Solution formation endothermic process

In every solution in which there are ions, i.e. in every electrolyte, the ions are combined with molecules of the solvent, producing "solvate ions . Since the dissociation into solvated ions is usually an endothermic process, the formation of ions, free or solvated, is only possible if the process is associated with an increase in entropy. In actual fact, a diluted solution always has a considerable entropy, because the ions are distributed over a large volume of the solvent. It will be clear that dissociation into ions, at a given temperature and volume, will be more complete, the lower the total energy connected with the formation of the ions. 

Many times, the dissolving process generates heat, which is to say it is exothermic. Occasionally solution formation is endothermic. 

Heat of solution During the process of solvation, the solute must separate into particles. Solvent particles also must move apart in order to allow solute particles to come between them. Energy is required to overcome the attractive forces within the solute and within the solvent, so both steps are endothermic. When solute and solvent particles mix, the particles attract each other and energy is released. This step in the solvation process is exothermic. The overall energy change that occurs during the solution formation process is called the heat of solution. 

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. 

Two factors namely, enthalpy of solution and entropy of solution AS, govern the dissolution of a substance in a solvent. A negative enthalpy of solution is always favourable for solution formation. The entropy factor, AS, also plays an important role. In fact, solution formation is largely governed by this factor. Entropy is a measure of disorder or randomness. The process of solution is always accompanied by an increase in disorder or randomness. When enthalpy of solution is highly endothermic, no solution is formed i.e., the solute does not mix with the solvent. 

The overall solution-formation process is exothermic (Af/join < 0) when the heat given off in step 3 is greater than the sum of heat required for steps 1 and 2. The overall process is endothermic (A//soin > 0) when the heat given off in step 3 is less than the total required for steps 1 and 2. (Figure 13.3 depicts a solution formation that is endothermic overall.)... 

Solution formation may be endothermic or exothermic overall. An increase in entropy is the driving force for solution formation. Solute particles are surrounded by solvent molecules in a process called solvation. 

Cleavage of the tetrazole ring is generally an endothermic process in solution, so the azide-tetrazole equilibrium is ordinary shifted to the side of the tetrazole form. The higher temperatures favor the azido species, the formation of azide from tetrazole on heating can be ascribed to entropy. 

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. 

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