Reactions in Aqueous Solutions Glenn V. Lo Department of Physical Sciences Nicholls State University

The Solution Process What happens when a substance dissolves in a liquid such as water? The substance is called the solute Ions or molecules are dispersed The liquid is called the solvent The ions or molecules of the solute are dispersed; are called solute particles Each solute particle is surrounded by solvent molecules (“solvated”) If solvent is water, “solvation” is also called “hydration”

Test Yourself Which figure below best represents an aqueous solution of CaCl 2.

Electrolytes and Nonelectrolytes Pure water is a very poor electrical conductor Substances that, when dissolved in water, produce ions, lead to solutions that conduct electricity and are called electrolytes Ionic compounds are strong electrolytes Most molecular compounds are nonelectrolytes. Molecular compounds may react with water to produce ions. Acids produce H 3 O + ions Bases produce OH - ions

Acids Generic formula: “HX” Reaction with water: HX(aq) + H 2 O(l)  H 3 O + (aq) + X - (aq) Abbreviation: HX(aq)  H + (aq) + X - (aq) H 3 O + is called “hydronium ion” H + is a “proton.” An acid is a “proton donor” HX is said to be “ionized” in water. X - is the “conjugate base” of HX. Write the chemical equation for ionization of HF in water.

Bases Generic formula: “B” Reaction with water: B(aq) + H 2 O(l)  BH + (aq) + OH - (aq) B has a lone pair that “pulls” H + away from an H 2 O molecule. A base is a “proton acceptor.” B is said to be ionized in water. BH + is the “conjugate acid” of B. Write the chemical equation for reaction of NH 3 with water.

Strength of Acids and Bases Most acids and bases are “weak” At any given time, only a small of the molecules are ionized Evidence: most acids and bases are “weak electrolytes” Exceptions: six acids are completely ionized and are strong electrolytes HCl, HBr, HI, HNO 3, HClO 4, H 2 SO 4

Test Yourself Which figures below best represent aqueous solutions of HF and HCl? A. A for both, B. B for both, C. HF-B, HCl-A, D. HF-A, HCl-B

Chemical Equation for Dissolution in water Molecular Equation (or complete formula equation) Write formula of solute; indicate physical state Draw an arrow Write formula of solute; indicate (aq) Ionic Equation: preferred if solute is a strong electrolyte. If solute is “insoluble,” use a double arrow instead.

Chemical Equation for Dissolution in water Write the chemical equation for the dissolution of the following in water: solid barium chloride, solid glucose (C 6 H 12 O 6 ), solid potassium hydroxide, chalk (calcium carbonate), hydrogen chloride gas, liquid acetic acid, gaseous ammonia

Test Yourself Which of the following best represents the dissolution of CaCl 2 (s) in water? A. CaCl 2 (s)  Ca 2+ (aq) + 2 Cl - (aq) B. CaCl 2 (s)  CaCl 2 (aq) C. CaCl 2 (s)  Ca 2+ (aq) + Cl 2 2- (aq)

Test Yourself Which of the following best represents the dissolution of methanol, CH 3 OH(l), in water? A. CH 3 OH(l)  CH 3 OH(aq) B. CH 3 OH(l)  CH 3 + (aq) + OH - (aq) C. CH 3 OH(l)  CH 3 O - (aq) + H + (aq) D. CH 3 OH(l) + H 2 O(l)  CH 3 OH 2 + (aq) + OH - (aq)

Test Yourself Which of the following best represents the ionization of nitric acid in water? A. HNO 3 (aq)  H(aq) + NO 3 (aq) B. HNO 3 (aq)  H + (aq) + NO 3 - (aq) C. HNO 3 (aq)  H + (aq) + N 3- (aq) + 3 O 2- (aq) D. HNO 3 (aq)  H + (aq) + N 3- (aq) + O 3 - (aq)

Determining solubility Mix substance with water using A sample of the substance that’s about the size of a grain of rice About 1 teaspoon of water (5 mL) Shake well If mixture is cloudy: substance is insoluble If mixture is clear: substance is soluble. NOTE: clear means you can see through it; it doesn’t have to be colorless

Solubility Rules for Common Ionic Compounds Empirical solubility rules (based on experimental data) Varies with textbook depending on what author considers “common” Typical Rules tell us: What’s always soluble? What’s usually soluble and what are the exceptions? Everything else is generally insoluble, with a few exceptions Use the rules in order; stop when you find a rule that applies

Solubility Rules for Common Ionic Compounds Rule 1. What’s always soluble? Compounds containing sodium, potassium, ammonium, nitrate, nitrite, perchorate, acetate Which of the following are soluble based on Rule 1. NaC 2 H 3 O 2,, K 2 CO 3 CaCO 3 MgCl 2 ZnSO 4 PbSO 4 AgBr MgS CuI AgF CuS Ag 3 PO 4

Solubility Rules for Common Ionic Compounds Rule 2. If Rule 1 does not apply, what’s generally soluble? Chloride, bromide, iodide except with Ag +, Pb 2+, Hg 2 2+, Cu + Sulfate, except with Pb 2+, Sr 2+, Ba 2+, Ca 2+, Hg 2 2+ Fluoride, except with Pb 2+, Sr 2+, Ba 2+, Ca 2+, Mg 2+ For which of these is Rule 2 applicable? Classify the following as soluble or insoluble based on Rule 2. NaC 2 H 3 O 2,, K 2 CO 3 CaCO 3 MgCl 2 ZnSO 4 PbSO 4 AgBr MgS CuI AgF CuS Ag 3 PO 4

Solubility Rules for Common Ionic Compounds Rule 3. If Rules 1 and 2 do not apply, probably insoluble. Exceptions: sulfides of Group IIA are soluble, hydroxides of strontium, barium, and calcium are moderately soluble. For which of these is Rule 3 applicable? Classify the following as soluble or insoluble based on Rule 3. NaC 2 H 3 O 2,, K 2 CO 3 CaCO 3 MgCl 2 ZnSO 4 PbSO 4 AgBr MgS CuI AgF CuS Ag 3 PO 4

Test Yourself Which of the following is insoluble in water? A. Na 2 CO 3, B. MgSO 4, C. PbCl 2, D. CaS

Test Yourself Which of the following is insoluble in water? A. CuCl 2, B. Hg 2 Cl 2, C. HgBr 2, D. Ca(NO 3 ) 2

Test Yourself Which of the following is soluble in water? A. MgCO 3, B MgF 2, C. Mg(OH) 2, D. MgSO 4

Precipitation Reaction Occurs when solutions containing ions are mixed and it is possible for ions to combine to form an insoluble compound. Write the precipitation reaction that occurs when the following are mixed: Pb(NO 3 ) 2 (aq) and KI(aq)

Test Yourself Precipitation is expected from which mixture? A. Na 2 CO 3 (aq) + KNO 3 (aq), B. AgNO 3 (aq) + CaCl 2 (aq), C. Neither, D. Both

Test Yourself A student is given two test tubes and is told that one contains NaCl(aq), while the other test tube contain Ba(NO 3 ) 2 (aq). Which precipitating reagent, if added to both test tubes would allow the student to determine their contents? A. AgNO 3 (aq), B. K 2 SO 4 (aq), C. neither, D. both

Ionic Equations Chemical equation that shows only complete formulas of compounds involved in a reaction is a “molecular equation” An ionic equation is a better representation of the reaction when ions are involved. Write strong electrolytes in aqueous solutions as separate ions A net ionic equation is even better if some ions are not actually involved Omit ions that are not actually involved in the reaction (“spectator ions”)

Example Write ionic equation and net ionic equation for the “molecular equation” Pb(NO 3 ) KI  PbI 2 (s) + 2 KNO 3 (aq)

Example Write the ionic and net ionic equation for the dissolution of sodium oxide in water. The molecular equation is: Na 2 O(s) + H 2 O(l)  2 NaOH(aq)

Test Yourself What is the net ionic equation for the precipitation reaction that occurs when K 2 SO 4 (aq) and Pb(NO 3 ) 2 are mixed? A. K + (aq) + NO 3 - (aq)  KNO 3 (s) B. Pb + (aq) + SO 4 - (aq)  PbSO 4 (s) C. Pb 2+ (aq) + SO 4 2- (aq)  PbSO 4 (s)

Test Yourself What is the net ionic equation for the precipitation reaction that occurs when Na 2 CO 3 (aq) and AgNO 3 (aq) are mixed? A. Na + (aq) + NO 3 - (aq)  NaNO 3 (s) B. Ag + (aq) + CO 3 2- (aq)  Ag 2 CO 3 (s) C. Ag 2+ (aq) + CO 3 2- (aq)  AgCO 3 (s) D. 2 Ag + (aq) + CO 3 2- (aq)  Ag 2 CO 3 (s)

Reactions of Acids Common feature: H + from acid reacts with anion from the other reactant, or molecule with a lone pair (typically on a N atom) H + from acid + oxide or hydroxide ion  H 2 O H + from acid + anion  weak acid if acid is strong, no reaction with H + If weak acid is H 2 CO 3 : H 2 CO 3  CO 2 (g) + H 2 O(l) If weak acid is H 2 SO 3 : H 2 SO 3  SO 2 (g) + H 2 O(l) Anion from acid may react with cation from other reactant to yield a precipitate.

Reactions of Acids Acid + metal oxide or hydroxide  salt + water (H + from acid) plus (O 2- or OH - ) yields H 2 O. Write net ionic equation for: HCl(aq) + NaOH(aq) Ba(OH) 2 (aq) + H 2 SO 4 (aq) Ca(OH) 2 (aq) + HC 2 H 3 O 2 (aq) Ca(OH) 2 (s) + HCl(aq)

Reactions of Acids Acid + N-containing molecule  protonated molecule Write the net ionic equation for: HCl(aq) + NH 3 (aq)

Reactions of Acids Acid + salt  ??? (H + from acid) plus (anion) yields weak acid. If acid is not weak, no reaction with H +. Write net ionic equation for: NaHCO 3 (s) + HC 2 H 3 O 2 (aq) HNO 3 (aq) + K 2 S(aq) H 2 SO 4 (aq) + MgSO 3 (aq) HCl(aq) + BaSO 4 (aq)

Test Yourself For which of the following mixtures is the net ionic equation for neutralization given by: H + (aq) + OH - (aq)  H 2 O(l)? A. HBr(aq) + KOH(aq), B. HCl(aq) + NaOH(s), C. HC 2 H 3 O 2 (aq) + Ba(OH) 2 (aq), D. HI(aq) + NH 3 (aq)

Test Yourself What is the net ionic equation for the reaction between HF(aq) and Ca(OH) 2 (aq)? A. 2 HF(aq) + Ca(OH) 2 (aq)  2 H 2 O(l) + CaF 2 (s) B. 2 H + (aq) + 2 F - (aq) + Ca 2+ (aq) + 2 OH - (aq)  2 H 2 O(l) + 2F - (aq) + Ca 2+ (aq) C. 2 HF(aq) + Ca 2+ (aq) + 2 OH - (aq)  2 H 2 O(l) + 2F - (aq) + Ca 2+ (aq) D. 2 HF(aq) + Ca 2+ (aq) + 2 OH - (aq)  2 H 2 O(l) + CaF 2 (s)

Test Yourself What is the net ionic equation for the reaction between Mg(OH) 2 in milk of magnesia with HCl (in stomach acid)? The complete formula equation is: Mg(OH) HCl  MgCl H 2 O A. Mg OH H Cl -  Mg Cl H 2 O B. Mg(OH) H Cl -  Mg Cl H 2 O C. H + + OH -  H 2 O D. Mg(OH) H +  Mg H 2 O

Test Yourself What is the net ionic equation for the reaction between HNO 3 (aq) and NH 3 (aq)? A. HNO 3 (aq) + NH 3 (aq)  NH 4 NO 3 (aq) B. H + (aq) + NO 3 - (aq) + NH 3 (aq)  NH 4 + (aq) + NO 3 - (aq) C. H + (aq) + NH 3 (aq)  NH 4 + (aq)

Test Yourself Which of the following appears in the net ionic equation for the reaction of HNO 2 (aq) and Na 2 CO 3 (aq)? A. HNO 2 (aq), B. H + (aq), C. Na + (aq)

Test Yourself Which of the following does not appear in the net ionic equation for the reaction of Ca 3 (PO 4 ) 2 (s) and HCl(aq)? A. Ca 2+ (aq), B. PO 4 3- (aq), C. H + (aq), D. H 3 PO 4 (aq)

Test Yourself Which of the following does not appear in the net ionic equation for the reaction BaCO 3 (s) and H 2 SO 4 (aq)? A. Ba 2+ (aq), B. CO 2 (g), C. H 2 O(l), D. BaSO 4 (s)

Hydrolysis of Salts Solutions can be classified as acidic, basic, or neutral depending on the pH pH = 7 (neutral): equal concentration of H + and OH - pH < 7 (acidic): higher H + concentration pH > 7 (basic ): higher OH - concentration How to tell? Use a pH meter or pH paper Salt solutions may be acidic or basic due to hydrolysis. Cations of salts may react with water to produce H 3 O + Anions of salts may react with water to produce OH -

Hydrolysis of Cations Cations can be proton donors; weak Explain why why NH 4 Cl(aq) and ZnSO 4 (aq) are acidic.

Hydrolysis of Anions Conjugate bases of weak acids are weak bases (proton acceptors) Explain why NaF is a weak base.