Ionic Equilibria (1)

8/4/2019 Ionic Equilibria (1)

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Ionic Equilibria

Compiled by

Dr. L. Coo

Acids and Bases

1. Arrhenius Theory

(Svante Arrhenius,1884)

Acid = a substance that contains hydrogenand produces H+ in aqueous soln.

Base = a substance that contains the OH(hydroxyl) group and produces hydroxideions (OH-) in aq. soln.

(explains rxns bet protic acids and metal hydroxides)

Neutralization rxn:

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


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2. Bronsted-Lowry Theory

(J.N. Bronsted and T.M. Lowry, 1923)

Acid = a substance that donates a proton, H+

(proton donor)

Base = a substance that accepts a proton

(proton acceptor)

An acid-base rxn is the transfer of a proton

from an acid to a base.

p. 350

B-L acid-base rxns can be described in terms of

conjugate acid-base pairs


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1) H2O + HF H3O+ + F-

base1 acid 2 acid1 base2

2) NH3 + H2O NH4+ + OH-

base1 acid 2 acid1 base2

Acid Conj. BaseHCl __________

H2CO3 __________

HPO42- __________

HSO3- __________

NH4+ __________

CH3COOH __________


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Base Conj. Acid

NH3 __________SO4

2- __________

SO2- __________

PH3 __________

I- __________

3. Lewis Theory

(Gilbert N. Lewis, 1923)

Acid = any species that can accept a share

in an electron pair.

Base = any species that can make available,

or donate, a share in an electron pair


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Autoionization of water

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

ion product constant:

*Kw = [H+][OH-]

= 1.0 x 10-14

at 25o

C

*valid for aq solns and pure water

[H+] = [OH-] soln is neutral

[H+] = [OH-] = Kw = 1.0 x 10-7 M

[H+] > [OH-] soln is acidic

[H+] < [OH-] soln is basic


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A. Indicate whether soln is acidic, basic or

neutral

1. [H+] = 2.4 x 10-5 M

2. [OH-] = 3.0 x 10-9 M

B. Calculate the [OH-] when

[H+] = 2.45 x 10-6 M

H+ = a simple proton with no surroundingvalence electron

= small positively charged particle

interacts strongly with the lone pairelectrons of water molecules to formhydrated ion

H

H+ + :O-H H3O+

H+

and H3O+

: interchangeably used torepresent the hydrated proton

2H2O() H3O+

(aq) + OH-(aq)

Kw = [H3O+][OH-]

= 1.0 x 10-14 at 25oC


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For convenience in representing small

concentrations of H+ or OH-, the

p function was introduced:

pH = -log [H+]

pOH = -log [OH-]

pCa = -log [ __ ]

pF = -log [ __ ]

pKw = 14 = pH + pOH

pH value

Acidic < 7.00

Basic > 7.00

Neutral = 7.00

Strong Acid-strong electrolyte, completely ionizes insoln.

-readily gives up proton

binary ternary

HCl HNO3HBr H2SO4HI HClO4

HClO3 (uncommon)


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Strong Base-strong electrolyte, completely ionizes in soln.

-readily accepts proton

1. Ionic hydroxides of alkali metals:

Most common : NaOH, KOH

Uncommon: LiOH, RbOH, CsOH

2. Alkaline earth metals : except for Be

3. Oxide ion, O2-: NaO, CaO

4. Ionic hydrides and nitrides: N3-, H-

O2-(aq) + H2O() 2OH-(aq)

H-(aq) + H2O() H2(g) + OH-(aq)

N3-(aq) + 3H2O() NH3(aq) + 3OH-(aq)

Sr(OH)2 is a strong base, so it is 100%dissociated.

Sr(OH)2(aq) Sr2+

(aq) + 2OH

(aq)

0.0105 M 0.0105 M 2(0.0105) M

So [Sr2+] = 0.0105 Mand [OH-] =0.0210 M

(the strong base is the only source of [OH-])


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HClO3 is a strong acid, so it is 100% ionized.

HClO3(aq) + H2O(l) H3O+(aq) + ClO3(aq)

0.0105 M 0.0105 M 0.0105 M

So [H3O+] = 0.0105 Mand

[ClO3] = 0.0105 M

(the strong acid is the only source of [H+])

Strong acid Ka

HCl 103.9

HBr 105.8

HI 1010.4

HNO3 101.4

1. What is the pH of a 0.040 M HClO4?

2. pH of HNO3 is 3.45, what is [H+]?

3. What is pH of 0.011 M Ca(OH)2?


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Weak Acid/Base

= partially ionizes in soln

The extent to which the weak acid ionizes is

expressed through the equilibrium

constant for the ionization reaction.

( the strength of the acid)

Ka, acid dissociation constant

HA(aq) H+

(aq) + A-(aq)

Ka = [H+][A-]

[HA]


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Weak Bases

Two categories of weak bases

1. Neutral substance: contains an atom with a

lone pair of electrons that can serve as a

proton acceptor.

Ex: Nitrogen-containing cpds

2. Anions of weak acids

Ex: ClO- (source is a salt, NaClO)

ClO-(aq) + H2O HClO(aq) + OH-(aq) Kb = 3.3x10

-7

Kb, base dissociation constant

B(aq) + H2O() BH+

(aq) + OH-(aq)

Kb = [BH+][OH-]

[B]

(Kb always refers to the equilibrium in which abase reacts with H2O to form the conj acid andOH-)


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Monofunctional (HCl, HOAc) : Ka

(NaOH, NH3) : Kb

Polyfunctional (H2SO4, H3PO4): Ka1, Ka2

(Kb1, Kb2

Extent of dissociation is measured thru themagnitude of its Ka (dissociation constant ofthe acid) or its Kb (dissociation constant ofthe base).

What is the pH of a 0.25 M HF?

HF H+(aq) + F-(aq)

I 0.25M 0 0

C - x x x

E 0.25-x x x

Ka = [H+][F-]

HF


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Ka = 7.2 x 10-4

7.2 x 10-4 = X2

0.25 x

x2 = 1.8 x 10-4 7.2 x 10-4x

x2 + 7.2 x 10-4x 1.8 x 10-4 = 0

Solving quadratic equations:

For a general quadratic equation

y = x2 + bx + c, x2 + bx + c = 0

the roots of the equation (i.e., the value of x when

the equation is zero, y = 0), the real value (+) =0.013 M

[H+] = [F-] = 0.013 M

% ionization[H+] x 100

[HF]i

0.013 M x 100 = 5.2 %

0.25 M


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What is the pH of a 0.10 M HOAc?

HOAc H+(aq) + OAc-(aq)

I 0.10M 0 0

C - x x x

E 0.10-x x x

Ka = [H+][OAc-]

[HOAc]

Ka = 1.8 x 10-5

1.8 x 10-5 = x2

0.100 x

Simplify eqn, the small value of Ka indicatesthat the acid is only slightly ionized, suchthat x


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NH3(aq) + H2O(l) NH4+

(aq) + OH

(aq)

0.25 x M x M x M

Kb = [NH4+][OH- ] (x)2

[NH3] = (0.25 -x) = 1.8 x 105

assumex


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Appln: Calcn of Ka from % ionization

In 0.0100 M soln HOAc is 4.2% ionized.Calc its Ka.

Polyprotic Acid

= acid that contains more than one ionizable

hydogen atom per formula unit

H2SO4

H3

PO4

H2C2O4

Stepwise ionization of

polyprotic acid

H2C2O4 H+ + HC2O4

- Ka1 = 5.9 x 10-2

HC2O4- H+ + C2O4

-2 Ka2 = 6.4 x 10-5


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Calculate all species present in

0.100 M H2C2O4

H2C2O4 H+ + HC2O4- Ka1 = 5.9 x 10-2

I 0.100M 0 0

C -x +x +x

E 0.100-x x x

5.9 x 10-2 = x2

0.100-x

Solve by quadratic eqn, Ka1 is too large to

neglect x relative to 0.100 M

x2 + (5.9 x 10-2)x - 5.9 x 10-3 = 0

x = [H+]1st = [HC2O4-] = 0.053 M

HC2O4- H+ + C2O4

-2 Ka2 = 6.4 x 10-5

0.053-y 0.053 +y y

Ka2 = (0.053 + y)(y)

0.053 -y

6.4 x 10

-5

= (0.053 + y)(y)0.053 y

y > [H+]2nd = 6.4 x 10

-5 M


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Relationship of Ka and Kb

HOAc H+(aq) + OAc-(aq) Ka = 1.8 x 10-5

OAc- + H2O HOAc + OH- Kb = ?

H2O H+ + OH- Kw = Ka x Kb

Therefore KbOAc- = Kw

KaHOAc

Pyridine, C5H5N has a Kb value of 1.5 x 10-9.

What is the Ka of C5H5NH+ ?

If Ka for HNO2 is 5.1 x 10-4, what is Kb for

NO2-?

Pyridine : used as pesticide, toxic by ingestion andinhalation (tolerance is 5 ppm in air), explosivein air at 1.8 2.4%

Acid-Base Properties of Salt Solns

Salt : ionic cpd with a cation other than H+

and an anion other than OH- or O2-

Dilute solns of soluble salts are assumed to

completely ionize in water


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The acid/base properties of salt solns

are due to the behavior of their cations

and anionsAqueous solns produced from salts of:

1. a strong acid and a strong base is neutral

2. a strong acid and a weak base is acidic

3. a weak acid and a strong base is basic

4. a weak acid and a weak base will

depend on the extent of hydrolysis of

each ion.

1.Salts of strong acids and strong bases.

Ex:

KNO3salt of nitric acid and potassium hydroxide

CaSO4salt of sulfuric acid and calcium hydroxide

2. Salts of weak acids and strong bases.Ex:

NaF

salt of hydrofluoric acid and sodium hydroxide

Ba(NO2)2

salt of nitrous acid and barium hydroxide


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3. Salts of strong acids and weak bases.

Ex:(NH4)2SO4

salt of sulfuric acid and aqueous ammonia

C5H5NHCl pyridiniumCl

salt of hydrochloric acid and pyridine

Al(NO3)3 (Al3+ + H2O Al(OH)

2+ + H+)

4. Salts of weak acids and weak bases.

Ex:

(NH4)2CO3salt of carbonic acid and aqueous ammonia

CH3NH3F methylammoniumF

salt of hydrofluoric acid and methylamine

pH depends on the extent of hydrolysis of each ion

1. Which is more acidic?

a) NaNO3, Fe(NO3)3

b) KBr, KBrO

c) CH3NH3Cl, BaCl

d) NH4NO2, NH4NO3

2. List the ff solns in order of increasing pH

0.10M Co(ClO4)2, RbCN, Sr(NO3)2, KOAc


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Predict whether the salt of Na2HPO4 will

form an acidic or basic soln on dissolutionin water.

Common Ion Effect

HOAc H+ + OAc-

What happens if a salt, NaOAc is added to

the HOAc soln?

In general, the dissociation of a weakelectrolyte is decreased by adding to the

soln a strong electrolyte that has an ion in

common with the weak electrolyte.


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1. What is the pH of a soln made by adding

0.30 mol of HOAc and 0.30 mole of

NaOAc to enough water to make 1.0 L ofsoln?

2. Calculate the [F-] and pH of a soln

containing 0.10 mol HCl and 0.20 mol HF

in 1.0 L.

Buffer

A soln that resists change in pH when a

smallamount of an acid or base is added

or when the soln is diluted.

Useful in maintaining the pH for a reaction

at an optimum value

Consists of a mixture of a weak acid andits conj base or a weak base and its conj

acid at predetermined concs or ratios.

Which of the following solns are buffer

systems?

a) KH2PO4/H3PO4

b) NaClO4/HClO4

c) C5H5N/C5H5NHCl

(pyridine, Kb= 1.7x10-9)

d) KF/HF

e) KBr/HBr

f) Na2CO3/NaHCO3


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Henderson-Hasselbalch Eqn

pH = pKa + log [conj base]

[acid]

pOH = pKb + log [conj acid]

[base]

Buffer capacity,

Measure of how well a buffered soln resists

changes in pH when strong acid or base is

added

= no of moles of acid(base)/pH unitchange

highest when ?

Buffer Range= 2 pH range

= the maximum range to which a buffer soln

should be exposed

effective buffer range

HOAc/NaOAc 3.7 5.7

NH3/NH4Cl 8.3 10.3


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A weak acid with Ka = 1.0 x 10-5 was usedto prepare a buffer.

What is the pH of a buffer that is 0.12 M in

benzoic acid and 0.20 M in sodium

benzoate? Ka = 6.5 x 10-5

1. Calculate the pH of a buffer soln preparedby dissolving 12.51 g NaB (121.135) and4.55 g HB (157.596) in 1.00 L water.

Ka,HB = 8.5 x 10-9

2. If we add 12.0 mL of 1.00 M HCl to the

above soln, what will be its new pH?


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Describe how you would prepare a

phosphate buffer with a pH of about 7.40

Given:H3PO4 H2PO4

- Ka1= 7.5 x 10-3

H2PO4-HPO4

2- Ka2 = 6.2 x 10-8

HPO42-PO4

3- Ka3 = 4.8 x10-13

Other buffer soln preparation

Buffers can also be prepared by adding

carefully measured quantity of strong base

to a soln of a weak acid (or strong acid to

a weak base)

AS LONG AS THE WEAK ACID OR WEAK

BASE REMAINS IN EXCESS.

Examples:

1. Na+OH- + HOAc OAc- + H2O

I 0.1 0.2 0

C -0.1 -0.1 + 0.1

E 0 0.1 0.1

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Ionic Equilibria (1)