Diploma_I_Applied science(chemistry)U-III Acid & bases

Course: Diploma

Subject: Applied science(Chemistry)

Unit: III

When we think of acids and bases we tend to think of science labs and chemicals…but did you know

Acids cause:

•Lemons to be sour

•Acid rain to eat away at sculptures

•Framed paintings to be damaged

•Cavities in your teeth

•Food to digest in your stomach

•Ants and bees use it to sting

Acids and Bases

http://en.wikipedia.org/wiki/Formic_acid

Acids and Bases

Acids and BasesAcids and Bases

3

Properties of Acids and BasesProperties of Acids and Bases

Acids ◦ turn blue litmus red ◦ taste sour ◦ Acids corrode metals

◦ positively charged hydrogen ions (H+)Bases ◦ turn red litmus blue◦ taste bitter ◦ Negatively charged hydroxide ions (OH–)◦ Feel slippery◦ Most hand soaps and drain cleaners are bases◦ Strong bases are caustic

Acid-Base ConceptsAcid-Base ConceptsIn the first part of this chapter we will

look at several concepts of acid-base theory including:

– The Arrhenius concept– The Bronsted Lowry concept– The Lewis concept

This chapter expands on what you learned in Chapter 3 about acids and bases.

Arrhenius Concept of Acids and Arrhenius Concept of Acids and BasesBasesAccording 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+).

– Chemists often use the notation H+(aq) for the H3O+(aq) ion, and call it the hydrogen ion.

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

Arrhenius Concept of Acids and Arrhenius Concept of Acids and BasesBases

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+).

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

Arrhenius Concept of Acids and Arrhenius Concept of Acids and BasesBasesA base, in the Arrhenius concept, is a

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

Arrhenius Concept of Acids and Arrhenius Concept of Acids and BasesBasesIn the Arrhenius concept, a strong acid is a

substance that ionizes completely in aqueous solution to give H3O+(aq) and an anion.

– Other strong acids include HCl, HBr, HI, HNO3 , and H2SO4.

– An example is perchloric acid, HClO4.

)()()()( 4324 aqClOaqOHlOHaqHClO −+ +→+

Arrhenius Concept of Acids and Arrhenius Concept of Acids and BasesBases

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

– Other strong bases include LiOH, KOH, Ca(OH)2, Sr(OH)2, and Ba(OH)2.

– An example is sodium hydroxide, NaOH.

)aq(OH)aq(Na)s(NaOH OH 2 −+ +→

Arrhenius Concept of Acids and Arrhenius Concept of Acids and BasesBasesMost other acids and bases that you encounter

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

– Ammonium hydroxide, NH4OH, is a weak base.

)aq(OH)aq(NH )aq(OHNH 44−+ +

(aq)OHC(aq)OH 2323−+ +

– An example is acetic acid, HC2H3O2.

)l(OH)aq(OHHC 2232 +

Arrhenius Concept of Acids and Arrhenius Concept of Acids and BasesBasesThe Arrhenius concept is limited in that it

looks at acids and bases in aqueous solutions only.

– In addition, it singles out the OH- ion as the source of base character, when other species can play a similar role

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

Brønsted-Lowry Concept of Acids Brønsted-Lowry Concept of Acids and Basesand Bases

A base is the species accepting the proton in a proton-transfer reaction.

– In any reversible acid-base reaction, both forward and reverse reactions involve proton transfer.

• According to the Brønsted-Lowry concept, an acid is the species donating the proton in a proton-transfer reaction.


Consider the reaction of NH3 and H20.

)aq(OH )aq(NH )l(OH )aq(NH 423−+ ++


Consider the reaction of NH3 and H2O.

– In the forward reaction, NH3 accepts a proton from H2O. Thus, NH3 is a base and H2O is an acid.


H+

base acid



– In the reverse reaction, NH4+ donates a

proton to OH-. The NH4+ ion is the acid and

OH- is the base.


H+

baseacid



– A conjugate acid-base pair consists of two species in an acid-base reaction, one acid and one base, that differ by the loss or gain of a proton.


base acid

– The species NH4+ and NH3 are a conjugate

acid-base pair.



– The Brønsted-Lowry concept defines a species as an acid or a base according to its function in the proton-transfer reaction.


base acid

– Here NH4+ is the conjugate acid of NH3 and

NH3 is the conjugate base of NH4+.

Brønsted-Lowry Concept of Acids Brønsted-Lowry Concept of Acids and Basesand BasesSome species can act as an acid or a base.

– For example, HCO3- acts as a proton donor (an acid) in

the presence of OH-

)l(OH)aq(CO)aq(OH)aq(HCO 22

33 +→+ −−−

–H+

– An amphoteric species is a species that can act either as an acid or a base (it can gain or lose a proton).

Brønsted-Lowry Concept of Acids Brønsted-Lowry Concept of Acids and Basesand BasesSome species can act as an acid or a base.

– An amphoteric species is a species that can act either as an acid or a base (it can gain or lose a proton).

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

)aq(F)aq(COH)aq(HF)aq(HCO 323−− +→+

H+

Brønsted-Lowry Concept of Acids Brønsted-Lowry Concept of Acids and Basesand BasesThe amphoteric characteristic of water is

important in the acid-base properties of aqueous solutions.

– Water reacts as an acid with the base NH3.

)aq(OH)aq(NH)l(OH)aq(NH 423−+ +→+

H+

Brønsted-Lowry Concept of Acids Brønsted-Lowry Concept of Acids and Basesand BasesThe amphoteric characteristic of water is

important in the acid-base properties of aqueous solutions.

– Water can also react as a base with the acid HF.

)aq(OH)aq(F)l(OH)aq(HF 32+− +→+

H+

Classes of Bronsted Acids & BasesClasses of Bronsted Acids & Bases

1. Monoprotic acids: one protonsE.g., HF, CH3COOH

2. Polyprotic acids: two or more protonsE.g., H2S, Oxalic acid

Bases: 1. Monoprotoc Bases: HS-, H2O

2. Polyprotic bases: SO4-2, PO4

-3

Lewis Concept of Acids and BasesLewis Concept of Acids and Bases

acid : electron pair acceptor base : electron pair donor.

Creates covalent bond : complex

–

Lewis Concept of Acids and BasesLewis Concept of Acids and Bases

The reaction of boron trifluoride with ammonia is an example.

– Boron trifluoride accepts the electron pair, so it is a Lewis acid. Ammonia donates the electron pair, so it is the Lewis base.

+ N

H

H

H:

::

: B

F

F

F

: :

::

::

::

: B

F

F

F

: :

::

:: N

H

H

H

Examples of Lewis reactionsExamples of Lewis reactions

1. Between H+ and NH3

2. H+ and OH-

3. H2O and CH3

4. BF3 and NH3

5. Hydration of Al+3

6. H- and BH3

Self-ionization of WaterSelf-ionization of WaterSelf-ionization is a reaction in which two like

molecules react to give ions.

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

)aq(OH)aq(OH )l(OH)l(OH 322−+ ++

– The equilibrium-constant expression for this system is:

22

3c ]OH[

]OH][OH[K

−+

=

Self-ionization of WaterSelf-ionization of Water

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

]OH][OH[K]OH[ 3c2

2−+=

constant

Self-ionization of WaterSelf-ionization of Water

– We call the equilibrium value for the ion product [H3O+][OH-] the ion-product constant for water, which is written Kw.

]OH][OH[K 3w−+=

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

– Like any equilibrium constant, Kw varies with temperature.

Self-ionization of WaterSelf-ionization of WaterSelf-ionization is a reaction in which two like

molecules react to give ions.

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

]OH][H[Kw−+=

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

Self-ionization of WaterSelf-ionization of WaterThese ions are produced in equal numbers in

pure water, so if we let x = [H+] = [OH-]

– Thus, the concentrations of H+ and OH- in pure water are both 1.0 x 10-7 M.

– If you add acid or base to water they are no longer equal but the Kw expression still holds.

C 25at )x)(x(100.1 o14 =× −

714 100.1100.1x −− ×=×=

Understanding the pH ScaleUnderstanding the pH Scaleo pH stands for (presence of Hydrogen)

o Numbered from 0 to 14.

o The lower the pH number – the higher Acido That means more Hydrogen Ions (H+)

o The higher the pH - the higher the Baseo That means less Hydrogen Ions (H+)

2

The pH of a SolutionThe pH of a SolutionAlthough you can quantitatively describe

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

– The pH of a solution is defined as the negative logarithm of the molar hydrogen-ion concentration.

]Hlog[pH +−=

The pH of a SolutionThe pH of a SolutionFor a solution in which the hydrogen-ion

concentration is 1.0 x 10-3, the pH is:

– Note that the number of decimal places in the pH equals the number of significant figures in the hydrogen-ion concentration.

00.3)100.1log( 3 =×−= −pH

The pH of a SolutionThe pH of a Solution In a neutral solution, whose hydrogen-ion

concentration is 1.0 x 10-7, the pH = 7.00.

• For acidic solutions, the hydrogen-ion concentration is greater than 1.0 x 10-7, so the pH is less than 7.00.

• Similarly, a basic solution has a pH greater than 7.00.

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

The pH ScaleThe pH Scale

A Problem to ConsiderA Problem to Consider

A sample of orange juice has a hydrogen-ion concentration of 2.9 x 10-4 M. What is the pH?

]Hlog[pH +−=

)109.2log(pH 4−×−=

54.3pH =

A Problem to ConsiderA Problem to Consider

The pH of human arterial blood is 7.40. What is the hydrogen-ion concentration?

)pHlog(anti]H[ −=+

)40.7log(anti]H[ −=+

M100.410]H[ 840.7 −−+ ×==

The pH of a SolutionThe pH of a SolutionA measurement of the hydroxide ion

concentration, similar to pH, is the pOH.

– The pOH of a solution is defined as the negative logarithm of the molar hydroxide-ion concentration.

]OHlog[pOH −−=

The pH of a SolutionThe pH of a SolutionA measurement of the hydroxide ion

concentration, similar to pH, is the pOH.

– Then because Kw = [H+][OH-] = 1.0 x 10-14 at 25 oC, you can show that

00.14pOHpH =+

A Problem to ConsiderA Problem to ConsiderAn 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:

72.2)109.1log(pOH 3 =×−= −

Then the pH is:

28.1172.200.14pH =−=

Buffer SolutionsBuffer SolutionsDEFINITION: A buffer solution contains a weak acid mixed

with its conjugate base (or weak base and conjugate acid)Buffers resist changes in pH when a small amount of a

strong acid or base is added to it.

HA ∏ H+ + A-

If a small amount of a strong acid (H+) is added eqm shifts to the left as [H+] increases so system adjusts to increase [HA] and reduce [H+] again.

HA ∏ H+ + A-

A small amount of a strong base will react with H+ to form H2O and eqm will shift to the right to increase [H+] again.

HA ∏ H+ + A-

Buffering agent pKa useful pH range

Citric acid 3.13, 4.76, 6.40 2.1 - 7.4

Acetic acid 4.8 3.8 - 5.8

KH2PO4, 7.2 6.2 - 8.2

CHES 9.3 8.3–10.3

Borate 9.24 8.25 - 10.25

http://en.wikipedia.org/wiki/Citric_acid

http://en.wikipedia.org/wiki/Acetic_acid

http://en.wikipedia.org/wiki/Potassium_dihydrogenphosphate




http://en.wikipedia.org/wiki/N-Cyclohexyl-2-aminoethanesulfonic_acid

http://en.wikipedia.org/wiki/Borate

Acid Base IndicatorAcid Base IndicatorAcid Base titration: add base from burette in to

an acid.Amount of base and acid are equal : equivalence

point or end point.End point: shown by color change of indicator.‘An Acid base indicator : organic dye that

signals the end point by a visual change in color.’

Phenolphthalein – pink in basePhenolphthalein – colorless in acidMethyl orange-red in acidMethyl orange – yellow in base.

ReferencesReferences1.Essentials of Physical Chemistry by Bahl

and tuli2.https://sites.google.com/a/wyckoffschools.

org/ems-biochemistry/acids-and-bases3. https://www.mhhe.com

http://www.mhhe.com/physsci/chemistry/chang7/esp/folder_structure/cr/m3/s3/

Diploma_I_Applied science(chemistry)U-III Acid & bases

Education

Transcript of Diploma_I_Applied science(chemistry)U-III Acid & bases