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 acidacid is a substance that produces H+ ions in water solutions

HCl H+ + Cl-

A basebase 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+

The Proton in WaterThe Proton in WaterWhen HCl dissolves in water we write:

HCl(g) H+(aq) + Cl-(aq)

Reality for the Hydronium ionH5O2

+

H9O4+

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.HClHCl(aq)(aq) + H + H22OO(aq)(aq) H H33OO++

(aq)(aq) + Cl + Cl--(aq)(aq)

or or

HClHCl(aq)(aq) H H++(aq)(aq) + Cl + Cl--

(aq)(aq)

Nitrogen compounds are Bronsted acids when they are

protonatedprotonated.

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 conjugateacid base

Is Water an Acid?

NH3(aq) +H2O(aq) NH4+

(aq) + OH-(aq)

Is Water a Base?

HC2H3O2(aq) + H2O(aq) H3O+(aq) +C2H3O2

1-(aq)

The auto ionization of waterThe reaction occurs to a very small extent;

about 1 in 108 molecules is ionized at any given moment

HH

HH

OO:: ::

HH

HH

OO:: ::+ HH

HH

OO::HH

+

+ HHOO:....

Dissociation of Water, pH ScaleH2O(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:

a) [H+] = 2 x 10-5 M

b) [OH-] = 0.010 M

c) [OH-] = 1.0 x 10-7 M

Calculate the concentration of H+(aq) in

(a)a solution in which the [OH-] is 0.020M

(b)a solution in which the [OH-] = 2.5 x 10-6 M.

Indicate whether the solution is acidic or basic

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 isAntilog -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 speciesHCl(aq) + H2O(aq) H3O+

(aq) + Cl-(aq)

or

HCl(aq) H+(aq) + Cl-

(aq)

HC2H3O2(aq) + H2O(aq) H3O+(aq) +C2H3O2

1-(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 strongstrong 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 3.2M HNO3?

What about H2SO4

H2SO4 → H+ + HSO41- (Strong)

HSO41- → H+ + SO4

2- (Weak)

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 0.10 M solution of HC2H3O2 (Ka

= 1.8 x 10-5)? pH? % ionization? HC2H3O2 H+ + C2H3O2

1-

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? Ka = 4.9 x 10-10

Acid-Base Equilibria: Strong BasesAcid-Base Equilibria: Strong BasesThe 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?

AminesAnions of Weak Acids

NH3 + H2O NH4+ + OH-

ICE

Dealing with Weak BasesDealing with Weak Bases

Weak base + H2O conjugate acid + OH-

NH3 (aq) + H2O (l) NH4 +

(aq) + OH-(aq)

Kb =[NH4

+] [OH-][NH3]

The base dissociation constant Kb refers to the equilibrium in which a base reacts with H2O to

form the conjugate acid and OH-

Calculate the [OH-] in a 0.15 M solution of NH3.

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 AcidsAnions of Weak Acids

HCHC22HH33OO2(aq)2(aq) + H + H22OO(aq)(aq) H H33OO++(aq)(aq) + + CC22HH33OO22

-- (aq)(aq)

Bronsted base

Bronsted acid

Conjugateacid

Conjugatebase

A second class of weak base is composed of the anions of weak acids Anions of weak acids can be incorporated into salts.

NaCNaC22HH33OO22 Na Na++(aq) + (aq) + CC22HH33OO22-- (aq)(aq)

C2H3O2- + H2O HC2H3O2 + OH- Kb = 5.6 x 1010

NaOHNaOH(aq)(aq)+HC+HC22HH33OO2(aq2(aq→H→H22O+O+NaCNaC22HH33OO22

NaCNaC22HH33OO22 Na Na++(aq)(aq) + C + C22HH33OO22

--(aq)(aq)

NaNa++(aq)(aq) + H + H22O O ← NaOH← NaOH(aq) (aq) + H+ H++

(aq)(aq)

CC22HH33OO22--(aq)(aq)+H+H22O O HCHC22HH33OO2 (aq)2 (aq)+OH+OH- -

(aq)(aq)

NH4Cl → NH4+ + Cl-

NH4+ NH3 + H+

NH4Cl

NaC2H3O2

NH4C2H3O2

ClO- + H2O HClO + OH-

I

C

E

Calculate the pH of a 0.01 M solution of sodium hypochlorite (NaClO)

Anions of Weak AcidsAnions of Weak Acids

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

KKaa and K and Kbb

NH4+

(aq) NH3(aq)+ H+(aq)

NH3(aq)+ H2O NH4+

(aq)+ OH-(aq)

KKaa = =[H[H++][NH][NH33]]

[NH[NH44++]]

KKbb = =[NH[NH44][OH][OH-- ] ]

[NH[NH33]]

NH4+(aq) NH3(aq) + H+ (aq)

NHNH33(aq) + H(aq) + H22O(l)O(l) NH NH44++(aq) + OH(aq) + OH-- (aq) (aq)

HH22O O H H++(aq) + OH(aq) + OH--(aq)(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

KKaa x K x Kbb = K = KwwpKa + 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

Now it’s your turn

Calculate the pKb for carbonic acid (Ka = 4.3 x 10-7)

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

•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-(aq) + H2O H3O+ + PO4

3-(aq)

HPO4- (aq) + H2O H2PO4

2-(aq) + OH-(aq)

K3 = 4.2 x 10-13HPO4

- acting like an acid

HPO4- acting like an base

So HPO- is the conjugate base of H2PO4-.

Since the K2 of H2PO4- = 6.2 x 10-8 then:

Kb =Ka

=Kw

1.0 x 10-14

6.2 x 10-8

= 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

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

two things to consider polarity difference and strength of the bond

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