Precipitation Reactions and

Acid-Base Reactions

Chemistry 142 B

James B. Callis, Instructor

Autumn Quarter, 2004

Lecture #11

The Solubility of Ionic Compounds in Water

The solubility of Ionic Compounds in water depends upon the relative strengths of the electrostatic forces between ions in the ionic compoundand the attractive forces between the ions and water molecules in the solvent. There is a tremendous range in the solubility of ionic compounds in water! The solubility of so called “insoluble” compoundsmay be several orders of magnitude less than ones that are called“soluble” in water, for example:

Solubility of NaCl in water at 20oC = 365 g/LSolubility of MgCl2 in water at 20oC = 542.5 g/LSolubility of AlCl3 in water at 20oC = 699 g/LSolubility of PbCl2 in water at 20oC = 9.9 g/LSolubility of AgCl in water at 20oC = 0.009 g/LSolubility of CuCl in water at 20oC = 0.0062 g/L

Compare with Table 4.1 of Zumdahl

Precipitation Reactions: A solid product is formed

When ever two aqueous solutions are mixed, there is the possibility of forming an insoluble compound. Let us look at some examples to see how we can predict the result of adding two different solutions together.

Pb(NO3) (aq) + 2 NaI(aq)

When we add these two solutions together, the ions can combine in the way they came into the solution, or they can exchange partners. In this case we could have lead nitrate and sodium iodide, or lead iodide andsodium nitrate formed, to determine which will happen we must look atthe solubility rules to determine what could form. Rule 3 in Table 4.1 (lecture) indicates that lead iodide will be insoluble, so a precipitate will form! The remainder (sodium nitrate) is soluble.

Pb(NO3)2 (aq) + 2 NaI(aq)

Precipitation Reactions: Will a precipitate form?

If we add a solution containing potassium chloride to a solution containing ammonium nitrate, will we get a precipitate?

KCl(aq) + NH4NO3(aq) = K+ (aq) + Cl-(aq) + NH4+(aq) + NO3

-(aq)

By exchanging cations and anions we see that we could have potassiumchloride and ammonium nitrate, or potassium nitrate and ammonium chloride. In looking at the solubility rules, Table 4.1:

If we mix a solution of sodium sulfate with a solution of barium nitrate,will we get a precipitate? Table 4.1 shows that barium sulfate is insoluble - therefore we expect:

Na2SO4(aq) + Ba(NO3)2(aq)

Problem 11-1: Predicting Whether a Precipitation Reaction Occurs (see Table 4.1); Writing Equations:

a) calcium nitrate and sodium sulfate solutions are added together.

Ca(NO3)2(aq) + Na2SO4(aq)

Ca2+(aq) + 2 NO3-(aq) +2 Na+(aq) + SO4

2-(aq)

Molecular Equation

Total Ionic Equation

Net Ionic Equation

Spectator Ions are

b) ammonium sulfate and magnesium chloride are added together.

Testing the Reactivity of the Cations Ag+, Ba2+, and Fe3+

with the Anions Cl-, SO42-, and OH-

Cation NaCl(aq)

(Cl-)

Na2SO4(aq) (SO4

2-)NaOH(aq)

(OH-)

Ag+ White ppt (AgCl)

No Reaction White ppt that turns brown

(AgOH -> Ag2O)

Ba2+ No Reaction White ppt

(BaSO4)

No Reaction

Fe3+ Yellow solution, but no solid

No Reaction Reddish Brn ppt Fe(OH)3

A Scheme for Selective Precipitation

Acid - Base Reactions : Neutralization Rxns.

An Acid is a substance that produces H+ (H3O+) ions when dissolved in water.A Base is a substance that produces OH- ions when dissolved in water.

Acids and Bases are electrolytes, and their strength is categorized in terms of their degree of dissociation in water to make hydronium orhydroxide ions, resp. Strong acids and bases dissociate completely, and are strong electrolytes. Weak acids and bases dissociate only to a small extent (<<100%) and are weak electrolytes.

The generalized reaction between an Acid and a Base is:

HX(aq) + MOH(aq) MX(aq) + H2O(l)

Acid + Base = Salt(aq) + Water

Selected Acids and Bases

Acids Bases

Strong (100% of H+) Strong (100% of OH-) Hydrochloric, HCl Sodium hydroxide, NaOH Hydrobromic, HBr Potassium hydroxide, KOH Hydroiodoic, HI Calcium hydroxide, Ca(OH)2

Nitric acid, HNO3 Strontium hydroxide, Sr(OH)2

Sulfuric acid, H2SO4 Barium hydroxide, Ba(OH)2

Perchloric acid, HClO4

Weak (low % of H+) Weak (low % yield of OH-) Hydrofluoric, HF Ammonia, NH3

Phosphoric acid, H3PO4

Acetic acid, CH3COOH (or HC2H3O2)

Problem 11-2: Writing Balanced Equations for Neutralization Reactions - I

Problem: Write balanced chemical reactions (molecular, total ionic, and net ionic) for the following Chemical reactions: a) calcium hydroxide(aq) and hydroiodic acid(aq) b) lithium hydroxide(aq) and nitric acid(aq) c) barium hydroxide(aq) and sulfuric acid(aq)Plan: These are all strong acids and bases, therefore they will make water and the corresponding salts.Solution:

a) Ca(OH)2(aq) + 2HI(aq) CaI2(aq) + 2H2O(l)

Ca2+(aq) + 2 OH -(aq) + 2 H+(aq) + 2 I-(aq) Ca2+(aq) + 2 I-(aq) + 2 H2O(l)

2 OH -(aq) + 2 H+(aq) 2 H2O(l)

Problem 11-2: Writing Balanced Equations for Neutralization Reactions - II

b) LiOH(aq) + HNO3(aq)

c) Ba(OH)2(aq) + H2SO4(aq)

Finding the Concentration of Acid from an Acid - Base TitrationVolume (L) of base (difference inburet readings) needed to titrate

Moles of base needed to titrate

Moles of acid which were titrated

M (mol/L) of original acid soln.

M (mol/L) of base

molar ratio

volume (L) of acid

Potassium Hydrogenphthalate KHC8H4O4

C

O

O K+

C

O

H

O

K+

O

C

O

O

C

O

H+

= “KHP” = a common acid used to titrate bases (M = 204.2 g/mol)

Problem 11-3: Finding the Concentration of Base from an Acid - Base Titration - I

Problem: A titration is performed between sodium hydroxide andKHP (204.2 g/mol) to standardize the base solution, by placing 50.00 mg of solid potassium hydrogenphthalate in a flask with a few drops of an indicator. A buret is filled with the base, and the initial buret reading is 0.55 mL; at the end of the titration the buret reading is 33.87 mL. What is the concentration of the base?Plan: Use the molar mass of KHP to calculate the number of moles of the acid, from the balanced chemical equation, the reactionis equal molar, so we know the moles of base, and from the difference in the buret readings, we can calculate the molarity of the base.Solution: The rxn is: KHP(aq) + OH-(aq) --> KP-(aq) + H2O(l) or

HKC8H4O4(aq) + OH-(aq) KC8H4O4-(aq) + H2O(l)

Problem 11-3: Finding the Concentration of Base from an Acid - Base Titration - II

moles KHP = 50.00 mg KHP

Volume of base = Final buret reading - Initial buret reading =

one mole of H+ = one mole of OH-; therefore ____________ moles of KHP will titrate _____________ moles of NaOH.

molarity of NaOH = = moles L

molarity of base =

Answers to Problems in Lecture #111. (a) Net ionic equation: Ca2+(aq) + SO4

2-(aq) = CaSO4 (s) (b) No reaction occurs.

2. (b) Total ionic reaction: Li+(aq) + OH-(aq) + H+(aq) + NO3

-(aq) = Li+ (aq) + NO3-(aq) + H2O(l)

(c) Total ionic reaction: Ba2+(aq) + 2 OH-(aq) + 2 H+(aq) + SO4

2-(aq) = BaSO4(s) + 2 H2O(l)3. 0.07349 M