Reactions involving pure substances

Law of Mass Action for Reactions Involving Pure Substances and Multiple Phases... 

Most of the reactions considered in Chapter 3 involved pure substances reacting with each other. However, most of the reactions you will carry out in the laboratory or hear about in lecture take place in water (aqueous) solution. Beyond that, most of the reactions that occur in the world around you involve ions or molecules dissolved in water. For these reasons, among others, you need to become familiar with some of the more important types of aqueous reactions. These include—... 

The mass action law assumes that the reaction medium is homogeneous. In heterogeneous reactions (involving different substances in multiple phases), the densities and effective concentrations of pure condensed phases (liquids or solids) are constant. The concentrations of such species are set to unity in the equilibrium constant expression for such reactions. For example, given the following decomposition,... 

Since the last two terms in Eq. (11.74) can be calculated from heat capacities and heats of reaction, the only unknown quantity is ASS, the change in entropy of the reaction at 0 K. In 1906, Nernst suggested that for all chemical reactions involving pure crystalline solids, ASS is zero at the absolute zero the Nernst heat theorem. In 1913, Planck suggested that the reason that ASS is zero is that the entropy of each individual substance taking part in such a reaction is zero. It is clear that Planck s statement includes the Nernst theorem. 

The reaction represented by this equation, and most of the reactions we have discussed to this point in the book, involve pure substances as reactants and products. However, many of the reactions done in laboratories, and most of those that go on in our bodies, take place between substances dissolved in a solvent to form solutions. In our bodies, the solvent is almost always water. A double-replacement reaction of this type done in laboratories is represented by the following equation ... 

We have previously established (i) that, at finite temperatures, ionic and electronic point defects are required as local chemical excitations at equiUbrium, (ii) that we can write ideal mass action laws in all cases for low concentrations of defects, and (iii) that we know what parameters influence our mass action constants. We can now turn to a specific consideration of defect chemistry. Let us consider first internal defect reactions and pure single crystals By internal defect reactions in pure substances we mean processes that occur as a consequence of nonzero temperature in the otherwise perfect crystal without neighbouring phases being involved. (For two of these reaction tjrpes, however, we will need surfaces as sinks or sources of monomeric units, i.e. of lattice molecules.) In binary systems such processes leave the composition within the sohd undhanged. If we refer to the Dalton composition , we also speak of the intrinsic case. 

A wide variety of materials, both pure substances and mixtures, are made by processes that involve one or more gas-phase reactions. Among these is one of the most important industrial chemicals, ammonia. 

Some reactions in solution involve the solvent as a reactant or product. When the solution is very dilute, the change in solvent concentration due to the reaction is insignificant. In such cases, the solvent is treated as a pure substance and ignored when writing K. In other words,... 

This is an expression of Nernst s postulate which may be stated as the entropy change in a reaction at absolute zero is zero. The above relationships were established on the basis of measurements on reactions involving completely ordered crystalline substances only. Extending Nernst s result, Planck stated that the entropy, S0, of any perfectly ordered crystalline substance at absolute zero should be zero. This is the statement of the third law of thermodynamics. The third law, therefore, provides a means of calculating the absolute value of the entropy of a substance at any temperature. The statement of the third law is confined to pure crystalline solids simply because it has been observed that entropies of solutions and supercooled liquids do not approach a value of zero on being cooled. 

For the purposes of this investigation-rather than adopting any single definition of a reactive chemicaT-CSB focuses on the broadest range of practices to identify reactive hazards and to manage the risk of reactive incidents. A reactive chemical may include any pure substance or mixture that has the capability to create a reactive incident. CSB defines a reactive incident as a sudden event involving an uncontrolled chemical reaction-with significant increases in temperature, pressure, or gas evolution-that has caused, or has the potential to cause, serious harm to people, property, or the environment. 

The techniques above are very useful for looking at pure substances or alloys of a known composition. However, to obtain enthalpies of formation it is necessary to examine some form of reaction which involves two or more starting materials. Some form of reaction between these materials occurs and there is a subsequent heat evolution/absorption associated with the reaction. 

To calculate the total free energy change of a reaction, AG, it is necessary to know the standard molar free energy of formation, AG°, of each component involved, i.e. the energy required to form one mole of a substance from its stable elements under standard conditions. For a solid, the standard state refers to a pure substance in its most stable form under reference conditions of pressure and temperature, usually 0.1 MPa and25°C (298.15 K). 

The third law of thermodynamics says that the entropy of pure, perfect crystalline substance is zero at absolute zero. But, in actual practice, it has been found that certain chemical reactions between crystalline substance, do not have DS = 0 at 0°K, which indicates that exceptions to third law exist. Such exceptional reactions involve either ice, CO, N2O or H2. It means that in the crystalline state these substances do not have some definite value of entropy even at absolute zero. This entropy is known as Residual Entropy. At 0°K the residual entropies of some crystalline substances are... 

Examples have not infrequently been found of reactions which involve the intervention of some impurity in the system, not at first imagined to be playing any part in the chemical change. For example, the rate of decomposition of hydrogen peroxide in aqueous solution is very variable, and Rice and Kilpatrick traced the cause of this behaviour to the fact that the decomposition is mainly determined by the catalytic action of dust particles. As a result, the view has sometimes been held that pure substances are in general very unreactive, and that velocity measurements have no absolute significance, because the reaction mechanism is quite different from what it appears to be, and involves the participation of accidental impurities. Among such impurities water occupies the most prominent position. 

The condition of equilibrium is also applicable to changes of state that involve heterogenous reactions, and the same methods used for homogenous reactions to obtain expressions of the equilibrium constant are used for heterogenous reactions. One difference is that in many heterogenous reactions one or more of the substances taking part in the change of state is a pure phase at equilibrium. In such cases the standard state of the substance is chosen as the pure phase at the experimental temperature and pressure. The chemical potential of the pure substance in its standard state still appears in Y.k vkPk but the activity of the substance is unity and its activity does not appear in the expression for the equilibrium constant. 

The postulates of Nernst are those that are required when we wish to determine equilibrium conditions for chemical reactions from thermal data alone. In order to calculate the equilibrium conditions, we need to know the value of AGe for the change of state involved. We take the standard states of the individual substances to be the pure substances at the chosen temperature and pressure. The value of AH° can be determined from measurements of the heat of reaction. We now have... 

We consider a change of state involving a chemical reaction, and wish to calculate the equilibrium conditions. We assume that the change of state is isothermal and that the standard states of all of the reacting substances are the pure substances at the temperature and 1 bar pressure. When the pressure is not 1 bar, the appropriate corrections are easily made. When the change of state is written as v,, - = 0, the condition of equilibrium becomes... 

The standard Gibbs free energy change, AG°, is most commonly used. The symbol0 designates a reaction involving reactants and products in their standard states (pure substances in their most stable states at 25 °C and 1 atm pressure). The relationship between A G° and Kcq is given by the expression... 

This collection begins with a series of three procedures illustrating important new methods for preparation of enantiomerically pure substances via asymmetric catalysis. The preparation of 3-[(1S)-1,2-DIHYDROXYETHYL]-1,5-DIHYDRO-3H-2.4-BENZODIOXEPINE describes, in detail, the use of dihydroquinidine 9-0-(9 -phenanthryl) ether as a chiral ligand in the asymmetric dihydroxylation reaction which is broadly applicable for the preparation of chiral dlols from monosubstituted olefins. The product, an acetal of (S)-glyceralcfehyde, is itself a potentially valuable synthetic intermediate. The assembly of a chiral rhodium catalyst from methyl 2-pyrrolidone 5(R)-carboxylate and its use in the intramolecular asymmetric cyclopropanation of an allyl diazoacetate is illustrated in the preparation of (1R.5S)-()-6,6-DIMETHYL-3-OXABICYCLO[3.1. OJHEXAN-2-ONE. Another important general method for asymmetric synthesis involves the desymmetrization of bifunctional meso compounds as is described for the enantioselective enzymatic hydrolysis of cis-3,5-diacetoxycyclopentene to (1R,4S)-(+)-4-HYDROXY-2-CYCLOPENTENYL ACETATE. This intermediate is especially valuable as a precursor of both antipodes (4R) (+)- and (4S)-(-)-tert-BUTYLDIMETHYLSILOXY-2-CYCLOPENTEN-1-ONE, important intermediates in the synthesis of enantiomerically pure prostanoid derivatives and other classes of natural substances, whose preparation is detailed in accompanying procedures. 

The crude trimethylolnitromethane from the condensation commonly contains a small amount. of mono- and dimethylolnitro methane from reactions involving one and two molecules of formaldehyde respectively. It is recrystalHzed from water to a melting point of 150, and is then nitrated. Stettbacher reports that the pure substance after many recrystallizations melts at 164-165 .,.The nitratiop is carried out either with the same mix acid as is used for the nitration of glycerin nitric acid,... 

In the crystalline state, regular atomic order persists over distances which are very large in comparison with interatomic distances. However, even in the most perfect of crystals there are some small and usually random departures from regularity. These imperfections result in minor changes to physical properties, such as resistance and conductivity, but are a feature of solid state materials in general. It is noteworthy that reactions of materials often involve the whole crystal lattice, imperfections and all, rather than just the atoms of the pure substance. 

In Chapter 4, we dealt with the thermodynamic, physical and chemical properties of pure liquids. However, in most instances solutions of liquids are used in chemistry and biology instead of pure liquids. In Chapter 5, we will examine the surfaces of mainly nonelectrolyte (ion-free) liquid solutions where a solid, liquid or gas solute is dissolved in a liquid solvent. A solution is a one-phase homogeneous mixture with more than one component. For a two-component solution, which is the subject of many practical applications, the major component of the solution is called the solvent and the dissolved minor component is called the solute. Liquid solutions are important in the chemical industry because every chemical reaction involves at least one reactant and one product, mostly forming a single phase, a solution. In addition, the understanding of liquid solutions is useful in separation and purification of substances. 

---