Acid-Base Equilibria: Acids and Bases What makes an Acid an Acid? An acid possess a sour taste An acid dissolves active metals magnesium An acid causes certain vegetable dyes to turn characteristic colors What makes a Base a Base? A bases possess a bitter taste A base feels slippery to the touch A base causes certain vegetable dues to turn a characteristic color

7 strong acids and 8 strong bases Acids - HI, HBr, HCl, HClO3, HClO4, H2SO4, HNO3 Bases – LiOH, NaOH, KOH, RbOH, CsOH, Ca(OH)2, Sr(OH)2, Ba(OH)2

The Arrhenius Definition of an Acid and a Base An acid is a substance that produces H+ ions in water solutions HCl  H+ + Cl- A base is a substance that produces OH- ions in a water solution NaOH  Na+ + OH-

acid dissociation equations HC6H5O3  C6H5O31- + H+ Fe(H2O)63+  Fe(H2O)5(OH)2+ + H+ CH3CH2NH31+  CH3CH2NH2 + H+

When HCl dissolves in water we write: HCl(g)  H+(aq) + Cl-(aq) The Proton in Water When HCl dissolves in water we write: HCl(g)  H+(aq) + Cl-(aq)

Reality for the Hydronium ion H5O2+

Acidic solutions are formed by a chemical reaction in which and acid transfers a proton (H+) to water, so we can write them either way. HCl(aq) + H2O(aq)  H3O+(aq) + Cl-(aq) or HCl(aq)   H+(aq) + Cl-(aq)

Nitrogen compounds are Bronsted acids when they are protonated. NH4Cl NH4+  NH3 + H+ CH3)2NH2+  (CH3)2NH + H+

The Bronsted-Lowry definition for Acids and Bases Acids may be defined as a substance that is capable of donating protons Bases may be defined as substance that accepts protons. HCl + NH3  NH4+ + Cl- acid base conjugate conjugate acid base

Is Water an Acid? NH3(aq) +H2O(aq)  NH4+(aq) + OH-(aq)

Is Water a Base? HC2H3O2(aq) + H2O(aq)  H3O+(aq) +C2H3O21-(aq)

The auto ionization of water The reaction occurs to a very small extent; about 1 in 108 molecules is ionized at any given moment H O : +  ..

Dissociation of Water, pH Scale H2O(l)  H+(aq) + OH-(aq) K =[H+] [OH-] [H2O] since water is a liquid and its concentration is therefore constant, this expression may be written as: Kw = [H+] [OH-] = 1.0 x 10-14 [H+] = [OH-] = 1.0 x 10-7 M

Sample exercise: Indicate whether each of the following solutions is neutral, acidic, or basic: [H+] = 2 x 10-5 M [OH-] = 0.010 M [OH-] = 1.0 x 10-7 M

Calculate the concentration of H+(aq) in a solution in which the [OH-] is 0.020M a solution in which the [OH-] = 2.5 x 10-6 M. Indicate whether the solution is acidic or basic

If pH is 3.8 the H+ concentration is The pH Scale pH = -log [H+] If [H+] = 2. 5 x 105 the pH is? pH = -log [2. 5 x 10-5 ] = 4.6 If pH is 3.8 the H+ concentration is Antilog -3.8= 1.58 x 10-4 M

In a sample of lemon juice, [H+] = 3.8 x 10-4 M. What is the pH? A commonly available window cleaner has a [H+] = 5.3 x 10-9 M. What is the pH? In a sample of freshly pressed apple juice has a pH of 3.76. Calculate the [H+]

What if we took the –log of the Kw expression Kw = [H+] [OH-] = 1.0 x 10-14 pKw = pH + pOH = 14

What is the pH, [H+], [OH-], of a solution with a pOH of 2.5? Is the solution acidic or basic?

Major species HCl(aq) + H2O(aq)  H3O+(aq) + Cl-(aq) or HCl(aq)   H+(aq) + Cl-(aq) HC2H3O2(aq) + H2O(aq)  H3O+(aq) +C2H3O21-(aq) Pb(NO3)2 + NaCl →NaNO3 + PbCl2

Indicators

What is the pH of 0.010 M solution of HCl? If it ionizes completely which is what strong means then take the negative log of the concentration. HCl(aq)  H+(aq) + Cl-(aq) .01M .01M .01M pH = 2

What is the pH of a solution made from 20mL of 2. 0M HCl and 35mL of 3 What is the pH of a solution made from 20mL of 2.0M HCl and 35mL of 3.2M HNO3?

H2SO4 → H+ + HSO41- (Strong) HSO41- → H+ + SO42- (Weak) What about H2SO4 H2SO4 → H+ + HSO41- (Strong) HSO41- → H+ + SO42- (Weak)

HX(aq)  H+(aq) + X-(aq), then What about weak acids HX(aq)  H+(aq) + X-(aq), then Ka = [H+][X-] [HX] The smaller the value of the acid dissociation constant Ka, the weaker the acid

What is the Ka of a 0.10 M solution of formic acid (HCHO2) which has a pH = 2.38? HCHO2  H+ + CHO21- I 0.10 0 0 C .00417 .00417 .00417 E .0958 .00417 .00417 Ka = (.00417)2 = 1.8 x 10-4 .0958

What is the concentration of H+ ions in a 0 What is the concentration of H+ ions in a 0.10 M solution of HC2H3O2 (Ka = 1.8 x 10-5)? pH? % ionization? HC2H3O2  H+ + C2H3O21- I .10 0 0 C -X +X +X E .10 – X X X 1.8 x 10-5 = X2 X = 1.3 x 10-3 .10 – X pH = 2.87

percent dissociation 1.3 X 10-3 x 100 = 1.3% .10

What is the pH and percent ionization of a 0. 20 M solution of HCN What is the pH and percent ionization of a 0.20 M solution of HCN? Ka = 4.9 x 10-10

Acid-Base Equilibria: Strong Bases The most common soluble strong Bases are the hydroxides of group IA and Ca, Ba and Sr What is the pH of a 0.010 M solution of Ba(OH)2?

Anions of Weak Acids Amines

Dealing with Weak Bases The base dissociation constant Kb refers to the equilibrium in which a base reacts with H2O to form the conjugate acid and OH- Weak base + H2O  conjugate acid + OH- NH3 (aq) + H2O (l)  NH4 +(aq) + OH-(aq) [NH4+] [OH-] Kb = [NH3] Calculate the [OH-] in a 0.15 M solution of NH3. NH3 + H2O  NH4+ + OH- I C E

H2SO3(aq)  H+(aq) + HSO3-(aq) HSO3-(aq)  H+(aq) + SO32-(aq) Polyprotic Acids H2SO3(aq)  H+(aq) + HSO3-(aq) Ka1 = 1.7 x 10-2 HSO3-(aq)  H+(aq) + SO32-(aq) Ka2 = 6.4 x 10-8 Calculate the pH of a .1M solution

Anions of Weak Acids HC2H3O2(aq) + H2O(aq)  H3O+(aq) + C2H3O2- (aq) Bronsted acid Bronsted base Conjugate acid Conjugate base A second class of weak base is composed of the anions of weak acids Anions of weak acids can be incorporated into salts. NaC2H3O2  Na+(aq) + C2H3O2- (aq) C2H3O2- + H2O  HC2H3O2 + OH- Kb = 5.6 x 1010

NaOH(aq)+HC2H3O2(aq→H2O+NaC2H3O2 NaC2H3O2  Na+(aq) + C2H3O2-(aq) Na+(aq) + H2O ← NaOH(aq) + H+(aq) C2H3O2-(aq)+H2O HC2H3O2 (aq)+OH- (aq)

NH4Cl → NH4+ + Cl- NH4+  NH3 + H+

NH4Cl NaC2H3O2 NH4C2H3O2

Calculate the pH of a 0.01 M solution of sodium Anions of Weak Acids Calculate the pH of a 0.01 M solution of sodium hypochlorite (NaClO) ClO- + H2O  HClO + OH- I C E

Now it’s you turn: the Kb for BrO- is 5.0 x 10-6. Calculate the pH of a 0.050 M solution of NaBrO

Ka and Kb NH4+(aq)  NH3(aq)+ H+(aq) Ka x Kb = Kw [NH4][OH- ] NH3(aq)+ H2O NH4+ (aq)+ OH-(aq) NH4+(aq)  NH3(aq)+ H+(aq) [NH4][OH- ] [H+][NH3] Ka = Kb = [NH4+] [NH3] NH4+(aq)  NH3(aq) + H+ (aq) NH3(aq) + H2O(l) NH4+(aq) + OH- (aq) H2O  H+(aq) + OH-(aq) When two reactions are added to give a third reaction, the equilibrium constant for the third reaction reaction is given by the product of the equilibrium constants for the two added reactions Ka x Kb = Kw pKa + pKb = pKw

Calculate the (a) base-dissociation constant, Kb, for the fluoride ion, is the pKa of HF = 3.17 pKa = -log Ka 3.17 = -log Ka Antilog -3.17 = 6.76 x 10-4 Since Ka x Kb = Kw (6.76 x 10-4)x Kb = 1.0 x 10-14 Kb = 1.0 x 10-14/ 6.76 x 10-4 = 1.5 x 10-11

Calculate the pKb for carbonic acid (Ka = 4.3 x 10-7) Now it’s your turn

Acid-Base Properties of Salt Solutions Anions of weak acids, HX, are basic and will react with H2O to produce OH- X- (aq) + H2O (l)  HX(aq) + OH-(aq) Anions of strong acids, such as NO3-, exhibit no basicitiy, these ions do not react with water and consequently do not influence the pH Anions of polyprotic acids, such as HCO3-, that still have ionizable protons are capable of acting as either proton donors or acceptors depending upon the magnitudes of the Ka or Kb

Predict whether the salt Na2HPO4 will form an acidic or Anions of polyprotic acids, such as HCO3-, that still have ionizable protons are capable of acting as either proton donors or acceptors depending upon the magnitudes of the Ka or Kb Predict whether the salt Na2HPO4 will form an acidic or basic solution on dissolvingin water. Na2HPO4  2Na+ (aq) + HPO4- HPO4- acting like an acid K3 = 4.2 x 10-13 HPO4-(aq) + H2O  H3O+ + PO43-(aq) HPO4- acting like an base 1.0 x 10-14 HPO4- (aq) + H2O  H2PO42-(aq) + OH-(aq) Kw = Kb = 6.2 x 10-8 Ka So HPO- is the conjugate base of H2PO4-. Since the K2 of H2PO4- = 6.2 x 10-8 then: 1.6 x 10-7 = Since Kb is larger than Ka, HPO4- will act like a base

Salt derived from a strong base and a strong acid will have a pH of 7 Salt derived from a strong base and a weak acid will have a pH above 7 Salt derived from a weak acid and a weak base depends upon whether the dissolved ion acts as an acid or a base as determined by the size of the Ka or Kb

Acid-Base Character and Chemical Structure two things to consider polarity difference and strength of the bond HF > HCl > HBr > HI (most polar least) Based on electronegativity difference HF is the most polar but a weak acid because the bond is so strong

Acid strength of oxyacids The more oxygen's the stronger the acid because of the oxygen pulling the electrons towards themselves. HClO4> HClO3> HClO2> HOCl