Pure substances reactant involvement

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,... 

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... 

When a solvent is a reactant or product in an equilibrium, its concentration is omitted from the equilibrium-constant expression, provided the concentrations of reactants and products are low, so that the solvent is essentially a pure substance. Applying this guideline to an equilibrium involving water as a solvent,... 

The number written before a formula in an equation, the coefficient, gives the relative amount of the substance involved. The subscripts give the composition of the pure substances. Changing the coefficient changes only the amount of the element or compound involved, whereas changing a subscript would change the identity of the reactant or product. For example, 2 CO represents two molecules of carbon monoxide, whereas GO represents a molecule of carbon dioxide, a very different substance formed in the complete combustion of carbon-containing materials ... 

Recall that the coefficients in a balanced equation give the relative number of moles of reactants and products, aexs (Section 3.6) To use this information, we must convert the masses of substances involved in a reaction into moles. When dealing with pure substances, as we did in Chapter 3, we use molar mass to convert between grams and moles of the substances. This conversion is not valid when working with a solution because both solute and solvent contribute to its mass. However, if we know the solute concentration, we can use molarity and volume to determine the number of moles (moles solute = M X F). T Figure 4.17 summarizes this approach to using stoichiometry for the reaction between a pure substance and a solution. 

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 ... 

The process involves the complete conversion of stoichiometric amounts of pure, unmixed reactants in their standard states to stoichiometric amounts of pure, unmixed products in their standard states. The Gibbs energy change for this process can be obtained from tabulated thermochemical data of pure substances in their standard states (see Appendix D). To see how this is done, consider the following general equation representing the process above. 

In the reactions described so far, all the reactants and products have been gaseous the equilibrium systems are homogeneous. In certain reactions, at least one of the substances involved is a pure liquid or solid the others are gases. Such a system is heterogeneous, because more than one phase is present. Examples include... 

The problem can be tackledby considering reaction 2.2, where all reactants and products are the pure species in their standard states at 298.15 K, and evaluating Ar//°(2.2) from data, which are easily found in thermochemical compilations. These data are the standard enthalpies of formation of the substances involved. 

This possibility of intimate association of rhodium with the aromatic ring suggests further experiments. A logical extension of asymmetric syntheses involving prochir-al reactants is a kinetic resolution with related chiral reactants under similar conditions. In the one case of hydroboration-amination where this has been applied, it has proved to be very effective. The reactant was prepared directly by a Heck reaction on 1,2-dihydronaphthalene, and under the standard conditions of catalytic hydrobora-tion gave >45% of both enantiomerically pure recovered alkene with (after oxidative work-up) the alcohol of opposite hand, mainly as the trans-isomer. This procedure forms a simple and potentially useful route to pharmacologically active substances, demonstrated by the racemic synthesis shown [105] (Scheme 34). 

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. 

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