Acids and BasesAcids and Bases

Acids, Bases and Acids, Bases and EquilibriumEquilibrium When an acid is dissolved in water, When an acid is dissolved in water,

the Hthe H++ ion (proton) produced by the ion (proton) produced by the acid combines with water to produce acid combines with water to produce the hydronium ion, Hthe hydronium ion, H33OO++

HCl and other strong electrolytes HCl and other strong electrolytes ionize completely in waterionize completely in water

Weak acids like acetic acid ionize Weak acids like acetic acid ionize only to a very small extent in wateronly to a very small extent in water

Acids, Bases and Acids, Bases and EquilibriumEquilibrium The equilibrium concept is used The equilibrium concept is used

to describe to what extent an acid to describe to what extent an acid or base ionizes in wateror base ionizes in water

Ionization constants: K>1 Ionization constants: K>1 indicates a strong acid or base, indicates a strong acid or base, K<1 refers to a weak acid or baseK<1 refers to a weak acid or base

Strong ElectrolytesStrong Electrolytes Strong Acids – HCl, HBr, HI, HNOStrong Acids – HCl, HBr, HI, HNO33, ,

HClOHClO44, H, H22SOSO44 (for first H (for first H++ only) only) Strong Bases – LiOH, NaOH, KOH, Strong Bases – LiOH, NaOH, KOH,

RbOH, CsOH, Sr(OH)RbOH, CsOH, Sr(OH)22, Ba(OH), Ba(OH)22 All others are probably weakAll others are probably weak

Arrhenius Acids and Arrhenius Acids and BasesBases An acid is a substance that, when An acid is a substance that, when

dissolved in water, increases the dissolved in water, increases the concentration of hydrogen ions, concentration of hydrogen ions, H+, in the waterH+, in the water

A base is a substance that, when A base is a substance that, when dissolved in water increases the dissolved in water increases the concentration of hydroxide ion, concentration of hydroxide ion, OH-, in the waterOH-, in the water

*Bronsted-Lowry Acids and *Bronsted-Lowry Acids and BasesBases

An acid is any substance that can An acid is any substance that can donate a proton to any other donate a proton to any other substancesubstance

Examples of Bronsted acids – Examples of Bronsted acids – molecular compounds (HNOmolecular compounds (HNO33), ), cations (NHcations (NH44++), hydrated metals ), hydrated metals (Fe(H(Fe(H22O)O)663+3+), or anions (H), or anions (H22POPO44--))

Theory is not restricted to Theory is not restricted to compounds in watercompounds in water

Bronsted-Lowry Acids and Bronsted-Lowry Acids and BasesBases

A Bronsted base is a substance A Bronsted base is a substance that can accept a proton from any that can accept a proton from any other substanceother substance

Bronsted bases can be molecular Bronsted bases can be molecular compound (NHcompound (NH33), anions (CO), anions (CO332-2-), ), or cations (Al(Hor cations (Al(H22O)O)55(OH)(OH)2+2+) )

Polyprotic Acids + Polyprotic Acids + BasesBases

Acid FormAcid Form Amphiprotic Amphiprotic FormForm

Base FormBase Form

HH22SS HSHS-- SS2-2-

HH33POPO44 HH22POPO44--

HPOHPO442-2-

POPO443-3-

HH22COCO33 HCOHCO33-- COCO332-2-

HH22CC22OO44 HCHC22OO44-- CC22OO442-2-

Conjugate Acid-Base Conjugate Acid-Base PairsPairs

A pair of compounds or ions that A pair of compounds or ions that differ by the presence of one H+ differ by the presence of one H+ ion is called a conjugate acid-ion is called a conjugate acid-base pairbase pair

Every reaction between a Every reaction between a Bronsted acid and Bronsted base Bronsted acid and Bronsted base involves H+ transfer and has two involves H+ transfer and has two conjugate acid-base pairsconjugate acid-base pairs

*Conjugate Acid-Base *Conjugate Acid-Base PairsPairs

Acid 1Acid 1 Base 2Base 2 Base 1Base 1 Acid 2Acid 2HClHCl + H+ H22OO ClCl-- + H+ H33OO++

HCOHCO33-- + H+ H22OO COCO332-2- + H+ H33OO++

CHCH33COCO22HH + H+ H22OO CHCH33COCO22-- + H+ H33OO++

HH22OO + NH+ NH33 OHOH-- + NH+ NH44++

HH22OO + CO+ CO332-2- OHOH-- + HCO+ HCO33--

HH22OO + H+ H22OO OHOH-- + H+ H33OO++

*Water and the pH *Water and the pH scalescale Water autoionizes to a small Water autoionizes to a small

extent, producing low extent, producing low concentrations of Hconcentrations of H33OO+ + and OHand OH-- ions (water conducts electricity)ions (water conducts electricity)

The equilibrium for autoionization The equilibrium for autoionization of water lies far to the leftof water lies far to the left

At 25At 25ooC, Kw=1.0 x 10C, Kw=1.0 x 10--1414 (the (the ionization constant for water)ionization constant for water)

*Water and the pH *Water and the pH scalescale Kw increases with temperature Kw increases with temperature

because the autoionization of water because the autoionization of water is endothermicis endothermic

Kw is valid in pure water and any Kw is valid in pure water and any aqueous solutionaqueous solution

In pure water and dilute aqueous In pure water and dilute aqueous solutions, the concentration of water solutions, the concentration of water is considered to be constant at 55.5 is considered to be constant at 55.5 MM

*Water and the pH *Water and the pH scalescale

[H[H33OO++] = [OH] = [OH--] = 1.0 x 10] = 1.0 x 10-7-7 M in M in pure water, a neutral solution pure water, a neutral solution

In acidic solution, [HIn acidic solution, [H33OO++]>[OH]>[OH--]] In basic solution, [HIn basic solution, [H33OO++]<[OH]<[OH--]]

*Water and the pH *Water and the pH scalescale pH = -log[HpH = -log[H33OO++]] pOH = -log[OHpOH = -log[OH--]] pKw = pH + pOH = 14.00pKw = pH + pOH = 14.00

Relationship between hydronium and Relationship between hydronium and hydroxide ion concentrations, pH and hydroxide ion concentrations, pH and pOHpOH

*Equilibrium Constants *Equilibrium Constants for Acid and Basesfor Acid and Bases The strength of acids and bases The strength of acids and bases

of the same concentration can be of the same concentration can be compared by measuring pHcompared by measuring pH

The relative strength of an acid The relative strength of an acid can be expressed with an can be expressed with an equilibrium constantequilibrium constant

KKaa = [H = [H33OO++][A][A--]/[HA]]/[HA]

Equilibrium Constants Equilibrium Constants for Acid and Basesfor Acid and Bases KKbb = [BH = [BH++][OH][OH--]/[B]]/[B] Weak acid, KWeak acid, Kaa<1, pH>2, small [H<1, pH>2, small [H33OO++]] Weak base, Kb<1, pH<12, small Weak base, Kb<1, pH<12, small

[OH[OH--]] A large value of K indicates A large value of K indicates

ionization products are strongly ionization products are strongly favored; small K value indicates favored; small K value indicates reactants are favoredreactants are favored

Equilibrium Constants Equilibrium Constants for Acid and Basesfor Acid and Bases Weak acids have strong conjugate Weak acids have strong conjugate

bases; small Ka corresponds with bases; small Ka corresponds with large Kblarge Kb

Consider the acids and conjugate Consider the acids and conjugate bases in bases in table 15.2 on page 668table 15.2 on page 668

Notice trends: as acid strength Notice trends: as acid strength declines in a series, the relative declines in a series, the relative conjugate base strength increases conjugate base strength increases

*Equilibrium Constants *Equilibrium Constants for Acid and Basesfor Acid and Bases

Use Tables 15.3 on page 671, 15.4 on Use Tables 15.3 on page 671, 15.4 on page 679, and 15.5 on page 683page 679, and 15.5 on page 683

Which is the stronger acid, HWhich is the stronger acid, H22SOSO44 or H or H22SOSO33?? Is benzoic acid stronger or weaker than Is benzoic acid stronger or weaker than

acetic acid?acetic acid? Which has the stronger conjugate base, Which has the stronger conjugate base,

acetic acid or formic acid?acetic acid or formic acid? Which is the stronger base, ammonia or Which is the stronger base, ammonia or

methylamine?methylamine? Which has the stronger conjugate acid, Which has the stronger conjugate acid,

ammonia or methylamine?ammonia or methylamine?

Calculations with Calculations with Equilibrium ConstantsEquilibrium Constants The principles of the equilibria can The principles of the equilibria can

be applied to aqueous solutions of be applied to aqueous solutions of weak acids and bases. The weak acids and bases. The equilbrium constants Ka and Kb can equilbrium constants Ka and Kb can be determined if the concentrations be determined if the concentrations of the various species present in of the various species present in the solution are known. These are the solution are known. These are often determined by measuring pH.often determined by measuring pH.

*Calculations with *Calculations with Equilibrium ConstantsEquilibrium Constants If the acid or base is weak, and If the acid or base is weak, and

the initial concentration of acid the initial concentration of acid (or base) is at least 100x Ka (or (or base) is at least 100x Ka (or Kb), then the approximation that Kb), then the approximation that

[acid][acid]initialinitial = [acid] = [acid]equilibriumequilibrium is valid. Otherwise the quadratic is valid. Otherwise the quadratic equation must be solved. equation must be solved.

Calculations with Calculations with Equilibrium ConstantsEquilibrium Constants

(ex) For the weak acid:(ex) For the weak acid:HA (aq) + HHA (aq) + H22O (l) O (l) H H33OO+ + (aq) + A- (aq) + A-

(aq) Ka = [H(aq) Ka = [H33OO+ + ][A-]/[HA] and ][A-]/[HA] and because [Hbecause [H33OO+ + ] = [A-], Ka = ] = [A-], Ka = [H[H33OO+ + ]]22/[HA]/[HA]

The assumption that [HA]The assumption that [HA]equil equil = [HA]= [HA]initialinitial is valid if [HA]is valid if [HA]initialinitial > 100 x Ka > 100 x Ka

*Calculations with *Calculations with Equilibrium ConstantsEquilibrium Constants

Calculating a Ka value from a measured pHCalculating a Ka value from a measured pH A solution prepared from 0.055 mol butanoic A solution prepared from 0.055 mol butanoic

acid dissolved in sufficient water to give 1.0 L acid dissolved in sufficient water to give 1.0 L of solution has a pH of 2.72. Determine Ka for of solution has a pH of 2.72. Determine Ka for butanoic acid. The acid ionizes according to butanoic acid. The acid ionizes according to the balanced equation:the balanced equation:

CHCH33CHCH22CHCH22COCO22H + HH + H22O O H H33OO++ + H + H33CHCH22CHCH22COCO22--

CHCH33CHCH22CHCH22COCO22H + HH + H22O O H H33OO++ + H + H33CHCH22CHCH22COCO22--

Calculate initial molarity of butanoic Calculate initial molarity of butanoic acidacid

Calculate equilibrium molarity of Calculate equilibrium molarity of hydronium ion from pHhydronium ion from pH

Construct ice tableConstruct ice table Write equilibrium constant Write equilibrium constant

expression and substitute in values expression and substitute in values from ice tablefrom ice table

Calculate KaCalculate Ka

*Calculating Equilibrium *Calculating Equilibrium Concentrations and pH from KaConcentrations and pH from Ka

What are the equilibrium concentrations of What are the equilibrium concentrations of acetic acid, acetate ion, and hydronium ion for acetic acid, acetate ion, and hydronium ion for a 0.10 M solution of acetic acid (Ka = 1.8 x 10a 0.10 M solution of acetic acid (Ka = 1.8 x 10--55)? What is the pH of the solution?)? What is the pH of the solution?

Write chemical equation for ionization of acetic Write chemical equation for ionization of acetic acid in wateracid in water

ICE it upICE it up Write equilibrium constant expression, Write equilibrium constant expression,

substitute values from ice table, solve for xsubstitute values from ice table, solve for x Use x to find equilibrium concentrationsUse x to find equilibrium concentrations Use concentration of hydronium ion to solve Use concentration of hydronium ion to solve

for pHfor pH

*Calculating the pH of a salt *Calculating the pH of a salt solutionsolution What is the pH of a 0.015 M solution of What is the pH of a 0.015 M solution of

sodium acetate?sodium acetate? Write equation for ionization reaction of Write equation for ionization reaction of

acetate ion in water (it is a base!)acetate ion in water (it is a base!) Put it on ICEPut it on ICE Write equilibrium constant expression Write equilibrium constant expression

and substitute ICE valuesand substitute ICE values Solve for x, solve for concentration of Solve for x, solve for concentration of

hydroxide ionhydroxide ion Solve for concentration of hydronium Solve for concentration of hydronium

ion, solve for pHion, solve for pH

*Calculating the pH after *Calculating the pH after the reaction of an acid with the reaction of an acid with a basea base Calculate the pH after mixing 15 mL of Calculate the pH after mixing 15 mL of

0.12 M acetic acid with 15 mL of 0.12 M 0.12 M acetic acid with 15 mL of 0.12 M NaOH. What are the major species in NaOH. What are the major species in solution at equilibrium (besides water) solution at equilibrium (besides water) and what are their concentrations?and what are their concentrations?

Write balanced equationsWrite balanced equations Stoichiometry problem to solve for Stoichiometry problem to solve for ““initialinitial”” concentration of acetate anion concentration of acetate anion

ICE, equilibrium constant express Kb, ICE, equilibrium constant express Kb, pOH and suchpOH and such

*Aqueous Solutions of Salts*Aqueous Solutions of SaltsCationCation AnionAnion pH of SolutionpH of SolutionFrom strong From strong base (Na+)base (Na+)

From strong From strong acid (Cl-)acid (Cl-)

=7 neutral=7 neutral

From strong From strong base (K+)base (K+)

From weak From weak acid (CHacid (CH33COCO22-)-)

>7 basic>7 basic

From weak From weak base (NHbase (NH44+)+)

From strong From strong acid (Cl-)acid (Cl-)

<7 acidic<7 acidic

From any From any weak base weak base (BH+)(BH+)

From any weak From any weak acid (A-)acid (A-)

Depends on Depends on relative relative strengths of acid strengths of acid and baseand base

*For each of the following salts in *For each of the following salts in water, predict whether the pH water, predict whether the pH will be greater than, less than, or will be greater than, less than, or equal to 7equal to 7 KBrKBr NHNH44NONO33 AlClAlCl33 NaNa22HPOHPO44

*Polyprotic acids and *Polyprotic acids and basesbases

The pH of many inorganic polyprotic The pH of many inorganic polyprotic acids depends primarily on the acids depends primarily on the hydronium ion generated in the first hydronium ion generated in the first ionization stepionization step

Each successive loss of a proton is Each successive loss of a proton is about 10about 1044-10-1066 more difficult than the more difficult than the previous stepprevious step

The hydronium ion produced in the The hydronium ion produced in the second step can be neglected; calculate second step can be neglected; calculate using the k of the first ionizationusing the k of the first ionization

*Molecular Structure, *Molecular Structure, Bonding, and Acid Bonding, and Acid StrengthStrength In a series of acids, as bond strength In a series of acids, as bond strength

decreases, acid strength increasesdecreases, acid strength increases Adjacent electronegative atoms that Adjacent electronegative atoms that

pull electrons from a hydrogen increase pull electrons from a hydrogen increase the strength of an acid (inductive the strength of an acid (inductive effect) effect)

Acids that have resonance structures Acids that have resonance structures are stronger because they are more are stronger because they are more stable after they lose a proton than stable after they lose a proton than acids without resonanceacids without resonance

http://www.mhhe.com/physsci/chemistry/chang7/ssg/http://www.mhhe.com/physsci/chemistry/chang7/ssg/chap15_9sg.htmlchap15_9sg.html

*Lewis Acids and *Lewis Acids and BasesBases A Lewis acid is a substance that A Lewis acid is a substance that

can accept a pair of electrons from can accept a pair of electrons from another atom to form a new bondanother atom to form a new bond

A Lewis base is a substance that A Lewis base is a substance that can donate a pair of electrons to can donate a pair of electrons to another atom to form a new bondanother atom to form a new bond

This is also know as coordinate This is also know as coordinate covalent chemistrycovalent chemistry

Lewis Acids and BasesLewis Acids and Bases A + B: A + B: B:A B:A acid + base acid + base acid-base adduct acid-base adduct The acid must have an empty orbital The acid must have an empty orbital

available or be able to make one available or be able to make one availableavailable

A base must have a pair of A base must have a pair of nonbonding electronsnonbonding electrons

The formation of hydronium ion is an The formation of hydronium ion is an example of this type of reaction example of this type of reaction

Lewis Acids and BasesLewis Acids and Bases Good Lewis bases include hydroxide Good Lewis bases include hydroxide

ion and ammonia and waterion and ammonia and water Metal cations are good Lewis acids; Metal cations are good Lewis acids;

transition metals form complexes transition metals form complexes known as coordination complexesknown as coordination complexes

Nonmetal oxides such as carbon Nonmetal oxides such as carbon dioxide are Lewis acidsdioxide are Lewis acids

Some metal hydroxides are Some metal hydroxides are amphoteric – acting like an acid in the amphoteric – acting like an acid in the presence of a base and a base in the presence of a base and a base in the presence of an acid (ex. Al(OH)presence of an acid (ex. Al(OH)33))

* pKa = -log Ka* pKa = -log Ka A logarithmic scale is used to A logarithmic scale is used to

report and compare acid report and compare acid strengthsstrengths

The pKa value becomes smaller The pKa value becomes smaller as the acid strength increasesas the acid strength increases

KaKb = Kw for an acid and its KaKb = Kw for an acid and its conjugate base conjugate base

Also, pKw = pKa + pKbAlso, pKw = pKa + pKb

*Equilibrium Constants *Equilibrium Constants and Acid-Base and Acid-Base ReactionsReactions All proton transfer reactions proceed All proton transfer reactions proceed

from the stronger acid and base to from the stronger acid and base to the weaker acid and base.the weaker acid and base.

Write the net ionic equation for the Write the net ionic equation for the possible reaction between acetic acid, possible reaction between acetic acid, Ka = 1.8 x 10Ka = 1.8 x 10-5-5 and sodium hydrogen and sodium hydrogen sulfite, NaHSOsulfite, NaHSO3, 3, Kb = 8.3 x 10Kb = 8.3 x 10-13-13. Does the . Does the equilibrium lie to the left or right?equilibrium lie to the left or right?

Types of Acid-Base Types of Acid-Base ReactionsReactions

Reaction of a Strong acid with a Strong Reaction of a Strong acid with a Strong BaseBase

Net ionic equation: Net ionic equation: HH33OO++ + OH + OH-- 2H 2H22OO K = 1/Kw = 1.0 x 10K = 1/Kw = 1.0 x 101414

Mixing equal molar quantities of a Mixing equal molar quantities of a strong base with a strong acid produces strong base with a strong acid produces a neutral solution (pH=7, at 25a neutral solution (pH=7, at 25oo ) )

Types of Acid-Base Types of Acid-Base ReactionsReactions Unless both the acid and base Unless both the acid and base

involved in the neutralization involved in the neutralization reaction are strong, the pH of the reaction are strong, the pH of the solution that results will not be solution that results will not be neutral.neutral.

Types of Acid-Base Types of Acid-Base ReactionsReactions Reaction of a Weak Acid with a Strong BaseReaction of a Weak Acid with a Strong Base (ex.) net ionic equation:(ex.) net ionic equation: CHCH33COCO22H + OHH + OH-- H H22O + CHO + CH33COCO22--

Adding these two reactions give the net Adding these two reactions give the net ionic equation above;ionic equation above;

CHCH33COCO22H + HH + H22O O H H33OO++ + CH + CH33COCO22- - Ka=1.8x10Ka=1.8x10-5-5

HH33OO++ + OH- + OH- 2 H 2 H22O Ka=1/Kw=1.0x10O Ka=1/Kw=1.0x101414

The K for the net reaction is the product of The K for the net reaction is the product of the two equilibrium constants the two equilibrium constants KKneutneut=Ka x 1/Kw=1.8x10=Ka x 1/Kw=1.8x1099

Types of Acid-Base Types of Acid-Base ReactionsReactions Mixing equal molar quantities of a Mixing equal molar quantities of a

strong base with a weak acid produces strong base with a weak acid produces a salt whose anion is the conjugate a salt whose anion is the conjugate base of the weak acid. The solution is base of the weak acid. The solution is basic, with the pH depending on Kb for basic, with the pH depending on Kb for the anion.the anion.

In the example above, the salt that In the example above, the salt that results from the reaction is acetate, the results from the reaction is acetate, the conjugate base of a weak acid. conjugate base of a weak acid. Therefore the solution will be basic at Therefore the solution will be basic at the equivalence point.the equivalence point.

Types of Acid-Base Types of Acid-Base ReactionsReactions

Reaction of a Strong Acid with a Weak Reaction of a Strong Acid with a Weak BaseBase

(ex.) net ionic equation:(ex.) net ionic equation: HH33OO++ + NH + NH33 NH NH44++ + H + H22O O Adding these two reactions give the net Adding these two reactions give the net

ionic equation above:ionic equation above: NHNH33 + H + H22O O NH NH44 + OH- Kb = 1.8x10 + OH- Kb = 1.8x10-5-5

HH33OO++ + OH- + OH- 2 H 2 H22O K = 1/Kw = 1.0x10O K = 1/Kw = 1.0x101414

The K for the net reaction is the product The K for the net reaction is the product of the two equilibrium constants of the two equilibrium constants K = Kb x 1/Kw = 1.8x10 K = Kb x 1/Kw = 1.8x1099

Types of Acid-Base Types of Acid-Base ReactionsReactions

Mixing equal molar quantities of a Mixing equal molar quantities of a strong acid and a weak base produces strong acid and a weak base produces a salt whose cation is the conjugate a salt whose cation is the conjugate acid of the weak base. The solution is acid of the weak base. The solution is acidic, with the pH depending on the acidic, with the pH depending on the Ka for the cation.Ka for the cation.

In the above example, the solution at In the above example, the solution at the equivalence point contains the the equivalence point contains the ammonium ion, the conjugate acid of a ammonium ion, the conjugate acid of a weak base, and the solution is acidic.weak base, and the solution is acidic.

Types of Acid-Base Types of Acid-Base ReactionsReactions Reaction of a Weak Acid with a Weak BaseReaction of a Weak Acid with a Weak Base (ex.) net ionic equation:(ex.) net ionic equation: CHCH33COCO22H + NHH + NH33 NH NH44++ + CH + CH33COCO22--

The reaction is product-favored because The reaction is product-favored because acetic acid is stronger than ammonium acetic acid is stronger than ammonium ion and ammonia is a stronger base than ion and ammonia is a stronger base than acetate ion (use table)acetate ion (use table)

K = Ka x Kb / Kw (what is the K for this K = Ka x Kb / Kw (what is the K for this ex?)ex?)

Types of Acid-Base Types of Acid-Base ReactionsReactions When a weak acid reacts with a When a weak acid reacts with a

weak base, the pH of the solution at weak base, the pH of the solution at the equivalence point depends upon the equivalence point depends upon which is the stronger, the acid or the which is the stronger, the acid or the base base

If equal molar solutions are mixed If equal molar solutions are mixed the resulting solution contains the resulting solution contains ammonium acetate; is it acidic or ammonium acetate; is it acidic or basic? (use table)basic? (use table)

Types of Acid-Base Types of Acid-Base ReactionsReactions Mixing equal molar quantities of a Mixing equal molar quantities of a

weak acid and a weak base weak acid and a weak base produces a salt whose cation is produces a salt whose cation is the conjugate acid of the weak the conjugate acid of the weak base and whose anion is the base and whose anion is the conjugate base of the weak acid. conjugate base of the weak acid. The solution pH depends on the The solution pH depends on the relative Ka and Kb values. relative Ka and Kb values.