) 205.2

Cl2(g) 223.1

LET’S THINK Ionic Compounds

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.


LiF(s) 35.6

NaF(s) 51.5

KF(s) 66.6


BeO(s) 13.8

MgO(s) 27.0

CaO(s) 38.1

LET’S THINK Molecular Complexity

The table lists values of Sfo for substances in the same state of matter,

with similar molecular mass, but different molecular complexity

• How would you explain the differences in standard molar en-tropy values?

Share your ideas with a classmate and justify your reasoning.


N2(g) 191.6

CO(g) 197.7

C2H4(g) 219.3


FACING THE CHALLENGE Primordial AtmosphereThe origin of life on Earth seems to have occurred in a relatively short period of time in our plan-et’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 organiza-tion 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 at-mospheric composition of our planet.

H2, H2O, CH4, CO, CO2, and NH3 are the most abundant molecu-lar gases in our solar sys-tem, and this was prob-ably the case during the 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 atmo-sphere derived from outgassing (gases from volca-nic emissions), at temperatures between 300 and 1500 oC. The chemical composition of the early mantle should have then determined the chemi-cal 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 neg-ligible.

The main components in the primordial atmo-sphere could have reacted with each other to form other chemical compounds. For example, the fol-lowing processes proceed to a large extent due to the thermodynamic stability of the products:

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 atmo-

sphere is somewhat linked to the development of organisms that produced organic matter (represent-ed 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 thermo-dynamically favored processes in which oxygen is consumed occur at a lower rate:

4 FeO(s) + O2(g) 2 Fe2O3(s)C(s) + O2(g) CO2(g)

H2(g) + O2(g) 2 H2O(g)

Existing evidence suggests that there were two dis-tinct periods in the history of Earth in which O2 amounts rose considerably. In each case, accumu-lation was due to the burial of chemical substances that otherwise may have reacted with O2 to form more thermodynamically stable products.

By NASA/Jenny Mottar

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Let’s ApplyChemistry on the Primitive EarthAnalysis of the thermodynamic stability of potential reactants and products is critical for build-ing 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) CH4(g) + 2 H2O(g)CO(g) + 3 H2(g) CH4(g) + H2O(g)

C(s) + 2 H2(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.


Oxygen Sinks

The following chemical reactions represent processes that may have played an im-portant role in hindering the accumulation of oxygen, O2(g), in Earth’s atmosphere:

4 FeO(s) + O2(g) 2 Fe2O3(s) 4 FeS2(s) + 15 O2(g) + 8 H2O(l) 2 Fe2O3(s) + 8 H2SO4(aq)

C(s) + O2(g) 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.



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Hydrogen Sources

Some models propose that H2(g) was a major component of Earth’s primordial atmo-sphere. 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 DSo

rxn (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:

Chemical Reaction DHrxn (kJ mol-1) DSorxn (J mol-1 K-1)

C(s) + 2 H2O(g) 2 H2(g) + CO2(g) 90.1 91.9

S(s) + 4 H2O(l) 3 H2(g) + H2SO4(aq) 233.9 98.5

2 FeO(s) + H2O(l) H2(g) + Fe2O3(s) 5.6 26.5

• 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:

• Based on your analysis of their composition and structure, arrange these amino acids in order of increasing standard molar entropy.


Glycine

Alanine

Leucine

Proline

Serine

Valine


SmeltingChemical 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.

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Let’s Apply

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.


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 DSo

rxn (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 DHrxn (kJ mol-1) DSorxn (J mol-1 K-1)

2 C(s) + O2(g) 2 CO(g) –221.0 178.8

3 Fe2O3(s) + CO(g) 2 Fe3O4(s) + CO2(g) –47.2 45.9

Fe3O4(s) + CO(g) 3 FeO(s) + CO2(g) 441.9 52.5

FeO(s) + CO(g) 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.



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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)

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)

• Analyze the composition and structure of reactants and products in these two reac-tions and then predict the signs of DHrxn and DSo

rxn in each case.


Metals and their Oxides

The following table lists the standard molar entropy of formation of several metals and of com-mon metallic oxides found in minerals.

• 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?



Fe(s) 27.3Ni(s) 29.9Zn(s) 41.6


Cu(s) 33.2Ag(s) 42.6Hg(l) 75.9


Al(s) 28.3Sn(s) 51.2Pb(s) 64.8


Fe3O4(s) 146.4NiO(s) 38.0ZnO(s) 43.7


CuO(s) 42.6Ag2O(s) 121.3HgO(s) 70.3


Al2O3(s) 50.9SnO(s) 57.2PbO(s) 68.7

how do we predict chemical change? -...

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