Molecular view

As we have seen, the third law of thermodynamics is closely tied to a statistical view of entropy. It is hard to discuss its implications from the exclusively macroscopic view of classical themiodynamics, but the problems become almost trivial when the molecular view of statistical themiodynamics is introduced. Guggenlieim (1949) has noted that the usefiihiess of a molecular view is not unique to the situation of substances at low temperatures, that there are other limiting situations where molecular ideas are helpfid in interpreting general experimental results ... 

Stratt R M and Maroncelli M 1996 Nonreactive dynamics in solution the emerging molecular view of solvation dynamics and vibrational relaxation J. Phys. Chem. 100 12 981... 

In Chapter 1 we saw that a major achievement of the first half of the twentieth cen tury was the picture of atomic and molecular structure revealed by quantum mechan ICS In this the last chapter we examine the major achievement of the second half of that century—a molecular view of genetics based on the structure and biochemistry of nucleic acids... 

Each of the pictures below shows a molecular view of a system undergoing a change. In each case, indicate whether heat is absorbed or given off by the system, whether expansion work is done on or by the system, and predict the signs of q and iv for the process. 

Yao N, Reichert P, Taremi SS, Prosise WW, Weber PC (1999) Molecular views of viral polyprotein processing revealed by the crystal structure of the hepatitis C virus bifunctional protease-heUcase. Structure 7 1353-1363... 

The atomic-molecular view is so ingrained that chemists often take it for granted. As you study chemistry, strive to attain this point of view. Example provides some practice. 

After a drop of ink is added to a beaker of water (left), the ink diffuses slowly through the liquid (center) until eventually the ink is distributed uniformly (right). The molecular views indicate that the motion of ink molecules and water molecules is responsible for this diffusion. Ink molecules (red-violet circles) and water molecules (blue circles) move about continually, even after they are well mixed. 

Atoms and molecules are always moving, even when no visible changes take place. In our ink example, ink molecules move randomly in all directions. As the molecular view in Figure 2-8c indicates, however, the total number of ink molecules and water molecules in any region of the liquid does not change once the molecules are evenly distributed. As a result, there is no further change in color. 

Notice that the molecular view in Figure 3-20 includes Ions as well as neutral molecules. Molecular view means a view of how matter looks at the atomlc/molecular level. Thus, such Illustrations may contain Ions and individual atoms as well as molecules. 

Molecular view of an aqueous solution of ammonium nitrate. Ammonium ions separate from nitrate ions, but both these species remain intact as polyatomic clusters. 

A molecular view of a limiting reactant situation for the ammonia synthesis. To make 4 molecules of INH3 and 6 molecules of... 

One of the most fundamental chemical reactions is the combination of a hydroxide ion (OH ) and a hydronium ion (H3 0+) to produce two molecules of water OH" (a g) + H3 (a g) 2 H2 O (/) A molecular view of this reaction (Figure 4-7f shows that the hydroxide anion accepts one hydrogen atom from the hydronium cation. Taking account of charges, it is a hydrogen cation (H ) that is transferred. The reaction occurs rapidly when H3 O and OH ions collide. The hydroxide anion accepts a hydrogen cation from the hydronium cation, forming two neutral water molecules. 

Any acid that undergoes quantitative reaction with water to produce hydronium ions and the appropriate anion is called a strong acid. Table gives the structures and formulas of six common strong acids, all of which are supplied commercially as concentrated aqueous solutions. These solutions are corrosive and normally are diluted for routine use in acid-base chemistry. At the concentrations normally used in the laboratory, a solution of any strong acid in water contains H3 O and anions that result from the loss of a proton. Example shows a molecular view of the proton transfer reaction of a strong acid. 

In one type of titration, a solution of a strong base such as sodium hydroxide is added slowly to a solution that contains an unknown amount of an acid. Each hydroxide ion added to the acid solution accepts one proton from a molecule of acid. As the titration proceeds, fewer and fewer acid molecules remain in the acid solution, but the solution is still acidic. At the stoichiometric point, just enough hydroxide ions have been added to react with every acidic proton present in the acid solution before the titration was started. The hydroxide ions in the next drop of titrant do not react because acid molecules are no longer present in the solution. Before the stoichiometric point, the solution contains excess acid. After the stoichiometric point, the solution contains excess OH". Figure 4-11 shows a titration setup and molecular views illustrating titration of a strong acid by a strong base. 

When a strip of zinc metal is added to a solution of copper(II) sulfate, the blue color slowly fades, and the zinc metal is replaced by copper metal (Figure 4-13). As copper ions in the solution are reduced to copper metal, zinc atoms are oxidized to Zn cations. This is an example of a metal displacement reaction, in which a metal ion in solution (Cu ) is displaced by another metal (Zn) by means of a redox reaction. Figure 4-13 also shows molecular views of this displacement reaction. 

The ideal gas equation and the molecular view of gases lead to several useful applications. We have already described how to cany out calculations involving P-V-n-T relationships. In this section, we examine the use of the gas equation to determine molar masses, gas density, and rates of gas movement. 

Because these are nonmetal oxides, they cannot be described as metallic. Neither oxide contains ions, so they must be network or molecular. The melting points provide the information needed to categorize the oxides, and the molecular views support the identifications. 

Ceramic oxide superconductors have distinct atomic layers. The Cu-containing superconductors contain planes of copper and oxygen atoms, as the molecular view shows. These planes alternate with layers containing oxygen and the other metals that make up the superconductor. Superconductivity takes place in the Cu—O planes. 

Views of the iron crystal (a) A molecular view, with the lattice representation (b) the unit cell (c) six unit cells stacked to form part of the crystal. 

Figure 12-10 is a molecular view showing that the equilibrium concentration of a dissolved gas varies with the partial pressure of that gas. An increase in the partial pressure of gas results in an increase in the rate at which gas molecules enter the solution. This increases the concentration of gas in solution. The increased concentration in solution, in turn, results in an increase in the rate at which gas molecules escape from the solution. Equilibrium is reestablished when the solute concentration is high enough that the rate of escape equals the rate of capture. 

Molecular view of a gasnsolution equilibrium, (a) At equilibrium, the rate of escape of gas molecules from the solution equals the rate of capture of gas molecules by the solution, (b) An increase in gas pressure causes more gas molecules to dissolve, throwing the system out of equilibrium, (c) The concentration of solute increases until the rates of escape and capture once again balance. 

Figure 12-11 is a molecular view of how a solute changes this liquid-vapor equilibrium of the solvent. The presence of a solute means that there are fewer solvent molecules at the surface of the solution. As a result, the rate of solvent evaporation from a solution is slower than the rate of evaporation of pure solvent. At equilibrium, the rate of condensation must be correspondingly slower than the rate of condensation for the pure solvent at equilibrium with its vapor. In other words, the vapor pressure drops when a solute is added to a liquid. A solute decreases the concentration of solvent molecules in the gas phase by reducing the rates of both evaporation and condensation. 

This molecular view of Figure 12-11 suggests that the extent of vapor pressure lowering will depend on the fraction of solvent molecules that has been replaced. In other words, the vapor pressure should be proportional to the mole fraction of the solvent. The molecular view also suggests that this effect does not depend on the nature of the solute, but only on its mole fraction. Experiments show that this is often the case, particularly for dilute solutions. A simple equation, Raoult s law, expresses this proportionality between vapor pressure and mole fraction V V /Jpuj-g solvent Raoulfs law states that the vapor pressure of a solution is the... 

Molecular views of the rates of solid-liquid phase transfer of a pure liquid and a solution at the normal freezing point. The addition of solute does not change the rate of escape from the solid, but it decreases the rate at which the solid captures solvent molecules from the solution. This disrupts the dynamic equilibrium between escape and capture. 

Cross-sectional molecular views of the structures that can form when surfactant molecules are placed in water. 

Chemists would like to invent molecules that assemble themselves into specific, we 11-organized arrays. Recently there have been some exciting successes. Our inset is a molecular view of a self-assembled molecular wreath. This organized sfructure forms when four molecular chains weave themselves together in the presence of 12 copper cations. Self-assembling molecular systems could lead to new materials with useful properties. They may also shed light on how life originated. 

The second part of Figure 14-1 shows a molecular view of what happens in the two bulbs. Recall from Chapter 5 that the molecules of a gas are in continual motion. The NO2 molecules in the filled bulb are always moving, undergoing countless collisions with one another and with the walls of their container. When the valve between the two bulbs is opened, some molecules move into the empty bulb, and eventually the concentration of molecules in each bulb is the same. At this point, the gas molecules are in a state of dynamic equilibrium. Molecules still move back and forth between the two bulbs, but the concentration of molecules in each bulb remains the same. 

A molecular view of the isomerization of ds-2-butene illustrates that the rate of isomerization decreases as the number of ds-2-butene molecules decreases. 

A molecular view of the formation of N2 O4 from NO2. The rate of reaction decreases as the number of... 

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