Equilibrium Constant Determination General Solubilities of Common Compounds in Water Guidelines for Identifying Electrolytes and Non-electrolytes Guidelines for Writing Net Ionic Equations Ionization Constants for Acids and Bases Problem Set Handouts

Aqueous Equilibria Electrolytic Character Edward A. Mottel Department of Chemistry Rose-Hulman Institute of Technology

7/15/2015 Electrolytes Reading Assignment: Chang: Chapter 14.5; Lab manual Exp. F The degree that a compound dissociates into ions is a measure of its strength, not its concentration. Strong electrolytes - dissociate virtually 100% into ions, at all concentrations Weak electrolytes - dissociate less than 50% into ions Non-electrolytes - do not form ions when dissolved in a solvent.

7/15/2015 Strong Electrolytes Strong Acids HCl, HBr, HI, HNO 3, H 2 SO 4, HClO 4, etc. Strong Bases Group IA hydroxides: NaOH, KOH Ca(OH) 2, Sr(OH) 2, Ba(OH) 2 Most ionic compounds

7/15/2015 Strong Electrolytes Soluble Salts Binary compounds of metal cations with nonmetal anions Compounds of metal cations with polyatomic anions Ammonium cation with nonmetal or polyatomic anions. Which of these are soluble, strong electrolytes? KCl, Zn(NO 3 ) 2, CuSO 4, NH 4 I

7/15/2015 Strong Electrolytes Slightly soluble salts Compounds which have a solubility product constant (K sp ) are strong electrolytes, but are not very soluble. AgCl, BaSO 4, Mg(OH) 2, etc. This indicates that AgCl isn’t very soluble. (sparingly soluble) AgCl (s)AgCl (aq)Ag + (aq) + Cl – (aq) This indicates that the amount of AgCl which dissolves ionizes extensively. (strong electrolyte)

7/15/2015 Weak Electrolytes Weak acids, weak bases, and other soluble compounds which do not ionize extensively. A weak electrolyte will usually have a dissociation constant associated with it such as K a or K b.

7/15/2015 Weak Electrolytes Weak Acids HCOOH, CH 3 COOH, HF, HCN, HNO 2, H 3 PO 4, H 2 S, HS –, H 2 CO 3, HCO 3 –, etc. Weak Bases NH 3, CH 3 NH 2, N 2 H 4, etc.

7/15/2015 Non-electrolytes  Compounds which dissolve but do not form a significant number of ions.  Most covalent compounds: compounds of two or more nonmetals excluding those which are strong or weak electrolytes. water, sugar, methanol, ethylene glycol, etc.

7/15/2015 Solubility is related to the concentration of the solute in the solvent Dissociation is related to the ability of the compound to form ions in solution Categorize each of the following compounds in terms of its solubility and tendency to form ions. oil NaCl sugar acetic acid Ca(OH) 2

7/15/2015 Electrolytic Character  Solubility Examples Oil -- not very soluble, doesn’t ionize NaCl -- very soluble and very conductive Sugar -- very soluble but non-conducting Acetic acid -- very soluble but poorly conducting Calcium hydroxide -- slightly soluble but the amount that dissolves, completely ionizes in water

7/15/2015 Solubility is related to the concentration of the solute in the solvent Dissociation is related to the ability of the compound to form ions in solution soluble slightly soluble strong weak non- electrolyte oil NaClCa(OH) 2 sugar acetic acidCO 2 The solubility of a compound does not predict its electrolytic character.

7/15/2015 Equation Notation  Each compound in solution is considered in its “predominant” form with proper annotation: HCl(aq)H + (aq) + Cl – (aq) KOH (aq)K + (aq) + OH – (aq) Strong Electrolytes as completely dissociated.

7/15/2015 Equation Notation  Each compound in solution is considered in its “predominant” form with proper annotation: Weak Electrolytes and Non-electrolytes as molecular compounds HF(aq)H + (aq) + F – (aq) CH 3 OH (aq)H + (aq) + CH 3 O – (aq)

7/15/2015 Group Activity  Identify the predominant form and write a dissociation reaction for the following compounds nitric acid hydrofluoric acid potassium chlorate aqueous ammonia sodium hydroxide

7/15/2015 Equilibrium Arrows HNO 3 (aq)H + (aq) + NO 3 – (aq) HF(aq)H + (aq) + F – (aq) KClO 3 (aq)K + (aq) + ClO 3 – (aq) NH 3 (aq) + H 2 O (l)NH 4 + (aq) + OH – (aq) NaOH (aq)Na + (aq) + OH – (aq)

Buff Colored Sheet Competency Assignment Complete Experiment F in the laboratory manual

7/15/2015 Ionization Constants  Acid dissociation constants, K a The larger the constant, the stronger the acid.  Base dissociation constants, K b The larger the constant, the stronger the base.  Function of temperature

7/15/2015 Acid Strength acetic acid phosphoric acid nitrous acid Rank the following compounds in terms of acid strength (strongest to weakest).

7/15/2015 Acid Strength H 3 PO x 10 – 3 Rank the following compounds in terms of acid strength (strongest to weakest). HNO x 10 – 4 CH 3 COOH1.8 x 10 – 5

7/15/2015 Numeric Values of Ionization Constants  Describe this reaction in a sentence. F – (aq) + H + (aq)HF(aq)  How is the equation listed in the Ionization Constants of Aqueous Acids table?  What is the numerical value of the reaction?  Is the arrow notation correct?

7/15/2015 Numeric Values of Ionization Constants F – (aq) + H + (aq)HF(aq) H + (aq) + F – (aq) K a = 7.2 x 10 –4 M K = 1 / 7.2 x 10 –4 M –1 = 1.4 x 10 3 M –1

7/15/2015 Numeric Values of Equilibrium Constants  For carbonic acid write the acid dissociation reaction. determine the mass-action expression. find the numeric value of the equilibrium constant. confirm the equilibrium arrow assignment.

7/15/2015 Carbonic Acid H 2 CO 3 (aq)H + (aq) + HCO 3 – (aq) [H + ] [HCO 3 – ] [H 2 CO 3 ] = K a,1 = 4.2 x 10 – 7 M

7/15/2015 Carbonic Acid H 2 CO 3 (aq)H + (aq) + HCO 3 – (aq) K a,1 = 4.2 x 10 – 7 HCO 3 – (aq)H + (aq) + CO 3 2 – (aq) K a,2 = 4.8 x 10 – 11 Write the mass-action expression formass-action expression the second ionization of carbonic acid. Carbonic acid has more than one acidic proton, and each ionization has its own constant.

7/15/2015 Carbonic Acid HCO 3 – (aq) H + (aq) + CO 3 2 – (aq) [H + ] [CO 3 2 – ] [HCO 3 – ] = K a,2 = 4.8 x 10 – 11 M

7/15/2015 Determining Unlisted Equilibrium Constants H 2 CO 3 (aq)2 H + (aq) + CO 3 2– (aq) What is the numeric value of the equilibrium constant equilibrium constant for this reaction?

7/15/2015 Determining Unlisted Equilibrium Constants H 2 CO 3 (aq)2 H + (aq) + CO 3 2– (aq) The equilibrium constant is equal to the mass-action expression at equilibrium, so write the mass-action expression for this reaction.mass-action expression What is the numeric value of the equilibrium constant for this reaction?

7/15/2015 Determining Unlisted Equilibrium Constants How is this ratio related to the other mass-action expressions? H 2 CO 3 (aq)2 H + (aq) + CO 3 2– (aq) [H + ] 2 [CO 3 2 – ] [H 2 CO 3 ] = K

7/15/2015 Determining Unlisted Equilibrium Constants [H + ] 2 [CO 3 2 – ] [H 2 CO 3 ] [H + ] [HCO 3 – ] [H 2 CO 3 ] = 4.2 x 10 –7 [H + ] [CO 3 2 – ] [HCO 3 – ] = 4.8 x 10 –11 What is similar, what is different?

7/15/2015 Determining Unlisted Equilibrium Constants [H + ] 2 [CO 3 2 – ] [H 2 CO 3 ] = [H + ] [HCO 3 – ] [H 2 CO 3 ] [H + ] [CO 3 2 – ] [HCO 3 – ]  = 4.2 x 10 –7 4.8 x 10 –11  = 2.0 x 10 –17 M 2 H 2 CO 3 (aq)2 H + (aq) + CO 3 2– (aq)

7/15/2015 Chemical Equations Can Be Treated Algebraically H 2 CO 3 (aq)H + (aq) + HCO 3 – (aq) HCO 3 – (aq)H + (aq) + CO 3 2– (aq) H 2 CO 3 (aq)2 H + (aq) + CO 3 2– (aq) target equation

7/15/2015 Numeric Values of Dissociation Constants  Describe the reaction in a sentence.  How is the listed K b equation related to this reaction? NH 4 + (aq)NH 3 (aq) + H + (aq)

7/15/2015 Dissociation of Ammonium Ion NH 4 + (aq)NH 3 (aq) + H + (aq) (target)

7/15/2015 Dissociation of Ammonium Ion NH 4 + (aq)NH 3 (aq) + H + (aq) (target) NH 4 + (aq) + OH – (aq)NH 3 (aq) + H 2 O(l) What other equation can be added to the first equation to give the target equation? H 2 O(l)OH – (aq) + H + (aq) What is the numeric value of the equilibrium constant?

Assignments Equilibrium Constant Determination Problem Set Complete Experiment F in the laboratory manual

7/15/2015 Group Activity hydrogen peroxide water ammonium ion Rank the following species in terms of acid strength (strongest to weakest). ammonium ion hydrogen peroxide water

7/15/2015