Endothermic dissolving

Because the enthalpy of solution is positive, there is a net inflow of energy as heat when the solid dissolves (recall Fig. 8.23b). Sodium chloride therefore dissolves endothermically, but only to the extent of 3 kj-mol-1. As this example shows, the overall change in enthalpy depends on a very delicate balance between the lattice enthalpy and the enthalpy of hydration. 

The enthalpy of solution of ammonium nitrate in water is positive, (a) Does NH4N05 dissolve endothermically or exothermically (b) Write the chemical equation for the dissolving process, (c) Which is larger for NH4NO , the lattice enthalpy or the enthalpy of hydration ... 

Dichloromethane dissolves endothermically in cone, nitric acid to give a detonable solution. 

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. 

Boric acid dissolves endothermically in water, its solubility increasing greatly with increasing temperature. Its solutions are mildly acidic, which is a result of the electron-acceptor character of boron and not a tendency toward proton donation. Boric acid reacts with water, as given by Eq. (1), to give an equilibrium concentration of the [B(OH)4] anion. The equilibrium constant... 

FIGURE 8.22 The exothermic dissolution of lithium chloride (left) is shown b> the rise in temperature above that of the original water (center) in contrast, ammonium nitrate (right) dissolves endothermically. 

The heat of chemisorption, which must be low in order to enable catalysis to take place, may even be negative. In various sections we have seen that endothermic chemisorption may play an important role (Secs. V,9, VI,3,4,5, and X,4). Figure 40 shows that surface contaminations can promote endothermic chemisorption. In nickel, as in iron, hydrogen atoms can be dissolved endothermically. It is highly probable that dissolved hydrogen atoms react from the metal phase with chemisorbed hydrocarbons. 

Which chemical dissolved endothermically Which chemical dissolved exothermically ... 

Why do some solids dissolve exothermically, while other solids dissolve endothermically What factors may be involved Research two factors lattice energies and solvation energies. Go to the web site above. Go to Science Resources, then to Chemistry 11 to find out where to go next. Report your results to the class. 

Many ionic solids dissolve by endothermic processes. Their solubilities in water usually increase as heat is added and the temperature increases. For example, KCl dissolves endothermically. 

Notice that the heat in an endothermic process is written as if it were a reactant because it must be added to the substances that will form the products. To increase the solubility of NH4NO3, add more heat, which increases the temperature. This forces the process toward the production of more aqueous ions. When working with a solute that dissolves endothermically, you may notice that the mixture becomes cooler during the dissolving. The heat required for the process is taken from the solution, which therefore cools off. 

Calcium chloride is found in salt mixtures that are used to melt ice on roads in the winter. The dissolving of CaCl2 is exothermic. Provide two reasons why CaCl2 is a good choice for this application as compared to a salt such as NaCl or a salt that dissolves endothermically. 

Instant hot and cold packs create aqueous solutions that form exothermically or endothermically and therefore release or absorb heat. A hot pack generates heat when a salt such as calcium chloride dissolves in water that is stored in the pack. The calcium chloride dissolves exothermically. A cold pack absorbs heat when a salt such as ammonium nitrate dissolves in water. The ammonium nitrate dissolves endothermically. 

Which of the solids that you tested dissolve exothermically Which dissolve endothermically ... 

MiniLab 1 Which factor—entropy, added heat, or both—promotes the dissolution in water of a solid that spontaneously dissolves endothermically Which factor promotes dissolution in water of a solid that spontaneously dissolves exothermically ... 

The answer is D. All the solubilities shown in the graph are endothermic. In fact most compounds (solids) dissolve endothermically. The graph shows that as the temperature increases, the solubility of the salts increases. In other words, as the temperature increases, more and more of the salt present is dissociated into ions. This is exactly how an endothermic process works. 

At high temperatures the potassium was dispersed in atomic form, but on slow coohng to lower temperatures it could be condensed into colloidal aggregates. The halogens and hydrogen dissolved endothermically in halide lattices, the heats of solution being... 

Ti, Ta, but endothermic for Cu, Fe, Co, and Ni. From the slopes of log (solubility)-1/T curves (24) (Fig. 42) the heats of solution given in Table 40 have been calculated. In some of the metals, notably those in which hydrogen dissolves endothermically, there is no appreciable alteration in the lattice constants on solution of the gas in the metal. On the other hand, when hydrogen dissolves in palladium one may have at saturation 10 % expansion of the lattice. In the metals Ti, V, Zr, Th, and Ta, where great quantities of hydrogen are... 

This method was applied for the first time by von Steinwehr (1901) and Brbnsted (1906). This measuring procedure is best illustrated by the experiment performed by Brbnsted. The device shown schematically in Figure 1.3 served for the measurement of the endothermic heat of solution of a salt in water. An amount of salt is dissolved endothermically in a Dewar vessel containing water, and the contents are steadily mixed by means of a stirrer. An electric heater yields a heat output that is regulated so as to leave the solution temperature unchanged. If the voltage Lf(f) and the current I(t) are constantly recorded, then AQ = f U(t) I(t)dt (the electrically generated compensatory heat) equals the heat of solution of the salt. 

If, say, a salt dissolves endothermically in a solvent, the temperature of the calorimeter vessel can be kept constant by a controlled supply of electric energy (heating). The total amount of energy thus introduced equals the heat of solution of... 

Most soluble salts dissolve endothermically and spontaneously ... 

There are many examples of reactions which are spontaneous. The vast majority of these reactions are exothermic. Hence it appears that the enthalpy change, AH, is a reliable guide to which direction a reaction will go. However, there are examples of endothermic reactions that occur without the need for heat to initiate the reaction, for example, the reaction between citric acid and a solution of sodium hydrogencarbonate. Some salts dissolve endothermically in water. Chapter 15 introduces a factor, known as entropy, that, in conjunction with enthalpy and temperature, determines whether or not reactions occur at a specified temperature. 

Draw an energy cycle for the dissolving (dissolution) of ammonium chloride. It dissolves endothermically. 

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