SOLUTIONS Properties of Solutions (electrolytes) Chapter 9.1 Concentration (Section 9.5, 13.3) · · · · · molarity mass % mole fraction ppm molality M % Xi m Solution process (energy changes) Solubility • Saturation • supersaturation Factors that Affect Solubility • temperature • intermolecular interactions • pressure Mary J. Bojan Chem 110 1 SOLUTIONS: start with definitions Homogeneous mixture: is uniformly mixed on a molecular level. Solute: the component of a mixture that is present in smaller quantity. Solvent: the component of a mixture that is present in greatest quantity. Aqueous solutions: water is the solvent Mary J. Bojan Chem 110 2 What is an electrolyte? DEMO to see which solutions can conduct electricity · For bulb to light, some minimum amount of current is needed. · amount of current is related to the CONDUCTIVITY of the solution. CONDUCTIVITY: ability to conduct electricity Which Bulbs Light Up? Bulb Wattage 7.5 25 40 Distilled H2O Tap H2O NaCl(aq) 0.1M HCl(aq) 0.1M CH3COOH sugar(aq) CH3OH Mary J. Bojan Chem 110 3 Why do electrolytes conduct electricity? · Strong electrolytes – Good conductors NaCl(s) + H2O ! Na+(aq) + Cl!(aq) + H2O HCl(aq) + H2O ! H3O +(aq) + Cl!(aq) · Weak electrolytes – Poor conductors NH3(aq) + H2O ! NH4+(aq) + OH!(aq) CH3COOH(aq)+H2O ! CH3COO!(aq)+H3O+(aq) · Nonelectrolytes – Non-conductors (do not conduct electricity) (glucose) C6H12O6(s)+H2O ! C6H12O6(aq)+ H2O Mary J. Bojan Chem 110 4 Electrolytes are an acid, a base or salt. Mary J. Bojan Chem 110 5 Electrolytes are an acid, a base or salt. A. Is the compound ionic or molecular? Ionic: Note: Molecular ??? B. If it is molecular, is it an acid or a base? Yes: No: Examples of electrolytes (strong or weak) Acids donate H+(aq) Bases accept H+(aq) Salts formed by replacing one or more H+ of an acid with another cation Examples of non-electrolytes: Mary J. Bojan Alcohols Carbohydrates (sugar) Aldehydes, ketones Chem 110 6 FYI: Ionic vs. Molecular Ionic compounds: compound that consist of + and ! ions Eg. Metal + nonmetal (usually) NaCl K2SO4 Mg(NO3)2 NH4Cl Ionic compounds have high melting points and are brittle. Molecular compounds: compounds consisting of individual molecules All nonmetals or nonmetals and metalloids. Eg. HCl H 2O CH3COOH NH3 Molecular compounds usually have low melting points Mary J. Bojan Chem 110 7 How do you identify strong and weak electrolytes? • Assume all salts are strong electrolytes. • Memorize strong acids (Table 9.1) and bases • If a compound is an acid or a base, but NOT one of the strong acids or bases, then it MUST be a weak electrolyte. Common misconception: Only ionic compounds are electrolytes: this is NOT CORRECT!!! The TRUTH: · Strong electrolytes are ionic or molecular compounds that ionize completely in solution. · Weak electrolytes are molecular compounds that are partially ionized in solution. • Nonelectrolytes are molecular compounds that do not ionize in solution Mary J. Bojan Chem 110 8 Expressing Concentration mass fraction = mass of component total mass weight % = mass fraction "100% mole fraction = Xi = moles of component total moles parts per million (ppm) = Xi " 106 = mass fraction "106 parts per billion (ppb) = Xi " 109 parts per trillion (ppt) = Xi " 1012 molarity M = moles of solute liters of solution Mary J. Bojan molality m = moles of solute mass (kg) of solvent Chem 110 9 When a solution is diluted, the number of moles remains constant. DILUTION Making a weaker solution by adding solvent. M = concentration (molarity) of solution & moles of solute # M = molarity = $$ !! % vol.of solution (L) " V = volume of solution (L) Number of moles of solute remains the same or: Moles of solute = M V SO# Moles of solute (initial) = moles of solute (final = dilute) Minit Vinit = Mfinal Vfinal Mary J. Bojan Chem 110 10 Solution process means solute molecules or ions separate and are surrounded by solvent Mary J. Bojan Chem 110 11 Focus on the energy associated with the solution process 1. break solute-solute interactions 2. break solventsolvent interactions ! COSTS ENERGY 3. Form solute-solvent interactions ! ENERGY IS RELEASED Mary J. Bojan Chem 110 12 The enthalpy change of the overall process depends on !H for each of these steps. Demonstrate the energy changes in solution CuSO4(s) NH4NO3(s) Mary J. Bojan Chem 110 13 Why Do Endothermic Processes Occur? NH4NO3(s) Things do not tend to occur spontaneously unless" Mary J. Bojan Chem 110 14 Enthalpy is only part of the picture Processes occur spontaneously when: CCl4 energy is released (exothermic) C6H14 disorder increases (increase in entropy) CCl4 + C6H14 Mary J. Bojan Chem 110 15 Solution process involves dissolution and crystallization of solute by the solvent. When rates are equal DYNAMIC EQUILIBRIUM Saturated solution: Supersaturated solution: Mary J. Bojan Chem 110 16 SOLUBILITY: FACTORS THAT AFFECT SOLUBILITY Temperature – Effect of Temperature on solubility of ionic solids – Effect of temperature on solubility of gases Type of intermolecular interactions and their strengths (LIKE DISSOLVES LIKE) Pressure Mary J. Bojan Chem 110 17 How does solubility of ionic solids change with temperature? Mary J. Bojan Chem 110 18 How does solubility of gases change with temperature? Mary J. Bojan Chem 110 19 How do intermolecular forces affect solubility? In general: like dissolves like – polar solvents dissolve polar (and ionic solutes) – nonpolar solvents dissolve non-polar solutes If intermolecular forces broken and formed are ~ the same, dissolution is probable. Which one of the following will be most soluble in benzene (C6H6(! ))? 1. 2. 3. 4. 5. Mary J. Bojan H2O(!) CH3OH(!) HCl(!) CH3CH2OH(!) heptane Chem 110 20 Like dissolves like As length of hydrocarbon chain increases, solubility of alcohol in water decreases. Alcohol solubility in water at 25°C (g/100g of H2O) CH3OH total CH3CH2OH total CH3CH2CH2OH total CH3CH2CH2CH2OH 8.06 CH3CH2CH2CH2CH2OH 2.82 CH3CH2CH2CH2CH2CH2OH 0.62 Mary J. Bojan Chem 110 21 Put these in order of increasing solubility in water. Mary J. Bojan Chem 110 22 Demonstration: Like dissolves like I2(s) purple CuSO4(s) blue H2O(!) heptane(!) Which solid is likely to dissolve in H2O(!) heptane(!) Is water soluble in heptane? (like dissolves like?) Mary J. Bojan Chem 110 23 Chem 110 24 Which vitamin is soluble in fat? Which vitamin is water soluble? Fat soluble vitamins can be stored in the body Water soluble vitamins are not stored in appreciable amounts Mary J. Bojan How does solubility of gases change with pressure? Increase Psolute(gas); increase solubility Decrease Psolute(gas); decrease solubility HENRY S Law C g = k Pg solubility partial pressure Cg Pg amount of gas dissolved (Cg) is proportional to the partial pressure of that gas (Pg) above the solution. k = Henry s Law constant Mary J. Bojan Chem 110 25 Deep Sea Diving Solubility increases with increasing P # amount of N2 in blood increases as depth increases. As divers rise, N2 bubbles out of blood causing painful bends . Solution: use He/O2 mixture when diving deep. He is not as soluble in blood. ! Mary J. Bojan Chem 110 26 Ammonia Fountain H2O + phenolphthalein (pink in base) Mary J. Bojan Chem 110 27 Test yourself To increase the solubility of N2 in water 1. 2. 3. 4. increase T decrease T increase P decrease P How will the solubility of KClO3 be effected by an increase in pressure? 1. increase solubility 2. decrease solubility 3. no effect on solubility Mary J. Bojan Chem 110 28