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Acid and Base

Dr. Wing Fat Chan wfchan@cuhk.edu.hk

Department of ChemistryThe Chinese University of Hong Kong

Reference Material for 5th IJSO Training Internal Use Only – Do not duplicate or distribute

( 酸和鹼 )

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How much do you know about acid and base?

Have you watched the ETV science program “Acid & Base” before?

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Definitions of Acid and Base ( 酸和鹼的定義 )

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I. Arrhenius Theory (1884)

- are substances that dissociate in water to give hydrogen ions.

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

Acids

Bases

- are substances that dissociate in water to give hydroxide ions.

NaOH (s) Na+ (aq) + OH- (aq)

Acid + Base Salt + Water

Arrhenius Acid-base Reaction:

(Swedish)

( 阿列紐斯學說 )

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II. Bronsted-Lowry Theory (1923)

- are substances that donate protons Acids

HCl (g) + H2O (l) H3O+ (aq) + Cl- (aq)

- are substances that accept protons Bases

Conjugate Acid-Base Pair: consists of two substances related to each

other by accepting or donating a proton

Bronsted-Lowry Acid-base Reaction:

(Bronsted – Danish)(Lowry – English)

AcidAcid BaseBase Conjugate Acid

Conjugate Acid

Conjugate Base

Conjugate Base

( 布侖斯特 - 路里學說 )

( 共軛酸鹼對 )

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III. Lewis Theory (1923)

Acids

Bases

- are electron-pair acceptors

- are electron-pair donors

Example of Lewis Acid-Base Reaction:

Ag+ + 2 :NH3 [ H3N: Ag :NH3 ]+

(American)

AcidAcid BaseBase

( 路易斯學說 )

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Acids:

Mineral acids

Organic Acids

Hydrochloric acid (HCl)

Sulphuric acid (H2SO4)

Nitric acid (HNO3)

Phosphoric acid (H3PO4)

Ethanoic acid (acetic acid)

Citric acid

Tartaric acid

Oxalic acid

( 檸檬酸 )

( 酒石酸 )

( 草酸 )

( 醋酸 )

( 磷酸 )

( 硝酸 )

( 硫酸 )

( 氫氯酸 )

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Concentrated Acid

Dilute Acid

The usual molarity of three common mineral acids in the laboratory

HCl HNO3 H2SO4

Dilute 2M 2M 1M

Concentrated

11M 16M 18M

Concentration of acid or base can sometimes affect very much its properties.

Concentration of acid or base can sometimes affect very much its properties.

Concentration

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Molarity:

Concentration Unit

L

mol

SolutionofLitres

SoluteofMoleMMolarity )(

e.g. one mole of CO2 = ?

g = 12 + 16 x 2

= 44 g

One mole ( 摩爾 ) of substance is the amount corresponding to its formula mass expressed in gram unit.

One mole ( 摩爾 ) of substance is the amount corresponding to its formula mass expressed in gram unit.

( 摩爾濃度 )

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Calculate the amount of NaCl needed to prepare 100 mL of 1M NaCl solution. (Atomic masses of Na: 23 and Cl: 35.5)

litre

moleyM

1.01

moley 1.0

yneededNaClofAmount )5.3523(

1.05.58 x

grams85.5

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Properties of Dilute Acids

• a sour taste (never taste any substance in the

laboratory)

• turn blue litmus paper to red

• conduct electricity

• react with metals higher than copper in the reactivity

series and give hydrogen gas

• react with metal oxides and hydroxides and give salt

and water

• react with carbonates and hydrogencarbonates and

give salt, carbon dioxide and water

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Basicity ( 鹽基度 ) of an acid:

e.g.

monobasic

dibasic

tribasic

HCl (aq) H+ (aq) + Cl- (aq)HNO3 (aq) H+ (aq) + NO3

- (aq)CH3COOH (aq) H+ (aq) + CH3COO-

(aq)H2SO4 (aq) 2H+ (aq) + SO4

2- (aq)H2CO3 (aq) 2H+ (aq) + CO3

2- (aq)H2C2O4 (aq) 2H+ (aq) + C2O4

- (aq)

H3PO4 (aq) 3H+ (aq) + PO43-

(aq)

the number of ionizable hydrogen atom in one molecule of the acid.

the number of ionizable hydrogen atom in one molecule of the acid.

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Properties of Concentrated Acids• highly corrosive

Conc. HNO3: • strong oxidizing agent

Mg (s) + 2NO3- (aq) + 4H+ Mg2+ + 2NO2 (g) +

2H2O (l)C (s) + HNO3 (aq) CO2 (g) + 4NO2 (g) +

2H2O (l)

Conc. H2SO4:

• hygroscopic properties

• dehydrating properties

• strong oxidizing agent

Cu (s) + 2H2SO4 (l) CuSO4 (s) + SO2 (g) + 2H2O (l)

C12H22O11 (s) 12C (s) + 11H2O (l)conc. H2SO4

( 吸濕性 )

( 脫水性 )

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Bases:

Metal oxides

Metal hydroxides

Ammonia

Common alkalis:

Sodium hydroxide NaOH

Potassium hydroxide KOH

Calcium hydroxide Ca(OH)2

Ammonia NH3

Alkalis : bases that are soluble in water

Alkalis : bases that are soluble in water

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Properties of Aqueous Alkalis• usually bitter taste (never try to check this yourself)

• a soapy feel (conc. KOH & NaOH attack and burn the skin)

• turn red litmus paper to blue

• conduct electricity

• react with acid to give salt and water

• react with carbon dioxide to give carbonate and water

• react with metal ions in aqueous solutions to give insoluble metal hydroxides

e.g. Ca(OH)2 (aq) + CO2 (g) CaCO3 (s) + H2O (l) (limewater

test)

Cu+2 (aq) + 2OH- (aq) Cu(OH)2 (s)

Mg+2 (aq) + 2OH- (aq) Mg(OH)2 (s)

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Strength ( 強度 ) of Acids and Bases

Strong acid- fully or highly ionizes in water

Weak acid

- ionizes only slightly in water

CH3COOH (aq) H+ (aq) + CH3COO- (aq)

e.g. HNO3. H2SO4

e.g. H2CO3, most organic acids

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

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Strength of Acids and BasesStrong base

- fully or highly ionizes in water

Weak base

- ionizes only slightly in water

NH3 (aq) + H2O (l) NH4+ (aq) + OH-

(aq)

e.g. KOH, Ca(OH)2

NaOH (aq) + water Na+ (aq) + OH-

(aq)

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e.g.

1M HCl strong dilute

12M HCl strong

concentrated

1M CH3COOH weak dilute

12M CH3COOH weak

concentrated

Don’t mix up the terms strengths and concentration for acids and bases!

Don’t mix up the terms strengths and concentration for acids and bases!

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How to prepare dilute acid or base solution from the concentrated solution?

Never add water to the concentrated solution and add slowly stirring the concentrated solution to water in order to avoid splashing of solution due to local heating

What to do if any of the concentrated acid or base is split on skin?

Always handle concentrated HCl and HNO3 in the fume hood as corrosive fumes will come out.

Safely Handling of Acid and Base:

Wash immediately with plenty of water on the affected skin and then looking for any medical treatment!

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Amphoteric Substance:- Substance that behaves either as acid or as base.

e.g. H2O (l) + H2O (l) H3O+ (aq) + OH-

(aq)At 25oC,

[H3O+] = [OH-], = 1.0 x 10-7 M

In all aqueous solutions, Kw = 1.0 x 10-14 M2

When [H3O+] = [OH-], it is neutral.When [H3O+] > [OH-], it is acidic.When [H3O+] < [OH-], it is basic or alkaline.

Kw = [H3O+] x [OH-] = 1.0 x 10-14 M2 Water ion-product constant

( 酸鹼兩性的 )

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Acidity and Alkalinity

pH value- is equal to –log [H3O+]

For water at 25oC,

pH = -log [1.0 x 10-7] = 7 (neutral)

When an acid is added to the water,

[H3O+] > [OH-], pH < 7 (acidic)When a base is added to the water,

[H3O+] < [OH-], pH > 7 (basic)

[H3O+] = [OH-], = 1.0 x 10-7 M

How can we compare the acidity or alkalinity of a solution?

How can we compare the acidity or alkalinity of a solution?

( 酸鹼度 )

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Calculate the pH values for the following solutions,

(i) 0.1M HCl

(ii) 1M HCl

(iii) 0.1M NaOH

(iv) 1M NaOH

(i) = –log [0.1] = – (–1) = 1

(ii) = –log [1] = – 0 = 0

(iii) = –log [(1.0 x 10-14)/0.1] = – (–13) = 13

Kw = [H3O+] x [OH-] = 1.0 x 10-14 M2

(iv) = –log [(1.0 x 10-14)/1] = – (–14) = 14

Is pH value of 0.1M CH3COOH equal to that of 0.1M HCl?

Is pH value of 0.1M CH3COOH equal to that of 0.1M HCl?

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pH scale and the pH values of some common substances

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Acidity increases Alkalinity increases

1M HCl

0.1M HCl

lemon juice

vinegar

0.1M CH3COOH

soft drinks rain

waterpure water

NaHCO3 solution

Milk of MagnesiaHousehol

d Ammonia

NaCO3 solution

Limewater

0.1M NaOH

1M NaOH

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How to measure the pH value of solution?

Red & blue litmus papers - test only acidic or

basic

Acid-base indicators

Phenolphthalein

pH: 1 112 3 4 5 6 7 8 9 10

OH-

H+

OH-

H+C

C

OH

O

OH

OH

O-

Colorless

C

C

O

O-

O-

O

Pink

C

C

O

O

OH

OH

Colorless

Phenolphthalein

pH: 1 112 3 4 5 6 7 8 9 10pH: 1 112 3 4 5 6 7 8 9 10

OH-

H+

OH-

H+

OH-

H+

OH-

H+C

C

OH

O

OH

OH

O-

Colorless

C

C

OH

O

OH

OH

O-

Colorless

C

C

O

O-

O-

O

Pink

C

C

O

O-

O-

O

Pink

C

C

O

O

OH

OH

Colorless

C

C

O

O

OH

OH

Colorless

( 酸鹼指示劑 )

( 石蕊試紙 )

( 酚酞 )

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Acid-base indicators

Methyl Orange

pH: 1 112 3 4 5 6 7 8 9 10

OH-

H+N N-O3S N(CH3)2

+

H

Pink

N N-O3S N(CH3)2

Yellow

Methyl Orange

pH: 1 112 3 4 5 6 7 8 9 10pH: 1 112 3 4 5 6 7 8 9 10

OH-

H+

OH-

H+N N-O3S N(CH3)2

+

H

Pink

N N-O3S N(CH3)2+

H

Pink

N N-O3S N(CH3)2

Yellow

N N-O3S N(CH3)2

Yellow

Thymol Blue

pH: 1 112 3 4 5 6 7 8 9 10

pK1=1.7 pK2=8.9OH

SO3-

HO+

Red

OH

SO3-

O

Yellow

O-

SO3-

O

Blue

Thymol Blue

pH: 1 112 3 4 5 6 7 8 9 10pH: 1 112 3 4 5 6 7 8 9 10

pK1=1.7pK1=1.7 pK2=8.9pK2=8.9OH

SO3-

HO+

Red

OH

SO3-

HO+

Red

OH

SO3-

O

Yellow

OH

SO3-

O

Yellow

O-

SO3-

O

Blue

O-

SO3-

O

Blue

Only get a rough idea of the acidity or alkalinity of a solution!

Only get a rough idea of the acidity or alkalinity of a solution!

( 甲基橙 )

( 百里酚藍 )

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Universal indicators- a mixture of several indicators

- different colors at different pH

- in solution or paper ( pH paper) form

Exact pH value:

Video

pH meter

( 通用指示劑 )

(pH 計 )

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Applications of Acids and Base to Dairy Life

(I) Control Soil pH:

- add powdered limestone CaCO3 or slaked lime Ca(OH)2

For alkaline soil:

- add alums AlK(SO4)2.12H2O or acidic fertilizers (NH4)2SO4

For acidic soil: (due to acid rain or extensive uses of acidic fertilizers)

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Applications of Acids and Base to Daily Life

(II) Neutralization of Excess Acid in Stomach:

- excess acid in stomach causing stomach pain

Mg(OH)2 (s) + 2HCl (aq) MgCl2 (aq) + 2H2O (l)weak base

Milk of Magnesia ( 鎂奶 ) can help to neutralize the excess acid.

Milk of Magnesia ( 鎂奶 ) can help to neutralize the excess acid.

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(III) Treatment of Insect Sting

Acidic ant and Bee Stings:

Relief of uneasy feeling due to insect stings:

- using a weakly alkaline solution to swollen part

Alkaline wasp stings - alkaline

- neutralize with a weakly acidic a solution.

Applications of Acids and Base to Dairy Life

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Neutralizationacid + base salt + water

neutralization

e.g. HCl (aq) + NaOH (aq) NaCl (aq) + H2O (l)

H+ (aq) + Cl- (aq)

Na+ (aq) + OH- (aq)

Na+ (aq) + Cl- (aq)

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

Insoluble base:

e.g. CuO (s) + H2SO4 (aq) CuSO4 (aq) + H2O (l)

CuO (s) + 2H+ (aq) Cu2+ (aq) + H2O (l)

H+ (aq) + O2- (s) H2O (l)

Neutralization is an exothermic reaction

Neutralization is an exothermic reaction

( 中和作用 )

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Neutralization Titration:- to analyze quantitatively the amount of acid

or base in a sample solution

Procedure:(i) Preparation of standard solution

(ii)Add standard solution (titrant) to the sample solution (analyte) or vice versa

(iii)Determine the volume of titrant added at the equivalent point

Equivalent point for neutralization titration:

- occurs when equal amounts of acid and base are reacted.

( 中和作用滴定法 )

( 等效點 )

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Titration Curve:

0123456789

1011121314

0 10 20 30 40 50

pH

Volume of titrant added

Equivalent point

Strong acid titrated with strong base

( 滴定曲線 )

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How to determine the equivalent point?

(i) Acid/base Indicators

Indicator Acid ColorTransition Range (pH)

Base Color

Thymol blue Red 1.2 – 2.8 yellow

Methyl Orange

Red 3.1 – 4.4 yellow

Litmus Red 5.0- 8.0 Blue

Bromothymol Blue

Yellow 6.0 – 7.6 Blue

Thymol Blue Yellow 8.0 – 9.6 Blue

Phenolphthalein

Colorless 8.0 – 9.6 Red

Nitramine Colorless 10.8 – 13.0Orange-Brown

Fountain experiment:

Video 1

Video 2

Video 3

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0123456789

1011121314

0 10 20 30 40 50

pH

Volume of titrant added

Equivalent point

Strong acid titrated with strong base

Phenolphthalein

Methyl orange

Color Change for Indicators

Thymol blue

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0123456789

1011121314

0 10 20 30 40 50

pH

Volume of titrant added

Equivalent point

Weak acid titrated with strong base

Phenolphthalein

Methyl orange

Color Change for Indicators

(Endpoint: an observable physical change at or near the equivalent point)

Using indicators, we can only detect the endpoint and not the equivalent point.

Using indicators, we can only detect the endpoint and not the equivalent point.

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0123456789

1011121314

0 10 20 30 40 50

Weak base titrated with strong acid

Plot the graph for:

pH

Volume of titrant added

Equivalent point

Phenolphthalein

Methyl orange

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Titration:

(i) Preparation of standard solutionVideo

(ii) Transfering a fixed amount of solution

(iii) Performing the titration

Video

Video

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(ii) pH meter

0123456789

1011121314

0 10 20 30 40 50

pH

Volume of titrant added

Equivalent point

Strong acid titrated with strong base

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Titration Calculations:0.1000 M NaOH was used to titrate with 25.00 mL HCl solution of unknown concentration. 28.25 mL of NaOH was needed to reach the endpoint. What is the molarity of the HCl solution?

HCl (aq) + NaOH (aq) NaCl (aq) + H2O (l)1 mole 1 mole

Remember:

L

mol

SolutionofLitres

SoluteofMoleMMolarity )(

Mole = (Molarity).(Litres of solution)

= C.V

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HCl (aq) + NaOH (aq) NaCl (aq) + H2O (l)

1 mole : 1 mole

C1V1 C2V2

1

221

.

V

VCC

00.25

25.281000.0 x

M1130.0

Calculation:

The molarity of HCl solution is 0.1130 M.

The molarity of HCl solution is 0.1130 M.

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(i) What happens if H2SO4 is used instead of HCl?

(ii) What happens if Na2CO3 is used instead of NaOH?

Equation:

H2SO4 (aq) + 2NaOH (aq) Na2SO4 (aq) + 2H2O (l)

1 mole : 2 mole

C1V1 C2V2

2

1

22

11 VC

VC

2HCl (aq) + Na2CO3 (aq) 2NaCl (aq) + CO2 (g) + H2O (l)

2 mole : 1 mole

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Buffer solutions- solutions that resist sharp change in pH with the

addition of small amount of acid or base

Applications:

(i) The pH of blood plasma is buffered around pH 7.35-7.45

due to H2CO3 and HCO3-.

(ii)Buffer solutions can help to keep enzymes working in

organisms

(iii)Fermentation processes

(iv)Colouring of fabric needs a correct pH condition.How does the buffer solution work?

How does the buffer solution work?

( 緩衝溶液 )

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HA (aq) + H2O (l) H3O+ (aq) + A- (aq)

Recall: Bronsted-Lowry Theory for Acid and Base Reaction

HCl (g) + H2O (l) H3O+ (aq) + Cl- (aq)Acid

Conjugate BaseBase

Conjugate Acid

Buffer solution- Consists of weak acid and its conjugate base (more common)

- or weak base and its conjugate acid (less common)

Acid Conjugate Base

e.g.

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CH3COOH (aq) + H2O (l) H3O+ (aq) + CH3COO- (aq)

Addition of base:H3O+ is consumed. The equilibrium of system will shift

to the right to produce more H3O+.

Addition of acid: A- is protonated to give HA. The equilibrium of system

will shift to the left and the pH of the system won’t change much.

e.g. Acetic acid / sodium acetate

HA (aq) + H2O (l) H3O+ (aq) + A- (aq)Acid Conjugate

Base

HA can only produce small amount of A-. In order to have enough A- in the solution, soluble salt of A- has to be added.

HA can only produce small amount of A-. In order to have enough A- in the solution, soluble salt of A- has to be added.

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Acid Rain:

- defined as any type of precipitation with an unusual low pH (generally with pH < 5)

- formed when the oxides of nitrogen (NOx) and sulphur (SO3) dissolve in water droplets in the air.

Where did those oxides come from?

Mainly from:

Power Plants

Automobile Emissions

( 酸雨 )

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NOx (g) + H2O (l) + O2 (g) HNO3 (aq)

SO3 (g) + H2O (l) H2SO4 (aq)

Effects of Acid Rain

• cause severe damage to aquatic environmemt

• cause death of leaves, needles and sometimes

trees

• cause deterioration of buildings and monuments

• affect the human health from particulates in air

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Solutions to acid rain

(i) To remove the oxides of sulphur from coal-

burning emissions by installing scrubbers

(ii)To dump the powdered limestone to the lake to

increase its buffer capacity

(iii) To reach international agreement on the

reduction of sulphur emissions

CaCO3 (aq) + H2O (aq) HCO3- (aq) + Ca2+

(aq)