CHEM 1120 1st Edition Lecture 4 Outline of Last Lecture I Continuing with Colligative Properties Boiling Point Elevation II Freezing Point Depression III Osmosis Outline of Current Lecture I Colligative Properties of Electrolyte Solutions A van t Hoff Factor II Colloids A Definitions and types B Hydrophillic vs Hydrophobic Colloids Current Lecture I Electrolytes often do not dissociate all the way in solutions This is where the van t Hoff Factor works its way into chemistry The van t Hoff Factor i is equal to the moles of particles in a solution over the moles of solute dissolved in a solution For salts it is pretty simple You just take the number of ions over the formula units This seems complicated but let me show you an example For NaCl it dissolves into two ions and the formula units is equal to one so two divided by one is two This is the same process for most dissociating salts However many ions it dissolves into is equal to the expected van t Hoff Factor for that solution Another example is K2SO4 Potassium sulfate dissociates into three ions therefore making the van t Hoff Factor for that solution equal to three Although salts dissociate and there van t Hoff Factor is the number of ions this is not true for molecules that do not dissociate For these other molecules including sucrose and others similar the van t Hoff Factor is usually just one These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute The actual van t Hoff Factors are slightly different than the expected values because of something called ion pair formation II A As we remember from earlier lectures a solution is a homogeneous mixture of two or more substances Example Na and Cl 0 1 1 particle diameter We need to remember this definition to compare solutions to suspensions and colloids Suspension a heterogeneous mixture with dispersed particles large enough to be seen Example corn starch 1000 nm particle diameter Colloid a mixture with dispersed particles larger than a solution s but smaller than a suspension s Example fog clouds 1 1000 nm particle diameter Colloids may not always be liquids and are found everywhere Additionally when talking about colloids the Tyndall Effect comes up as well The Tyndall Effect is a phenomenon that states that colloidal suspensions can scatter rays of light SOLUTION COLLOID SUSPENSION PARTICLE SIZE 0 1 1 nm 1 1000nm 1000 nm PARTICLE TYPE ions and small molecules particles and large molecules large particles PARTICLES do not settle do not settle settle FILTERABLE no no yes UNIFORMITY homogeneous intermediate heterogeneous TYNDALL EFFECT no yes yes LIGHT does not scatter scatters scatters B Hydrophilic and Hydrophobic Colloids Hydrophilic water loving Hydrophobic water fearing Remember like dissolves like so polar molecules will dissolve in water because water is polar and octane a nonpolar molecule will be insoluble in polar water Hydrophilic Colloids Dispersed particles are large molecules the core may be hydrophobic the surface is hydrophilic surface groups hydrogen bond to water Hydrophobic Colloids must be stabilized by ions absorbed on the particle s surface or by hydrophilic groups on the particle s surface one nonpolar particle has a greater attraction for another nonpolar particle than it does for water absorbed ions interact with water and particles repel each other the tail of a hydrophobic colloid is nonpolar hydrophobic and the head i s polar hydrophilic Colloids can help to emulsify fats and oils in aqueous solutions An emulsifier causes something that normally does not dissolve in a solvent to do so The motion of colloids is mainly due to the numerous amount of collisions with the much waller solvent particles molecules Brownian Motion