Expt 4-Ionic Equilibria

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Chemistry 18.1 - General Chemistry Laboratory II

Text of Expt 4-Ionic Equilibria

Experiment 4-Ionic EquilibriaCrystle T. Cotingting Reinavelle Jeunesse D. Mateo

Objectives Distinguish acidic from basic compounds Distinguish a strong acid/base from a weak acid/base Determine experimentally the ionization constant of a weak electrolyte Determine experimentally the solubility product constant of a slightly soluble substance

Review: Acids and Bases Arrhenius Acids: substances that contain hydrogen and dissociate in water to yield hydronium ions Bases: substances that contain hydroxyl groups and dissociate in water to yield hydroxide ions Bronsted-Lowry - expanded the definition of a base - Base: specie with an electron pair available to accept a transferred proton.

Review: Acids and Bases Lewis - Acids: electron pair acceptors - Bases: electron pair donators

Review: Strength of Acids & Bases Acids & Bases electrolytes Strong acids & bases almost completely dissociate or ionize in water Weak acids & bases dissociate only slightly

Ionic Equilibrium observed when ionization takes place in a solvent medium.

Ionization or Dissociation Constant equilibrium constant, K, for the ionization of a weak acid or base. tells us how far to the right the reaction will proceed to reach the equilibrium

Ionization Constant In an acid, HA(aq)+ H2O(l) A+ H3O+(aq) (aq) it has the general form Kacid = [H3O+] [A-] [HA]

Solubility Product Constant Ionic solid + pure water dissolution at a relatively rapid initial rate As the concentration of dissolved ions increases, so does the rate of formation. Soon, rateformtion = ratedissolution a (state of equilibrium and saturation of solution)

Solubility Product Constant At equilibrium, no net dissolution of the solid. If more of the solid is added to the mixture of solid and solution at equilibrium, none will dissolve. If solid is removed, the concentration of ions in solution will stay the same.

Solubility Product Constant Presence of solid - no effect on the equilibrium concentrations of ions in the saturated solution Therefore, the equilibrium constant expression for equation does not include a term referring to the solid. Solubility product constant is defined as such: + Ksp = [Am ]n[Bn- ]m

Solubility Product Constant A measure of how far to the right the dissolution proceeds at equilibrium Important in explaining phenomena like solubility and precipitation of compounds

ExperimentalMethodology

Part 1. Strong and Weak Electrolytes Put pH paper to solutions A-G Determine pH of each solution Classify if strong or weak acid or base or neutral

Part 2. Ionization Constant of Acetic Acid1.1 ml 1M acetic acid 1 drop methy l violet

Match color1.0 ml distille d water 1 drop methyl violet 0.01M HCl from syring e

Part 3. Solubility Product Constant of Benzoic Acid, C6H5COOH50 ml distille d water

Pinch of benzoic acid crystals

40C0.01M NaOH titration to light pink end point

2 drops phenolp hthalein

Results and Discussion

Part 1. Strong and Weak Electrolytes (Result) Observed pH for Various Solutions Solution Observed pHA B C D E F G 1 4 13 7 8 6 6

Part 1. Strong and Weak Electrolytes (Discussion) Classification System Devised for Strong or Weak Acid and Strong or Weak Base pH Classification0-2.5 2.5-6 7 8-11 12-14 Strong acid Weak acid Neutral Weak base Strong acid

Part 1. Strong and Weak Electrolytes (Discussion) Final Classification of SolutionsSolution A B C D E F G Observed pH 1 4 13 7 8 6 6 Classification Strong acid Weak acid Strong base Neutral Weak base; basic salt Weak acid; acidic salt Weak acid; acidic salt

Part 1. Strong and Weak Electrolytes (Discussion)Solution Observed pH A B C D E F G 1 4 13 7 8 6 6 Classification Strong acid Weak acid Strong base Neutral Weak base; basic salt Weak acid; acidic salt Weak acid; acidic salt Slight ionization Slight ionization Slight ionization Ionization complete Slight ionization Ionization complete

Ka of solution A > Ka of solutions B, F, and G.

Part 2. Ionization Constant of Acetic Acid (Result) Volume of HCl added (to match light purple color of acetic acid) - 0.6mL of HCl

Acetic Acid

Part 2. Ionization Constant of Weak Acid (Discussion)Methyl violet a pH indicator to test pH ranging from 0 to 1.6. - acid end (pH below 0.0): yellow - alkaline end (above pH 1.6): bluish violet

Part 2. Ionization Constant of Weak Acid (Discussion) Addition of Methyl Violet 1.1mL 1M acetic acid: light purple color

solution has a pH about or above 1.6 acetic acid weak acid About 0.6mL of 0.01M HCl was added to 1.0mL of distilled water until the solution turned the same color as acetic acid

2 solutions with same pH indicator are the same color about the same pH

Part 2. Ionization Constant of Weak Acid (Discussion)pH of acetic acid = pH of HCl solution Since pH = -log [H3O+], [H3O+] in HCl solution = [H3O+] in the acetic acid (HOAc) solution [H3O+] in HCl solution = molarity of the HCl because HCl is a known strong acid and strong acids dissociate almost completely into their respective ions

Part 2. Ionization Constant of Weak Acid (Discussion)Calculation for Molarity of HCl Note: Cant immediately assume to be 0.01M because it has been diluted by the 1.0 mL distilled water C1V1 = C2V2 C1 = 0.01 M original concentration of HCl V1 = 0.6 mL volume of HCl added V2 = 1.6 mL 1.0mL H2O + 0.6mL HCl

(0.01M)(0.6mL) = (C2)(1.6mL) C2 = (0.01M)(0.6mL) (1.6mL) C2 = 0.00375 M diluted concentration of HCl

Part 2. Ionization Constant of Weak Acid (Discussion) C2 = 0.00375 M C2 = [HCl] = 0.00375 M [HCl] = [H3O+] in HCl solution = 0.00375 M Because it is a strong acid

[H3O+] in HCl solution = [H3O+] in the acetic acid (HOAc) solution Because same color with same pH indicator

Therefore, [H3O+] in the acetic acid solution = 0.00375M

Part 2. Ionization Constant of Acetic Acid (Discussion)Chemical equation for the ionization of acetic acid (HOAc) HOAc(aq) + H2O(l) OAc-(aq) + H3O+(aq)

Part 2. Ionization Constant of Acetic Acid (Discussion) Calculation of [OAc-] and [HOAc].HOAc(aq) + H2O(l) OAc-(aq) + H3O+(aq) Initial Change Equilibrium 1M -x 1M-x 0 +x x 0 +x x

At equilibrium, x = [H3O+] [H3O+] = 0.00375 M. Since x = [OAc-] = [H3O+] [OAc-] = 0.00375 M. At equilibrium, [HOAc] = 1M x = 1M 0.00375M = 0.99625 M

Part 2. Ionization Constant of Acetic Acid (Discussion) Summary of ValuesVolume of HCl used [H3O+] in HCl solution H3O+] in HOAc solution Equilibrium Concentration of HOAc solution 0.6 mL 0.00375 M 0.00375 M 0.99625 M

From here, we can solve for Ka. Ka = [OAc-] [H3O+] [HOAc] = (0.00375 M) (0.00375M) (0.99625 M) = 1.4115 x 10-5 The theoretical Ka for acetic acid at 25C is 1.77x10-5

Part 2. Ionization Constant of Acetic Acid (Conclusion) Calculated Ka value of acetic acid is 1.41x10-5 - dissociates only slightly weak acid

Part 3. Solubility Product Constant of Benzoic Acid (Results) 19.7 mL of the titrant, 0.01M NaOH was needed to reach the light pink end point of titration

Part 3. Solubility Product Constant of Benzoic Acid (Discussion) Calculate concentration of H3O+ using the following equation Macid Vacid =Mbase Vbase . Macid Vacid =Mbase Vbase Mbase = 0.01M Vbase = 19.7mL Vacid = 10mL (Macid )(10mL) = (0.01M)(19.7mL) Macid = 0.0197M

Part 3. Solubility Product Constant of Benzoic Acid (Discussion) At end point, solution being titrated is neutralized by the titrant. Therefore, [OH-] of titrant = [H3O+ ] of acid being titrated In this experiment, [OH-] NaOH = [H3O+] benzoic acid = 0.0197 M

Part 3. Solubility Product Constant of Benzoic Acid (Discussion) Get value of [C6H5COO ] using the initial-change-equilibrium table.

[C6H5COO-] = x [H3O+] =x = 0.0197 M too [C6H5COO-] = [H3O+]

Part 3. Solubility Product Constant of Benzoic Acid (Discussion) Calculating the solubility product constant Ka = [C6H5COO- ][ H3O+] [C6H5COOH] Ka[C6H5COOH] = [C6H5COO-] [H3O+] Ksp = [C6H5COO-][H3O+] = (0.0197M) (0.0197M) = 0.00038809 ~ 3.9 x 10-4

Part 3. Solubility Product Constant of Benzoic Acid (Discussion)Calculating the solubility of benzoic acid

Ksp = [C6H5COO-] [H3O+]

*S solubility

0.00038809 = S x S 0.00038809 = S2 S = 0.0197 M Theoretical value (25C): 0.02785 M

Part 3. Solubility Product Constant of Benzoic Acid (Conclusion) Ksp value for benzoic acid is 0.003889. Similarly, since Ksp is a measure of how far to the right the dissolution proceeds at equilibrium, a compound with a higher Ksp value is more soluble than this system.

Recommendations

Recommendations The group recommends the use of a pH meter instead of pH paper to determine the pH of the seven solutions. Or, if a pH meter is not available we recommend the use of other indicators, so as to more accurately estimate the pH. Comparing our data with other groups, we noticed that the pH we got were different from the pH they observed, probably because comparing the colors in the pH paper is very subjective to the user as well as the lighting of the room.

Recommendations Also, in determining the ionization constant of acetic acid, titrating with a base would prove more effective than matching colors, because comparing colors again is quite subjective to the one performing the experiment.

The End.