Reactions between ions in solution

There is very little variation in the magnitude of A for a given reaction in different solvents. 

When reaction occurs between ions of like charge the activated complex will have a larger charge than either reactant. All three will be solvated, but the activated complex can orientate the solvent considerably more than can the reactant ions. Orientation is equivalent to an increase in order for solvent molecules, which results in an increase in order for the solvent on activation, manifesting itself as a decrease in entropy on activation. Comparison with reactions for uncharged molecules in the gas phase predicts a decreased value for the entropy of activation, and hence low A factors. 

When reaction is between ions of unlike sign the activated complex has a total net charge less than the reactants, and so the total degree of orientation of the solvent around the activated complex is now less than before activation. This leads to release of solvent molecules giving an increase in disorder of the solvent, leading to an increase in entropy on forming the activated complex. The A 5 and the A factor are predicted to be larger than the values found for the reaction of molecules in the gas phase. 

These predictions are confirmed by the typical values given in Table 7.1. 

For reactions between ions of like sign, solvation effects give a decrease in entropy on activation. Consideration of the internal structure leads again to a decrease in entropy on activation. The two effects reinforce each other, and also are in the same direction as predicted by the electrostatic treatment as given in Section 7.4.5. This would indicate p factors of less than unity. 

---

Diflfiision-controlled reactions between ions in solution are strongly influenced by the Coulomb interaction accelerating or retarding ion diffiision. In this case, die dififiision equation for p concerning motion of one reactant about the other stationary reactant, the Debye-Smoluchowski equation. 

In the next chapter, you will extend your knowledge of equilibria involving aqueous ions. You will learn how to calculate the pH at an equivalence point, so you can select an appropriate indicator for any acid-hase titration. You will also learn why equilihrium is important to the solubility of compounds that are slightly soluble, and how to predict whether a precipitate will form as the result of a reaction between ions in solution. 

Equation (4.40) is the starting point, with the corresponding expression for reaction between ions in solution being Equation (7.26). 

Chemical systems often seem difficult to deal with simply because there are many components. Solving a problem involving a solution in which several components are present is simplified if you think about the chemistry involved. The key to success is to write down all the components in the solution and to focus on the chemistry of each one. We have been emphasizing this approach in dealing with the reactions between ions in solution. Make it a habit to write down the components of solutions before trying to decide which reaction(s) might take place. 

Every chemical reaction requires some time for its completion, but some reactions are very fast and some are very slow. Reactions between ions in solution without change in oxidation state are usually extremely fast. An example is the neutralization of a strong acid by a strong base, which proceeds as fast as the solutions can be mixed. Presumably nearly every time a hydronium ion collides with a hydroxide ion reaction occurs, and the number of collisions is very great, so that there is little delay in the reaction. 

Ion exchange is a chemical reaction between ions in solution and ions in the solid. Typically, the ion in solution replaces an atom in an insoluble solid, the ion exchanger, and that solid atom is turned into an ion in solution. An example of such a process is the replacement of ( a- ions in solution with Na ions. The sodium ions exist in combination with an organic polymer, R, to form the ion exchanger NutR. The reaction between Ca + ions and the sorbent is representative of a water-softener process,... 

The Gibbs energy of reaction between ions in solution is calculated by summation of the Gibbs energy of... 

The majority of reactions between ions in solution, particularly between simple ions of opposite charge, occur so rapidly that until recently it was impossible to measure the rates of these reactions. Relaxation techniques such as those described in Section 32.19 are now used to determine the rate of reactions such as HjO + OH 2 H2O. The rate constant... 

Ion exchange involves a reversible reaction between ions in solution and ions held on the resin. An example of monovalent cation exchange is the removal of sodium ions from the resin using hydrochloric acid. 

Ion-exchange processes are basically chemical reactions between ions in solution and ions in an insoluble solid phase. The techniques used in ion exchange so closely resemble those used in adsorption that for the majority of engineering purposes ion exchange can be considered as a special case of adsorption. 

A. Reaction rate and ionic strength, also known as the primary salt effect. For reactions between ions in solution, the presence of other ions influences the reaction rate. The Debye-Hiickel theory provides an explanation for both the direction and the magnitude of the effect. It stems from the stabilization of an ion by a cloud of oppositely charged ions. The stabilization of an ion of charge z scales as where /u. is the ionic strength, /u = (1 /2) c z due to the other... 

We will base our discussion of reactions between ions in solution on the effects of electrostatic interactions between species of charges and Z e in a solvent of dielectric constant, e. Initially we consider the ions at an infinite distance, and then calculate the energy needed to bring them together to a contact distance, forming the double sphere as shown in Figure 9.10. 

A net ionic equation represents a reaction between ions in solution in such a way that all nonparticipant (spectator) ions are eliminated from the equation. The equation must be balanced both atomically and for net electric charge. 

---