AcidsBases&Buffers

(1)

Acids, Bases &

Buffers

(2)

Introduction

•• Sourness in foods is

Sourness in foods is

caused by

caused by acids

acids

,,

molecules that release

protons.

•• The chemical opposite,

The chemical opposite,

bases

(3)

Some Substances that are Acids

Foods Foods Vin

Vinegaegar r MusMustartard d PicPickle kle JuJuiceice (all contain(all contain acetic acid acetic acid HHCC₂₂HH₃₃OO₂₂))

O

Orraannggees s LLeemmoonns s GGrraappeeffrruuitits s GGeellaattinins s MMoosst t ssooddaass

(all contain

(all contain citric acid citric acid HH₃₃CC₆₆HH₅₅OO₇₇))

Sour

Sour Milk Milk Buttermilk Buttermilk YogurtYogurt (all contain(all contain lactic acid lactic acid HHCC₃₃HH₅₅OO₃₃))

Cranberry Juice

Cranberry Juice (contains(contains benzoic acid benzoic acid HHCC₇₇HH₅₅OO₂₂))

Apples

Apples Watermelons Watermelons Grapejuice Grapejuice WineWine (all contain(all contain malic acid malic acid H

H₂₂CC₄₄HH₄₄OO₅₅))

Spinach Tomatoes

Spinach Tomatoes (both contain(both contain oxalic acid oxalic acid HH₂₂CC₂₂OO₄₄))

G

Grraappeejjuuiicce e WWiinnee (both contain(both contain tartaric acid tartaric acid HH₂₂CC₄₄HH₄₄OO₆₆)) All acids above are

(4)

Some Substances that are Acids

Other Common Acids

STRONG STRONG STRONG

STRONG WEAKWEAK_WEAK_WEAK

Hydrocloric acid

Hydrocloric acid ((HHClCl)) Carbonic AcidCarbonic Acid ((HH₂₂COCO₃₃))

Nitric Acid

Nitric Acid ((HHNONO₃₃)) Phosphoric AcidPhosphoric Acid ((HH₃₃POPO₄₄))

Sulfuric acid

Sulfuric acid ((HH₂₂SOSO₄₄)) Hydrofluoric AcidHydrofluoric Acid ((HHFF))

Perchloric Acid

Perchloric Acid ((HHClOClO₄₄)) Hydrocyanic AcidHydrocyanic Acid ((HHCNCN Hydrobromic acid

Hydrobromic acid ((HHBrBr)) Sulfurous acidSulfurous acid ((HH₂₂SOSO₃₃)) Hydroiodic Acid

Hydroiodic Acid ((HHII))

How do we recognize an acid by its c

How do we recognize an acid by its chemical formula?hemical formula?

Ans:

(5)

Some Substances that are Bases

Household Products

Household Ammonia

Household Ammonia (contains ammonia ((contains ammonia (NHNH₃₃) in water)) in water)

Drain and Oven Cleaners

Drain and Oven Cleaners (contains(contains sodium hydroxide,sodium hydroxide, NaNaOHOH))

Antacids

Antacids (Rolaids, Tums, Alka Seltzer, Maalox (contains(Rolaids, Tums, Alka Seltzer, Maalox (contains Al(Al(OHOH))₃₃ ))

Milk of Magnesia

Milk of Magnesia (contains(contains Mg(Mg(OHOH))₂₂ ))

Baking Soda

Baking Soda (contain(contains NaHCOs NaHCO₃₃ ))

Toothpaste

Soap

Most cleaning products are

Most cleaning products are basic

basic

(a few glass cleaning

products contain acetic acid or HCl)

(6)

Some Substances that are Bases

Other Common Bases

STRONG STRONG STRONG

STRONG WEAKWEAK_WEAK_WEAK

Lithium hydroxide

Lithium hydroxide ((LiLiOHOH)) AmmoniaAmmonia ((NHNH₃₃))

Sodium hydroxide

Sodium hydroxide ((NaNaOHOH)) AminesAmines (e.g(e.g CHCH₃₃CHCH₂₂NHNH₂₂,, (CH3)

(CH3)₂₂NH, and CNH, and C₅₅HH₅₅N)N)

Potassium hydroxide

Potassium hydroxide ((KKOHOH))

Cesium hydroxide

Cesium hydroxide ((CsCsOHOH))

Calcium hydroxide

Calcium hydroxide ((Ca(Ca(OHOH))₂₂))

Barium hydroxide

Barium hydroxide ((Ba(Ba(OHOH))₂₂ ))

Amines are ORGANIC BASES Amines are ORGANIC BASES

How can we generally recognize a base by its chemical formula?

Ans:

Ans: OH or OH’s at the end of the formula & a metal at theOH or OH’s at the end of the formula & a metal at the beginning of the formula.

(7)

(8)

General Properties of Aqueous Acids and Bases

Aqueous

means dissolved in water

A

Acciidds s hhaavve e a a ssoouur r ttaasstte e BBaassees s hhaavve e a a bbiitttteer r ttaassttee A

Acciidds s aarre e eelleeccttrroollyyttees s BaBassees s aarre e eelleeccttrroollyytteess

(Electrolytes conduct electricity) (Electrolytes conduct electricity)

(9)

Definitions of Acids & Bases

Arrhenius

Acid:

H

++

_{or H}

3

O

++

producer in water

(ex: HCl, HNO(ex: HCl, HNO33, H, H22SOSO44))

Base:

OH

--

_{producer in water}

_(ex:_{(ex: NaOH,}_{NaOH, LiOH,}_{LiOH, Ca(OH)}_Ca(OH)

2 2))

Bronsted-Lowry

Acid:

proton (H

++

_{) donor}

_{(ex: NH}_{(ex: NH}

4

4++, HSO, HSO44--, all Arrhenius acids), all Arrhenius acids)

Base:

proton acceptor

(ex: NH(ex: NH₃₃, H, H₂₂O, COO, CO₃₃2-2-₎₎

Lewis

Acid:

electron-pair acceptor

(ex: H(ex: H++_{, positive ions --Al}_{, positive ions --Al}3+3+_{, Fe}_{, Fe}2+2+₎₎

Base:

(10)

An Acid-Base Reaction

Neutralization

Neutralization is the reaction of an H

is the reaction of an H

++

_(H

3

O

++

) ion from the

acid and the OH

--

_{ion from the base to form water, H}

2 2

O.

O. The neutralization reaction is exothermic and releases approx.

The neutralization reaction is exothermic and releases approx.

56 kJ per mole of acid and base.

H H++ (aq) (aq) + + OHOH --(aq) (aq) HH22OO(l)(l) HH 0 0 rxn rxn = -55.9 kJ= -55.9 kJ

(11)

(12)

Strong and Weak

Acids & Bases

(13)

Strong

Strong acid: acid: HA(HA( g g or or l l ) + H) + H₂₂O(O(l l ) ) HH₂₂OO++₍₍_aq_aq_{) + A}_{) + A}--₍₍_aq_aq₎₎

The Extent of Dissociation for Strong Acids

Complete dissociation

(14)

Weak

Weak acid: acid: HA(HA( aq aq) + H) + H₂₂O(O(l l ) ) HH₂₂OO++₍₍_aq_aq_{) + A}_{) + A}--₍₍_aq_aq₎₎

The Extent of Dissociation for Weak Acids

Partial Dissociation

(15)

Strong & Weak Acids

Strong acids dissociate completely into ions in water:

HA HA_{(g or l)}_{(g or l)} + + HH₂₂OO_(l)_(l) HH₃₃OO++ (aq) (aq) + + AA --(aq) (aq) Nitric a

Nitric acid cid is an is an example: example: HNOHNO_{3 (l)}_{3 (l)} + + HH₂₂OO_(l)_(l) HH₃₃OO++ (aq) (aq) + + NONO33 --(aq) (aq)

Weak

acids d

issociate very

slightly into

ions in

water:

HA

HA_(aq)_(aq) + + HH₂₂OO_(aq)_(aq) HH₃₃OO++ (aq)

(aq) + + AA--(aq)(aq)

In a dilute

In a dilute solution of solution of a weak aca weak acid, the gid, the great majority of reat majority of HAHA molecules are undissociated:

(16)

In the same manner, bases are considered

either strong or weak.

(17)

Classifying the Relative Strengths of Acids and

Bases

Qualitative Classifications

((memorize these

memorize these

))

Strong acids.

Strong acids. There are two types of strong acids:There are two types of strong acids: 1.

1. HCl, HBrHCl, HBr, and, and HIHI

2.

2. Acids Acids containing containing O O in in which which the the # # of of O O atoms atoms exceeds exceeds thethe number of ionizable H atoms by two or more, such as

number of ionizable H atoms by two or more, such as HNOHNO₃₃,,

H

H₂₂SOSO₄₄, HClO, HClO₄₄ Weak acids.

Weak acids. Many more weak acids than strong ones. Four types are:Many more weak acids than strong ones. Four types are: 1.

1. HFHF

2.

2. Those Those acids acids in in which which H H is nis not ot bounded bounded to to O O or or to to a a halogen, halogen, suchsuch as

as HCNHCN andand HH₂₂SS

3.

3. Acids Acids containing containing O O in in which which the the # # of of O O atoms atoms exceeds exceeds by by oneone the number of ionizable H atoms, such as

the number of ionizable H atoms, such as HClOHClO,, HNOHNO₂₂, and, and

H

H₃₃POPO₄₄

4.

(18)

Classifying the Relative Strengths of Acids and

Bases

Qualitative Classifications

((memorize these

memorize these

))

Strong bases.

Strong bases. Soluble compounds containing OSoluble compounds containing O2-2- _{or OH}_{or OH}-- _{ions are strong}_{ions are strong}

bases. The cations are usually those of the most active metals: bases. The cations are usually those of the most active metals: 1)

1) MM₂₂O or MOH, where M= Group 1A(1) metals (Li, Na, K, Rb, Cs)O or MOH, where M= Group 1A(1) metals (Li, Na, K, Rb, Cs) 2) MO or M(OH)

2) MO or M(OH)₂₂, where M = Group 2A(2) metals (Ca, Sr, Ba), where M = Group 2A(2) metals (Ca, Sr, Ba) [MgO and Mg(OH)

[MgO and Mg(OH)₂₂ are only slightly soluble, but the solubleare only slightly soluble, but the soluble portion dissociates completely.]

portion dissociates completely.]

Weak bases.

Weak bases. Many compounds with an electron-rich nitrogen are weak Many compounds with an electron-rich nitrogen are weak bases (none are Arrhenius bases). The common structural feature bases (none are Arrhenius bases). The common structural feature

is an N atom that has a lone electron pair in its Lewis structure. is an N atom that has a lone electron pair in its Lewis structure. 1) Ammonia (:NH

1) Ammonia (:NH₃₃))

2) Amines (general formula RNH

2) Amines (general formula RNH₂₂, , RR₂₂NH, RNH, R₃₃N), such asN), such as CH

(19)

The Hydronium Ion,

H

(20)

H

++

_{comes from acid and combines with H}

2

O

to

yield H

(21)

H

₃₃

O

++

_{Concentration, [H}

3 3

O

+ +

_]

•• The acidity of a solution is normally

The acidity of a solution is normally

specified by the concentration of H

₃₃

O

++

_in

moles per liter of solution, M

–

– Strong

Strong acids:

acids: acid

acid concentration

concentration =

= [H

[H

₃₃

O

++

]]

–

– Weak

Weak acids:

acids: acid

acid concentration

concentration >

> [H

[H

₃₃

O

++

]]

•• Pure water has a [H

Pure water has a [H

₃₃

O

++

] = 1 x 10

-7-7

M.

(22)

Autoionization of Water

H

H₂₂OO_(l)_(l) + + HH₂₂OO_(l)_(l) HH₃₃OO++ (aq)

(aq) + + OHOH--(aq)(aq)

K K _cc == [H[H33OO + +_][OH_][OH--_]] [H [H₂₂O]O]22

The ion-product for water,

K

_w_w

::

K

_cc

[H

₂₂

O]

22

₌

_= K

_K

w

=

[H

33

O

+

_][OH

--

_]

_{= 1.0 x 10}

₌

_{1.0 x 10}

-14-14

_{(at 25}

_°°

_C)

For pure water the concentration of hydroxyl and

hydronium ions must be equal:

[H

[H₃₃OO++_{] = [OH}_{] = [OH}--_]_{] =}₌ _1.0_{1.0 x}_{x 10}₁₀-14-14 ₌_{= 1.0}_{1.0 x}_{x 10}₁₀ -7-7_M_M _{(at 25}_{(at 25}_°°_C)_C)

The

The molarity molarity of of pure pure water water is: is: 1000g/L1000g/L = = 55.555.5 M M

18.02 g/mol 18.02 g/mol

(23)

Acid and Base Character and the pH

Scale

H

++ (aq) (aq)

+

+ H

H

22

O

(l)(l)

H

33

O

+ + (aq) (aq)

[H

+ +

_{] = [H}

3 3

O

+ +

_]]

A scale called the

A scale called the pH scale

pH scale

is used express the H

++

_ion

conc’n in water:

pH = - log[H

₃₃₃₃

O

_O

++++

_]

_]]]

What is the pH of a solution that is 10

What is the pH of a solution that is 10-12-12_M_M_{in hydronium ion ?}_{in hydronium ion ?}

pH = -log[H

pH = -log[H₃₃OO++_{] = (-1)log 10}_{] = (-1)log 10}-12-12 _{= (-1)(-12) =}_{= (-1)(-12) =}

₁₂

What is the pH of a solution that is 7.3 x 10

What is the pH of a solution that is 7.3 x 10-9-9_M_M_in_{in H}_H 3

3OO++ ??

pH = -log(7.3 x 10

pH = -log(7.3 x 10-9-9₎_{) =}₌

_8.14

pH of a neutral solution = 7.00

[H

₃₃

O

++

_{] = [OH}

--

_]

pH of an acidic solution < 7.00

[H

₃₃

O

++

_{] > [OH}

--

_]

pH

(24)

Relationship b/w [H

₃₃

O

++

_{] and [OH}

--

_{] and}

the relative acidity of solutions

(25)

The pH Values of

Some Familiar

Aqueous

Solutions

pH + pOH = 14

For every change of 1 unit

on the pH scale, [H

₃₃

O+ ]

changes by a factor of 10.

(26)

Calculating [H

₃₃

O

++

_{], pH, [OH}

--

_{], and pOH}

Problem

ydi

_ydi

A chemist dilutes concentrated hydrochloric

acid to make two solutions: (a) 3.0

M

and

(b) 0.0024

M

. Calculate the [H

₃₃

O

++

_{], pH, [OH}

--

_],

and pOH of the two solutions at 25

(27)

Calculating [H

₃₃

O

++

_{], pH, [OH}

--

_{], and pOH}

Plan:

Plan: We know that hydrochloric acid is a strong acid, so it

We know that hydrochloric acid is a strong acid, so it

dissociates completely in water; therefore [H

₃₃

O

++

_{] = [HCl]}

init. init.

..

We use the [H

₃₃

O

++

_{] and K}

w

to calculate the [OH

--

_{] and pH as}

well as pOH.

Solution:

(a) [H

(a) [H₃₃OO++_]_{] =}₌ _3.0_3.0_M_M _{pH = -log[H}_{pH = -log[H} 3 3OO + +_{] = -log(3.0) =}_{] = -log(3.0) =} _-0.477_-0.477 [OH [OH--_]_{] =}₌ K K ww ₌₌ ₌₌ _{3.333 x 10}_{3.333 x 10}-15-15_M_M [H [H₃₃OO++_]] 1 x 10 1 x 10-14-14 3.0 3.0 pOH = - log(3.333 x 10 pOH = - log(3.333 x 10-15-15₎_{) =}₌ _14.477_14.477 (b) [H (b) [H₃₃OO++_]_{] =}₌ _0.0024_0.0024_M_M _{pH = -log[H}_{pH = -log[H} 3 3OO + +_{] = -log(0.0024) =}_{] = -log(0.0024) =} _2.62_2.62 [OH [OH--_]_{] =}₌ ₌₌ ₌₌ _{4.167 x 10}_{4.167 x 10}-12-12_M_M pOH = -log(4.167 x 10 pOH = -log(4.167 x 10-12-12₎_{) =}₌ _11.38_11.38 K K _w_w [H [H₃₃OO++_]] 1 x 10 1 x 10-14-14 0.0024 0.0024

(28)

pH (indicator) paper pH (indicator) paper

pH meter pH meter

Methods for Measuring the pH of an

Aqueous Solution

(29)

Acid

Acid Rain:

Rain: Fossil

Fossil Fuel

Fuel Combustion

Combustion

Acid

Rain:

Fossil

Fuel

Combustion

•• SO

SO

22

and NO

22

formed during fossil fuel combustion combine

with atmospheric water to form acid rain.

•• Unpolluted rain is slightly acidic due to atmospheric carbon

Unpolluted rain is slightly acidic due to atmospheric carbon

dioxide.

(30)

(31)

Acid

Acid Rain:

Rain: The

The Effects

Effects

Acid

Rain:

The

Effects

•• The environment into which acid rain falls

The environment into which acid rain falls

determines its fate.

•• In some cases naturally occurring geography can

In some cases naturally occurring geography can

serve to neutralize the acid.

•• Rapid acidification occurs when neutralization is

Rapid acidification occurs when neutralization is

not possible.

(32)

•• Lakes and Streams

Lakes and Streams

–

– Approx. 2000 lakes and streams in the eastern U.S.

Approx. 2000 lakes and streams in the eastern U.S.

have elevated pH.

–

– Some aquatic species cannot survive.

Some aquatic species cannot survive.

–

– U.S. emissions have contaminated Canadian lakes.

U.S. emissions have contaminated Canadian lakes.

•• Building Materials

Building Materials

–

– Acids dissolve stone, marble, paint

Acids dissolve stone, marble, paint

–

– Rusting of steel is accelerated

Rusting of steel is accelerated

•• Forests and Reduced Visibility

Forests and Reduced Visibility

–

– Trees cannot grow and fend off disease.

Trees cannot grow and fend off disease.

–

– Sulfate aerosols account for 50% of visibility

Sulfate aerosols account for 50% of visibility

problems in the eastern U.S.

(33)

Clean Air Act Amendments

•• Cut SO

Cut SO

₂₂

emissions to half of 1980 levels by

2010

–

– Use low-sulfur coal, rremove sulfur before burning

Use low-sulfur coal, rremove sulfur before burning

–

– Use flue gas scrubbers

Use flue gas scrubbers

–

– Conservation and efficiency of customers

Conservation and efficiency of customers

•• SO

SO

₂₂

emission allowances

–

– Can be traded among utilities but congress reduces

Can be traded among utilities but congress reduces

number of allowances as per regulations

(34)

What is a Buffer ?

A solution that changes pH

only

slightly

small amounts of strong

acid or base are added to it .

(35)

The Effect of Addition of Acid or Base to

Unbuffered or Buffered Solutions

pH of

unbuffered

(top) changes

by a

large amount

;

pH of

(36)

3 Kinds of Buffer Solutions

•

• Acid Buffer

Acid Buffer

: a solution of weak acid & its salt

(conjugate base); its pH < 7.

Examples

HNO

₂₂

/NO

₂₂--

_{; HClO}

2 2

/ClO

22 --

_{; ; CH}

_CH

3 3

COOH/CH

33

CO

22

--•

• Base Buffer

Base Buffer

: a solution of weak base & its salt

(conjugate acid); its pH > 7.

Examples

NH

₄₄++

_/NH

3 3

; ; H

H

22

PO

44 --

_{/ HPO}

4 4 2 2 --

_{; (CH}

3 3

))

33

NH

+ +

_/

_/ (CH

_(CH

3 3

))

33

N

•

(37)

Why are Buffers Important ?

•• Human blood & other cell fluids buffered at pH =

Human blood & other cell fluids buffered at pH =

7.40

±±

0.05. A rise in pH above 7.45 (

0.05. A rise in pH above 7.45 ( alkalosis

alkalosis

) or a

drop in pH below 7.35 (

drop in pH below 7.35 ( acidosis

acidosis

) can be life-

) can be

life-threatening. (Intravenous solutions are buffered)

threatening. (Intravenous solutions are buffered)

•• Some chemical reactions can only be done within a

Some chemical reactions can only be done within a

narrow pH range;

•• pH meters have to be calibrated;

pH meters have to be calibrated;

•• Culturing (growing in prepared medium) of bacteria

Culturing (growing in prepared medium) of bacteria

done in specific small pH range;

•• Largest buffered system-the OCEAN-relies on

Largest buffered system-the OCEAN-relies on

presence of bicarbonates (HCO

(38)

How does a buffer resist

pH changes?

(39)

How a Buffer Works

•• Small quantities of H

Small quantities of H

33

O

+

_{(acid) or OH}

--

_{(base) added to}

buffer cause

buffer cause a small amount of one buffer component to

a small amount of one buffer component to

convert into the other

..

•• As long as amounts of H

As long as amounts of H

33

O

+

_{and OH}

--