Chemical Change. Section 9.1. Chapter 9. Electrolytes and Solution Conductivity. Goal 1. Electrical Conductivity

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Simon Gordon
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strong electrolytes, weak electrolytes, and nonelectrolytes.

1 Chapter 9 Chemical Change Section 9.1 Electrolytes and Solution Conductivity Goal 1 Electrical Conductivity Distinguish among strong electrolytes, weak electrolytes, and nonelectrolytes. Strong Electrolyte: A substance that, when dissolved in water, conducts electricity strongly Weak Electrolyte: A substance that, when dissolved in water, conducts electricity, but does so poorly 1

2 Solutions of Ionic Compounds Goal 2 When an ionic compound dissolves in water, its solution consists of water molecules and ions The

2 Nonelectrolyte: A substance that, when dissolved in water, does not conduct electricity Conducting solutions contain ions The extent of conductivity is related to the concentration of ions in the solution Section 9.2 Solutions of Ionic Compounds Goal 2 When an ionic compound dissolves in water, its solution consists of water molecules and ions The ions are identified by separating the compound into its ions Given the formula of an ionic compound (or its name), write the formula of the ions present when it is dissolved in water. NaCl(s) Na + + Cl 2

3 NaCl(s) Na + + Cl Polyatomic ions are treated in the same way in writing the formulas of ions in solution K 2 SO 4 (s) 2 K + + SO 4 2 Goal 3 Section 9.3 Strong and Weak Acids Explain why the solution of an acid may be a good conductor or a poor conductor of electricity. Goal 4 Acid: Hydrogen-bearing molecular compound made up of nonmetal elements that reacts with water to form a hydrated hydrogen ion and an anion O + HX > H 3 O + + X Given the formula of a soluble acid (or its name), write the major and minor species present when it is dissolved in water. 3

Acid solutions can be strong or weak electrolytes, depending on the extent to which

+ St Major species: ions Minor species: molecules Weak acid: Ionizes very slightly

the macroscopic and particulate levels Seven Common Strong Acids Two Well- Known

4 Acid solutions can be strong or weak electrolytes, depending on the extent to which the acid molecule ionizes in solution Strong acid: Ionizes almost completely HSt H + + St Major species: ions Minor species: molecules Weak acid: Ionizes very slightly HWe H + + We Major species: molecules Minor species: ions Strong and weak acids at the macroscopic and particulate levels Seven Common Strong Acids Two Well- Known Acids Three Group 7A/17 Acids Two Chlorine Oxyacids HNO 3 SO 4 HCl HBr HI HClO 3 HClO 4 4

5 Identifying the Major Species in a Solution Ions are the major species in the solutions of two kinds of substances: All soluble ionic compounds The seven strong acids Neutral molecules are the major species in solutions of everything else, primarily: Weak acids Weak bases Water Section 9.4 Net Ionic Equations: What They Are and How to Write Them Consider the reaction of solutions of silver nitrate and sodium chloride What is the particulate-level composition of each solution before the reaction occurs? Silver nitrate solution: Silver ions, Ag + Nitrate ions, NO 3 Water molecules How are these represented in a conventional equation? AgNO 3 + NaCl The problem with this conventional representation is that it disguises the true composition of the solutions Sodium chloride solution: Sodium ions, Na + Chloride ions, Cl Water molecules To avoid this misrepresentation, we can write the formulas of the reactants as they actually occur in solution Ag + + NO 3 + Na + + Cl We can perform a similar analysis with the products of the reaction Silver chloride precipitate: Solid silver chloride, AgCl(s) Sodium nitrate solution: Sodium ions, Na + Nitrate ions, NO 3 Water molecules 5

6 Conventional representation of products: AgCl(s) + NaNO 3 Products as they actually occur: AgCl(s) + Na + + NO 3 Combining reactants and products: AgNO 3 + NaCl AgCl(s) + NaNO 3 This is the conventional equation It is good for stoichiometry problems but lacks in revealing what really happens in solution Writing actual reactants and products: Ag + + NO 3 + Na + + Cl Note that some species appear as both reactant and product in the total ionic equation: Ag + + NO 3 + Na + + Cl AgCl(s) + Na + + NO 3 AgCl(s) + Na + + NO 3 This is the total ionic equation It represents each species as it actually occurs in solution NO 3 is both reactant and product; Na + is both reactant and product Neither ion undergoes a chemical change If we eliminate these non-reacting spectator ions, Ag + + NO 3 + Na + + Cl AgCl(s) + Na + + NO 3 What remains is the net ionic equation: Ag + + Cl AgCl(s) A net ionic equation describes what happens in a chemical change Aqueous silver ion reacts with aqueous chloride ion to yield solid silver chloride A net ionic equation ignores non-reacting species: Sodium ion Nitrate ion Water molecules 6

Procedure: How to write a net ionic equation 1. Write the conventional equation. Include state symbols. 2. Write the total ionic equation. Solids, liquids, and gases do not ionize.

7 Procedure: How to write a net ionic equation 1. Write the conventional equation. Include state symbols. 2. Write the total ionic equation. Solids, liquids, and gases do not ionize. Aqueous compounds may ionize: A. Ionic compounds and strong acids ionize B. Weak acids do not ionize Section 9.7 Double-Replacement Precipitation Reactions 3. Write the net ionic equation. Eliminate the spectators. Goal 7 Goal 8 Predict whether a precipitate will form when known solutions are combined; if a precipitate forms, write the net ionic equation. (Reference to a solubility table or solubility rules may or may not be allowed.) Given the product of a precipitation reaction, write the net ionic equation. Double-Replacement Precipitation Reactions Reactants: Reaction type: Equation type: Products: Solutions of two ionic compounds or a solution of an ionic compound and an acid solution Double-replacement precipitation AX + BY AY + BX Precipitate of ionic compound and second ionic compound or acid 7

8 Example: Calcium nitrate and potassium sulfate solutions are combined. Write the net ionic equation. Ca(NO 3 ) 2 + K 2 SO 4 CaSO 4 ( ) + KNO 3 ( ) 2 Conventional equation: Ca(NO 3 ) 2 + K 2 SO 4 CaSO 4 (s) + Total ionic equation: Ca NO K + + SO 4 2 2KNO 3 CaSO 4 (s) + 2 K NO 3 Net ionic equation: Ca 2+ + SO 4 2 CaSO 4 (s) Section 9.8 Double-Replacement Molecule-Formation Reactions Goal 9 Given reactants that yield a molecular product, write the net ionic equation. Ion Combinations That Form Molecules Double-Replacement Neutralization Reactions The reaction of an acid often leads to an ion combination that yields a molecular product, typically water or a weak acid Reactants: Reaction type: Equation type: Products: Acid (HX) and a hydroxide base (MOH) Double-replacement neutralization HX + MOH O + HX Water and a salt (MX) (an ionic compound) 8

9 Example: Write the net ionic equation for the reaction of solutions of hydrobromic acid and sodium benzoate (benzoate ion, C 7 ). Conventional equation: HBr + NaC 7 HC 7 + NaBr HC 7 is a weak acid NaBr: Group 1A salts are soluble Conventional equation: HBr + NaC 7 HC 7 + NaBr Total ionic equation: H + + Br + Na + + C 7 O 2 HC 7 + Na + + Br Net ionic equation: H + + C 7 O 2 HC 7 Section 9.9 Double-Replacement Reactions That Form Unstable Products Goal 10 Given reactants that form CO 3, SO 3, or NH 4 OH by ion combination, write the net ionic equation for the reaction. Three common unstable products: CO 3 O(l) + C (g) Example: Write the net ionic equation for the reaction that occurs when hydrochloric acid and sodium sulfite solutions are combined. SO 3 NH 4 OH O(l) + S NH 3 + O(l) Conventional equation: 2 HCl + Na 2 SO 3 SO 3 ( ) + 2NaCl( aq) O(l) + S Group 1A salts are soluble 9

10 Conventional equation: 2 HCl + Na 2 SO 3 O(l) + S + 2 NaCl Total ionic equation: 2 H Cl + 2 Na + + SO 3 2 O(l) + S + 2 Na Cl Net ionic equation: 2 H + + SO 3 2 O(l) + S 10

Session 8: LECTURE OUTLINE (SECTIONS I1 I4 pp F61 F67)

Session 8: LECTURE OUTLINE (SECTIONS I1 I4 pp F61 F67) I. Elecrolytes a. Soluble substances b. Insoluble substances c. Electrolytes d. Non-Electrolytes e. Ions and electrical conductivity f. Strong and