ACIDS AND BASES ACIDS AND BASES … … for it cannot be for it cannot be But I am pigeon-liver’d and lack gall But I am pigeon-liver’d and lack gall To make oppression bitter… To make oppression bitter… Hamlet Hamlet
Dec 26, 2015
ACIDS AND BASESACIDS AND BASES
……for it cannot be for it cannot be But I am pigeon-liver’d and lack gall But I am pigeon-liver’d and lack gall To make oppression bitter…To make oppression bitter…HamletHamlet
Learning objectivesLearning objectives
Name and write formulae for common acids and Name and write formulae for common acids and basesbases
Describe acids and bases according to Describe acids and bases according to Arrhenius and Brønsted definitionsArrhenius and Brønsted definitions
Identify conjugate acid-base pairsIdentify conjugate acid-base pairs
Distinguish between strength and concentration Distinguish between strength and concentration in acid-base terminologyin acid-base terminology
Calculate and estimate pH of acids and basesCalculate and estimate pH of acids and bases
Identify buffer solutions and explain actionIdentify buffer solutions and explain action
ACIDS AND BASESACIDS AND BASES
The meaning of acid and base has The meaning of acid and base has changed over the yearschanged over the years
Arrhenius acid is one that Arrhenius acid is one that generatesgenerates protons when dissolved in waterprotons when dissolved in water
Arrhenius base is one that Arrhenius base is one that generatesgenerates hydroxide ions when dissolved in waterhydroxide ions when dissolved in water
Hydronium ion is the active Hydronium ion is the active ingredient of an acidingredient of an acid
Protons do not exist in solutionProtons do not exist in solution
CHCH33COCO22H + HH + H22O = O = HH33OO++ + CH + CH33COCO22--
Vinegar in water produces hydronium ionsVinegar in water produces hydronium ions
Hydroxide ion is the active Hydroxide ion is the active ingredient of a baseingredient of a base
NHNH33 + H + H22O = NHO = NH44++ + + OHOH--
Ammonia, a base, dissolves in water and Ammonia, a base, dissolves in water and produces hydroxide ionsproduces hydroxide ions
NeutralizationNeutralization
The mixing of an acid with a base:The mixing of an acid with a base:
ACID + BASE = SALT + WATERACID + BASE = SALT + WATER
The reaction of carbonic acid (COThe reaction of carbonic acid (CO22 in H in H22O) O)
to give limestone:to give limestone:
HH22COCO33 + Ca(OH) + Ca(OH)22 = CaCO = CaCO33 + 2H + 2H22OO
ACID BASE WATERSALT
The essence of neutralizationThe essence of neutralization
Elimination of the components of acid and Elimination of the components of acid and base by combination to give Hbase by combination to give H22OO
– Reflection of strong O – H bondReflection of strong O – H bond
HH++ + OH + OH-- H H22OO
ACID BASE
BrBrøønsted and Lowrynsted and Lowry
A broader definition of acids and basesA broader definition of acids and bases
In the reaction NHIn the reaction NH33 + HCl = NH + HCl = NH44Cl has all Cl has all
the elements of acid-base neutralization the elements of acid-base neutralization but no Hbut no H22O as would be required in the O as would be required in the
Arrhenius definitionArrhenius definition
Brønsted acid Brønsted acid donatesdonates a proton a proton
Brønsted base Brønsted base acceptsaccepts a proton a proton
Brønsted acidBrønsted acid
HCl + HHCl + H22O = HO = H33OO++ + Cl + Cl--
Brønsted baseBrønsted base
NHNH33 + H + H22O = NHO = NH44++ + OH + OH--
waterwater
NHNH33 + HCl = NH + HCl = NH44++ClCl--
No waterNo water
Substances can be both acids and Substances can be both acids and bases – depends on environmentbases – depends on environment
Note that in one instance HNote that in one instance H22O behaves O behaves
like a base – like a base – acceptingaccepting protons, and in protons, and in another, behaves like an acid – another, behaves like an acid – donatingdonating protonsprotons
HCl + HCl + HH22OO = H = H33OO++ + Cl + Cl--
In presence of an acid HIn presence of an acid H22O is a baseO is a base
NHNH33 + + HH22OO = NH = NH44++ + OH + OH--
In presence of a base HIn presence of a base H22O is an acidO is an acid
The products are themselves acids The products are themselves acids and basesand bases
Equilibrium: solution contains Equilibrium: solution contains mixture of all componentsmixture of all components
Conjugate Conjugate acids and basesacids and bases
Conjugate acid-base pair differs by HConjugate acid-base pair differs by H++
HHAA + + BB ↔ ↔ AA-- + H + HBB++
Conjugate acid-base pair Conjugate acid-base pair
Conjugate baseConjugate acidbase
acid
Strength and concentrationStrength and concentration
Not all acids completely donate the Not all acids completely donate the protons to water molecules in solutionprotons to water molecules in solution
HA + HHA + H22O O A A-- + H + H33OO++
The degree of ionization is described by The degree of ionization is described by strengthstrength
The total number of moles per unit volume The total number of moles per unit volume is described by concentrationis described by concentration
Equilibrium for a weak acidEquilibrium for a weak acid
Measuring acid strength with KMeasuring acid strength with Kaa
– Weak acid has small KWeak acid has small Kaa
– Strong acid has large KStrong acid has large Kaa
AcOHOHHAc 32
HAc
AcOHKa
3
Changing concentration does not Changing concentration does not change strengthchange strength
StrengthStrength refers to degree of ionization: refers to degree of ionization:– Strong is completely ionized (100 %)Strong is completely ionized (100 %)– Weak is partly ionized (1 % - 1:10Weak is partly ionized (1 % - 1:1066))
ConcentrationConcentration refers to number of moles refers to number of moles per unit volumeper unit volume
An acid (or base) can be strong and An acid (or base) can be strong and concentrated, weak and concentrated, concentrated, weak and concentrated, strong and dilute, weak and dilutestrong and dilute, weak and dilute
Relative strength of conjugate pairRelative strength of conjugate pair
The conjugate base of a very strong acid The conjugate base of a very strong acid is itself very weakis itself very weak
The conjugate acid of a very strong base The conjugate acid of a very strong base is itself very weakis itself very weak
AmphotericityAmphotericity
A substance that behaves as an acid and A substance that behaves as an acid and a base is amphoteric. Water is a good a base is amphoteric. Water is a good exampleexample
Ionization of waterIonization of water
Even in pure water, a fraction of the Even in pure water, a fraction of the molecules are ionized and the molecules are ionized and the concentrations of OHconcentrations of OH-- and H and H33OO++ are equal are equal
HH22O + HO + H22O = HO = H33OO++ + OH + OH--
[H[H33OO++] = [OH] = [OH--]]
Concentration
In all aqueous solutions, product of In all aqueous solutions, product of concentrations is a constantconcentrations is a constant
[H[H33OO++][OH][OH--] = constant] = constant
Increasing [HIncreasing [H33OO++] decreases [OH] decreases [OH--] ]
(acidic conditions)(acidic conditions)
Increasing [OHIncreasing [OH--] decreases [H] decreases [H33OO++]]
(basic conditions)(basic conditions)
The pH scale – reduces large The pH scale – reduces large range of numbers to smallrange of numbers to small
In water KIn water KWW = [H = [H33OO++][OH][OH--] = 10] = 10-14-14
pH = - logpH = - log1010[H[H33OO++]]
Range of [HRange of [H33OO++] 10 M – 10] 10 M – 10-14-14 M M
Range of pH -1 to +14 Range of pH -1 to +14 Low pH = acid; high pH = basicLow pH = acid; high pH = basic
pH = 7 = neutralpH = 7 = neutral
Relating pH to [HRelating pH to [H33OO++]]
For pH, take exponent of [HFor pH, take exponent of [H33OO++], change ], change
signsign
– 1010-1-1 M (0.1 M) HCl has pH = M (0.1 M) HCl has pH = 11
– Pure water has [HPure water has [H33OO++] = 10] = 10-7-7 M, pH = M, pH = 77
– Ammonia has [HAmmonia has [H33OO++] =10] =10-11-11 M, pH = M, pH = 1111
Note: change of 1 unit in pH is factor of tenNote: change of 1 unit in pH is factor of ten
Acid strength and pHAcid strength and pH
pH is determined by the ratio of the weak pH is determined by the ratio of the weak acid:conjugate base and the value of Kacid:conjugate base and the value of Kaa
HAc
AcOHKa
3
Ac
KHAcOH a
3
Estimating pHEstimating pH
Estimating pH is often more useful than Estimating pH is often more useful than doing exact calculationsdoing exact calculations
Smaller pH value means larger HSmaller pH value means larger H++ concentrationconcentration
Estimating pHEstimating pH
Smaller pH value means larger HSmaller pH value means larger H++ concentrationconcentration
pH = - logpH = - log1010[H[H33OO++]]
[H[H33OO++] = 10] = 10-4-4
pH = - logpH = - log1010(10(10--44))
= = 44
[H[H33OO++] = ] = 44 x 10 x 10--44
pH = - logpH = - log1010((44 x 10 x 10--44))
= - log= - log101044 - log - log1010(10(10--44))
= = 44 – log – log101044
==44 – – 0.6020.602
= 3.4= 3.4
Concentration 1 x 10Concentration 1 x 10-4-4 M MpH = 4pH = 4
Concentration 4 x 10Concentration 4 x 10-4-4 M MpH = 3.4pH = 3.4
pH and pOHpH and pOH
In any aqueous solution, In any aqueous solution,
[H[H++] x [OH] x [OH--] = 10] = 10-14-14
Obtain [HObtain [H++] given [OH] given [OH--]]
pOH = -logpOH = -log1010[OH[OH--]]
pH + pOH = 14pH + pOH = 14
Relationships between [HRelationships between [H++], pH and [OH], pH and [OH--]]
Buffer solutions control pHBuffer solutions control pH
In biological systems it is often critical for In biological systems it is often critical for stable function to maintain a specific pH stable function to maintain a specific pH environment.environment.
Buffer solutions perform this function – Buffer solutions perform this function – they resist large changes in pH when they resist large changes in pH when either acids or alkalis are addedeither acids or alkalis are added
Buffer solution contains a Buffer solution contains a combination of a weak acid and its combination of a weak acid and its
saltsalt
Mechanism follows Le Chatelier’s Mechanism follows Le Chatelier’s principleprinciple
– Addition of HAddition of H++ converts CH converts CH33COCO22-- to CH to CH33COCO22HH
CHCH33COCO22-- + H + H++ CH CH33COCO22HH
– Addition of OHAddition of OH-- converts CH converts CH33COCO22H to CHH to CH33COCO22--
CHCH33COCO22H + OHH + OH-- CH CH33COCO22-- + H + H22O O
– In general, In general,
Add OHAdd OH- - shifts equilibrium to right (HA shifts equilibrium to right (HA A A--))
HA + HHA + H22O O A A-- + H + H33OO++
Add HAdd H++ shifts equilibrium to left (A shifts equilibrium to left (A-- HA) HA)