Heat of Solution
The heat of solution is the heat that is absorbed when one substance dissolves in another to formation a homogeneous mixture. A homogeneous mixture is one which a sample of the mixture that contains a substantial number of molecules has the same composition as any other sample: the components of the mixture are distributed uniformly throughout. In common usage, the heat of solution is used in place of the more precise term the enthalpy of solution, which has the symbol ()Hsoln. The enthalpy of solution is the heat of solution for dissolution of one mole of a substance in another substance such that the final solution is one molar and three conditions are fulfilled: (1) the pressure remains constant, (2) the only possible work that occurs is expansion against the atmosphere (so-called P()V work) and (3) the temperature remains constant during the process. The enthalpy of solution of a substance is only valid for dissolution of the substance in its pure state into another pure substance.
Enthalpies of solution for a large number of substances can be measured directly when the resulting solution is liquid, the substance that is dissolved (the solute) is either a solid or liquid and the substance into which the solute is dissolved (the solvent) is liquid. If the solute is dissolved slowly, a very nearly constant temperature can be maintained. The heat absorbed in gas-liquid dissolution processes is often readily measured, but if the measurements are made using a constant volume apparatus, the P() work must be accounted for. Some gas-solid solution processes, such as the dissolution of hydrogen in certain metals, occur under conditions that allow measurements to be made readily, but many do not. As in the case of gas-liquid solutions, the effect of the decreased volume as the gas is absorbed must be accounted for in determining the enthalpy of solution.
Gases that do not react with each other readily mix without substantial absorption or evolution of heat, so gas-gas enthalpies of solution are generally small. Enthalpies of solution for solid-solid solutions are difficult to measure accurately because the time required for dissolution is typically very long.
For those cases in which it is not feasible to carry out a dissolution process without increasing the temperature of the system, the enthalpy of solution can be determined by subtracting the heat absorbed to increase the temperature of the resulting solution from the total heat absorbed in the process. The heat absorbed by the solution is equal to the change in temperature multiplied by the specific heat capacity of the solution and the mass of the solution in grams.
Enthalpies of solution are often specifically designated to show the conditions under which they apply. The temperature is designated by a subscript, for example, ()Hsoln, 350 K for a process in which one substance at 350 K is dissolved in another also at 350 K and no temperature change occurs. If no temperature is specified, the temperature is typically 298 K or 25°C. For some types of solutions, particularly gas-liquid and gas-solid solutions, the heat of solution as well as the solubility can depend on pressure. If there is a substantial effect of pressure, it should also be included in the subscript. The units typically used for heats of solution are kilojoules per mole or kilocalories per mole.
One of the most common types of heat of solution is that for the dissolution of a substance, most commonly salts, in water. The enthalpy of solution for aqueous solutions is called the enthalpy of hydration or, simply the heat of hydration, and has the symbol ()Hhyd, where hyd is the abbreviation for hydration. Enthalpies of hydration for individual ions have been calculated from the enthalpies of hydration of a large number of salts containing the ions. The solubilities of salts are related to their enthalpies of hydration, as the solubilities of other solutes in a particular solvent are related to their enthalpies of solution in those solvents. A more negative enthalpy increases the tendency for a solute to dissolve.
Enthalpies of solution can be positive or negative: the dissolution process may be endothermic, with heat being absorbed, or exothermic, with heat being evolved. If the energy of attraction of the solute molecules or ions with each other and the energies of attraction of the solvent molecules or ions with each other are greater than the energy of attraction of solute and solvent particles, heat will be required to make the solute disperse in the solvent. Heat will then be absorbed in the process and the heat of solution will be positive. In this case, the solubility of the material increases with temperature. The heat of solution for formation of aqueous solutions of most salts is positive. The absorption of heat as a result of a mixing process is the basis of commercial "cold-packs" used by athletic trainers to treat minor injuries. If the energy of attraction of solute molecules or ions and solvent molecules or ions is greater than the energies of attraction of solute particles with each other and solvent particles with each other, heat will be released as the solute dissolves. For these exothermic processes, the solubility decreases with an increase in temperature.
This is the complete article, containing 865 words
(approx. 3 pages at 300 words per page).
