1 acids and bases. 2 acid-base concepts antoine lavoisier was one of the first chemists to try to...

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Acids and Bases

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Acid-Base Concepts

Antoine Lavoisier was one of the first chemists to try to explain what makes a substance acidic.

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Acid-Base Concepts

In the first part of this chapter we will look at several concepts of acid-base theory including:

Common CharacteristicsAcids Bases

SOLUTION

NO Current

Conducts Current

Nonelectrolytic solution

Electrolytic Solution

Molecular Solution

Acid, Base or Salt Solution

Molecule = no net chargeEXCEPTIONS:

Covalent / Hydrohalic Acids

Ion = charged particle

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Arrhenius Concept of Acids and Bases According to the Arrhenius concept of

acids and bases, an acid is a substance that, when dissolved in water, increases the concentration of hydronium ion (H3O+).

– Remember, however, that the aqueous hydrogen ion is actually chemically bonded to water, that is, H3O+.

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Arrhenius Concept of Acids and Bases

The H3O+ is shown here hydrogen bonded to three water molecules.

According to the Arrhenius concept of acids and bases, an acid is a substance that, when dissolved in water, increases the concentration of hydronium ion (H3O+).

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Arrhenius Concept of Acids and Bases A base, in the Arrhenius concept,

is a substance that, when dissolved in water, increases the concentration of hydroxide ion, OH-(aq).

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Theory of Ionization

Svente Arrhenius 1884 Water

Acid

Base

Neutralization

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Arrhenius Concept of Acids and Bases In the Arrhenius concept, a strong acid

is a substance that ionizes completely in aqueous solution to give H3O+(aq) and an anion. (See Animation: Acid Ionization Equilibirum)

– Other strong acids include

– An example is perchloric acid, HClO4.

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Arrhenius Concept of Acids and Bases

In the Arrhenius concept, a strong base is a substance that ionizes completely in aqueous solution to give OH-(aq) and a cation.

See Base Ionization animation

– Other strong bases include

– An example is sodium hydroxide,.

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Arrhenius Concept of Acids and Bases Most other acids and bases that you

encounter are weak. They are not completely ionized and exist in reversible reaction with the corresponding ions.

– An example is acetic acid,

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Arrhenius Concept of Acids and Bases The Arrhenius concept is limited in

that it looks at acids and bases in aqueous solutions only.

– Broader definitions of acids and bases are discussed in the next sections.

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Consider…

What is the acid?

What is the base?

Production of NH4

)( )( )( )( 423 aqOHaqNHlOHaqNH

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Brønsted-Lowry Concept of Acids and Bases

A base is

– In any reversible acid-base reaction,

• According to the Brønsted-Lowry concept, an acid is

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Consider the reaction of NH3 and H2O.

H+

base acid

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)aq(OH )aq(NH )l(OH )aq(NH 423

H+

baseacid

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base acid

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base acid

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Bronsted – Lowry Theory

Acid

Base

When an acid loses a proton, its conjugate base is formed.

When a base accepts a proton, its conjugate acid is formed

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Conjugate Acids and Bases

H2O + NH3 NH4+ + OH-

acid base

CAN donate H+

conjugate acid

CAN donate OH-

conjugate base

Conjugate Acid – Base Pairs

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What’s the Deal With Water?H2O + NH3 NH4

+ + OH-

acid base

H2O + HCl H3O+ + Cl-

base acid Amphoteric

Autoionization Protolysis

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– HCO3- acts as a proton donor (an acid) in the

presence of OH-

–H+

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– HCO3 can act as a proton acceptor (a base) in the presence of HF.

H+

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Brønsted-Lowry Concept of Acids and Bases In the Brønsted-Lowry concept:

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Lewis Concept of Acids and Bases The Lewis concept defines an acid

as an electron pair acceptor and a base as an electron pair donor.

– The Lewis concept embraces many reactions that we might not think of as acid-base reactions.

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Lewis Theory 1923 Extended the acid–base theory

electron pair donor =

electron pair receiver =

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4 ammonia molecules forming a complex ion with cupric ion

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Characteristics of Lewis… Acids

Bases

Lone e- pair

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Neutralization = Coordinate covalent

bond

Both shared e- donated by the same atom / ion

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Nesting Theories

LewisLewis

Bronsted - LowryBronsted - Lowry

ArrheniusArrhenius

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Strength in Water

Depends on degree of ionization Ease of bond breakage

Stability of resulting ions

Higher the [H+] or [OH-]

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And the Winner is…

Hydronium H3O+ is the

Hydroxide OH- is the

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Water is a leveling solvent;.

HCl, HBr, HI

NaOH, Ca(OH)2

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ACIDS & BASES H2SO4 v H2SO2 NaOH v S(OH)6

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H3PO4 weaker than HNO3

+5 +5

HNO2 weaker than H3PO3

+3 +3

N

O O

H

PO O

O

H H

H

Trigonal planar Tetrahedral

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Relative Strength of Acids and Bases The Brønsted-Lowry concept

introduced the idea of conjugate acid-base pairs and proton-transfer reactions.

– We consider such acid-base reactions to be a competition between species for hydrogen ions.

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Relative Strength of Acids and Bases

Consider the equilibrium below.

(aq)OHC(aq)OH 2323 )l(OH)aq(OHHC 2232

acid acidbase base

conjugate acid-base pairs

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Relative Strength of Acids and Bases

Consider the equilibrium below.

(aq)OHC(aq)OH 2323 )l(OH)aq(OHHC 2232

acid acidbase base

conjugate acid-base pairs

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Molecular Structure and Acid Strength Two factors are important in

determining the relative acid strengths.

– The H atom should have a partial positive charge:

XH

+ -

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Molecular Structure and Acid Strength Two factors are important in

determining the relative acid strengths.

XH d+ d-

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Molecular Structure and Acid Strength Consider a series of binary acids

from a given column of elements.

– You can predict the following order of acidic strength.

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Molecular Structure and Acid Strength As you go across a row of

elements, the polarity of the H-X bond becomes the dominant factor.


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Molecular Structure and Acid Strength Consider the oxyacids. An oxyacid

has the structure:

YOH

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Molecular Structure and Acid Strength

YOH


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has the structure:

YOH

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has the structure:

YOH

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Molecular Structure and Acid Strength Consider polyprotic acids and their

corresponding anions.

– Therefore the acid strength of a polyprotic acid and its anions decreases with increasing negative charge.

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Self-ionization of Water

Self-ionization is a reaction in which two like molecules react to give ions. (See Animation: Self-ionization of Water to Form H+ and OH- in Equilibrium)

– In the case of water, the following equilibrium is established.

– The equilibrium-constant expression for this system is:

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– The concentration of ions is extremely small, so the concentration of H2O remains essentially constant. This gives:

constant

Self-ionization is a reaction in which two like molecules react to give ions. [i.e. H2O]

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– .

– At 25 oC, the value of Kw is 1.0 x 10-14.

Self-ionization is a reaction in which two like molecules react to give ions.

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– Because we often write H3O+ as H+, the ion-product constant expression for water can be written:

– Using Kw you can calculate the concentrations of H+ and OH- ions in pure water.

Self-ionization is a reaction in which two like molecules react to give ions.

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These ions are produced in equal numbers in pure water, so if we let x = [H+] = [OH-]

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Solutions of Strong Acid or Base In a solution of a strong acid you

can normally ignore the self-ionization of water as a source of H+(aq).

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Solutions of Strong Acid or Base As an example, calculate the

concentration of OH- ion in 0.10 M HCl.Because you started with 0.10 M HCl (a strong acid) the reaction will produce 0.10 M H+(aq).

– Substituting [H+]=0.10 into the ion-product expression, we get:

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concentration of OH- ion in 0.10 M HCl.Because you started with 0.10 M HCl (a strong acid) the reaction will produce 0.10 M H+(aq).

– Substituting [H+]=0.10 into the ion-product expression, we get:

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Solutions of Strong Acid or Base Similarly, in a solution of a strong

base you can normally ignore the self-ionization of water as a source of OH-(aq).

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concentration of H+ ion in 0.010 M NaOH.

Because you started with 0.010 M NaOH (a strong base) the reaction will produce 0.010 M OH-(aq).

– Substituting [OH-]=0.010 into the ion-product expression, we get:

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concentration of H+ ion in 0.010 M NaOH.

Because you started with 0.010 M NaOH (a strong base) the reaction will produce 0.010 M OH-(aq).

– Substituting [OH-]=0.010 into the ion-product expression, we get:

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Solutions of Strong Acid or Base By dissolving substances in water,

you can alter the concentrations of H+(aq) and OH-(aq).

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Solutions of Strong Acid or Base At 25°C, you observe the following

conditions.

– In an acidic solution, [H+]– In a neutral solution, [H+]– In a basic solution, [H+]

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The pH of a Solution

Although you can quantitatively describe the acidity of a solution by its [H+], it is often more convenient to give acidity in terms of pH.

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For a solution in which the hydrogen-ion concentration is 1.0 x 10-3, the pH is:

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In a neutral solution, whose hydrogen-ion concentration is 1.0 x 10-7, the pH = 7.00.

• For acidic solutions,

• Similarly, a basic

• Figure 16.6 shows a diagram of the pH scale and the pH values of some common solutions.

Figure 16.8: The pH Scale

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A Problem to Consider A sample of orange juice has a hydrogen-ion

concentration of 2.9 x 10-4 M. What is the pH?

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A Problem to Consider The pH of human arterial blood is 7.40. What is

the hydrogen-ion concentration?

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A measurement of the hydroxide ion concentration, similar to pH, is the pOH.

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A measurement of the hydroxide ion concentration, similar to pH, is the pOH.

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What is the [H3O+] of a .050 M Sr(OH)2 solution?

Sr(OH)2 ↔ Sr2+ + 2OH-

.050 M 0 0(strong base) 0 .050 M .100 M

[OH-] = .100 M2H2O ↔ H3O+ + OH–

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Negligibility

Applies only to addition and subtraction calculations

Variable very small 10-3, 10-4, 10-5 and Less than 5% of the number to

which it is added / subtracted

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pH and pOH

p = -log

pKw = -logKw

pH pOH

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A Problem to Consider

An ammonia solution has a hydroxide-ion concentration of 1.9 x 10-3 M. What is the pH of the solution?

You first calculate the pOH:

Then the pH is:

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If and acid solution has a [H3O+] of .05 M, what are its pH and pOH ?

pH = -log [H3O+] = =

Kw = = = =

pOH = pOH =

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pH + pOH =

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Calculate the pH and pOH of a household ammonia solution that contains 2.5 mol of NH3 per liter of solution. Assume 10% ionization.

NH3 + H2O ↔ NH4+ + OH-

Initial

Change

@equilib

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The pH of a solution can accurately be measured using a pH meter (see Figure 16.9).

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Net Ionic Equation Real physical state of every component of the reaction

Strong acids and strong bases in ionic form Soluble salts in ionic form Pure substances, oxides, gases and solids in

molecular formCaCO3(s) + HCl ?

Molecular Equation:

Total

Ionic Equation:

Net Ionic Equation:

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Salts

Product of acid – base rxn.

Types1. Normal

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2. Acidic

3. Basic

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Indicators Organic dyes Change color over pH range

Acidic Neutral Basic

pH<7 7 pH>7

Red [Litmus] Blue

YellowYellow [Bromothymol Blue] Blue

ColorlessColorless [Phenolphthalein] Deep Pink

Red [Methyl Orange] YellowYellow

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Animation: Acid Ionization Equilibrium

Return to Slide 7

(Click here to open QuickTime animation)

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Animation: Self-Ionization of Water to Form H+ and OH- in Equilibrium

Return to Slide 38

(Click here to open QuickTime animation)

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Figure 16.9: A digital pH meter.Photo courtesy of American Color.

Return to Slide 60

1 acids and bases. 2 acid-base concepts antoine lavoisier was one of the first chemists to try to...

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