Acids and Bases. Acid/Base Definitions Arrhenius Model Acids produce hydrogen ions in aqueous solutions Bases produce hydroxide ions in aqueous

Acids and Acids and BasesBases

Acid/Base Definitions Arrhenius Model

Acids produce hydrogen ions in aqueous solutions

Bases produce hydroxide ions in aqueous solutions

Bronsted-Lowry Model Acids are proton donors Bases are proton acceptors

Lewis Acid Model Acids are electron pair acceptors Bases are electron pair donors

WhoTheory:

Acid=When

Arrhenius increases H+ 1880’s

Brønsted proton donor 1923

Lowry ditto 1923

LewisElectron-pair acceptor

1923

Three definitions of acid

Some Definitions

• Arrhenius acids and bases– Acid: Substance that, when dissolved

in water, increases the concentration of hydrogen ions (protons, H+).

– Base: Substance that, when dissolved in water, increases the concentration of hydroxide ions.

Some Definitions• Brønsted–Lowry: must have both

1. an Acid: Proton donor

and

2. a Base: Proton acceptor

If it can be either…

...it is amphiprotic.

HCO3–

HSO4 –

H2O

What Happens When an Acid Dissolves in Water?

• Water acts as a Brønsted–Lowry base and abstracts a proton (H+) from the acid.

• As a result, the conjugate base of the acid and a hydronium ion are formed.

Movies…

Conjugate Acids and Bases:• From the Latin word conjugare, meaning “to

join together.”• Reactions between acids and bases always

yield their conjugate bases and acids.

Acid and Base Strength

• Strong acids are completely dissociated in water.– Their conjugate bases are

quite weak.

• Weak acids only dissociate partially in water.– Their conjugate bases are

weak bases.


• Substances with negligible acidity do not dissociate in water.– Their conjugate bases are

exceedingly strong.

Acid DissociationAcid Dissociation

HA H+ + A-

Acid Proton Conjugate

base

][

]][[

HA

AHKa

Alternately, H+ may be written in its hydrated form, H3O+ (hydronium ion)

Ka > 1 = product favored

Dissociation of Strong AcidsDissociation of Strong AcidsStrong acids are assumed to dissociate completely in solution.

Large Ka or small Ka?Reactant favored or product favored?

Dissociation Constants: Strong Dissociation Constants: Strong AcidsAcids

AcidFormul

aConjugate

BaseKa

Perchloric HClO4 ClO4- Very large

Hydriodic HI I- Very large

Hydrobromic HBr Br- Very large

Hydrochloric HCl Cl- Very large

Nitric HNO3 NO3- Very large

Sulfuric H2SO4 HSO4- Very large

Hydronium ion H3O+ H2O 1.0

Dissociation of Weak AcidsDissociation of Weak AcidsWeak acids are assumed to dissociate only slightly (less than 5%) in solution.

Large Ka or small Ka?Reactant favored or product favored?

Dissociation Constants: Weak Dissociation Constants: Weak AcidsAcids

Acid FormulaConjugate Base

Ka

Iodic HIO3 IO3- 1.7 x 10-1

Oxalic H2C2O4 HC2O4- 5.9 x 10-2

Sulfurous H2SO3 HSO3- 1.5 x 10-2

Phosphoric H3PO4 H2PO4- 7.5 x 10-3

Citric H3C6H5O7 H2C6H5O7- 7.1 x 10-4

Nitrous HNO2 NO2- 4.6 x 10-4

Hydrofluoric HF F- 3.5 x 10-4

Formic HCOOH HCOO- 1.8 x 10-4

Benzoic C6H5COOH C6H5COO- 6.5 x 10-5

Acetic CH3COOH CH3COO- 1.8 x 10-5

Carbonic H2CO3 HCO3- 4.3 x 10-7

Hypochlorous HClO ClO- 3.0 x 10-8

Hydrocyanic HCN CN- 4.9 x 10-10


Acetate is a stronger base than H2O, so the equilibrium favors the left side (K<1).

The stronger base “wins” the proton.

HC2H3O2(aq) + H2O H3O+(aq) + C2H3O2–(aq)

Self-Ionization of WaterSelf-Ionization of Wateraka “autoionization”aka “autoionization”

H2O + H2O H3O+ + OH-

At 25, [H3O+] = [OH-] = 1 x 10-7

The ion-product constant for water at 25 C:

Kw = [H3O+][OH-]

Kw = (1 x 10-7)(1 x 10-7) = 1 x 10-14

Calculating pH, Calculating pH, pOHpOHpH = -log (H3O+)

pOH = -log (OH-)

Relationship between pH and Relationship between pH and pOHpOH pH + pOH = 14

Finding [HFinding [H33OO++], [OH], [OH--] from pH, pOH] from pH, pOH

[H3O+] = 10-pH

[OH-] = 10-pOH

Acidsand

Bases

pH

• In pure water,

Kw = [H3O+] [OH–] = 1.0 10-14

• Because in pure water [H3O+] = [OH-],

[H3O+] = (1.0 10-14)1/2 = 1.0 10-7

Acidsand

Bases

pH• Therefore, in pure water,

pH = –log [H3O+] = –log (1.0 10-7) = 7.00

• An acid has a higher [H3O+] than pure water, so its pH is <7

• A base has a lower [H3O+] than pure water, so its pH is >7.

Other “p” Scales

• The “p” in pH tells us to take the negative log of the quantity (in this case, hydronium ions).

• Some similar examples are– pOH –log [OH-]

– pKw –log Kw

If you know one, you know them

all:

[H+][OH-]pH

pOH

pH and pOH Calculations

H + O H -

pH pO H

[O H -] = 1 x 10 - 1 4

[H + ]

[H + ] = 1 x 10 - 1 4

[O H -]

p O H = 14 - p H

p H = 14 - p O H

pOH

= -l

og[O

H- ]

pH =

-log

[H+

]

[OH

- ] = 1

0-pO

H

[H+

] = 1

0-pH

pH pH ScaleScale

You need to know

the acidity of common

substances!

Acidsand

Bases

How Do We Measure pH?

Litmus paper• “Red” paper turns

blue above ~pH = 8• “Blue” paper turns

red below ~pH = 5An indicator

• Compound that changes color in solution.

Acidsand

Bases

Strong Acids• You will recall that the seven

strong acids are HCl, HBr, HI, HNO3, H2SO4, HClO3, and HClO4.

• These are strong electrolytes and exist totally as ions in aqueous solution.

• For the monoprotic strong acids,

[H3O+] = [acid].

Acidsand

Bases

Strong Bases

• Strong bases are the soluble hydroxides, which are the alkali metal (NaOH, KOH)and heavier alkaline earth metal hydroxides (Ca(OH)2, Sr(OH)2, and Ba(OH)2).

• Again, these substances dissociate completely in aqueous solution.

[OH-] = [hydroxide added].

Acidsand

Bases

Dissociation Constants• For a generalized acid dissociation,

the equilibrium expression is

• This equilibrium constant is called the acid-dissociation constant, Ka.

Acidsand

Bases

Calculating Ka from the pH

The pH of a 0.10 M solution of formic acid, HCOOH, at 25°C is 2.38. Calculate Ka for formic acid at this temperature.

To calculate Ka, we need all equilibrium concentrations.

We can find [H3O+], which is the same as [HCOO−], from the pH.

Acidsand

Bases

Calculating Ka from the pH

pH = –log [H3O+]

– 2.38 = log [H3O+]

10-2.38 = 10log [H3O+] = [H3O+]

4.2 10-3 = [H3O+] = [HCOO–]

Acidsand

Bases

Calculating Ka from pH

In table form:

[HCOOH], M [H3O+], M [HCOO−], M

Initially 0.10 0 0

Change –4.2 10-3 +4.2 10-

3

+4.2 10-3

At Equilibrium

0.10 – 4.2 10-3

= 0.0958 = 0.10

4.2 10-3 4.2 10 - 3

Acidsand

Bases

Calculating Ka from pH

[4.2 10-3] [4.2 10-3][0.10]

Ka =

= 1.8 10-4

A Weak Acid Equilibrium A Weak Acid Equilibrium ProblemProblem

What is the pH of a 0.50 M solution of acetic acid, HC2H3O2, Ka = 1.8 x 10-

5 ?Step #1: Write the dissociation equation

HC2H3O2 C2H3O2- + H+



5 ?Step #2: ICE it!


II

CC

EE

0.50 0 0

- x +x +x

0.50 - x xx



5 ?Step #3: Set up the law of mass action


0.50 - x xxEE

)50.0()50.0(

))((108.1

25 x

x

xxx



5 ?Step #4: Solve for x, which is also [H+]


0.50 - x xxEE

)50.0(108.1

25 x

x [H+] = 3.0 x 10-3 M



5 ?Step #5: Convert [H+] to pH


0.50 - x xxEE

52.4)100.3log( 5 xpH

Acidsand

Bases

Calculating Percent Ionization

In the example:

[A-]eq = [H3O+]eq = 4.2 10-3 M

[A-]eq + [HCOOH]eq = [HCOOH]initial = 0.10 M

Acidsand

Bases

Calculating pH from Ka

Calculate the pH of a 0.30 M solution of acetic acid, C2H3O2H, at 25°C.

Ka for acetic acid at 25°C is 1.8 10-5.Is acetic acid more or less ionized than

formic acid (Ka=1.8 x 10-4)?

Acidsand

Bases


The equilibrium constant expression is:

Acidsand

Bases


Use the ICE table:

[C2H3O2], M [H3O+], M [C2H3O2−], M

Initial 0.30 0 0

Change –x +x +x

Equilibrium 0.30 – x x x

Acidsand

Bases


Use the ICE table:

[C2H3O2], M [H3O+], M [C2H3O2−], M

Initial 0.30 0 0

Change –x +x +x

Equilibrium 0.30 – x x x

Simplify: how big is x relative to 0.30?

Acidsand

Bases


Use the ICE table:

[C2H3O2], M [H3O+], M [C2H3O2−], M

Initial 0.30 0 0

Change –x +x +x

Equilibrium 0.30 – x ≈ 0.30 x x


Acidsand

Bases


Now,

(1.8 10-5) (0.30) = x2

5.4 10-6 = x2

2.3 10-3 = x

Check: is approximation ok?

Acidsand

Bases


pH = –log [H3O+]pH = – log (2.3 10−3)pH = 2.64

Acidsand

Bases

Polyprotic Acids

Have more than one acidic proton.

If the difference between the Ka for the first dissociation and subsequent Ka values is 103 or more, the pH generally depends only on the first dissociation.

Dissociation of Strong Dissociation of Strong BasesBases

Strong bases are metallic hydroxidesGroup I hydroxides (NaOH, KOH) are

very solubleGroup II hydroxides (Ca, Ba, Mg, Sr)

are less solublepH of strong bases is calculated

directly from the concentration of the base in solution

MOH(s) M+(aq) + OH-(aq)

Reaction of Weak Bases with Reaction of Weak Bases with WaterWater

The generic reaction for a base The generic reaction for a base reacting with water, producing its reacting with water, producing its conjugate acid and hydroxide ion:conjugate acid and hydroxide ion:

B + H2O BH+ + OH-

[ ][ ]

[ ]b

BH OHK

B

(all weak bases do this)(all weak bases do this)

Weak Bases

Bases react with water to produce hydroxide ion.

Weak Bases

The equilibrium constant expression for this reaction is

where Kb is the base-dissociation constant.

Reaction of Weak Bases with Reaction of Weak Bases with WaterWater

The base reacts with water, producing The base reacts with water, producing its conjugate acid and hydroxide ion:its conjugate acid and hydroxide ion:

CH3NH2 + H2O CH3NH3+ + OH- Kb = 4.38 x 10-

4

4 3 3

3 2

[ ][ ]4.38 10

[ ]b

CH NH OHK x

CH NH

KKbb for Some Common Weak for Some Common Weak BasesBases

Base FormulaConjugat

e AcidKb

Ammonia NH3 NH4+ 1.8 x 10-5

Methylamine CH3NH2 CH3NH3+ 4.38 x 10-4

Ethylamine C2H5NH2 C2H5NH3+ 5.6 x 10-4

Diethylamine (C2H5)2NH (C2H5)2NH2+ 1.3 x 10-3

Triethylamine (C2H5)3N (C2H5)3NH+ 4.0 x 10-4

Hydroxylamine HONH2 HONH3+

1.1 x 10-8

Hydrazine H2NNH2 H2NNH3+

3.0 x 10-6

Aniline C6H5NH2 C6H5NH3+

3.8 x 10-10

Pyridine C5H5N C5H5NH+ 1.7 x 10-9

Many students struggle with identifying weak bases and their conjugate acids.What patterns do you see that may help you?

A Weak Base Equilibrium A Weak Base Equilibrium ProblemProblem

What is the pH of a 0.50 M solution of ammonia, NH3, Kb = 1.8 x 10-5 ?

Step #1: Write the equation for the reaction

NH3 + H2O NH4+ + OH-



Step #2: ICE it!

II

CC

EE

0.50 0 0

- x +x +x

0.50 - x xx



Step #3: Set up the law of mass action

0.50 - x xxEE

)50.0()50.0(

))((108.1

25 x

x

xxx




Step #4: Solve for x, which is also [OH-]

0.50 - x xxEE

)50.0(108.1

25 x

x [OH-] = 3.0 x 10-3 M




52.4)100.3log( 5 xpOH

Step #5: Convert [OH-] to pH

0.50 - x xxEE



48.900.14 pOHpH

Acidsand

Bases

pH of Basic Solutions

What is the pH of a 0.15 M solution of NH3?

[NH4+] [OH−]

[NH3]Kb = = 1.8 10-5

Acidsand

Bases


Tabulate the data.

[NH3], M [NH4+], M [OH−], M

Initial 0.15 0 0

Equilibrium 0.15 - x 0.15 x x


Acidsand

Bases


(1.8 10-5) (0.15) = x2

2.7 10-6 = x2

1.6 10-3 = x2

(x)2

(0.15)1.8 10-5 =

Check: is approximation ok?

Acidsand

Bases


Therefore,

[OH–] = 1.6 10-3 M

pOH = –log (1.6 10-3)

pOH = 2.80

pH = 14.00 – 2.80

pH = 11.20

Acidsand

Bases

Ka and Kb are linked:

Combined reaction = ?

Acidsand

Bases

Ka and Kb are linked:

Combined reaction = ?

Acidsand

Bases

Ka and Kb

Ka and Kb are related in this way:

Ka Kb = Kw

Therefore, if you know one of them, you can calculate the other.

Acid-Base Properties of SaltsAcid-Base Properties of Salts

Type of Salt Examples

Comment pH of solution

Cation is from a strong base, anion from a strong acid

KCl, KNO3

NaClNaNO3

Both ions are neutral

Neutral

These salts simply dissociate in water:

KCl(s) K+(aq) + Cl-(aq)




Cation is from a strong base, anion from a weak acid

NaC2H3O2

KCN, NaF

Cation is neutral,Anion is basic

Basic

C2H3O2- + H2O HC2H3O2 + OH-

base acid acid base

The basic anion can accept a proton from water:




Cation is the conjugate acid of a weak base, anion is from a strong acid

NH4Cl,

NH4NO3

Cation is acidic,Anion is neutral

Acidic

NH4+(aq) NH3(aq) + H+(aq)

Acid Conjugate Proton base

The acidic cation can act as a proton donor:




Cation is the conjugate acid of a weak base, anion is conjugate base of a weak acid

NH4C2H3O2

NH4CN

Cation is acidic,Anion is basic

See below

IF Ka for the acidic ion is greater than Kb for the basic ion, the solution is acidic

IF Kb for the basic ion is greater than Ka for the acidic ion, the solution is basic

IF Kb for the basic ion is equal to Ka for the acidic ion, the solution is neutral

Acid-Base Properties of SaltsAcid-Base Properties of SaltsType of Salt Exampl

esComment pH of

solutionCation is a highly charged metal ion; Anion is from strong acid

Al(NO3)2

FeCl3

Hydrated cation acts as an acid;Anion is neutral

Acidic

Step #1:AlCl3(s) + 6H2O Al(H2O)6

3+(aq) + Cl-(aq)Salt water Complex ion anion

Step #2:Al(H2O)6

3+(aq) Al(OH)(H2O)52+(aq) + H+(aq)

Acid Conjugate base Proton

Reactions of Anions with Water

• Anions are bases.

• As such, they can react with water in a hydrolysis reaction to form OH– and the conjugate acid:

X–(aq) + H2O(l) HX(aq) + OH–(aq)

Reactions of Cations with Water• Cations with acidic protons

(like NH4+) lower the pH of a

solution by releasing H+.

• Most metal cations (like Al3+) that are hydrated in solution also lower the pH of the solution; they act by associating with H2O and making it release H+.

Reactions of Cations with Water

• Attraction between nonbonding electrons on oxygen and the metal causes a shift of the electron density in water.

• This makes the O-H bond more polar and the water more acidic.

• Greater charge and smaller size make a cation more acidic.

Effect of Cations and Anions

1. An anion that is the conjugate base of a strong acid will not affect the pH.

2. An anion that is the conjugate base of a weak acid will increase the pH.

3. A cation that is the conjugate acid of a weak base will decrease the pH.

Effect of Cations and Anions

4. Cations of the strong Arrhenius bases will not affect the pH.

5. Other metal ions will cause a decrease in pH.

6. When a solution contains both the conjugate base of a weak acid and the conjugate acid of a weak base, the affect on pH depends on the Ka and Kb values.

What effect on pH? Why?An anion that is the conjugate base of a strong acid does not affect pH.

= very weak base

An anion that is the conjugate base of a weak acid increases pH. = strong base

A cation that is the conjugate acid of a weak base decreases pH. = strong acid

Cations of the strong Arrhenius bases (Na+, Ca2+) do not affect pH.

= very weak acid(not really acidic at all)

Other metal ions cause a decrease

in pH.= moderate bases(cations)

Weak acid + weak base Depends on Ka and Kb

Factors Affecting Acid Strength

• The more polar the H-X bond and/or the weaker the H-X bond, the more acidic the compound.

• Acidity increases from left to right across a row and from top to bottom down a group.


In oxyacids, in which an OH is bonded to another atom, Y,

the more electronegative Y is, the more acidic the acid.


For a series of oxyacids, acidity increases with the number of oxygens.


Resonance in the conjugate bases of carboxylic acids stabilizes the base and makes the conjugate acid more acidic.

Lewis Acids

• Lewis acids are defined as electron-pair acceptors.

• Atoms with an empty valence orbital can be Lewis acids.

• A compound with no H’s can be a Lewis acid.

Lewis Bases

• Lewis bases are defined as electron-pair donors.• Anything that is a Brønsted–Lowry base is also a

Lewis base. (B-L bases also have a lone pair.)• Lewis bases can interact with things other than

protons.

Documents

Acids and Bases. Acid/Base Definitions Arrhenius Model Acids produce hydrogen ions in aqueous solutions Bases produce hydroxide ions in aqueous