Acid-base reactions are the chemical reactions that occur when acids and bases are mixed together The Brønsted-Lowry theory of acids and bases discusses them
Bases form hydroxide ions in aqueous solution Examples of Arrhenius acids (in water): HCl, H2SO4, etc Examples of Arrhenius bases (in water): NaOH, NH3,
Acid/base reactions represent an example of a fundamental class of chemical reactions The process involves the transfer of a hydrated proton from a donor
Dyes and many other chemicals are made with sulfuric acid and nitric acid, and corn syrup, which is added to a variety of foods, is processed with hydrochloric
A- = the conjugate base Examples: HCl + water; carbonate + water; H2S in Water Note that water can act as either an acid or a base
depending on the presence of an acid or a base, and the chemistry behind the Some examples of acids are orange juice, tomatoes, and battery acid
30 oct 2015 · can dissociate into cations (acid) and anions (base) • The classic example is water: • Now we can say that sulfuric acid is an acid because
Water is an example of a Lewis base Carbocations are examples of Lewis acids When water reacts with a carbocation as shown below, one
6 juil 2009 · Acid-base indicators Indicators = substances (like natural dyes) that change colors in acidic or basic (alkaline) solutions Examples:
Organic acids are covered in more detail in organic chemistry Page 2 189 Inorganic acids are generally composed of nonmetallic elements The polyatomic ions
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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.
2 lectures dealing with some core chemistry :
àacid/base reactions
àthepH concept.
We will study these concepts in more detail during the main lecture course later on. We 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
Chemistry Preliminary Course 20113
Required Reading Material.
Silberberg, Chemistry, 4th edition.
àChapter 18.
ŻAcid/base equilibria. pp.766-813.
àChapter 19.
ŻIonic equilibriain aqueous systems. pp.814-862.
Kotz, Treicheland Weaver, 7thedition.
àChapter 17&18, pp.760-859.
Burrows et al. Chemistry3(OUP), 2009.Ch.6, pp.263-300. Lecture notes available after course on School of Chemistry website located at: http://www.tcd.ie/Chemistry/outreach/prelim/
Useful websites
http://www.shodor.org/unchem/basic/ab/
http://chemistry.about.com/od/acidsbases/
http://www.chem.neu.edu/Courses/1221PAM/
acidbase/index.htm
http://dbhs.wvusd.k12.ca.us/webdocs/AcidBa
se/AcidBase.html
http://www.sparknotes.com/chemistry/acids
bases/fundamentals/section1.html
Chemistry Preliminary Course 20114
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
Chemistry Preliminary Course 20116
Acid and Bases
Chemistry Preliminary Course 20117
Acid and Bases
Chemistry Preliminary Course 20118
Acid and Bases
Chemistry Preliminary Course 20119
Chemistry Preliminary Course 201110
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.
Chemistry Preliminary Course 201111
Arrhenius (or Classical) Acid-Base Definition
An acidis a neutral substance that contains hydrogen and dissociates or ionizes in water to yield hydrated protons or hydroniumions H3O+.
A baseis a neutral substance that contains the hydroxyl group and dissociates in water to yield hydrated hydroxide ions OH-.
Neutralizationis the reaction of an H+(H3O+) ion from the acid and the OH -ion from the base to form water, H2O.
These definitions although correct are limited in that they are not very general and do not
Give a comprehensive idea of
what acidity and basicityentails. )()( )()( aqOHaqNaNaOH aqClaqHHCl o o
OHNaClNaOHHCl2 o
acidbasesaltwater
Chemistry Preliminary Course 201112
Arrhenius acid is a substance that produces H+ (H3O+) in water Arrhenius base is a substance that produces OH-in water
Chemistry Preliminary Course 201113
Acids and bases:
Bronsted/Lowry definition.
Bronsted/Lowry Acid (HA):
àAn acid is a species which donatesa proton
Bronsted/Lowry Base (B):
àA base is a species which acceptsa proton.
These definitions are quite general and refer to the reaction between an acid and a base.
An acid must contain H in its formula; HNO3and H2PO4-are two examples, all Arrhenius acids are Brønsted-Lowry acids.
A 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.
Chemistry Preliminary Course 201114
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.
Chemistry Preliminary Course 201115
A Brønstedacidis a proton donor
A Brønstedbaseis a proton acceptor
acidbaseacidbase 15.1 acidconjugate basebaseconjugate acid
Chemistry Preliminary Course 201116
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.
Chemistry Preliminary Course 201117
Quantifying acid/base strength.
How 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.
Chemistry Preliminary Course 201118
Battery acid
Vinegar
H2SO4
CH3COOH
Chemistry Preliminary Course 201119
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
Chemistry Preliminary Course 201120
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)
Chemistry Preliminary Course 201121
Reactivity of strong and weak acids.
1M HCl(aq)1M CH3COOH(aq)
Strong acid:
Extensive H2evolution
Weak acid:
H2evolution
Not very extensive
Chemistry Preliminary Course 201122
Weak acids/bases.
We 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.
These 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.
Chemistry Preliminary Course 201123
Weak acids.
HClCH3CO2HCH3CO2H
Chemistry Preliminary Course 201124
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
Chemistry Preliminary Course 201125
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
Chemistry Preliminary Course 201126
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
Chemistry Preliminary Course 201127
Acid/base equilibria.
23HA H O H O A
2B H O BH OH
Weak acid solution
at equilibrium
Weak base solution
at equilibrium
Chemistry Preliminary Course 201128
Mathematical interlude :
the logarithm Paul Monk, Maths for Chemistry, Oxford University Press, 2006.
http://en.wikipedia.org/wiki/Logarithm
The 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.
If 10x= y then log10y = x, e.g. 102=10x10=100, then log10(100)=2.
The antilogarithmfunction is another name for the inverse of the logarithmic function. It is written antilog b(n) and means the same as bn.
Logarithms 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 .
Chemistry Preliminary Course 201129
Mathematical interlude :
the logarithm
The 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.
The 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).
Chemistry Preliminary Course 2011
log2xlogex log10x 30
http://mathworld.wolfram.com/Logarithm.html
Chemistry Preliminary Course 201131
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
Chemistry Preliminary Course 201132
Acid strength : the acid dissociation constant KA.
It is easy to quantify the strength
of strong acids since they fully dissociate to ions in solution.
The situation with respect to weak
acids is more complex since they only dissociate to a small degree in solution.
The question is how small is small?
We 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
Chemistry Preliminary Course 201133
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
Chemistry Preliminary Course 201134
Ionization Constants of Weak Acids and Bases
Chemistry Preliminary Course 201135
Acid dissociation constants.
Chemistry Preliminary Course 201136
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
Chemistry Preliminary Course 201137
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
Chemistry Preliminary Course 201138
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
Chemistry Preliminary Course 201139
The pH concept.
The best quantitative measure of acidity or alkalinity rests in the determination of the concentration of hydrated protons [H3O+] present in a solution.
The [H3O+] varies in magnitude over quite a large range in aqueous solution, typically from 1 M to 10-14M.
Hence to make the numbers meaningful [H3O+] is expressed in terms of a logarithmic scale called the pH scale.
The 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