Reactions Involving Ions

All of the discussion in Chapter 12 applies to reactions involving ions. 

Example 13.1 Estimate the equilibrium concentration of hydrogen ions (H ) and hydroxyl ions (OH ) in equilibrium with pure water at 25 °C. 

Reading the necessary values from Table A.8, we find 

The activity of any pure liquid at its standard state is 1.00, and here the water is practically pure and at its standard T and P, so 

This is the result given in all elementary chemistry books, that in water at 25 °C, the product of the hydrogen ion and hydroxyl ion concentrations, both expressed in molality (=moI/kg of solvent) = 10 . For dilute solutions, molality moI/L = molarity. In tables like Table A.8, the standard states of ions are all based on electrochemical measurements, which can only give values for ion pairs (e.g., H and OH ), never a direct measurement for one ion alone. The convention adopted is to take the standard state Gibbs energy of H, as 0.0000 at a concentration of 1 molal at 25 °C and 1.00 bar. With that convention, the 

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Reactions involving ions can be favored to occur in the organic phase by use of phase-transfer catalysts. Thus the conversion of 1-chlorooctane to 1-cyanooctane with aqueous NaCN is vastly accelerated in the organic phase by 1.3 percent of tributyl (hexadecyl) phosphonium bromide in the aqueous phase. (Starks and Owens, J. Am. Chem. Soc., 95, 3613 [1973]). A large class of such promotions is known. 

The emphasis is on writing and balancing chemical equations for these reactions. All of these reactions involve ions in solution. The corresponding equations are given a special name net ionic equations. They can be used to do stoichiometric calculations similar to those discussed in Chapter 3. 

Net ionic equation A chemical equation for a reaction involving ions in which only those species that actually react are included, 79-80... 

The application of TST to account for the effects on the rate constants of reactions involving ions is one of its notable successes. We shall make use of a previously derived equation [Eq. (9-28)] to relate changes in the rate constant to the activity coefficients ... 

J 2 Construct balanced complete ionic and net ionic equations for reactions involving ions (Example 1.1). 

When balancing the chemical equation for a redox reaction involving ions, the total charge on each side must be balanced. 

Predict what will happen when the following pairs of substances are allowed to react. Write a balanced chemical equation for each reaction. When the reaction involves ions, write a net ionic equation. Identify each reaction as precipitation, as acid-base, or as redox, (a) AgN03(a q) and NaCl(a q) (b)... 

Kennedy and Thomas attempted to construct a formal kinetic theory to account for these phenomena. They started from the assumption, for which there is no evidence, that the propagation reaction takes place with free ions only, but that the chain breaking reactions involve ion-pairs. 

Estimation Procedures. There are basically two ways which have been developed to deal with the fact that heat capacity terms are large in reactions involving ions. One is based on empirical relationships (the entropy correspondence principle) between ionic entropies at different temperatures which Criss and Cobble (62) developed and checked to 200 C. Lewis (63) has checked a number of its predictions against available experimental evidence and has found the method reasonably satisfactory for several... 

Division of these values by the measured equilibrium constants gives the rate constants for reactions involving ion pairs. 

Since this reaction involves ions, electrons, and gas molecules in three separate phases, the edge of the o/y interface that makes contact with the gas phase /3 is often described as the three-phase (or triplephase) boundary (TPB). The concept of the TPB actually dates to the 1920s, when workers studying the oxidation of H2 on platinum introduced this concept to explain why Pt must be exposed simultaneously to both solution and gas to get significant reaction. This type of electrode, which Schmid called die diffusiongaselektrode or gas-diffusion electrode (GDE), is still called this today by workers studying solution- or polymer-based fuel cells. As... 

The previous chapters have demonstrated that liquid-liquid extraction is a mass transfer unit operation involving two liquid phases, the raffinate and the extract phase, which have very small mutual solubihty. Let us assume that the raffinate phase is wastewater from a coke plant polluted with phenol. To separate the phenol from the water, there must be close contact with the extract phase, toluene in this case. Water and toluene are not mutually soluble, but toluene is a better solvent for phenol and can extract it from water. Thus, toluene and phenol together are the extract phase. If the solvent reacts with the extracted substance during the extraction, the whole process is called reactive extraction. The reaction is usually used to alter the properties of inorganic cations and anions so they can be extracted from an aqueous solution into the nonpolar organic phase. The mechanisms for these reactions involve ion pah-formation, solvation of an ionic compound, or formation of covalent metal-extractant complexes (see Chapters 3 and 4). Often formation of these new species is a slow process and, in many cases, it is not possible to use columns for this type of extraction mixer-settlers are used instead (Chapter 8). 

Based on equation 8.266, the molality change induced by T simultaneous reactions involving ion i is... 

Complex RedOx reactions involve ions. For example. 

This reaction involves ion diffusion in apatites and the reaction rate depends on the temperature, the size of the crystals and the porosity of the ceramics. At 900°C, the reaction is relatively fast and crystals of a few hundred microns can be totally exchanged in a few hours. The advantage of these processes is that they can be easily performed from existing HA powders or ceramics and... 

Reactions such as (3.129) suggest the utility of extending (3.106) to include reactions involving ions as reactants or products, for example,... 

In fact, this interpretation became imperative when it was found that lithium perchlorate in dimethyl formamide does not initiate the polymerisation in systems for which lithium chloride is an effident initiator. This proves that the reaction involves the negative ion, i. e. Cl- or CJ04, and not the positive Li+ ion, and while CT in dimethyl formamide is a sufficiently strong base capable of accepting a proton and initiating the process, the C104 apparently is not. Actually, one may question to what extent these salts are dissodated in dimethyl formamide. It is possible, therefore, that the reaction involves ion-pairs rather than free ions, and the Li+, Cl- ion-pair may be a more powerful proton acceptor than Li+, CI04-. 

One should mention here that in reactions involving ions, the effects of electrolytes can be pul into two principal categories (a) primary salt effect and lb) secondary salt effects. 

This reversal is of little practical value because organic reactions involving ions normally are not carried out in the gas phase. However, it should alert us to the tremendous role that solvents play in determining acidities by their abilities (some much more than others) to stabilize ions by the property known as solvation. (See Section 11-8A.)... 

Identity 5h 2 reactions of MeF + F- and MeCl + CP have been compared computationally with the reactions of MeF + LiF or NaF and of MeCl + LiCl.87 Calculations by new methods essentially confirm results obtained previously by other methods. Extension of the calculations to the corresponding ethyl systems gave lower barriers than for methyl systems in the reactions involving ion pairs as nucleophiles. 

These are useful when studying reactions involving ions. Again this can be illustrated by a problem. 

Answer. Conductance changes are ideal for reactions involving ions. 

In contrast to gas phase reactions, reactions involving ions, polar molecules and charged transition states occur readily in solution. 

However, modifications are required for reactions involving ions and charged or charge-separated activated complexes (Problem 4.5(ii) and Tables 7.1 to 7.3). 

Mechanism II Adsorption-Desorption Reaction Involving Ion Exchange with Two Protons. This mechanism was investigated similarly to mechanism I but did not appear to be operational. 

Mechanism III Adsorption-Desorption Reaction Involving Ion Exchange with Proton. This reaction for the slow process can be given as (Hachiya et al., 1979)... 

Mechanism V Two-Step Adsorption-Desorption Reactions Involving Ion Exchange with a Proton. Hachiya etal. (1979) felt that the existence of two relaxations in Fig. 4.12 suggested that the Pb2+ adsorption-desorption reactions consisted of two processes, one fast and the other slow. Mechanism III could thus be further divided into two elementary steps ... 

At basic pH values the rate of 3-MPA formation is reduced, but continues at measurable rates even at a pH value as high as 10. These results indicate that acrylate ion possesses significant reactivity, although the undissodated form is much more reactive. In the addic pH ranee, the rate of 3-MPA formation in seawater is similar to that in Milli-Q water, but at basic pH values, the rates in seawater are higher than those in Milli-Q water (Figure 4). In an ionic medium such as seawater, for reactions involving ions, the Bronsted-Bjerrum equation predicts that ionic interactions cause deviations from ideal-solution behaviour (Equation 5) (451. 

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