Solutions and Colloids PDF

Solutions and Colloids David A. A Katz Department of Chemistry Pima Community College Solutions SOME SOLUTION TERMINOLOGY Solvent: the fraction of a solution in which the other components are dissolved. ((This is usually y the liquid)...

Solutions and Colloids David A. A Katz Department of Chemistry Pima Community College Solutions SOME SOLUTION TERMINOLOGY Solvent: the fraction of a solution in which the other components are dissolved. ((This is usually y the liquid) q ) Solute: a substance that is dissolved in a solvent to produce d a solution. l ti (This (Thi is usually the solid) Types yp of Solutions Appearance Solute Solvent off S Solution l ti Example Gas in Liquid Liquid Carbonated water Liquid in Liquid Liquid Wine Solid in Liquid Liquid Salt water (saline solution) Gas in Gas Gas Air Solid in Solid Solid 14 Carat gold Characteristics of Solutions 1. The distribution of particles in a solution is uniform. (the mixture is homogeneous) 2. The components of a solution do not separate on standing. standing 3. The components of a solution cannot be separated by filtration. 4. For many solvent/solute combinations, it is possible to make solutions of many different compositions. 5 S 5. Solutions l i are transparent. (but (b may beb colored) l d) 6. Solutions can be separated into pure components; the separation is a physical change, not a chemical change.  Solubility y Solubility: the maximum amount of a solute that dissolves in a given amount of solvent at a given temperature. Solubility is a physical constant. For solids: – Each solid has a different solubility in every liquid: Those with low solubility are said to be insoluble – Those with higher solubility are said to be soluble. Solubility y For liquids: Some liquids are insoluble in each other, these liquids are said to be immiscible examples: gasoline and water; oil and vinegar. vinegar Some liquids have limited solubility in each other, these liquids are miscible example:l ether th in i water t (6 g/100 /100 g H2O) Some other liquids are completely soluble in each other,, these liquids q are completely p y miscible example: ethanol and water. Concentration terms Dilute solution: a solution that contains a small amount of a solute dissolved in the solvent. Concentrated solution: a solution that contains a large amountt off a solute l t di dissolved l d iin the th solvent. l t These definitions are too general to be of use Concentration terms Saturated solution: a solution that contains the maximum amount of a solute that can be dissolved at equilibrium at a given temperature. Unsaturated solution: a solution that contains less than the maximum amount of a solute that can be dissolved at a given temperature. Supersaturated solution: a solution that contains more than the maximum amount of a solute that can be dissolved under equilibrium conditions at a given temperature; p ; when this solution is disturbed in any y way, the excess solute separates and the equilibrium solubility is restored. Pouring a supersaturated solution of sodium acetate  Solubility y Solubility depends on several factors: Nature of the solvent and solute. The more similar two compounds are, the more likely it iis that th t one is i soluble l bl in i the th other. th Like dissolves like examples: benzene and carbon tetrachloride (two non-polar liquids) NaCl in water (an ionic substance in a polar solvent) table sugar (C12H22O11) in water (two polar substances)  Solubility y Solubility depends on several factors: Temperature. The solubility of solids in liquids generally increases as temperature increases increases. The solubility of gases in liquids almost always decreases as temperature increases.  Solubility of salts in water at various temperatures  Solubility y Pressure Pressure has little effect on the solubility of liquids or solids in each other. other The solubility of a gas in a liquid increases as pressure increases, as for example the solubility of CO2 in ca carbonated bo ated beverages.  Solubility of gases at different temperatures  Percent Composition p Percent composition: weight g of solute perp volume of solution (w/v); (w/v) ( ) a ); solution of 10 g of table sugar in 100 mL of solution, for example, has a concentration of 10 percent w/v. weight i ht off solute l t per weight i ht off solution l ti (w/w); ( / ) essentially the same as w/v except that the weight of the solution is used instead of its volume. volume of solute per volume of solution (v/v); example, a solution of 40 mL of ethanol in 100 mL of aqueous solution is 40 percent v/v.v/v amount of solute %= ×100 amount of solution  Parts Per Million (pp (ppm)) For very dilute solutions, less than 0.1%, we sometimes express p concentration in parts p per p million (ppm), or even parts per billion (ppb). amount of solute ppm= amount of solution This is the decimal equivalent of a small percent concentration. A 0.1% solution would be expressed, in decimal form, as 0.001. This is 1 ppt (one part per thousand) A00.0001% 0001% solution would be expressed, expressed in decimal form, as 0.000001. This is 1 ppm (one part per million)  Parts Per Million (pp (ppm)) Parts P t per million: illi may be either w/w, w/v, or v/v; which ever quantities are used used, the units in which each is reported must be the same. for example, 1 mg of lead ions per 1 kg of water is equivalent to 1 mg of lead per 1,000,000 mg of water; the concentration of lead is 1 ppm. Parts per billion: calculated in the same way way.  Molarity y Molarity Molarity:: moles of solute per liter of solution. Molarity (M ) = moles of solute (n) liter of solution (L) example: H How iis 2.0 2 0 L off a 0 0.15 15 M NaOH N OH solution l ti prepared? d? Determine the number of moles of NaOH required: q 0.15 mol N aOH x 2.0 L = 0.30 mol N aOH 1L C Convert t00.30 30 moll NaOH N OH to t g NaOH: N OH (MWNaOH = 40.0 g/mol) 40.0 g N aOH 0.30 mol N aOH x = 12.0 g N aOH 1 mol N aOH To prepare this solution, dissolve 12.0 g NaOH in water for a total volume of 2.0 L  Molarity y Molarity Molarity:: alternate equation (one-step): g solute ×1000 mL L Molarity ( M ) = MWsolute × mLsolution Example: E l calculate l l t the th molecular l l weight i ht off sodium di hydroxide - MW = 40 g/mol g solute ×1000 mL L 0.15M = g 40 mol × 2000mL Solve S l for f gNaOH g NaOH = 12.0 g NaOH To prepare this solution, dissolve 12.0 g NaOH in water for a total volume of 2.0 L solution Molarity y Problem: The concentration of NaCl in blood serum is approximately 0.14 M. What volume of serum contains 2.0 g of NaCl? (MWNaCl = 58.5 g/mol) g solute × ×1000 1000 mL L Molarity ( M ) = MWsolute × mLsolution 2.0 g ×1000 mL L 0.14 M = 58.5 g mol × mLsolution 2.0 g ×1000 mL L mL = = 244 mL 58 5 moll × 0.14 58.5 g 0 14 M  Molarity y - Dilution If we dilute a solution, the number of moles of solute remains the same after dilution as before dilution; we can then use this relationship: M1V1 = M2V2 problem: How do you prepare 200 mL of 3.5 M aqueous solution off acetic ti acid id if you have h a bottle b ttl off 6.0 6 0 M acetic ti acid. id 3.5 mol × 200 mL V1 = = 117 mL 6 0 mol 6.0 To prepare the desired solution, put 117 mL of 6.0 M acetic in a 200 mL volumetric flask and fill to the mark.  Water as a Solvent How water dissolves ionic compounds: Ionic compounds are a regular array of positive and negative ions. Water is a polar molecule, with positive and negative dipoles. p The negative ions attract the positive dipole of water, and the positive ions attract the negative dipole of water; each ion attracts two to four molecules of water Ions dissolved in water are said to be hydrated (surrounded by water molecules). Water of hydration: the attraction between ions and water is so strong that water molecules are a part of the crystal structure of many solids solids. Dissolving g sodium chloride in water Heating a hydrated salt. The blue colored hydrate CuSO4 5H hydrate, 5H2O, O can be seen in the middle of the crucible. The light colored anhydrous CuSO4 is visible around the sides of the crucible. Electrolytes y Ions in water can migrate from one place to another, maintaining their charge as they migrate. Cations (positive ions) migrate to the negative electrode (the cathode) cathode). Anions migrate to the positive electrode (the anode). The movement of ions constitutes an electric current. Electrolyte: a substance that conducts electric current when dissolved in water A substance that does not conduct electricity is called a nonelectrolyte nonelectrolyte..  Electrolytes y Conductance by an electrolyte Electrolytes y Strong electrolyte: a compound that dissociates completely to ions in an aqueous solution. Compound Dissociates to No. of ions per formula unit NaCl Na+ and Cl- 2 CaCl2 Ca2+ and 2 Cl- 3 K2SO4 2 K+ and SO42- 3 Mg3(PO4)2 3 Mg2+ and 2 PO43- 5 Ionic substances dissociate into the ions and polyatomic ions used in writing the chemical formulas of the compounds Weak electrolyte: a compound that only partially dissociates to ions in an aqueous solution. An example is acetic acid, HC C2H3O2, which exists as HC2H3O2 molecules, H+ and C2H3O2- in water solution Electrolytes y Nonelectrolyte: Nonelectrolyte: a compound that does not dissociate into ions in an aqueous solution. Examples are polar compounds such as sucrose, C12H22O11, and ethanol, C2H5OH, which exist as molecules in water solution  Water as a Solvent How water dissolves molecular compounds: In a few cases,, molecular compounds p dissolve in water because they react with water. An example is HCl, which reacts in the following way: HCl(g) + H2 O(l) Cl- (aq) + H3 O+ (aq) Hydron ium ion Polar covalent molecules dissolve because they are solvated by hydrogen bonding. Sucrose contains a number of polar OH groups which allow it to dissolve in water  Water as a Solvent How water dissolves molecular compounds: When the nonpolar p part of an organic p g molecule is considerably larger than the polar part, the molecule no longer dissolves in water. F example For l ethanol, th l CH3CH2OH iis soluble l bl in i water t but butanol CH3CH2CH2CH2OH is not  Colloids In true solutions, the maximum diameter of a solute p particle is about 1 nm. Colloid: a solution in which the solute particle diameter is between 1nm and 1000 nm. Colloid particles have very large surface areas, which accounts for these two characteristics of colloidal systems; they scatter light and, therefore, appear turbid, cloudy, or milky. milky they form stable dispersions; that is, they do not settle out. Types yp of Colloids Type of Colloid Example Gas in gas N one Gas in liq uid Whip ped cream Gas in solid Marshmallow s Liq uid in gas Clou ds, fog Liq uid in liq uid Milk, mayonn aise Liq uid in s olid Cheese b utter Cheese, Solid in gas Smoke Solid in liq uid Jelly Solid in s olid D ried paint  Colloids John Tyndall (1820-1893) Tyndall effect: a characteristic of colloids in which light passing through g the colloid is scattered (i.e., reflected off of colloidal particles). Examples of colloids that exhibit the Tyndall effect are smoke, serum, and fog. fog  Colloids Tyndall effect Why y is the sky y blue? Normal sky color Pale blue sky near horizon Colloids Robert Brown (1773-1858) In 1827 the English botanist Robert Brown noticed that pollen grains suspended in water jiggled about under the lens of the microscope, f ll i a zigzag following i path. th Brownian motion: the random motion of colloid-size colloid size particles particles.  Colloids Examples of Brownian motion are the motion of dust particles in the air; what we see are the dust p particles due to scattered light. Joseph Perrin 1908  Colloids Why do colloidal particles remain in solution despite p all the collisions due to Brownian motion? Most colloidal particles carry a large solvation layer; if the solvent is water, as in the case of protein molecules in the blood, the large number of surrounding g water molecules prevents p colloidal molecules from touching and sticking together. Because of their large surface area, colloidal particles acquire i charges h from f solution; l ti for f example,l they th allll may become negatively charged. When a charged colloidal particle encounters another particle of the same charge, they repel each other. Properties p of Mixtures Property S olutions Colloids Susp ensions Partricle size (nm) 0.1 - 1.0 1 - 1000 >1000 Filterable w ith No No Yes ord inaryy p pap p er Homogeneous Yes Borderline No Settles on s tandin g No No Yes Behavior to ligh t Trans paren t Tynd all Translu cent effect or opaq ue  Colligative g Properties p Colligative property: any property of a solution that depends p on the number of solute particles, p , and not on the nature of the particles. We studyy two colligative g properties: freezing-point depression and boiling point elevation osmosis  Freezing- Freezing g-Point Depression p One mole of any particle dissolved in 1000 grams of water lowers the freezing g point p of water by y 1.86°C. The nature of the particles does not matter, only the number of particles. For convenience, we will use Molarity for the number of moles of particles in solution ΔTf = kf M i where: ΔTf = is the number of degrees the freezing point is lowered kf = the freezing point depression constant for the solvent M = the Molarity of the solution i = the number of ions formed from the solute molecule  Boiling- Boiling g-Point Elevation One mole of any particle dissolved in 1000 grams of water raises the boiling g point p of water by y 0.52°C. The equation for boiling point elevation is the same as that for freezing point depression ΔTb = kb M i where: ΔTb = is the number of degrees the boiling point is increased kb = the boiling point elevation constant for the solvent M = the Molarity of the solution i = the number of ions the formed from the solute molecule Freezing- Freezing g-Point Depression p Each solvent has its own freezing point depression and boiling point elevation constants.  Freezing- Freezing g-Point Depression p Depression of freezing point has a number of practical applications: p pp We use NaCl and CaCl2 to melt snow and ice. We use ethylene glycol as antifreeze in automobile radiators. Freezing- Freezing g-Point Depression p Problem: What is the freezing point of a solution made by adding ddi 275 g off ethylene th l glycol, l l C2H6O2, to t 1000 g off water in a car radiator? Kf = 1.86°C/M Solution: Ethylene glycol is a molecular compound; it is a nonelectrolyte (i = 1) MWethylene glycol = 62.0 62 0 g/mol g solute ×1000 mL L Molarity ( M ) = MWsolute × mLsolution 275 g ×1000 mL L Molarity ( M ) = = 4.44 M 62 0 mol ×1000 g 62.0 g Freezing- Freezing g-Point Depression p Solution (continued): ΔTf = kf M i = 1.86°C/M x 4.44 M = 8.26 8 26°CC the freezing point of the solution will be lowered by 8.26°C to -8.26°C (17.2°F). Freezing- Freezing g-Point Depression p Problem: Problem: what will be the freezing point of a solution prepared by dissolving 1.00 mole of K2SO4 in 1000 grams of water? K2SO4 is an ionic solid and dissociates to ions when dissolved in water. water One mole of K2SO4 gives three moles of ions. The freezing g point p is lowered byy 3 x 1.86°C or 5.58°C. The solution will freeze at -5.58°C.  Osmosis Figure 7.14 Osmotic pressure. Osmosis Semipermeable membrane: a membrane with pores that are big enough to allow solvent molecules to pass through them them, but not big enough to allow the passage of larger solute molecules. Osmosis: the movement of solvent particles through a semipermeable i bl membrane b from f a region i off lower l solute concentration (higher solvent concentration) to a region of higher solute concentration (lower solvent concentration). t ti ) Osmotic pressure: the pressure necessary to prevent osmosis. Osmolarity (osmol): the molarity multiplied by the number of particles produced by each formula unit of solute. Osmosis Problem: an 0.89 percent w/v NaCl solution is referred to as physiological saline solution. What is the osmolarity (osmol) of this solution? 0.89 w/v NaCl = 8.9 g in 1.00 L of solution first we calculate the number of moles of NaCl in this solution: 8.9 g NaCl 1.00 mol NaCl 0.15 mol NaCl x = = 0.15 M NaCl 1L 58.5 g NaCl 1.00 L because each mole of NaCl dissolved in water di dissociates i t into i t two t ions, i the th osmolarity l it off the th solution is 0.15 x 2 = 0.30 osmol Osmosis Isotonic solutions: solutions with the same osmolarity. Isotonic solution: a term used primarily in the health sciences to refer to a solution with the same osmolarity as blood plasma and red blood cells. Hypotonic solution: a solution with lower osmolarity than blood plasma and red blood cells. Hemolysis: y the swellingg and burstingg of red blood cells because they cannot resist the increase in osmotic pressure when put into a hypotonic solution. Hypertonic solution: a solution with higher osmolarity than red blood cells. Hypertonic solution Hypotonic solution Dialysis y Dialysis: the separation of larger molecules, dissolved substances, or colloidal particles from smaller molecules,, substances, or colloidal particles by a semipermeable membrane.