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IONS in AQUEOUS SOLUTIONS I. Ionic Salts A. Dissociation: separation of ions that occur when an ionic compound dissolves • refers to ionic salts ONLY • the salt separates in solution (usually water) to form cations and anions • ex. ionic equation: NaCl(aq) ↔ Na+ + Cl- B. Forms electrolytes (ions), which enables the solution to conduct electricity Examples - Dissociation Ex: LiCl (s) Li +(aq) + Cl - (aq) What is the total number of moles of ions produced? _____ Ex. Ca(OH)2 (s) + What is the total number of moles of ions produced? _____ Ex. Al2(SO4)3 (s) + What is the total number of moles of ions produced? _____ II. Covalent Molecules A. Ionization: the formation of ions from molecular solutes by action of the solvent – No ions existed before contact with the solvent – Refers to covalent molecules ONLY – Acids are good examples! Examples - Ionization • Ex: HCl (aq) H+ (aq) + Cl- (aq) • Ex: H3PO4 (aq) + B. Hydronium Ion (H3O+) H2O + HCl ↔ H3O+ + Cl- III. Net Ionic Equations: • Includes only those substances that undergo a chemical change in the reaction in an aqueous solution • Spectator Ions: ions that do not chemically change….not written in the net ionic equation • Ex. For….. • Cd (NO3)2 (aq) + (NH4)2S (aq) → Net Ionic Eq: Spectator Ions: •Ex. AgC2H3O2 (aq) + CaCl2 (aq) → Net Ionic Eq: Spectator Ions: •Ex. NaOH (aq) + Fe(ClO3)3 (aq) → Net Ionic Eq: Spectator Ions: IV. Molecular electrolytes: • Strong Electrolytes: – any solute that is present entirely as hydrated ions – A high degree of ionization or dissociation. All or almost all of the compound forms ions in solution. 100% or close to that of ionization or dissociation. ex. HCl, strong acids, ionic salts Strong acids include: HBr, HCl, HI Strong bases include: Gp IA and IIA hydroxides, except Be • Weak Electrolytes: – a solute that yields a relatively low concentration of ions in aqueous solution…partial ionization – A low degree of ionization or dissociation. ex. “weak” acids Weak acid = HF, acetic acid Weak base = NH4OH The ammeter measures the flow of electrons (current) through the circuit. •If the ammeter measures a current, and the bulb glows, then the solution conducts. •If the ammeter fails to measure a current, and the bulb does not glow, the solution is non-conducting. V. Properties of electrolytic solutions: A. Conductivity: the magnitude of the current depends on the solute ion concentration …Pure water is a poor conductor, but it does conduct some due to the self-ionization of water…making hydronium B. Colligative Properties: influence of electrolytes on the BP and FP …depend on concentration of solute (or ions, if it is an electrolytic solution) VI. Colligative Properties Properties that depend on the concentration of solute particles but not on their identity 4 colligative properties: Osmotic Pressure Vapor Pressure Lowering Freezing Point Depression (our focus) Boiling Point Elevation (our focus) The vapor pressure of a pure solvent is higher than a solution. A. Freezing Point Depression • Adding a solute lowers the FP of the solvent • The “new” FP is in between the solute and solvent FPs • Directly proportional to molal concentration • Kf…molal FP constant • Kf…for water…. -1.86 °C/m B. Boiling Point Elevation • Adding a solute raises the BP of the solvent • Kb…molal BP constant • Kb…for water… 0.51 °C/m C. Math: • ΔT = Kf m (#moles) • ΔT = Kb m(# moles) where K is in °C/m expanded…ΔT = K• molsolute kgsolvent Non-Electrolytic Solutions: Sugars and other hydrocarbons are nonpolar covalent and do not form ions in solution These substances are termed non- electrolytes. Sugars are not salts, acids or bases. Sugars are organic molecular compounds and do not dissociate or ionize. Ex. 1 What is the FPD of a 2.55 m glucose aqueous solution? Ex. 2 What is the concentration (molality) of a glucose solution that has a -3.25ºC FPD? Ex. 3 What is the BPE of a glucose solution with 1.25 mol of glucose dissolved in 0.500 kg of water? Ex. 4 What mass of glucose (C6H12O6) would be needed to cause a FPD of -1.14 ºC in 750. g of water? Ex. 5 What is the molar mass of a non-electrolytic substance if 155 g of the substance dissolved in 0.855 kg of water causes a 2.00 ºC BPE? Electrolytic Solutions: Ex. 6 What is the FPD of a 3.1 m NaCl aqueous solution? What is the new FP? Ex. 7 How many moles of NaCl are needed to lower the FP of 3.6 kg of water by 5.6 ºC? Ex. 8 What mass of KCl is needed to change the BP of 4.5 kg of water to 104.2 ºC?
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