Heats of Hydration Ionic Solids in Water

The heat of hydration is a key factor in dissolving an ionic solid. Breaking H bonds in water is more than compensated for by forming strong ion-dipole forces, so hydration of an ion is always exothermic. The A/Z ydr of an ion is defined as the enthalpy change for the hydration of 1 mol of separated (gaseous) ions  

The energy required to separate an ionic solute (A//so,ute) into gaseous ions is its lattice energy (A//iattice), and A/Z oiute is highly positive  

the heat of solution for ionic compounds in water combines the lattice energy (always positive) and the combined heats of hydration of cation and anion (always negative), 

The sizes of the individual terms determine the sign of the heat of solution. 

A NaCI. AHiattice is slightly larger than AHhydri AHsoin is small and positive. 

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

Hydrates often form as solutions of ionic compounds evaporate to dryness. All hydrates are solids. The water in a hydrate can be driven off with heating, leaving the water-free compound behind. 

These are ionic solids and can exist as the anhydrous salts (prepared by heating together sulphur with excess of the alkali metal) or as hydrates, for example Na2S.9HjO. Since hydrogen sulphide is a weak acid these salts are hydrolysed in water,... 

ChloricfVII) acid fumes in moist air and is very soluble in water, dissolving with the evolution of much heat. Several hydrates are known the hydrate HCIO4. H2O is a solid at room temperature and has an ionic lattice [HjO ] [CIO4]. 

Many ionic compounds can have water molecules incorporated into their solid structures. Such compounds are called hydrates. To emphasize the presence of discrete water molecules in the chemical structure, the formula of any hydrate shows the waters of hydration separated from the rest of the chemical formula by a dot. A coefficient before H2 O indicates the number of water molecules in the formula. Copper(II) sulfate pentahydrate is a good example. The formula of this beautiful deep blue solid is C11SO4 5 H2 O, indicating that five water molecules are associated with each CuSOq unit. Upon prolonged heating, CuSOq 5 H2 O loses its waters of hydration along with its color. Other examples of hydrates include aluminum nitrate nonahydrate, A1 (N03)3 9 H2 O,... 

Consider the heat of solution of a process in which an ionic compound is the solute and water is the solvent. For example, what happens when solid NaCl dissolves in water In solid NaCl, the Na and CP ions are held together by strong positive-negative (electrostatic) forces, but when a small crystal of NaCl dissolves in water, the three-dimensional network of ions breaks into its individual units. (The structure of solid NaCl is shown in Figure 2.12.) The separated Na+ and CP ions are stabilized in solution by their interaction with water molecules (see Figure 4.2). These ions are said to be hydrated. In this case water plays a role similar to that of a good electrical insulator. Water molecules shield the ions (Na+ and CP) from each other and effectively... 

In aqueous systems, the typically trivalent rare earths show strong ionic character. Ce (IV) is the only tetrapositive rare earth species that is stable in aqueous solution as well as in solids. The trivalent rare earths form salts with a large number of anions. There is a wide variability in solubility of such salts. Rare earths containing thermally unstable ions like OH-, COs, or 204, convert, when heated first, into the basic derivative, and finally into the oxide (Gupta and Krishnamurthy 2005). Chlorides, bromides, nitrates, bromates, and perchlorate salts of rare earths are all water soluble. When crystallizing as a result of evaporation, they all form hydrated crystalline salts (Gupta and Krishnamurthy 2005). 

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