Spontaneity of the Dissolution Process

In Section 6-1, Part 5, we listed the solubility guidelines for aqueous solutions. Now we investigate the major factors that influence solubility. 

A substance may dissolve with or without reaction with the solvent. For example, when metallic sodium reacts with water, bubbles of hydrogen are produced and a great deal of 

If the resulting solution is evaporated to dryness, sohd sodium hydroxide, NaOH, is obtained rather than metalhc sodium. This, along with the production of hydrogen bubbles, is evidence of a reaction with the solvent. Reactions that involve oxidation state changes are usually considered to be chemical reactions and not dissolutions. 

Solid sodium chloride, NaCl, on the other hand, dissolves in water with no evidence of chemical reaction. 

Evaporation of the water from the sodium chloride solution yields the original NaCl. In this chapter we will focus on dissolution processes of this type, in which no irreversible reaction occurs between components. 

---

There are two factors which control the spontaneity of a dissolution process (1) the amount of heat absorbed or released and (2) the amount of increase in the disorder, or randomness of the system. All dissolution processes are accompanied by an increase in the disorder of both solvent and solute. Thus, their disorder factor is invariably favorable to solubility. Dissolution will always occur if the dissolution process is exothermic and the disorder term increases. Dissolution will occur when the dissolution process is endothermic if the disorder term is large enough to overcome the endothermicity, which opposes dissolution. 

NOJ(aqf). This includes solid ammonium nitrate, the water in which it dissolves, and the aquated ions that are the products of the dissolution process. The surroundings include the flask or beaker in which the system is held, the air above the system, and other neighboring materials. The dissolution of ammonium nitrate is spontaneous after any physical separation (such as a glass wall or a space of air) between the water and the ammonium nitrate has been removed. 

Because electrons are neither products nor reactants in chemical reactions, the two processes are interdependent and neither can occur alone. The zinc metal dissolution must furnish electrons for the copper metal plating. The reaction of zinc and copper sulfate solution is a spontaneous reaction involving a transfer of electrons, i.e., is a spontaneous redox process. The spontaneity of the reaction is commonly explained by saying that zinc loses electrons more readily than copper or, alternatively, that Cu2+ ions gain electrons more readily than Zn2+ ions. 

The amount of heat released or absorbed in the dissolution process is important in determining whether the dissolution process is spontaneous, meaning, whether it can occur. What is the other important factor How does it influence solubility ... 

We have seen examples of endothermic processes that are spontaneous, such as the dissolution of ammonium nitrate in water. (Section 13.1) We learned in our discussion of the solution process that a spontaneous process that is endothermic must be accompanied by an increase in the entropy of the system. However, we have also encountered processes that are spontaneous and yet proceed with a decrease in the entropy of the system, such as the highly exothermic formation of sodium chloride from its constituent elements. (Section 8.2) Spontaneous processes that result in a decrease in the system s entropy are always exothermic. Thus, the spontaneity of a reaction seems to involve two thermodynamic concepts, enthalpy and entropy. 

The overall corrosion process is a chemical reaction representing spontaneous dissolution of the metal (M) by reaction with the environment. This can be described schematically by one of the following processes ... 

Polycondensation reactions (eqs. 3 and 4), continue to occur within the gel network as long as neighboring silanols are close enough to react. This increases the connectivity of the network and its fractal dimension. Syneresis is the spontaneous shrinkage of the gel and resulting expulsion of Hquid from the pores. Coarsening is the irreversible decrease in surface area through dissolution and reprecipitation processes. 

In contrast to the equilibrium electrode potential, the mixed potential is given by a non-equilibrium state of two different electrode processes and is accompanied by a spontaneous change in the system. Besides an electrode reaction, the rate-controlling step of one of these processes can be a transport process. For example, in the dissolution of mercury in nitric acid, the cathodic process is the reduction of nitric acid to nitrous acid and the anodic process is the ionization of mercury. The anodic process is controlled by the transport of mercuric ions from the electrode this process is accelerated, for example, by stirring (see Fig. 5.54B), resulting in a shift of the mixed potential to a more negative value, E mix. 

The dissolution of salt in water (2) is endothermic (AH > 0)—i. e., the liquid cools. Nevertheless, the process still occurs spontaneously, since the degree of order in the system decreases. The Na"" and Cl ions are initially rigidly fixed in a crystal lattice. In solution, they move about independently and in random directions through the fluid. The decrease in order (AS > 0) leads to a negative -T AS term, which compensates for the positive AH term and results in a negative AG term overall. Processes of this type are described as being entropy-driven. The folding of proteins (see p. 74) and the formation of ordered lipid structures in water (see p. 28) are also mainly entropy-driven. 

---