[PDF] ACID-BASE REACTIONS/ THE PH CONCEPT

2431_8pHandAcid_BaseReactions.pdf

Chemistry Preliminary Course 20111

ACID-BASE REACTIONS/

THE PH CONCEPT.

Dr Mike LyonsSchool of ChemistryTrinity College Dublin.melyons@tcd.ie

Chemistry Preliminary Course 20112

Lecture topics.

ƒ2 lectures dealing with some core chemistry :

àacid/base reactions

àthepH concept.

ƒWe will study these concepts in more detail during the main lecture course later on. ƒWe will address the following questions/ideas:

àWhat are acids and bases?

àCan we provide a general definition of acid and base?

àHow can we quantify acidity and basicity?

àCan we classify acid and base strength?

àpH concept and pH scale.

àAcid/base reactions: neutralization

àHow can we monitor an acid/base reaction in real time?

ŻAcid/base titrations

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Required Reading Material.

ƒSilberberg, Chemistry, 4th edition.

àChapter 18.

ŻAcid/base equilibria. pp.766-813.

àChapter 19.

ŻIonic equilibriain aqueous systems. pp.814-862.

ƒKotz, Treicheland Weaver, 7thedition.

àChapter 17&18, pp.760-859.

ƒBurrows et al. Chemistry3(OUP), 2009.Ch.6, pp.263-300. ƒLecture notes available after course on School of Chemistry website located at: http://www.tcd.ie/Chemistry/outreach/prelim/

Useful websites

ƒhttp://www.shodor.org/unchem/basic/ab/

ƒhttp://chemistry.about.com/od/acidsbases/

ƒhttp://www.chem.neu.edu/Courses/1221PAM/

acidbase/index.htm

ƒhttp://dbhs.wvusd.k12.ca.us/webdocs/AcidBa

se/AcidBase.html

ƒhttp://www.sparknotes.com/chemistry/acids

bases/fundamentals/section1.html

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Chemistry Preliminary Course 20115

Acids Have a sour taste. Vinegar owes its taste to acetic acid. Citrus fruits contain citric acid. React with certain metals to produce hydrogen gas. React with carbonates and bicarbonates to produce carbon dioxide gas

Have a bitter taste.

Feel slippery. Many soaps contain bases.

Bases 4.3

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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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Acid etching

The inside surfaces of these light bulbs

are etched with HF. Acids are used to wash away oxides of silicon and metals during the production of computer chips.

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Arrhenius (or Classical) Acid-Base Definition

ƒAn acidis a neutral substance that contains hydrogen and dissociates or ionizes in water to yield hydrated protons or hydroniumions H3O+.

ƒA baseis a neutral substance that contains the hydroxyl group and dissociates in water to yield hydrated hydroxide ions OH-.

ƒNeutralizationis the reaction of an H+(H3O+) ion from the acid and the OH -ion from the base to form water, H2O.

ƒThese definitions although correct are limited in that they are not very general and do not

ƒGive a comprehensive idea of

what acidity and basicityentails. )()( )()( aqOHaqNaNaOH aqClaqHHCl   o o

OHNaClNaOHHCl2o

acidbasesaltwater

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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

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Acids and bases:

Bronsted/Lowry definition.

ƒBronsted/Lowry Acid (HA):

àAn acid is a species which donatesa proton

ƒBronsted/Lowry Base (B):

àA base is a species which acceptsa proton.

ƒThese definitions are quite general and refer to the reaction between an acid and a base.

ƒAn acid must contain H in its formula; HNO3and H2PO4-are two examples, all Arrhenius acids are Brønsted-Lowry acids.

ƒA base must contain a lone pair of electrons to bind the H+ion; a few examples are NH3, CO32-, F -, as well as OH -. Brønsted-Lowry bases are not Arrhenius bases, but all Arrhenius bases contain the Brønsted-Lowry base OH-.

‡In the Brønsted-Lowry perspective:

one species donates a proton and another species accepts it: an acid-base reaction is a proton transfer process.

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HA(aq) + H2O (l)H3O+ (aq) + A- (aq)

B (aq) + H2O (l)HB+ (aq) + OH- (aq)

Proton transfer

BL acidBL base

BL base

BL acid

‡Proton donation and acceptance are dynamicprocesses for all acids and bases. Hence a proton transfer equilibriumis rapidly established in solution. ‡The equilibrium reaction is described in terms of conjugate acid/base pairs. ‡The conjugate base (CB) of a BL acid is the base which forms when the acid has donated a proton. ‡The conjugate acid (CA) of a BL base is the acid which forms when the base has accepted a proton. ‡A conjugate acid has one more proton than the base has, and a conjugate base one less proton than the acid has. ‡If the acid of a conjugate acid/base pair is strong (good tendency to donate a proton) then the conjugate base will be weak (small tendency to accept a proton) and vice versa.

HA (aq) + B (aq) BH+ (aq) + A- (aq)

Proton transfer

ABCACB

Acid : proton donor

Base : proton acceptor

BL acid/base equilibria.

Water can function

both as an acid and a base depending on the circumstances.

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A Brønstedacidis a proton donor

A Brønstedbaseis a proton acceptor

acidbaseacidbase 15.1 acidconjugate basebaseconjugate acid

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The Conjugate Pairs in Some Acid-Base Reactions

BaseAcid+AcidBase+

Conjugate Pair

Conjugate Pair

Reaction 4H2PO4-OH-+

Reaction 5H2SO4N2H5++

Reaction 6HPO42-SO32-+

Reaction 1HFH2O+

F-H3O++

Reaction 3NH4+CO32-+

Reaction 2HCOOHCN-+

HCOO-HCN+

NH3HCO3-+

HPO42-H2O+

HSO4-N2H62++

PO43-HSO3-+

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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Quantifying acid/base strength.

ƒHow can acid and base strength be

quantified? à¶6PURQJ· MŃLGV vs¶RHMN· MŃLGV

à¶6PURQJ· NMVHV vs¶RHMN· NMVHV

àKey concept is extent or degree of

ionization/dissociation. àCorrelation exists between acid/base strength, degree of ionization in solutionand extent to which solution exhibits ionic conductivity.

Strong and weak acids.

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Battery acid

Vinegar

H2SO4

CH3COOH

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The Extent of

Dissociation for

Strong and

Weak Acids

Key concept :

Acid/base strength

quantified in terms of extentor degree of dissociation.

An acid or base is

classified as strong if it is fully ionized in solution (e.g. HCl,

NaOH).

An acid or base is

classified as weakif only a small fraction is ionized in solution (e.g. CH3COOH, NH3).

Complete ionization

Partial ionization

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Strong Electrolyte ²100% dissociation

NaCl (s)Na+(aq) + Cl-(aq)H2O

Weak Electrolyte ²not completely dissociated

CH3COOHCH3COO-(aq) + H+(aq)

Strong Acidsare strong electrolytes

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

HNO3(aq)+ H2O (l)H3O+(aq)+ NO3-(aq)

HClO4(aq)+ H2O (l)H3O+(aq)+ ClO4-(aq)

H2SO4 (aq)+ H2O (l)H3O+(aq)+ HSO4-(aq)

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Reactivity of strong and weak acids.

1M HCl(aq)1M CH3COOH(aq)

Strong acid:

Extensive H2evolution

Weak acid:

H2evolution

Not very extensive

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Weak acids/bases.

ƒWe can quantify the extent of dissociation of a weak acid or a weak base in aqueous solution by introducing:

àthe acid dissociation constant Kaor

àthe base dissociation constant Kb.

ƒThese are numbers which reflect acid or base strength and are computed by determining the equilibrium concentrations of all relevant species in the solution, and inputting this data into a theoretical expression for the relevant dissociation constant.

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Weak acids.

HClCH3CO2HCH3CO2H

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Weak Acids

Ka== 1.8x10-5

[CH3CO2H] [CH3CO2-][H3O+] pKa= -log(1.8x10-5) = 4.74 glycine H2NCH2CO2H lactic acid CH3CH(OH)CO2H C OH O R

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HF (aq)+ H2O (l)H3O+(aq)+ F-(aq)

Weak Acidsare weak electrolytes

HNO2(aq)+ H2O (l)H3O+(aq)+ NO2-(aq)

HSO4-(aq)+ H2O (l)H3O+(aq)+ SO42-(aq)

H2O (l)+ H2O (l)H3O+(aq)+ OH-(aq)

Strong Basesare strong electrolytes

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

KOH (s) K+(aq)+ OH-(aq)H2O

Ba(OH)2(s) Ba2+(aq)+ 2OH-(aq)H2O

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F-(aq)+ H2O (l)OH-(aq)+ HF (aq)

Weak Basesare weak electrolytes

NO2-(aq)+ H2O (l) OH-(aq)+ HNO2(aq)

Conjugate acid-base pairs:

The conjugate base of a strong acid has no measurable strength. H3O+is the strongest acid that can exist in aqueous solution. The OH-ion is the strongest base that can exist in aqeous solution. 15.4

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Acid/base equilibria.

23HA H O H O A

2B H O BH OH

Weak acid solution

at equilibrium

Weak base solution

at equilibrium

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Mathematical interlude :

the logarithm ƒPaul Monk, Maths for Chemistry, Oxford University Press, 2006.

ƒhttp://en.wikipedia.org/wiki/Logarithm

ƒThe logarithmis the mathematicaloperation that is the inverseof exponentiation(raising a constant, the base, to a power). The logarithm of a number xin base bis the number nsuch that x= bn. It is usually written as logb(x)=n.

ƒIf 10x= y then log10y = x, e.g. 102=10x10=100, then log10(100)=2.

ƒThe antilogarithmfunction is another name for the inverse of the logarithmic function. It is written antilog b(n) and means the same as bn.

ƒLogarithms can reduce multiplication operations to addition, division to subtraction, exponentiation to multiplication, and roots to division. Therefore, logarithms are useful for making lengthy numerical operations easier to perform .

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Mathematical interlude :

the logarithm

ƒThe most widely used bases for logarithms are 10, the mathematical constant e§ 2B71828BBB MQG 2B JOHQ ORJ LV RULPPHQ RLPORXP M NMVH bmissing from log b), the intent can usually be determined from context:

ànatural logarithm(loge) in mathematical analysis

àcommon logarithm(log10) in engineeringand when logarithm tablesare used to simplify hand calculations

àbinary logarithm(log2) in information theoryand musical intervals.

ƒThe notation "ln(x)" invariably means loge(x), i.e., the natural logarithm of x, but the implied base for "log (x)" varies by discipline:

àMathematicians generally understand both "ln(x)" and "log(x)" to mean loge(x) and write "log10(x)" when the base-10 logarithm of xis intended.

àEngineers, biologists, and some others write only "ln(x)" or "loge(x)" when they mean the natural logarithm of x, and take "log (x)" to mean log10(x) or, sometimes in the context of computing, log2(x).

àOn most calculators, the LOG button is log10(x) and LN is loge(x).

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log2xlogex log10x 30
http://mathworld.wolfram.com/Logarithm.html

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Operations with numbersLogarithmic identity

ablog . log( ) log( )ab a babbabalog log( ) log( )a b a blog logba b alog log( )ba a b.abab

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Acid strength : the acid dissociation constant KA.

ƒIt is easy to quantify the strength

of strong acids since they fully dissociate to ions in solution.

ƒThe situation with respect to weak

acids is more complex since they only dissociate to a small degree in solution.

ƒThe question is how small is small?

ƒWe quantify the idea of incomplete

dissociation of a weak acid HA by noting that the dissociation reaction is an equilibrium process and introducing the acid dissociation constant KA.

HA(aq)+H2O(l)H3O+ (aq) + A- (aq)

@>@ @>@ >@@>@ @HA

AOHOHKK

OHHA AOHK CA C   3 2 2 3

Acid dissociation

equilibrium

Acid dissociation

constant

KAis a measure of the acid strength.

When KAis large there is considerable

Dissociation and the acid is strong.

When KAis small there

is a small degree of dissociation, and the acid is weak.

KAvalues vary over a wide range

so it is best to use a log scale.

AAKpK10log

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The Relationship Between Kaand pKa

Acid Name (Formula) KAat 298 K pKA Hydrogen sulfate ion (HSO4-) 1.02 x 10-21.991 Nitrous acid (HNO2) 7.1 x 10-43.15 Acetic acid (CH3COOH) 1.8 x 10-54.74 Hypobromousacid (HBrO) 2.3 x 10-98.64 Phenol (C6H5OH) 1.0 x 10-1010.00

When KAis small pKAis large and the acid does not dissociate in solution to a large extent. A change in 1 pKA

unit implies a 10 fold change in KAvalue and hence acid strength.

AAKpK10log

p A A pK K

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Ionization Constants of Weak Acids and Bases

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Acid dissociation constants.

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O H H+O H HO H

HHOH-+[]+

Acid-Base Properties of Water

H2O (l)H+(aq)+ OH-(aq)

H2O + H2O H3O++ OH-

acidconjugate base baseconjugate acid autoionization of water

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H2O (l)H+(aq)+ OH-(aq)

The Ion Product of Water

Kc=[H+][OH-]

[H2O][H2O] = constant

Kc[H2O] = Kw= [H+][OH-]

The ion-product constant(Kw) is the product of the molar concentrations of H+and OH-ions at a particular temperature.

At 250C

Kw= [H+][OH-] = 1.0 x 10-14

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

Solution Is

neutral acidic basic

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BasicityConstant Kb.

YThe proton accepting

strength of a base is quantified in terms of the basicity constant Kb.

YThe larger the value of Kb,

the stronger the base.

YIf Kbis large then pKbwill be

small, and the stronger will be the base.

‡Solve weak base problems

like weak acids exceptsolve for [OH-] instead of [H+].@>@ @>@ >@@>@ @B

OHBHOHKK

OHB OHBHK Cb C   2 2

B(aq) + H2O (l)BH+ (aq) + OH- (aq)bbKpK10log Wba

Wba pKpKpK KKK 

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The pH concept.

ƒThe best quantitative measure of acidity or alkalinity rests in the determination of the concentration of hydrated protons [H3O+] present in a solution.

ƒThe [H3O+] varies in magnitude over quite a large range in aqueous solution, typically from 1 M to 10-14M.

ƒHence to make the numbers meaningful [H3O+] is expressed in terms of a logarithmic scale called the pH scale.

ƒThe higherthe [H3O+] , the more acidicthe solution and the loweris the solution pH. @>@pHOH OHpH    10 log 3 310

Linear and

logarithmic

Scales.



3)10log()10

1log()001.0log(

2)10log(10

1log)01.0log(

110log)10

1log()1.0log(

3 3 2 2 1