Ionic solids in water

The presence of ions in solution is what gives a sodium chloride solution the ability to conduct electricity. If positively and negatively charged wires are dipped into the solution, the ions in the solution respond to the charges on the wires. Chloride anions move toward the positive wire, and sodium cations move toward the negative wire. This directed movement of ions in solution is a flow of electrical current. Pure water, which has virtually no dissolved ions, does not conduct electricity. Any solution formed by dissolving an ionic solid in water conducts electricity. Ordinary tap water, for example, contains Ionic Impurities that make It an electrical conductor. 

Figure 2.9 The dissolution of an ionic solid in water, showing the hydration of...

To understand the dissolution of ionic solids in water, lattice energies must be considered. The lattice enthalpy, A Hh of a crystalline ionic solid is defined as the energy released when one mole of solid is formed from its constituent ions in the gas phase. The hydration enthalpy, A Hh, of an ion is the energy released when one mole of the gas phase ion is dissolved in water. Comparison of the two values allows one to determine the enthalpy of solution, AHs, and whether an ionic solid will dissolve endothermically or exothermically. Figure 1.4 shows a comparison of AH and A//h, demonstrating that AgF dissolves exothermically. 

The solubility of ionic solids in water covers a wide range of values. Knowing the concentration of ions in aqueous solution is important in medicine and in chemical analysis. In this section, you will continue to study equilibrium. You will examine the solubility equilibria of ionic compounds in water. 

Why is the dissolving of many ionic solids in water an endothermic process, whereas the mixing of most miscible liquids is an exothermic process ... 

When we say an ionic solid is insoluble in water, we do not mean that none of the solid dissolves. There are always some ions that can escape from the surface of an ionic solid in water and go into solution. Thus, when we say that calcium carbonate is insoluble in water, what we really mean is that the solubility is very low (0.0014 g CaC03 per 100 mL Fi20 at 25 °C). 

Solubility product constant the constant for the equilibrium expression representing the dissolving of an ionic solid in water. (16.1) Solute a substance dissolved ina liquid to form a solution. (4.2 11.1) Solution a homogeneous mixture. (1.9)... 

Getting back to the dissolution of an ionic solid in water, it is clear that going from the nicely ordered ionic solid to separate ions in solution, arranged in many different and random patterns, increases the entropy/disorder, and hence, solvation will be preferred unless the attraction in the solid is much stronger than the attractions felt by each ion from the water molecules that surround it. When the interaction energies in the solid and the solution are not too different, then endothermic dissolution can nevertheless happen, and it will be driven by the increase of entropy/disorder. This is the case of the dissolution of [NH4+Cl ]soiid in water. 

FIGURE 2.9 The dissolution of an ionic solid in water, showing the hydration of positive and negative ions by the very polar water molecules. The ions become surrounded by water molecules in all three dimensions, not just the two shown here. 

Figure 13.1 A schematic illustration of the solution process of an ionic solid in water, (a) The solid substance is hydrated by water molecules, with the ojq gen atoms of the water molecules oriented toward the cations and the hydrogens oriented toward the anions. 

Figure 19.8 Changes in degree of order in the ions and solvent molecules on dissolving an ionic solid in water. The ions themselves become more disordered, but the water molecules that hydrate the ions become less disordered. 

Heat of Solution Solution Cycles Heat of Hydration Ionic Solids in Water Solution Process and Entropy Change... 

The attraction of ions for water molecules is called hydration. Hydration of ions favors the dissolving of an ionic solid in water. Figure 12.9 illustrates the hydration of ions in the dissolving of a lithium fluoride crystal. Ions on the surface become hydrated and then move into the body of the solution as hydrated ions. 

For the dissociation of an ionic solid in water, any one of the following conditions may exist (1) the solution is unsaturated, (2) the solution is saturated, or (3) the solution is supersaturated. For concentrations of ions that do not correspond to equilibrium conditions, we use the reaction quotient (Q) [IH Section 15.2] to predict when a precipitate will form. Note that Q has the same form as K p except that the concentrations of ions are not equilibrium concentrations. For example, if we mix a solution containing Ag ions with one containing Q ions, we write... 

TABLE 12.5 Solublllty Rules for Ionic Solids in Water... 

A new Explore Your World Preparing Rock Candy is added to illustrate the formation of a saturated solution. Table 9.8 Solubility Rules for Ionic Solids in Water is updated. 

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