CHM1045 Exam 2 Study Guide Chapter 4 Reactions in Aqueous Solutions Solutions Solutions Homogeneous mixtures Solute The component of the solution that gets dissolved Solvent The component that dissolves the solute To describe solutions accurately we must describe how much of each component is present Concentration Quantitatively the relative amount of solute in the solution Ex Table salt mixed with water seems to disappear homogeneous If both components start in the same state the major component is the solvent Dilute Solutions Have a small amount of solute compared to solvent Concentrated Solutions Have a large amount of solute compared to solvent Solution Concentration Molarity Molarity Moles of solute per 1 liter of solution Used because it describes how many molecules of solute in each liter of solution molarity M amount amount of of solute in solution moles in L Preparing 1 L of a 1 00 M NaCl Solution 1 CHM1045 Exam 2 Study Guide o o o o o o Practice Problem 4 1 Find the molarity of a solution that has 3 21 g of magnesium iodide dissolved in 180 mL of solution 0 0641 M Most solutions are generally between 0 and 18 M Using Molarity in Calculations Molarity shows the relationship between moles of solute and liters of solution If a sugar solution concentration is 2 0 M then 1 liter of solution contains 2 0 moles of sugar 2 liters 4 0 moles sugar 0 5 liters 1 mole sugar 2 mol sugar 1 L solution 1 L solution 2 mole sugar 1 L solution 2 mole sugar Practice Problem 4 2 unknown 0 38 L Dilution o How many liters of a 0 4 M unknown solution contains 0 15 moles of the Solutions are often stored as concentrated stock solutions To make solutions of lower concentrations from these stock solutions more solvent is added o The amount of solute doesn t change just the volume of solution 2 CHM1045 Exam 2 Study Guide Moles solute in solution 1 moles solute in solution 2 The concentrations and volumes of the stock and new solutions are inversely proportional o A student needs to dilute 100mL of a concentrated 6 0 M LiOH aqueous solution to obtain a 1 5 M solution How much water does he have to add o M1 V1 M2 V2 Practice Problem 4 3 Solution Stoichiometry 0 3 L Since molarity relates the moles of solute to the liters of solution it can be used to convert between amount of reactants and or products in a chemical reaction Practice Problem 4 4 o What volume of 0 150 M Li2S is required to completely react with 125 mL of 0 150 M Co NO3 2 Li2S aq Co NO3 2 aq LiNO3 aq CoS aq Electrolytes and Nonelectrolytes 0 125 L Electrolytes Materials that dissolve in water to form a solution that will conduct electricity electricity Nonelectrolytes Materials that dissolve in water to form a solution that will not conduct Molecular View of Electrolytes and Nonelectrolytes In order to conduct electricity a material must have charged particles ions or electrons that are able to flow Electrolyte solutions all contain ions dissolved in water o Ionic compounds are mostly electrolytes because they tend to dissociate into their ions when they dissolve Nonelectrolyte solutions contain whole molecules dissolved in the water o Generally molecular compounds do not ionize when they dissolve in water o The notable exception molecular acids General rule ionic compounds dissolve to give electrolyte solutions and molecular compounds except acids give nonelectrolytes Ex NaOH electrolyte Ex C2H5OH nonelectrolyte 3 CHM1045 Exam 2 Study Guide When it enters the solution the ion is surrounded by water molecules insulating it from other ions Result is a solution with free moving charged particles able to conduct electricity Strong and Weak Electrolytes Strong Electrolytes Materials that dissolve completely as ions Weak Electrolytes Materials that dissolve mostly as molecules but partially as ions o Most ionic compounds and strong acids Solutions conduct electricity as well o Weak acids Solutions conduct electricity but not well o o When compounds containing a polyatomic ion dissolve the polyatomic ion stays together 2 aq o Na2SO4 aq 2 Na aq SO4 o HC2H3O2 aq H aq C2H3O2 aq Acids Acids ionize when they dissolve in water o The molecules are pulled apart by their attraction for the water o When acids ionize they form H cations and anions The percentage of molecules that ionize varies from one acid to another Strong Acids Acids that ionize virtually 100 o HCl aq H aq Cl aq Strong acids strong bases are strong electrolytes o Weak Acids Acids that only ionize a small percentage o HF aq H aq F aq o Weak acids weak bases are weak electrolytes 4 CHM1045 Exam 2 Study Guide Solubility of Ionic Compounds Some ionic compounds dissolve very well in water at room temperature Some ionic compounds dissolve hardly at all in water at room temperature o Ex NaCl o Ex AgCl Soluble Compounds Compounds that dissolve in a solvent Insoluble Compounds Compounds that don t dissolve in a solvent o NaCl soluble in water o AgCl insoluble in water The degree of solubility depends on the temperature Even insoluble compounds dissolve just not enough to be meaningful Solubility a spectrum lots of gray areas Solubility Rules Compounds that are Generally Soluble in Water Compounds Containing the Following Ions are Generally Soluble Li Na K NH4 C2H3O2 NO3 Cl Br I 2 SO4 Exceptions When combined with the ions on the left the compound is insoluble None None Ag Hg2 2 Pb2 Ag Ca2 Sr2 Ba2 Pb2 Compounds that are Generally Insoluble 5 CHM1045 Exam 2 Study Guide Compounds Containing the Following Ions are Generally Insoluble OH S2 CO3 2 PO4 3 CrO4 2 Exceptions When combined with the ions on the left the compound is soluble or slightly soluble Li Na K NH4 Ca2 Sr2 Ba2 Li Na K NH4 Li Na K NH4 Ca2 Sr2 Ba2 Practice Problem 4 5 o Strong weak or non electrolyte NaI Strong electrolyte Mg OH 2 Nonelectrolyte PbS Nonelectrolyte Li2S Strong electrolyte HNO2 Weak electrolyte Precipitation Reactions Precipitation Reactions Reactions between aqueous solutions of ionic compounds that produce an ionic compound that is insoluble in water Precipitate Insoluble product of a precipitation reaction 2 KI aq Pb NO3 2 aq PbI2 s 2 KNO3 aq No precipitate formation no reaction o KI aq NaCl aq KCl aq NaI aq All ions still present no reaction Predicting the Products of a Precipitation Reaction 1 Determine what ions each aqueous reactant has 2 Determine the formulas of possible products a Exchange ions i ion from one reactant with ion from other b Balance charges of combined ions to get formula of each product 3
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