CHAPTER 14 SOLUTIONS Spontaneity of the Dissolution Process The Dissolution 1 Process solute solvent solution 25 mL H2O 15 mL CH3CH2OH 125 mL CH3COCH3 solution solute note that the solvent will always be in much higher concentration than the solute will be in the 7 homogeneous possibilities solute solvent solute solid liquid gas liquid solid gas gas solute solid liquid solvent liquid liquid liquid solid solid solid gas solvent gas gas example salt water mixed drinks carbonated drinks dental amalgams alloys metal pipes air O H and N example dust in the air clouds and fog the 2 heterogeneous possibilities two major factors affect the dissolution of solutes 1 enthalpy a measure of heat Hsolution where H is exothermic dissolution is favored and H is endothermic dissolution is NOT favored 2 entropy a measure of randomness disorder Smixing where S is an increase in disorder dissolution is favored and S is a decrease in disorder dissolution is NOT favored NaCl s Na aq Cl aq S increasing entropy 0 favorable Na aq Cl aq NaCl s S decreasing entropy 0 not fav disorder almost always increases when mixing a solution S so S is almost always 0 the ideal situation is H exothermic H 0 S more disordered S 0 solutes nearly always become more disordered upon dissolution the main factors that determine the heat of solution Hsolution are solute solute attractions things like ion ion attraction dipole dipole attraction etc overcoming solute solute attractions requires the absorption of energy H endothermic Na Cl H2O breaking the bond between Na and Cl in NaCl solvent solvent attractions one example is hydrogen bonding in water overcoming solvent solvent attractions requires the absorption of energy H endothermic H2O H2O overcoming the attractive forces between the O s and the H s NaCl H2O H solvent solute attractions known as solvation this releases energy H exothermic if the solvation energy is greater than the sum of the solute solute and solvent solvent attractions then the dissolution is exothermic H or Hsolution 0 if the solvation energy is less than the sum of the solute solute and solvent solvent attractions then the dissolution is endothermic H or Hsolution 0 so Hsolution is a sum of the three processes described above and dissolution is favored when the first two factors are small and the third factor is large 2 Dissolution of Solids in Liquids The Dissolution Process many solids that are very soluble in water are ionic crystal lattice energy the energy released H exothermic when 1 00 mol of formula units of a solid is formed from its constituent ions molecules or atoms for non ionic solids in the gas phase a measure of the attractive forces in a solid g X M g M X crystal lattice energy M X s crystal lattice energy s M g X g note that if the first equation is an endothermic process then the second equation is an exothermic process also if the first equation is an exothermic process then the second equation is an endothermic process crystal lattice energy increases as charge density increases in the picture below the Li ion and the Cl ion in the bottom right are hydrated ions energy of solvation molar energy of hydration or hydration energy if the solvent is water the energy released when solute particles are dissolved when a solvent dissolves a solute in an exothermic dissolution mol of formula units in the form of gaseous ions becomes hydrated g xH2O M OH2 x n hydration energy for Mn g nH2O X H2O n y hydration energy for Xy hydration energy increases with increasing charge density the amount of energy absorbed when 1 00 Mn Xy ion K Ca2 Cu2 Al3 radius 1 33 0 99 0 72 0 50 charge radius ratio 0 75 2 02 2 78 6 00 heat of hydration 351 kJ mol 1650 kJ mol 2160 kJ mol 4750 kJ mol notice that the hydration energy increases as the charge density increases remember the trend for ionic radii K Ca2 Al3 N3 O2 F the like dissolves like rule says that polar molecules are soluble in polar solvents and that non polar molecules are soluble in non polar solvents London dispersion forces look like this Dissolution of Liquids in Liquids Miscibility The Dissolution 3 Process non polar molecules essentially slide in between each other fitting very close to one another this close fit enhances London dispersion forces which increases miscibility for example CCl4 is very miscible in C6H6 benzene 4 Dissolution of Gases in Liquids The Dissolution Process polar gases are more soluble in water than non polar gases polar gases are things like the strong acid gases HCl g HI g etc HF g is also a polar gas even though it is a weak acid due to hydrogen bonding remember this particular polar gas formula HI g MHCO3 or MCO3 salt H2O l CO2 g note that H2CO3 is not included because it is very unstable polar gases can hydrogen bond with water some polar gases enhance their solubility by reacting with water aq Br aq HBr g H2O l H3O strong acid SO2 aq H2O l H2SO3 aq H2SO3 aq H3O weak acid aq HSO3 aq a few non polar gases are soluble in water because they react with water CO2 aq H2O l H2CO3 aq H3O very weak acid aq HCO3 aq because gases have very weak solute solute interactions gases dissolve in liquids in exothermic processes 5 Rates of Dissolution and Saturation The Dissolution Process finely divided solids dissolve more rapidly than large crystals due to an increase in surface area the number of molecules of sugar exposed to water increase compare the dissolution of granulated sugar and sugar cubes in cold water saturated solutions dissolved in a given volume of solvent the maximum amount of solute that can be an equilibrium is established between dissolved and undissolved solutes symbolically this equilibrium is written as MX s M aq X aq note that in an equilibrium reaction the forward rate of reaction is equal to the reverse rate of reaction examples air that has 100 humidity and some solids dissolved in liquids supersaturated solutions of dissolved solutes a precipitate forms have higher than saturated concentrations 6 Effect of Temperature on Solubility The Dissolution Process LeChatelier s Principle equilibrium the system responds in a way that best relieves the stress since saturated solutions are at equilibrium this principle applies to them when a stress is applied to a system at equilibrium means that the rate of the formation of the product is equal to the rate of the decomposition of the product equilibrium lasts until a stress is applied to the equilibrium aq aq X MX s M any time you increase concentration on one side