Slide 1 Back Chapter 16-Acids and Bases Chapter 16 Acid-Base Equilibria AP CHEMISTRY Content from The Central Science, 9 th ed David P. White, revised

Chapter 16-Acids and Bases Slide 1

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Chapter 16Chapter 16AcidAcid--BaseBase Equilibria Equilibria

AP CHEMISTRY

Content from The Central Science, 9th ed David P. White,

revised 2004 by CMB


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• Acids: taste sour and cause dyes to change color.• Bases: taste bitter and feel soapy.• Arrhenius Definition:

• acids increase [H+]• bases increase [OH-] in solution.

• Arrhenius: acid + base salt + water.• Arrhenius limits us to aqueous solution.

Acids and Bases: A Brief Acids and Bases: A Brief ReviewReview

Aqueous Acids (strong and weak).mov Aqueous Bases (strong and weak).mov


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Hydronium: The H+ Ion in Water

• The H+(aq) ion is simply a proton with no electrons.

• Generally we use H+(aq) and H3O+

(aq) interchangeably.

• Brønsted-Lowry: acid donates H+ and base accepts H+.• Brønsted-Lowry base does not need to contain OH-.

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

(aq):

• HCl donates a proton to water. Therefore, HCl is an acid.

• H2O accepts a proton from HCl. Therefore, H2O is a base.

• Water can behave as either an acid or a base-its amphoteric.• Amphoteric substances can behave as acids and bases.

BrBrønsted-Lowry Acids and ønsted-Lowry Acids and Bases Bases

Example on next page

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

(aq):


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The Ion Product of Water• In pure water the following equilibrium is established

• at 25 C

• The above is called the autoionization of water.

• In neutral, pure water, [H3O+] = 1 x 10-7 M

• In neutral, pure water, [OH-] = 1 x 10-7 M

The The AutoionizationAutoionization of Water of Water

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

14-3

-3

22

22

-3

100.1]OH][OH[

]OH][OH[]OH[

]OH[

]OH][OH[

w

eq

eq

K

K

K


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“p” = -log• In most solutions [H+(aq)] is quite small.• We define

• In neutral water at 25 C, pH = pOH = 7.00.• In acidic solutions, [H+] > 1.0 10-7, so pH < 7.00.• In basic solutions, [H+] < 1.0 10-7, so pH > 7.00.• The higher the pH, the lower the pOH, the more basic the

solution.• There are no theoretical limits on the values of pH or pOH.

(e.g. pH of 2.0 M HCl is -0.301.)

The pH ScaleThe pH Scale

]OHlog[pOH ]Hlog[]OHlog[pH -3

Logarithm algebra is needed here


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• pH meter – measures conductivity as a function of [H3O+].

• Precise, but calibration is often needed for accuracy

• Indicators – dyes that change their bonding & color as a function of [H3O+].

• Less precise than pH meters, but quick and cheap

• Most indicators tend to be red in more acidic solutions.

The pH Scale, & The pH Scale, & Measuring pHMeasuring pH

Natural Indicators.mov

Indicators & the pH ScaleIndicators & the pH ScaleCarbonDioxide as an acid.MOV

Transition IntervalTransition Interval: : the range of pH that the range of pH that they are effective.they are effective.

EndpointEndpoint: the pH : the pH that produces a that produces a noticeable color noticeable color changechange


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Definition• Conjugate Base = Whatever is left of the acid after the

proton is donated.• Conjugate Acid = Whatever remains of the base after it

accepts a proton.• Consider

– After HA (acid) loses its proton it is converted into A- (base). Therefore HA and A- are conjugate acid-base pairs.

– After H2O (base) gains a proton it is converted into H3O+ (acid). Therefore, H2O and H3O+ are conjugate acid-base pairs.

• Conjugate acid-base pairs differ by only one proton.

Conjugate Acid-Base PairsConjugate Acid-Base Pairs

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


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Identify Conjugate pairs:Identify Conjugate pairs:

KClOHKOHHCl 2

0342432 HPOHPOHOHAcid

Acid

Base

Base Conjugate Acid

Conjugate Base

Conjugate Base

Conjugate Acid


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Relative Relative Strengths Strengths of Acids of Acids

and Basesand Bases

Strengths of Acids or Bases is determined by the extent to which a species dissociates, or ionizes in water.


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RulesRules: Relative Strengths of Acids and Bases• The stronger the acid, the weaker the conjugate base. • The stronger the base, the weaker the conjugate acid.• A “strong” acid’s conjugate base (e.g. Cl-) has negligible

acid-base properties.• A “strong” base’s conjugate acid has negligible acid-base

properties.• Conjugates of weak acids are weak bases.• Conjugates of weak bases are weak acids.

• Acid strength increases across a period and down a group.

• Base strength decreases across a period and down a group.

ConjugateConjugate Acids and Acids and Base Strength Base Strength


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Strong Acids and BasesStrong Acids and Bases*MEMORIZE**MEMORIZE*

• Six Strong Acids*:

• HClO4 HI HBr HCl H2SO4 HNO3HNO3(aq) H+

(aq) + NO3-(aq)

• Strong Bases:

• NaOH KOH Ba(OH)2 etc-OH (those soluble)

• Strong bases need not have OH- (e.g.: H-(aq) + H2O(l) H2(g) + OH-

(aq) )

• All others, consider to be “weak” (not completely dissociated)

• Strong acids & bases are strong electrolytes.

• “Weak” acids and bases = Ions are in equilibrium with unionized compounds.

• *Your book: HClO3 is also strong, but not on AP test.

Strong and weak acids-electrolytes.mov

]HA[]A][H[ -

aK


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The of the acid-base on the AP• Weak acids are only partially ionized in

solution.• Therefore, weak acids are in equilibrium:

• Ka is the acid dissociation constant.

• The larger the Ka the stronger the acid (i.e. the more ions are present at equilibrium relative to unionized molecules).

• If Ka >> 1, then the acid is completely ionized and the acid is a strong acid.

Weak AcidsWeak Acids


]HA[]A][OH[ -

3

aK


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Some Weak Acids:Some Weak Acids:


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• Weak acids are simply equilibrium calculations.• The pH gives the equilibrium concentration of H+.

• Using Ka, the concentration of H+ (and hence the pH) can be calculated:1. Write the balanced chemical equation clearly showing the

equilibrium.

2. Write the equilibrium expression. Find the value for Ka.

3. Write down the initial and equilibrium concentrations for everything except pure water. We usually assume that the change in concentration of H+ is x.

4. Substitute into the equilibrium constant expression and solve. Remember to turn x (at equilibrium) into pH if necessary.

We can also solve for Ka from pH or [H3O+] in this manner.

Calculating pH or [HCalculating pH or [H33OO++] from K] from Kaa


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100]HA[

]OH[ionization %

0

3

eqm

Using %Ionization to Calculate pH or Ka• Percent ionization is another method to assess acid strength.• For the reaction

• The higher percent ionization, the stronger the acid.• Percent ionization of a weak acid decreases as the molarity

of the solution increases.• For acetic acid, 0.05 M solution is

2.0 % ionized whereas a 0.15 M solution is 1.0 % ionized.

% Ionization% Ionization



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Polyprotic Acids have more than one ionizable H+

• The protons are removed in steps not all at once:

• It is always easier to remove the first proton in a polyprotic acid than the second.

• Therefore

• Ka1 > Ka2 > Ka3 etc.

Polyprotic AcidsPolyprotic Acids

H2SO3(aq) H+(aq) + HSO3-(aq) Ka1 = 1.7 x 10-2

HSO3-(aq) H+(aq) + SO3

2-(aq) Ka2 = 6.4 x 10-8


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• Weak bases remove protons from substances.

• There is an equilibrium between the base and the resulting ions:

• Example:

• The base dissociation constant, Kb, is defined as

Weak BasesWeak Bases

Weak base + H2O conjugate acid + OH-

NH3(aq) + H2O(l) NH4+(aq) + OH-(aq)

]NH[]OH][NH[

3

-4

bK


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Trends in Weak Bases• Bases generally have lone pairs or negative charges in order

to attack protons.• Most neutral weak bases contain nitrogen.• Amines are related to ammonia and have one or more N-H

bonds replaced with N-C bonds (e.g., CH3NH2 is methylamine).

• Anions of weak acids are also weak bases. Example: OCl- is the conjugate base of HOCl (weak acid):

Weak BasesWeak Bases

ClO-(aq) + H2O(l) HClO(aq) + OH-(aq) Kb = 3.3 x 10-7


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Some Weak BasesSome Weak Bases


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• When two reactions are added to give a third, the equilibrium constant for the third reaction is the product of the equilibrium constants for the first two:

• For a conjugate acid-base pair:

• The larger the Ka, the smaller the Kb. That is, the stronger the acid, the weaker the conjugate base.

• Taking negative logarithms:

Conjugate Acid/Base StrengthsConjugate Acid/Base Strengths

213 KKK

baw KKK

baw pKpKpK


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Ionic salts dissociate completely• Acid-base properties of salts are a consequence of the

reaction of their ions in solution.• The reaction in which ions produce H+ or OH- in water is

called hydrolysis.• Anions from strong acids are neutral.• Anions from weak acids are basic:

• Polyatomic cations with ionizable protons can be considered conjugate acids of weak bases:

Acid-Base Properties of Acid-Base Properties of SaltSalt Solutions Solutions

X-(aq) + H2O(l) HX(aq) + OH-(aq)

NH4+(aq) + H2O(l) NH3(aq) + H3O+(aq)


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Consider H-X:

For this substance to be an acid we need:• H-X bond to be polar• H-X bond must be weak enough to be broken• The conjugate base, X-, must be stable• HF is a weak acid because the bond energy is high and not

easily broken. H-I is a strong acid, easy to break H-I bond.• Acid strength increases across a period and down a group.• Base strength decreases across a period and down a group.

Binary Acid StrengthsBinary Acid Strengths


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Binary Acids

Acid-Base Behavior and Acid-Base Behavior and Chemical StructureChemical Structure


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Oxyacids contain O-H bonds• All oxyacids have the general structure Y-O-H.• The strength of the acid depends on Y and the relative

location of the electron density near Y:– If Y is a metal (low electronegativity), then the substance is a base.

– If Y has intermediate electronegativity (e.g. I, EN = 2.5), the substance is a weak oxyacid.

– If Y has a large electronegativity (e.g. Cl, EN = 3.0), the electrons are located closer to Y than O and the O-H bond is polarized to lose H+.

– More O atoms attached to Y increase the O-H bond polarity and the strength of the acid increases (e.g. HOCl is a weaker acid than HClO2 which is weaker than HClO3 which is weaker than HClO4 which is a strong acid).

Oxy-acidsOxy-acids


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Oxyacids



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Carboxylic Acids• Carboxylic acids all contain the COOH group.• All carboxylic acids are weak acids.• When the carboxylic acid loses a proton, it generate the

carboxylate anion, COO-.


RC

OH

O


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• Recall: Brønsted-Lowry acid is a proton donor.• Focusing on electrons: a Brønsted-Lowry acid can be

considered as an electron pair acceptor.• Lewis acid: electron pair acceptor.• Lewis base: electron pair donor.• Note: Lewis acids and bases do not need to contain protons.

• Lewis acids generally have an incomplete octet (e.g. BF3).

• Transition metal ions are generally Lewis acids.• Lewis acids must have a vacant orbital

(into which the electron pairs can be donated).

Lewis Acids and BasesLewis Acids and Bases

Lewis Acids and Bases.mov


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Hydrolysis of Metal Ions (can be tricky)• Alkali and Alkaline-earth metals form bases in solution.• Transition metal ions are positively charged and attract

water molecules (via the lone pairs on O).• The higher the charge, the smaller the metal ion and the

stronger the M-OH2 interaction.

• Hydrated metal ions (water-complexes) act as weak acids:

• [H3O+] increases as the size of the ion decreases (Ca2+ / Zn2+) and as the charge increases (Na+ / Ca2+ and Zn2+ / Al3+).

• Na+ = no acidic behavior, Al3+ = very acidic cation

Metals and pHMetals and pH

Fe(H2O)63+(aq) Fe(H2O)5(OH)2+(aq) + H+(aq)

Ka = 2 x 10-3


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Hydrolysis of Metal Ions

Lewis Acids and BasesLewis Acids and Bases

End of chapter 16 (phew)

Documents

Slide 1 Back Chapter 16-Acids and Bases Chapter 16 Acid-Base Equilibria AP CHEMISTRY Content from The Central Science, 9 th ed David P. White, revised