Effect of Temperature on Solubility Objectives: To state the effect of temperature and pressure on the solubility of a gas in a liquid. To interpret a graph that shows the effect of temperature on the solubility of a solid compound in water. To interpret a graph of temperature versus solubility and determine whether a solution is saturated, unsaturated, or supersaturated.

Solubility vs. Temperature 200 180 160 140 120 100 80 60 40 20 KI KNO3 Solubility (g solute / 100 g H2O) NaNO3 The general rule of thumb is that solubility of solids increases with increases in temperature. Maximum amount of a solute that can dissolve in a solvent at a specified temperature and pressure is its solubility. – Solubility is expressed as the mass of solute per volume (g/L) or mass of solute per mass of solvent (g/g) or as the moles of solute per volume (mol/L). – Solubility of a substance depends on energetic factors and on the temperature and, for gases, the pressure. • A solution that contains the maximum possible amount of solute is saturated. • If a solution contains less than the maximum amount of solute, it is unsaturated. When a solution is saturated and excess solute is present, the rate of dissolution is equal to the rate of crystallization. • Solubility increases with increasing temperature — a saturated solution that was prepared at a higher temperature contains more dissolved solute than it would contain at a lower temperature, when the solution is cooled, it can become supersaturated. Solubility of a substance generally increases with increasing temperature No relationship between the structure of a substance and the temperature dependence of its solubility Solubility may increase or decrease with temperature; the magnitude of this temperature dependence varies widely among compounds This variation of solubility with temperature is used to separate the components of a mixture by fractional crystallization, the separation of compounds based on their solubilities in a given solvent Fractional crystallization is a common technique for purifying compounds; the compound of interest must be more soluble at high temperature than at low temperature, so that lowering the temperature causes it to crystallize out of solution Solubility of gases in liquids decreases with increasing temperature Attractive intermolecular interactions in the gas phase are essentially zero for most substances When a gas dissolves, its molecules interact with solvent molecules and heat is released when these new attractive interactions form, therefore, dissolving most gases in liquids is an exothermic process (Hsoln < 0) Adding heat to the solution provides thermal energy that overcomes the attractive forces between the gas and the solvent molecules, thereby decreasing the solubility of the gas Na3PO4 NaCl 20 40 60 80 100 Temperature (oC) Timberlake, Chemistry 7th Edition, page 297

Gas Solubility Higher Temperature …Gas is LESS Soluble CH4 O2 2.0 O2 Higher Temperature …Gas is LESS Soluble CO Solubility (mM) 1.0 The general rule of thumb is that the solubility of gases decreases when temperature increases. Solubility of a substance generally increases with increasing temperature No relationship between the structure of a substance and the temperature dependence of its solubility Solubility may increase or decrease with temperature; the magnitude of this temperature dependence varies widely among compounds This variation of solubility with temperature is used to separate the components of a mixture by fractional crystallization, the separation of compounds based on their solubilities in a given solvent Fractional crystallization is a common technique for purifying compounds; the compound of interest must be more soluble at high temperature than at low temperature, so that lowering the temperature causes it to crystallize out of solution Solubility of gases in liquids decreases with increasing temperature Attractive intermolecular interactions in the gas phase are essentially zero for most substances When a gas dissolves, its molecules interact with solvent molecules and heat is released when these new attractive interactions form, therefore, dissolving most gases in liquids is an exothermic process (Hsoln < 0) Adding heat to the solution provides thermal energy that overcomes the attractive forces between the gas and the solvent molecules, thereby decreasing the solubility of the gas He 10 20 30 40 50 Temperature (oC)

Solubility

Solubility Rules General Rules for Solubility of Ionic Compounds (Salts) in Water at 25 oC Most nitrate (NO31-) salts are soluble. Most salts of Na+, K+, and NH4+ are soluble. Most chloride salts are soluble. Notable exceptions are AgCl, PbCl2, and Hg2Cl2. Most sulfate salts are soluble. Notable exceptions are BaSO4, PbSO4, and CaSO4. Most hydroxide compounds are only slightly soluble.* The important exceptions are NaOH and KOH, Ba(OH)2 and Ca(OH)2 are only moderately soluble. Most sulfide (S2-), carbonate (CO32-), and phos- phate (PO43-) salts are only slightly soluble. *The terms insoluble and slightly soluble really mean the same thing. Such a tiny amount dissolves that it is not possible to detect it with the naked eye. Ionic substances are most stable in polar solvents, and the higher the lattice energy, the more polar the solvent must be to overcome the lattice energy and dissolve the substance. • Water is the most common solvent for ionic compounds because of its high polarity. • Many ionic compounds are soluble in other polar solvents because these solvents consist of molecules that have large dipole moments and can interact favorably with the dissolved ions. • A more useful measure of the ability of a solvent to dissolve ionic compounds is its dielectric constant (), which is the ability of a bulk substance to decrease the electrostatic forces between two charged particles. Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 218

Solubility of Common Compounds Soluble compounds NO3- salts Na+, K+, NH4+ salts Except for those containing Cl-, Br-, I- salts Ag+, Hg22+, Pb2+ Except for those containing SO42- salts Ba2+, Pb2+, Ca2+ Insoluble compounds S2-, CO32-, PO43- salts Except for those containing OH1- salts Na+, K+, Ca2+ Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 218

Temperature-Solubility Relationships* 190 CsCl 180 NaNO3 170 160 NaC2H3O2 150 140 130 RbCl 120 LiCl 110 Solubility in grams per 100 g of water 100 90 80 NH4Cl 70 60 KCl 50 NaCl 40 30 Li2SO4 20 10 SO2 10 20 30 40 50 60 70 80 90 100 Temperature (oC) *Values on this graph are valid only when some undissolved solid is in contact with the solution. For SO2 values are valid when pressure due to SO2 and water vapor is 101.325 kPa.