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EQUILIBRIUM

By : Ainun Jariah Winda Hayati

Introduction

Glossary References

Supplementary QuestionsMaterialContents

*IntroductionFor centuries people had known that vinegar, lemon

juice,apple and many other food items taste sour. But they didn’t know that their sourness comes from their specific acids. The term acid comes from the Latin word “acere” ,which means sour. In the 17thcentury,the English Chemist Robert Boyle grouped substances as either acids or bases, but he couldn’t explain their behavior. The first logical definition wouldn’t be coined until 200 years later.

In this presentation ,we will focus on acid-base reactions in aqueous solutions. such solutions play important roles in our daily lives.

The Authors

Introduction

Glossary References


Content1. Acid-Base Theories

1.a. The Arrhenius Acid-Base Theory1.b. The BrФnsted-Lowry Acid-Base Theory1.c. The Lewis Theory1.d. General Properties of Acids1.e. General Properties of Bases

2. Ionization of Water3. The pH Scale4. Strength of Acids and Bases

4.a. Acid Strength4.b. Base Strength

5. Neutralization

Introduction

Glossary References


1. Acid-Base Theories

1.a. The Arrhenius Acid-Base TheoryIn 1884, Swedish chemist Svante Arrhenius defined acids as compounds that produce H+ ions in aqueous solutions,and base as compounds that produce OH-- ions in aqueous solutions.

For example,HCL and are HNO3 Arrhenius acids.HCL(aq) ------------→ Cl - (aq) + H+ (aq) HNO3 (aq) ---------- → NO

3- (aq) + H+ (aq)

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NaOH and Ba(OH)2 are example of Arrhenius bases. These bases release OH-- ions in aqueous media.

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

Ba(OH)2 (aq) ----------→ Ba 2+(aq) + 2OH - (aq)In water, the H+ ion of acid is attracted to a water(H2O) molecule to produce hydronium ion (H3O+ )

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• The Arrhenius acid-base theory is insufficient to explain the acidic or basic properties of some substances,such as SO2 and NH3

since these don’t have H + and OH- ions in their structures. For these molecules,another theory must be applied,since the Arrhenius acid-base theory can only be applied to aqueous solutions.

Introduction

Glossary References


1.Acid-Base Theories1.b. The The BrФnsted-Lowry Acid-Base Theory

In 1923,Johannes BrФnsted and his English counterpart Thomas Lowry independently developed a more general acid-base theory. According to the BrФnsted-Lowry model,an acid is a proton donor and a base is a proton acceptor. Each proton donor(acid) has its pair(conjugate) proton acceptor base.

For example:

NH3

(Base 1)H2O

(Acid 2)OH-

(Base 2)NH4

+

(Acid 1)

Conjugate acid-base pair

Conjugate acid-base pair

Introduction

Glossary References


Example

Solution:

H2S give H+ (proton) to H2O (proton accceptor), so it is an acid. In the same way, H2O accepts one H+ (proton) from H2S (proton donor). Thus it is a base.

H2S (g) + H2O (l) ↔ H3O+ (aq) + HS- (aq)

find the conjugate acid-base pairs for the above reaction.

Introduction

Glossary References


Example conjugate acid-base pair

H2S (g) + H2O (l) ↔ H3O+ (aq) + HS- (aq) acid 1 base 2 acid 2 base 1

conjugate acid-base pair

Introduction

Glossary References


1.Acid-Base TheoriesNH3 and NH4

+ form a conjugate acid-base pair,where NH3 is the conjugate base of NH4 (NH4

+ is the conjugate acid of NH3). Similarly, H2O and OH- are conjugates of each other.

A model of Acid-Base reaction between NH3 and H2O

HO

HH

NHH

HO

HN

HHH

Introduction

Glossary References


1.Acid-Base Theories

1.c. The Lewis Acid-Base Theoryin 1923, American chemist,Gilbert N.Lewis definited acid and base considered to chemical bond theory. According to Lewis opinion that acid is compound that accept the pair of free electron. And base is compound that donor the pair of free electron.the example of The Lewis Acid-Base theory:

B BF

FF

FF

F

F

F

Introduction

Glossary References


1.Acid-Base Theories

1.d. General Properties of AcidsThe General properties of Acids can be summarized as:

1.The aqueous solutions of acids conduct electricity2. Acids change the color of litmus paper to red3. Acids have a sour taste (such as vinegar and lemon)4. Acids are corrosive substances.5. Acids react with active metals and produce hydrogen gas.For example:2Al(s) + 3H2SO4(aq) → Al2(SO4)3 (aq) + 3H2(g)

Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2 (g)

Introduction

Glossary References


6. Acids react with bases to produce salt and water,this reaction is called a neutralization reaction.

For example:NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)

7.Inert metals (Cu,Hg,Ag,Pt,Au) do not react with binary acids(HCl,HBr,…). Cu,Hg,and Ag can react with strong

oxyacid such as HNO3 and H2SO4.

For example:3Cu(s) + 8HNO3(aq) →(dilute) → 3Cu(NO3)2 (aq) + 2NO (g) + 4H2O(l)

8. Acids react with carbonate salts of metal.For example:MgCO3(s) + 2HCl(aq) → MgCl2(aq) + CO2(g) + H2O(l)

Introduction

Glossary References


1.Acid-Base Theories1.e. General Properties of BasesThe General properties of Bases can be summarized as:1.The aqueous solutions of Bases conduct electricity2. Bases change the color of litmus paper to blue3. Bases have a bitter taste 4. Strong bases are corrosive5. Basic solutions cause slippery skin6. Bases do not react with most metals. However, strong bases(KOH,NaOH) react

with amphoteric metals such as aluminium(Al), zink(Zn),and produce hydrogen gas.

For example:2Al(s) + 6NaOH(aq) → 2Na3AlO3(aq) + H2(g)

Zn(s) + 2NaOH(aq) → Na2ZnO2(aq) + H2 (g)

Introduction

Glossary References


2. Ionization of WaterSome molecules and ions act as acids or bases depending on the

conditions of the reaction are called amphoteric.A water molecule acts as an acid when it donates a proton, and acts as a

base when it accepts a proton, as given below:H2O(l) +H2O(l) ↔ H3O+(aq) + OH-(aq)

OH

OH

OH

OHHHH

H

Introduction

Glossary References


2. Ionization of Water

The equilibrium sconstan expression for the reaction is:

Kw is the equilibrium constant used in equations to refer to ionization of water.

Kw= 1,0 . 10-14 at 250C[H3O+ ] = [OH-] for the neutral medium. Then,

1 . 10-14 = [H3O+ ] 2

1. 10-7 = [H3O+ ]

[H3O+ ] = [OH-] = 1. 10-7 M

Kw = [H3O+ ] [OH-]

Introduction

Glossary References


3. The pH ScaleThe acidity or basicity of a solution can be described in terms of its H3O+ or OH- ion

concentrations.These concentrations are usually small. For example,H3O+ ion concentration is 1.10-7 M

in the neutral solution. Thus,the concentration of H3O+ is generally expressed as the negative logarithm. This is known as pH. From now on, for simplicity H+ will be used instead of H3O+

and

In a similar,the way negative logarithm of [OH-] is pOH

and

pH = -log [H+]

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

[H+] = 10-pH

Introduction

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For pure water at 250C,[H+] = 1. 10-7 [OH-] = 1. 10-7 pH = - log (1. 10-7) pOH = - log (1. 10-7)pH = 7 pOH = 7

Since [H+] [OH-] = 1. 10-14 a

The pH value of a solution gives an idea about the condition of a solution as follows: If pH < 7,0 If pH = 7,0 solution is neutralIf pH > 7,0

pH + pOH = 14

Solution is acidic

Solution is basic

Introduction

Glossary References


pH is measured by using a pH meter. A pH meter translates H+ion concentrations in solutioninto an electrical signal that is converted into a digital display

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

pure water baking powderAcid in stomach vinegar coffe rain water blood soap household

lemon juice tomato bread milk sea water amonnia juice

pH scale

H+ CONCENTRATION, [H+] in mol/L100 10-1 10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 10-11 10-12 10-13 10-14

Introduction

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A less accurate way to mesure pH is to use indicators. An indicator is a compound such as phenolphthalein or methyl orange, that is changes color reversibility at different pH values. For exampe, phenolphthalein is a colourless substance in any solution with a pH value smaller than 8,3 it turns red-violet in solution with a pH value greater than 8,3.

Indicator Acidic Basic

Phenolphthalein Colorless Red-violetMethyl orange Red Orange-yellowLitmus Red Blue

Introduction

Glossary References


Example

Solution:a. KW = and = 1 . 10-4 M

=

For a solution in which = 1 . 10-4 M;a. What is the molarity of OH- ions ?b. What are the pH and pOH value ?c. Is the solution acidic or basic ?

Introduction

Glossary References


Example

= = 1 . 10-10

b. pH = -log pH + pOH =14= -log 1 . 10-4 4 + pOH = 14= 4 pOH = 14 – 4 = 10

c. pH < 7 , solution is acidic

Introduction

Glossary References


4. Strength of Acids and Bases4.a. Acid Strength

Acids are classified as strong and weak, depending upon theirdegree of ionization in water. A weak acidionizes in water reversibly to form H3O+ions. A weak acid is a weak electrolyte and its aqueous solution does not conduct electricity well. The dissociation reaction occurs to very small extent; usually,fewer than 1 percent of the HA molecules are ionized. The ionized of a weak acid is shown as follows:

HA(aq) + H2O(l) ↔H3O+(aq) + A-(aq)

weak acid conjugate baseThe dissociation constant Ka,of a given acid iswritten as ;

Ka refers to the acid dissociation constant which is measure of an acid’s strength. Some references call Ka the acid ionization constant

If Ka < 10-3, acid is generally said to be weak

If Ka = 1 to 10-3, acid is accepted as moderate

If Ka > 1 acid is strong.

Ka=[H3O+] [A-] / [HA]

Introduction

Glossary References


Some Ka values of acids are given in Table below.

Name of acid Formula Ka

Hydrochloric acid HCL 1.107

Nitric acid HNO3 1.103

Sulfuric acid H2SO4 22

Oxalic acid (COOH)2 6,5 .10-2

Phosphoric acid H3PO4 7,5 . 10-3

Lictic acid C2H5OCOOH 1,38 .10-4

Formic acid HCOOH 1,8.10-4

Nitrous acid HNO2 4. 10-4

Hydroflouric acid HF 7,2 .10-4

Acetic acid CH3COOH 1,8 . 10-5

Introduction

Glossary References


4. Strength of Acids and Bases4.2 Base Strength

The dissociation of a weak base in water is illustrated as follows ;B(aq) + H2O(l)↔ BH+(aq) + OH-(aq)

The equilibrium expression for the above weak base is

Kb is the base dissociation constant or base ionization constant, that measure a base’s strength.

Kb= [BH+] [OH-] / [B]

Introduction

Glossary References


Some Kb values of bases are given in Table below.

Name of base Formula Kb

Potassium hydroxide KOH 158,5

Sodium hydroxide NaOH 10

Methylamine CH3NH2 4,38 . 10-4

Ethylamine C2H5NH2 5,6 . 10-4

Ammonia NH3 1,8 . 10-5

Pyridine C5H5N 1,7 . 10-9

Aniline C6H5NH2 3,8 . 10-10

Introduction

Glossary References


Example

What is the Ka expression for acetic acid (CH3COOH) ?

Solution:CH3COOH(aq) ↔ H+ (aq) + CH3COO- (aq)

Ka =

Introduction

Glossary References


ExampleWrite the Kb expression of the weak base hydrazine (N2H4)!

Solution:Ionization of N2H4 in water is

N2H4(aq) + H2O (l) ↔ N2H5+

(aq) + OH-(aq)

Kb =

Introduction

Glossary References


4. Strength of Acids and Bases4.c. Relationship between Ka and Kb

Dissociation constants of weak acids and bases can be determined in the laboratory. However, it is easy to find the Ka or Kb values of acids or bases by using a simple mathematical expression between Ka and Kb of conjugate acid-base. The multiplication of Ka and Kb of conjugate acid-base is Kw.

Introduction

Glossary References


4. Strength of Acids and BasesKw = Ka . Kb = Kb . Ka = 1 . 10-14

For example NH4+ - NH3 is a conjugate acid base.

I. NH3 + H2O ↔ NH4+ + OH- K1 = Kb of NH3

II. NH4+ + H2O ↔ NH3 + H3O+ K2 = Ka of NH4

+

2H2O ↔ H3O+ + OH- Kw = K1 . K2

Introduction

Glossary References


Example

What is the Ka value of NH4+ if the Kb of NH3 is 1.8

x 10-5 ?

Solution:NH3 + H2O ↔ NH4

+ + OH-

NH4+ is the conjugate acid of NH3

Kb for NH3 is 1,8 x 10-5

Introduction

Glossary References


ExampleKa . Kb = Kw

Ka . 1.8 x 10-5 = 1 x 10-14

Ka =

Ka = 5.6 x 10-10

Introduction

Glossary References


5. NeutralizationThe reaction of an acid and a base solution to form salt

and water is called a neutralization reaction. Indeed,a neutralization reaction is a reaction hydrogen(H+) ions and hydroxide(OH-) ions to form water. In neutralization,all H+ and OH- ions coming from acid and bases turn into H2O. The reaction between HCl(aq) and NaOH(aq) is an example of a neutralization reaction. HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)

The net ionic equation for this reaction is H+(aq) + OH- (aq)→ H2O (l)

Introduction

Glossary References


The nature of the particular acid and base involved in a reaction determines the acidity or basicity of the resulting solution.

1 .Neutralization of a strong acid and strong base gives a neutral solution.HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)

The resulting NaCl salt is neutral2. Neutralization of a strong acid and a weak base gives a acidic solution.HCl(aq) + NH3(aq) → NH4Cl(aq) + H2O(l)

NH4Cl is called acidic salt

3. Neutralization of a weak acid and a strong base gives a basic solution.CH3COOH(aq) + NaOH(aq) → CH3COONa(aq) + H2O(l)

CH3COONa is called basic salt

4. Neutralization of a weak acid and a weak base gives a complex type of acid-base solution. If weak acid and weak base have the same strength,the solution is neutral. If the strength of an acid and base are not equal,the solution will be either acidic or basic,depending on the strength of either.

Introduction

Glossary References


Supplementary Questions

1. Find the conjugate acid-base pairs for the following reaction.a. CO3

2- + H2O ↔ HCO3- + OH-

b. HF + H2O ↔ H3O+ + F-

c. NH3 + H2O ↔ NH4+ + OH-

d. HSO4- + HCl ↔ H2SO4 + Cl-

Introduction

Glossary References


Supplementary Questions2. What is the pH of a solution prepared by

dissolving 0.8 g NaOH in water to make 200 mL solution ?

3. How many moles of HCl are found in 500 mL of HCl solution, whit a pH of 3 ?

4. How many millilitres of 0.01 M NaOH are required to neutralize 50 mL of 0.02 M HCl ?

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Glossary References


Supplementary Questions5.

I IIafter mixing the solutions, what would be the pH of the final solution ?

100 mL = 3 . 10-2 M

100 mL = 1 . 10-12 M

Introduction

Glossary References


GlossaryAcid : A substance that produces hydrogen ions in solution. A proton donor substance.Acid dissociation constans (Ka) : The equilibrium constant for weak acids.Aqueous solutions : solutions in which water is the solvent.Arrhenius concept : A concept stating that acids produce hydrogen ions and bases produc hydroxide ions in aqueous solution.

Introduction

Glossary References


Glossary

Base : A substance that produces hydroxide ions in aqueous solution. A proton acceptor substanceBase dissociation constans (Kb) : the equilibrium constans for the reaction of a base with water to produce the conjugate acid and hydroxide ion.Chemical bond : The force, or energy, that holds atoms together in a compound.

Introduction

Glossary References


Glossary

Conjugate acid : The specie formed when a proton is added to a base.Conjugat acid-base pair : two species related to each other by donating and accepting of a single proton.Conjugat base : What remains of an acid molecule after a proton is lost.

Introduction

Glossary References


Glossary

Equilibrium : The position where the rate of forwad and reverse reaction becomes equal.Indicator : A chemical that changes color and is used to mark the end-point of a titration.Ion-product constant (Kw) : the equilibrium constant for water.Lewis acid : An electron-pair acceptor

Introduction

Glossary References


Glossary

Lewis base : An electro-pair donor.pH curve (titration curve) : A plot showing the pH of a solution being analyzed as a function of the amount of tittration added.pH scale : A logaritmhmic scale based on 10 and equal to – log a way to represent solution acidity.

Introduction

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Glossary

Polyprotic acid : an acid with more than one acidic proton. It dissociates in a stepwise manner, one proton at a time.Salt brige : A U-shaped tube containing an electrolyte that connects the two compartments of a galvanic cell without extensive mixing of the different solutions.

Introduction

Glossary References


GlossarySolution : A homogeneous mixture.Strong base : A metal hydroxide salt that completely dissociates into its ions in water.Weak acid : An acid that dissociates only slightly in aqueous solution.Weak base : A base that reacts with water to produce hydroxide ions to only a slight extent ain aqueous solution.

Introduction

Glossary References


References• Nazli, Ayhan., Lise 2 kimya Ders Kitab.

Textbook of Chemistry for Lycee 2. Zambak Publishing, Istanbul : 2006

• McDuell B., A Level Chemistry. Letts Education, UK : 2000

• Earl B., Wilford L.D.R., IGCSE Chemistry. Hodder Murray, Dubai : 2005

Introduction

Glossary References


References

• Ratcliff B., Eccles H., and others, AS Level and A level Chemistry. Cambridge University Press, UK : 2005

• Oxtoby, D.W., Nachtrieb, N.H., Principals of Modern Chemistry. 3rd Ed., Saunders Collage Publising, USA : 1996

Introduction

Glossary References


References

• Sevenair, J.P., Burkett, A.R., Introductory Chemistry. Investigating the Molecular Nature of Matter. WBC Publishers, USA :1997

• Prescott, C.N., Chemistry A Course for “0” Level. Times Media Privat Limited, Singapore : 2000

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