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pH AND pOH

BY MS IRSHAD BEGUM

ASSISTANT PROFESSORCHEMISTRY DEPARTMENT

DA COLLEGE FOR WOMEN PHASE VIII

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Overview

Acid – Base Concepts

Arrhenius

Bronsted – Lowry

Lewis

Acid and Base Strengths

Self – Ionization of Water

The pH of a Solution

The pOH of a Solution

Calculations for pH and pOH

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Learning Goals and Objectives Students will learn key vocabulary and facts associated with pH, including acid, base.Be able to describe the different definitions of acids and base. Have a basic understanding of pH and its impact on life on earth. Be able to explain why the molecular structure of a molecule can affect its pH.Investigate the usefulness of pH scale, to describe the acidity or basicity of liquids.Describe qualitative relationships between pH and concentration of hydroxide and hydronium ions using the values and the relationships between: pH and hydronium ion concentrationHydronium ion concentration and Hydroxide ion concentrationpH and Hydroxide ion concentration

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ANALYSING

•Identify and classify a list of substances in terms of Acids and Bases or neutral?•Experiment different substances using litmus paper, pH paper and pH meter and analyse by making chart.•Investigate different combinations of strength/concentrations that result in same pH values.•Investigate the usefulness of pH scale, to describe the acidity or basicity of liquids. •Calculate the pH value for 0.001 M NaOH?

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EVALUATION

•Compare qualitatively the difference between Acids (strong and weak acids) and Bases (strong and weak) in terms of Ionization.•Explain how to test Hydrogen ion concentration (pH) and hence the relative acidity using indicator paper and pH scale?•Describe in words (graphs or molecular drawings) what it means if a: Concentrated solution of a weak acid (or base) or concentrated solution of a strong acid (or base) or other combinations.•Tell why the value for pH for 1x10-8 HCl solution will be 7 instead of 8?•Justify how NH3 will be classified as a Base?

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CREATING

•Construct a graph showing the change in pH by mixing different volumes of acid and alkali.

•Can you develop a proposal which would helpful in controlling pH in soils and how excess acidity can be treated with lime?

•Can you give a propose a household remedy for Indigestion?

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INTRODUCTIONWhy pH is important?Our world would be very different if we did not have acids and bases. Most of the food we eat is acid and our stomach produces very strong acids. The acids help us digest our food.Our blood has a pH of about 7.3 which helps our red blood cells to carry oxygen throughout our body. If the pH of water is too high or basic, minerals can settle out of the water causing our water pipes to clog and give us low water pressure. If the pH of our water is too low, or acidic, plumbing fixtures and our hot water heaters can be damaged.In this lesson, students will create their own representation of the pH scale.

pH

APPROACHES• Questioning• Observing and monitoring skills• Encouraging

SKILLS• Computational skills•Collaborative skills• Critical thinking•Creative and Intellectual Curiosity

TECHNIQUES• Experiment•Questions• Observation chart•Power Point Presentation•Worksheets

RESOURCES•Multimedia •Projector•Handouts

CHALLENGES• Availability of material•Availability of the computer lab and multimedia •Misuse of the resources• Discipline

SOLUTIONS• Proper planning•Teachers can bring their own Laptop• Allocation of time •Strict hold

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CONCEPT MAP Acids and Bases

Acid Base

produce produce

H + ions OH- ions

100% Small %

Strong acid

Weak acid

100% Small %

Strong base

Weak base

Ionization in water

gives

H+ OH-

product

[H+] x [OH-]

is

pH

undergo

Neutralization

to form

Salt & Water

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The Chemistry of Acids and BasesThe Chemistry of Acids and Bases

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PROPERTIES OF ACIDS

Acids Produce hydrogen ions (H+) in H2O.

Taste sour. Turn blue litmus (vegetable dye) red.Act as electrolytes in solution.Neutralise solutions containing hydroxide ions (OH -).React with several metals releasing H2(g)

React with carbonates releasing CO2(g).

Destroy body tissue.

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Common Acids HCl- hydrochloric- stomach acidH2SO4- sulfuric acid - car batteries

HNO3 – nitric acid - explosives

HC2H3O2- acetic acid - vinegar

H2CO3-carbonic acid – sodas

H3PO4- phosphoric acid -flavorings

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Bases Produce or cause an increase in hydroxide

ions (OH-) in H2O. Taste bitter .Act as electrolytes in solution .Neutralise solutions containing hydrogen ions(H +). Have a slippery, ‘soapy’ feel .pH greater than 7.Turns red litmus Blue.” Basic Blue”.

AlkaliA soluble base is called an Alkali and in aqueous solution it produces hydroxide ions (OH-) .

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Common BasesNaOH- sodium hydroxide (LYE) soaps, drain

cleanerMg (OH)2 - magnesium hydroxide-antacids

Al(OH)3-aluminum hydroxide- deodorants

NH4OH-ammonium hydroxide- “ammonia”

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ARRHENIUS CONCEPT OF ACID and Base

In the 1800’s chemical concepts were based on the reactions of aqueous solutions.

Svante Arrhenius developed a concept of acids and bases relevant to reactions in H2O.

Arrhenius acid – produces hydrogen ions in water.Arrhenius base – produce hydroxide ions in water.

(Problem: Some Bases don’t have (OH-) ion.)

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Arrhenius Acid: is a substance that produces H+(H3O+)in water.

Arrhenius Base: is a substance that produces (OH-) in water.

How Ammonia (NH3) will acts as a Base?

Aqueous ammonia (NH3) does not fit the Arrhenius definition of a base because it has no OH- to release:

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NH3 + OH-H2O

?

Answer:

The answer to this question is that the water is acting not only as a solvent, but also as a reactant:

The ammonia molecule is splitting the water molecule, holding onto the proton (H+), and releasing the hydroxyl ion (OH-).

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

BRONSTED-LOWRY CONCEPT

A broader ,more modern concept of acids and bases

was developed later.

Bronsted-Lowry acid: donates a proton in a reaction.

Bronsted-Lowry base: accepts a proton in a reaction.

What is meant by a proton here?

A “proton” is really just a hydrogen atom that has lost it’s electron!

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CONJUGATE ACID AND BASE

Conjugate acid: compound formed when an base gains a hydrogen ion.

Conjugate base: compound formed when an acid loses a hydrogen ion.

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A Brønsted-Lowry acid is a proton donor.A Brønsted-Lowry base is a proton acceptor.

acid conjugate base

base conjugate acid

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Conjugate Pairs

LEWIS ACID BASE CONCEPT

Lewis acid : a substance that

accepts an electron pair.

Lewis base : a substance that donates an electron pair.

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Lewis Acid/Base Reaction

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Basicity OF AN ACIDBASICITY OF AN ACID : refers to the number of replaceable hydrogen ion in one molecule of the acid.

Monobasic Acid: 1 molecule of an acid produce 1H+ ion upon ionization.Example: HCl, HNO3

HCl(aq) → H+ (aq) + Cl-(aq)

Dibasic Acid:1 molecule of an acid produce 2H+ ion upon ionization. Example: H2SO4 H2SO4(aq) → 2H+(aq) + SO4

2-(aq)

Tribasic Acid:1 molecule of an acid produce 3H+ ion upon ionization. Example: H3PO4

H3PO4(aq) → 3H+(aq) + PO43-(aq)

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ACIDITY OF A BASEACIDITYOF A BASE : refers to the number of replaceable hydroxyl ion in one molecule of the base.

Monoacid Base:1 molecule of a base produce 1(OH-) upon ionization.Example: NaOH,KOH

NaOH → Na+ (aq) + OH- (aq)

Diacid Base:1 molecule of a base produce 2(OH-) upon ionization.Example: Ca(OH)2

Ca(OH)2 → Ca2+ (aq) + 2OH- (aq)

Triiacid Base:1 molecule of a base produce 3(OH-) upon ionization.Example: Al(OH)

3 Al(OH)

3 → Al3+ (aq) + 2OH- (aq)

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HNO3, HCl, H2SO4 and HClO4 are among the only known strong acids.

Strong and Weak Acids/Bases

The strength of an acid (or base) is determined by the amount of IONIZATION.The strength of an acid (or base) is determined by the amount of IONIZATION.

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A strong acid essentially ionizes 100%.

An example of a strong acid is hydrochloric acid, HCl (aq)

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

0.10 mol 0.10 mol 0.10 mol

97-98 % ionization

few molecules many ions

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An example of a weak acid is acetic acid, CH3COOH.

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

(aq)

0.10 mol << 0.10 mol << 0.10 mol

5% ionization at 25Cmany molecules few ions

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A strong base ionizes 100%.

An example of a strong base is sodium hydroxide, NaOH.

NaOH(s) + H2O(l) Na +(aq) + OH-

(aq) 0.10 mol 0.10 mol 0.10 mol

97-98 % ionizationfew formula units (NaOH) many ions

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A weak base ionizes to a small extent.

An example of a weak base is NH3(g).

NH3(g) + H2O(l) NH4+

(aq) + OH-(aq)

0.10 mol <<0.10 mol <<0.10mol

5% ionization at 25C

many molecules few ions

33Timberlake, Chemistry 7th Edition, page 330

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H+ A - H+ A - HA

A - H+ A - H+ A –

H+ A - H+ A - H+

A - HA H+ A -

H+ A - H+ A - H+

HA HA HA HA

HA HA HA

H+ A - HA HA

HA HA H + A –

HA H + A – HA HA

H+ A- H+ A- H+ A- H+ A- HAA- H+ A- H+ A- H+ A- H+ A -

H+ A- HA H+ A- H+ A- H+ A-

A- H+ A- H+ A- H+ A- H+ A- H+ H+ A - H + A - H + A - HA H + A -

A- H+ A- H+ A- H+ A- H+ A–

H+ A- H+ A- H+ A- H+ A- H+ A- H+ A- H+ A- H+ A- H+ A-

HA A- H+ A- H+ A- H+ A- H+

HA HA H+ A- HA HA HA HA HA HA HA HA H+ A- H+ A- HA HA HA HA HA

HA HA H+ A- HA HA HA HA HA HA H+

A- HA HA H+ A- HA HA HA HA HA HA HA HA H+

A- HA HA H+ A- HA HA HA HA HA HA HA H+ A- HA HA HA

DILUTECONCENTRATED

STRO

NG

WEA

K

STRONG ACIDS

Dissociate nearly 100%

HA H1+ + A-

WEAK ACIDS

Dissociate very little

HA H1+ + A-

Acids: Concentration vs Strength

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pH Scale

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pH Scale pH scale was developed by (Dr. Sorenson) in the early1909.

pH Scale is used to measure the strength of an acid or base.

pH is defined as the –log [H+]

pH can use the concentration of hydronium ions or hydrogen ions.

From the French pouvoir hydrogene (“hydrogen power” or power

of hydrogen)

Copyright © 2009 by Pearson Education, Inc.

www.wikipedia.com

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Self-Ionization Of Water Amphoteric substance: A substance that can act as either an acid or base.

(e.g. H2O)

H2O An Acid: NH3 + H2O NH4+ + OH-

H2O As a Base: H2CO3 + H2O HCO3- + H3O+

The pure water conducts electricity. water self-ionizes

H2O + H2O H3O+ + OH-

Kw = [H3O+][OH-]

Kw - Ionic product of water Kw = 1.0 x 10-14 at 25 oC

This equilibrium constant is very important because it applies to all aqueous solutions - acids, bases, salts, and non-electrolytes - not just to pure water.

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OH

H

OH

HO

Self ionization reaction of water:

+O

HH

H

OHOHOH2 32

C)25(at10][OH]O[HK -143w

[OH-]

Kw]O[H3

H

+

+-

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pH

pH = -log [H+]

Kelter, Carr, Scott, Chemistry A World of Choices 1999, page 285

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pH and pOH

pH = - log[H3O+] [H3O+] = 10-pH

pOH = - log[OH-] [OH-] = 10-pOH

pKw = pH + pOH = 14.00

Neutral solution: [H3O+]=[OH-]=10–7 M pH = 7.0

Acidic solution: [H3O+] > 10-7 M pH < 7.0

Basic solution: [H3O+] < 10-7 M pH > 7.0

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Acid – Base Concentrations

pH = 3

pH = 7

pH = 11

OH-

H3O+OH-

OH-H3O+

H3O+

[H3O+] = [OH-] [H3O+] > [OH-] [H3O+] < [OH-]

acidicsolution

neutralsolution

basicsolution

conc

entr

ation

(mol

es/L

)

10-14

10-7

10-1

Timberlake, Chemistry 7th Edition, page 332.

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The pH scale is a way of expressing the strength of acids and bases. Instead of using very small numbers, use the NEGATIVE power of 10 on the Molarity of the H+ (or OH-) ion.

Below 7 = acid 7 = neutral

Above 7 = base

Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 515

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pH of Common Substances

Timberlake, Chemistry 7th Edition, page 335.

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Reactions with indicators

Indicator Acid color Neutral color

Base color

Phenolphthalein Colorless Faint pink Dark pink

Bromthymol blue Yellow Green Blue

Litmus Red ----- Blue

Methyl Orange Orange Red

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pH testing

There are several ways to test pH

Blue litmus paper (red = acid)

Red litmus paper (blue = basic)

pH paper (multi-colored)

pH meter (7 is neutral, <7 acid, >7 base)

Universal indicator (multi-colored)

Indicators like phenolphthalein

Natural indicators like red cabbage, radishes

Paper testingPaper tests like litmus paper and pH paperPut a stirring rod into the solution and stir.Take the stirring rod out, and place a drop

of the solution from the end of the stirring rod onto a piece of the paper

Read and record the color change. Note what the color indicates.

Use a small portion of the paper. Use one piece of paper for several tests.

pH meter

Tests the voltage of the electrolyte

Converts the voltage to pH

Very cheap, accurateMust be calibrated with

a buffer solution

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pH indicators Indicators are dyes that can be added

that will change color in the presence of an acid or base.

Some indicators only work in a specific range of pH.

Once the drops are added, the sample is ruined.

Some dyes are natural, like radish skin or red cabbage.

Universal Indicator

To measure pH, an indicator that has a different colour for each value of pH

Universal Indicator

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Since [H3O+], [H+], and [OH-] are so small, scientists use the concept of p when dealing with very small numbers, where p stands for - log.

pN = - log N

pH = - log [H3O+]= - log [H+]

pOH = - log [OH-]

pKw = - log Kw

pKa = - log Ka

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FORMULAe TO REMEMBER

pH = – log [H+]

pOH = – log [OH–]

[H+] = 10–pH

[OH–] = 10–pOH

pH + pOH = 14

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Calculating the pH

pH = - log [H+]

(Remember that the [ ] mean Molarity)

Example: If [H+] = 1 X 10-10

pH = - log 1 X 10-10

pH = - (- 10)

pH = 10

Example: If [H+] = 1.8 X 10-5

pH = - log 1.8 X 10-5

pH = - (- 4.74)

pH = 4.74

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pH calculations – Solving for [H+]pH calculations – Solving for [H+]If the pH of Coke is 3.12,Compute [H+] =?

Because pH = - log [H+] then

- pH = log [H+]

Take antilog (10x) of both sides and get

10-pH = [H+]

[H+] = 10-3.12 = 7.6 x 10-4 M

*** to find antilog on calculator, press“Shift” or “2nd function” and then the log button

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pH calculations – Solving for H+Q A solution has a pH of 8.5. What is the Concentration of hydrogen ions in

the solution?

Solution:

pH = - log [H+]

8.5 = - log [H+]

-8.5 = log [H+]

Antilog -8.5 = antilog (log [H+])

10-8.5 = [H+]

3.16 X 10-9 = [H+]

pH = - log [H+]

8.5 = - log [H+]

-8.5 = log [H+]

Antilog -8.5 = antilog (log [H+])

10-8.5 = [H+]

3.16 X 10-9 = [H+]

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pOHSince acids and bases are opposites, pH and pOH are

opposites!

pOH does not really exist, but it is useful for changing bases to pH.

pOH looks at the perspective of a basepOH = - log [OH-]

Since pH and pOH are on opposite ends,pH + pOH = 14

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[H3O+], [OH-] and pH

What is the pH of the 0.0010 M NaOH solution?

Solution :

[OH-] = 0.0010 (or 1.0 X 10-3 M)

pOH = - log 0.0010

pOH = 3

pH = 14 – 3 = 11

OR

Kw = [H3O+] [OH-]

[H3O+] = 1.0 x 10-11 M

pH = - log (1.0 x 10-11) = 11.00

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[OH-]

[H+] pOH

pH

10 -pOH

10 -pH-log[H+]

-log[OH

-]

14 -

pOH

14 -

pH

1.0

x 10-1

4

[OH

- ]

1.0

x 10-1

4

[H

+ ]

ExerciseIdentify each solution as

1. acidic 2. basic 3. neutral

A. _____ HCl with a pH = 1.5

B. _____ Pancreatic fluid [H+] = 1 x 10-8 M

C. _____ Sprite soft drink pH = 3.0

D. _____ pH = 7.0

E. _____ [OH- ] = 3 x 10-10 M

F. _____ [H+ ] = 5 x 10-12

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Solution:Identify each solution as

1. acidic 2. basic 3. neutral

A. _1__ HCl with a pH = 1.5

B. _2__ Pancreatic fluid [H+] = 1 x 10-8 M

C. _1__ Sprite soft drink pH = 3.0

D. _3__ pH = 7.0

E. _1__ [OH-] = 3 x 10-10 M

F. _2__ [H+] = 5 x 10-12

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APPLICATIONS OF pH ACID RAIN:Pollution in the air(sulfurdioxide,carbondioxide,nitrogendioxide)

combines with water to form various acids.

FISH TANKS AND PONDS:Rapid changes in pH can kill fish and other organisms in lakes and streams.

SOIL:Soil pH is affected and can kill plants and create sinkholes.

BLOOD: pH of Human blood is 7.3 maintained by bicarbonates,phosphates and complex protiens system.

DIGESTION OF FOODSWIMMING POOL

referenceswww.cottonchemistry.bizland.comwww.ndpteachers.orgwww.chalkbored.comwww.tutorvista.comwww.authorstream.comwww.wikipedia.comGeneral Chemistry by wittens ,DavisGeneral Chemistry by Hill , Petrucci