Acids and Bases Titrations

Acids and Bases - 3 Definitions Arrhenius Arrhenius Bronsted-Lowry Bronsted-Lowry Lewis Lewis

Arrhenius Definition Acid = proton donor HA = H + + A - Acid = proton donor HA = H + + A - Base = hydroxide donor BOH = B + + OH - Base = hydroxide donor BOH = B + + OH - Dilemma: NH 4 OH, HCl Dilemma: NH 4 OH, HCl

Bronsted-Lowry Definition Acid = proton donor Acid = proton donor HCl H + + Cl - Base = proton acceptor NH 3 + H + = NH 4 + Base = proton acceptor NH 3 + H + = NH 4 + Dilemna: ferrocene (organometallics) Dilemna: ferrocene (organometallics)

Acid and Base Strength Strong acids and Bases completely dissociate (ionize) in aqueous solution Strong acids and Bases completely dissociate (ionize) in aqueous solution EX: HCl aq -> H + aq + Cl - aq complete ionization EX: HCl aq -> H + aq + Cl - aq complete ionization Weak acids and Bases incompletely dissociate in aqueous solution Weak acids and Bases incompletely dissociate in aqueous solution EX: HC 2 H 3 O 2aq = H + aq + C 2 H 3 O 2 - aq partial ionization EX: HC 2 H 3 O 2aq = H + aq + C 2 H 3 O 2 - aq partial ionization

Strong Acids Most mineral acids Most mineral acids HCl HCl H 2 SO 4 H 2 SO 4 HNO 3 HNO 3 HClO 4 HClO Not: HF, H 3 PO 4, H 2 SO 3

Weak Acids Organic acids (usually have COOH) EXAMPLES: formic acid (HCOOH) acetic acid (HC 2 H 3 O 2 )

Scheme for Identification of Acids and Bases Proton donor or proton acceptor? If proton donor, then acid then acid Inorganic Strong except HF, HCN, H2S, H3PO 4 Weak acid Organic If proton acceptor, Then base Inorganic If IA or IIA, then strong If NH4OH or not IA and IIA Then weak Organic Weak base

ACID - BASE CHEMISTRY IN GENERAL What is an acid? What is an acid? What is a base? What is a base? How does HNO 3, a strong acid, behave in water? How does HNO 3, a strong acid, behave in water? How does HNO 2, a weak acid, behave in water? How does HNO 2, a weak acid, behave in water? hydrogen ion donor hydrogen ion acceptor By lone pair of electrons Strong electrolyte: HNO 3 ---> H + + NO % Weak electrolyte: HNO 2 H + + NO 2 - partial

Acidic Solution high [H + ] concentration high [H + ] concentration low pH value value below 7 low pH value value below 7 low [OH - ] concentration low [OH - ] concentration high pOH value value greater than 7 high pOH value value greater than 7

Basic Solution high [OH - ] concentration high [OH - ] concentration low pOH value value less than 7 low pOH value value less than 7 low [H + ] concentration low [H + ] concentration high pH value value greater than 7 high pH value value greater than 7

Acid – Base Salts There are four kinds of salts: There are four kinds of salts: salts of weak acids and strong base salts of weak acids and strong base NaOH + HC 2 H 3 O 2 NaC 2 H 3 O 2 + H 2 O sodium acetate sodium acetate salts of weak bases and strong acid salts of weak bases and strong acid NH 4 OH + HCl NH 4 Cl + H 2 O ammonium chloride ammonium chloride salts of strong acids and strong bases salts of strong acids and strong bases NaOH + HCl NaCl + H 2 O sodium chloride sodium chloride salts of weak acids and weak base salts of weak acids and weak base NH 4 OH + HC 2 H 3 O 2 (NH 4 )C 2 H 3 O 2 + H 2 O ammonium acetate ammonium acetate

Salts of Strong Acids and Bases Example: NaCl (Strong acid & Strong base) Example: NaCl (Strong acid & Strong base) NaCl + H 2 O -> Na + aq + Cl - aq + H 2 O NaCl + H 2 O -> Na + aq + Cl - aq + H 2 O pH  7 pH  7 These salts do not directly perturb the water equilibrium These salts do not directly perturb the water equilibrium

Salts of Weak Acids and Strong Bases Example: NaC 2 H 3 O 2 (Weak acid & Strong base) Example: NaC 2 H 3 O 2 (Weak acid & Strong base) NaC 2 H 3 O 2 + H 2 O = Na + + HC 2 H 3 O 2 + OH - NaC 2 H 3 O 2 + H 2 O = Na + + HC 2 H 3 O 2 + OH - pH = 0.5(14 + pK a + log[salt]) pH = 0.5(14 + pK a + log[salt]) solutions are weakly basic solutions are weakly basic

Salts of Weak Bases and Strong Acids Example: NH 4 Cl (Strong acid & Weak base) Example: NH 4 Cl (Strong acid & Weak base) NH 4 Cl + H 2 O = NH 4 OH + H + + Cl - NH 4 Cl + H 2 O = NH 4 OH + H + + Cl - Derivation Derivation pH = 0.5(14 - pK b - log[salt]) pH = 0.5(14 - pK b - log[salt]) solutions are weakly acidic solutions are weakly acidic

Salts of Weak Acids and Weak Bases Example: NH 4 C 2 H 3 O 2 (Weak acid & Weak base) NH 4 C 2 H 3 O 2 + H 2 O = NH 4 OH + HC 2 H 3 O 2 pH = 0.5(14 + pK a - pK b ) solution pH depends on the relative strength of the acid vs. the base

Titrimetry Purpose: Purpose: Determine concentration of an acid or base of unknown concentration (M A V A = M B V B ) Determine concentration of an acid or base of unknown concentration (M A V A = M B V B ) Identification of unknown acid or base based on pK a of weak acid or pH of strong acid. Identification of unknown acid or base based on pK a of weak acid or pH of strong acid. (pH = pK a at ½ volume at equivalence point) Method: volumetrically using biuret Method: volumetrically using biuret At endpoint: moles acid = moles base At endpoint: moles acid = moles base

Terminology Titrant = Strong acid or base delivered from a biuret Titrant = Strong acid or base delivered from a biuret Standardized = concentration made known both in terms of accuracy and precision Standardized = concentration made known both in terms of accuracy and precision Endpoint = pH at which visual indicator changes color as …… Endpoint = pH at which visual indicator changes color as …… Equivalence point = pH at which moles of acid equal moles of base Equivalence point = pH at which moles of acid equal moles of base

Types of Acid-Base Titrations 1) Strong acid - Strong base: Either of both can be used as titrant or titrand. 2) Strong acid – Weak base Strong acid is Titrant and weak base is Titrand 3) Weak acid – Strong base Strong base is Titrant and weak acid is Titrand

Titration Curves Give an idea about the route of reaction. Give an idea about the route of reaction. By measuring the pH of titrand with the regular little measured amounts of titrant. By measuring the pH of titrand with the regular little measured amounts of titrant. The obtaining curve explain the medium at which neutralization ocuurs. The obtaining curve explain the medium at which neutralization ocuurs. Strong acid + Strong base (NEUTRAL) Strong acid + Strong base (NEUTRAL) Strong acid + Weak base (ACIDIC) Strong acid + Weak base (ACIDIC) Weak acid + Strong base (BASIC) Weak acid + Strong base (BASIC)

INDICATORS Show us the route of reaction. Show us the route of reaction. Without measuring the pH during the titration process. Without measuring the pH during the titration process. Choosing the Proper Indicator Choosing the Proper Indicator color change range should be in area where titration curve is most vertical color change range should be in area where titration curve is most vertical The selective indicator is chosed according to the type of titration. The selective indicator is chosed according to the type of titration. Strong acid + Strong base (EITHER ACIDIC OR BASIC) Strong acid + Strong base (EITHER ACIDIC OR BASIC) Strong acid + Weak base (ACIDIC) Strong acid + Weak base (ACIDIC) Weak acid + Strong base (BASIC) Weak acid + Strong base (BASIC)

Acidic Indicators Indicator Behavior HIn + H 2 O H 3 O + + In - acid base color color [H 3 O + ][In - ] K a = [HIn] [HIn]

Basic Indicators Indicator Behavior In + H 2 O  InH + + OH - base acid base acid color color color color [InH + ][OH - ] K b = (2) [In] [In]

Indicators Organic weak acids that have different colors in their acid and conjugate base forms. Organic weak acids that have different colors in their acid and conjugate base forms. phenolphthalein HA = H + + A - colorless Pink phenolphthalein HA = H + + A - colorless Pink

phenolphthalein bromocresol green

8.0 phenolphthalein bromocresol green

TITRATION PROCEDURE 1) Washing of conical flask, pipette, burete with dist. water. 2) Washing of burete with titrant and pipette with titrand before the filling. - DON ’ T WASH THE CONICAL FLASK WITH ANY; JUST DIST. WATER.

Filling the solutions 1) Fill the pipette with the titrand. 2) Add two drops of the indicator and record the color of solution. 3) Start the titration by adding the titrant till the end point (COLOR OF INDICATOR AT THE END POINT)

Calculation No. of moles of titrant = No. of moles of titrand. No. of moles of titrant = No. of moles of titrand. No. of moles = Volume in liter x Molarity No. of moles = Volume in liter x Molarity (V x M) titrant = (V x M) titrand (V x M) titrant = (V x M) titrand

Titration of 0.1 M KHP With an unknown concentration of NaOH using phenolphethalein indicator before titration COLORLESS (ACIDIC) Titration of 0.1 M KHP With an unknown concentration of NaOH using phenolphethalein indicator before titration COLORLESS (ACIDIC)

Titration of KHP with NaOH using phenolphethalein indicator at the end point FAINT PINK (NEUTRAL)

Titration of KHP with NaOH using phenolphethalein indicator after the end point PINK (BASIC)

CALCULATIONS Method: volumetrically using biuret Method: volumetrically using biuret At endpoint: moles acid = moles base At endpoint: moles acid = moles base (M x V) known = (M x V) unknown (M x V) known = (M x V) unknown

EXAMPLE 1 5 mL of HCl with 0.2 M concentration is neutralized with 6.2 mL of NaOH. 5 mL of HCl with 0.2 M concentration is neutralized with 6.2 mL of NaOH. i) Select the suitable indicator? ii) What is the molar concentration of NaOH? iii) What is the strength of both HCl and NaOH solutions?

ANSWER 1 i) Either acidic or basic indicator can be used. ii) (M x V) HCl = (M x V) NaOH 0.2 x 5 = M x x 5 = M x 6.2 M NaOH = 0.2 x 5 = M M NaOH = 0.2 x 5 = M iii) Strength of HCl = M x Mol. Mass = 0.2 x 36.5 = 73 g /L = 0.2 x 36.5 = 73 g /L Strength of HCl = M x Mol. Mass Strength of HCl = M x Mol. Mass = x 40 = g /L = x 40 = g /L

EXAMPLE 2 10 mL of CH 3 COOH with 0.1 M concentration is neutralized with 6.5 mL of NaOH. 10 mL of CH 3 COOH with 0.1 M concentration is neutralized with 6.5 mL of NaOH. i) Select the suitable indicator? ii) What is the molar concentration of NaOH?

ANSWER 2 i) Basic indicator must be used. ii) (M x V) CH3COOH = (M x V) NaOH 0.1 x 10 = M x x 10 = M x 6.5 M NaOH = 0.1 x 10 = M M NaOH = 0.1 x 10 = M iii) Strength of NaOH = M x Mol. Mass s = x 40 = g /L s = x 40 = g /L