DRA14 Acid BaseConcepts

8/10/2019 DRA14 Acid BaseConcepts

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Acid and Bases


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Acid and Bases


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Acid and Bases


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AcidAcid--Base ConceptsBase Concepts

I. Unifying Concepts

A. The Acid-Base Concept

ere are many ac - a e e n on , eac a me

useful2) Acid-Base concepts are not facts or even theories,

but are useful generalizations for classification, andorganization

3 Acid-Base conce ts are ower ul wa s to ex laindata and predict trends

B. Arrhenius Concept

n ac orms n wa er; a ase orms - nwater

2) Applicable to aqueous solutions only

3) HCl + NaOH H+ + OH- + Na+ + Cl-


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C. Bronsted-Lowery Concept

1) Acid is a proton donor; Base is a proton

acceptor2) Conjugate acid/base pairs differ only by a

3) Reactions proceed to produce the weakest acid

and base4) H3O

+ + NO2- H2O + HNO2

5) Includes non-aqueous systems

NH4+ + NH2- 2 NH3


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BrnstedBrnsted--Lowr Acids and BasesLowr Acids and BasesThe HThe H

++Ion in WaterIon in Water


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Proton Transfer ReactionsProton Transfer Reactions Focus on the H+(aq).

Brnsted-Lowry: acid donates H+

and base accepts H+

. Brnsted-Lowry base does not need to contain OH-.

onsi er aq + 2 3+ aq + - aq :

HCl donates a proton to water. Therefore, HCl is.

H2O accepts a proton from HCl. Therefore, H2O isa base.

Water can behave as either an acid or a base. Amphoteric substances can behave as acids and

.


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Proton Transfer ReactionsProton Transfer Reactions


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BrnstedBrnsted--Lowry Acids and BasesLowry Acids and Bases

Conjugate AcidConjugate Acid--Base PairsBase Pairs

Whatever is left of the acid after the proton isdonated is called its conjugate base.

Similarly, whatever remains of the base after it.

Consider

After HA acid loses its roton it is converted intoA- (base). Therefore HA and A- are conjugate acid-base pairs.

er 2 ase ga ns a pro on s conver e n oH3O+ (acid). Therefore, H2O and H3O

+ areconjugate acid-base pairs.

Conjugate acid-base pairs differ by only one proton.


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Conjugate Acids and Bases:

Reactions between acids and bases always.


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D. Solvent System Concept

1) Useful for aprotic, non-aqueous systems

2) A lies to an solvent that can dissociate to

cation (acid) and anion (base)3) For water: 2 H2O H3O+ + OH-

a ny so ute increasing 3+ is an aci

HCl + H2O

H3O

+

+ Cl- -

NH3 + H2O NH4+ + OH-

4 A rotic non-a ueous e a e: 2 r BrF2+ + BrF4-

a) Acid: SbF5 + BrF3 BrF2+ + SbF6

-

b) Base: F- + BrF3 BrF4-


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5) The acid-base reaction:

b) Bronsted: acid1 + base2 = base1 + acid2

6) pKion = -log[acid][base]a) pKW = -log([H3O

+][OH-]) = -log[10-7][10-7] = 14

p H2SO4 = - og [ 3S 4+][ S 4

-] = .

c) The smaller the number, the more dissociation hasoccurred


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E. Lewis Conce t1) Acid = e- pair acceptor; Base = e- pair donor

2) Includes metal ions and non-aqueous systems;encompasses o er concep s

3) Metal ion Example: Ag+ + 2 :NH3 [Ag(NH3)2]+

-

b) If the acid is a metal ion, it is also called a

coordination compound or a coordination complexor a complex ion

4) A non-metal example: BF3 + NH3 H3N:BF3

or 3 3


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completely dissociatedin water.A. Their conjugate bases

are quite weak.

dissociate partially inwater.

A. Their conjugate basesare weak bases.


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Substances with

negligible acidity donot dissociate in.

- Their conjugate bases

are exceedingly strong.


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Consider H-X. For this substance to be an acid we

need: -X bond to be olar with + and X - i X is a

metal then the bond polarity is H - , X + and

the substance is a base) The H-X bond must be weak enough to be

broken

The conjugate base, X-, must be stable


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Acid StrengthAcid Strength is a measure for the tendency ofis a measure for the tendency ofan acid to accept an electron pair to forman acid to accept an electron pair to form

::the tendency increases.the tendency increases.

Base StrengthBase Strength is a measure for the tendency ofis a measure for the tendency ofa base to donate an electron pair to forma base to donate an electron pair to form

the tendency increases.the tendency increases.


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Th r n h f BTh r n h f B


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electrons to an acceptor depends on :electrons to an acceptor depends on :

The more negatively

-+

c arge e onor a om,the more effective the

THE NEGATIVETHE NEGATIVEII

THE DONOR ATOMTHE DONOR ATOM


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-

in Bond Multiplicity

+ Capacity of the GroupsAttached to the Donor

Polarizability of the Donor

PARTIAL CHARGE OFPARTIAL CHARGE OFTHE DONOR ATOMTHE DONOR ATOM

Concentration of the DonorElectron Pair


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Th r n h f A iTh r n h f A i

Emptyr a


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electrons from a donor depends on :electrons from a donor depends on :

The more ositivel char edthe acceptor atom, the moreeffective the chemical bond

THE POSITIF PARTIALTHE POSITIF PARTIALorme

ATOMATOM


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available in the acceptor

,

the chemical bond formed

THE EMPTY ORBITALSTHE EMPTY ORBITALSI II I

ACCEPTOR ATOMACCEPTOR ATOM


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B. Binary Hydrogen Compounds

1) Acidity increases down a column of the periodic table

a) H2Se > H2S > H2O

b) HI > HBr > HCl > HF

density, thus a smaller attraction for H+

a) NH3 < H2O < HF

b) The more electronegative the conjugate base is, theeas er s or o ssoc a e


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HF is a weak acid because the bond energy is high.

The electronegativity difference between C and H isso small that the C-H bond is non-polar and CH4 isneither an acid nor a base.


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C. Inductive Effects (electron pulling/pushing through

1) Electronegative substituents increase acidity anddecrease basicity

Basicity: :PF3 < :PH3

2) Electron Donating substituents decrease acidity andncrease as c y

Basicity: NMe3 > NHMe2 > NH2Me > NH3

xyac s: e more unpro ona e xygens, estronger the acid

a) Acidity: HOClO3 > HOClO2 > HOClO > HOCl

b) The electronegative Os pull e- away from theHO bond

base


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Oxyacids contain O-H bonds.

- - .

The strength of the acid depends on Y and the.

If Y is a metal (low electronegativity), then

. If Y has intermediate electronegativity (e.g.

I EN = 2.5 the electrons are between Y

and O and the substance is a weak oxyacid.


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,with the number of oxygens.


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Organic AcidsOrganic Acids

Carboxylic AcidsCarboxylic Acids These are organic acids which contain a COOH group

RC

OH

O

When the proton is removed, the negative charge is

de oca ized o er the carbo ate anion:

C

O

C

O

The acid strength increases as the number ofe ectronegative groups on R increases.


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esonance n t e con ugate ases ocarboxylic acids stabilizes the base and

.


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D. Cations in Aqueous Solution

1) Cationic metal ions are generally Lewis acids inwater solutions

[Fe(H2O)6]3+ + H2O [Fe(H2O)5(OH)]2+ + H3O+3 Lar e char e and small radii increase acidit

a) Alkali metals are not acidic (Na+); Alkaline

Earths are weakly acidic (Ca2+

)b) 2+ Transition Metals are weak acids; 3+Transition Metals are strong acids

MxOy


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Cations with acidic protons+

of a solution.

Most metal cations that arehydrated in solution alsolower the pH of the solution.

Attraction between nonbondingelectrons on oxygen and the

electron density in water.

This makes the O-H bond more

polar and the water more acidic. Greater charge and smaller size

ma e a ca on more ac c.


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4) The stronger acid the cation is, the less soluble thehydroxide complex is. OH- cant dissociate to dissolveecause o s rong c arge a rac on.

We can use this property to estimate the acidstren th o the cation


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E. Steric Effects

1) Steric bulk can repel an acid-base partner,mo y ng e ac - ase s reng

2) The order of basicity can scramble dependingon bulk of the acid


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.

1) Solvation is interaction with solvent molecules

asicity in water: e2 > 2 e > e3 >

NH3 ,

should be the most basic

b) This amine has less Hs to interact withwater


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G. Non-aqueous Solvents

e eve ng ect: t e strongest ac poss e na solvent is the solvent cation; the strongest basepossible in a solvent is the solvent anion.

a) H2SO4 + H2O H3O+ + HSO4

- (100%

dissociation) + -

dissociation)

c) H2SO4 + HOAc

H2OAc+

+ HSO4-

(< 100%)NH3 + HOA NH4+ + OAc- 100

2) HNO3, H2SO4, HClO4, HCl are all equally acidic inwa er

3

3) HClO4 > HCl > H2SO4 > HNO3 in HOAc

4) Hydrocarbon Solvents dont level acids or bases


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DRA14 Acid BaseConcepts

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