Acidsand

Bases

Chapter 14Acids and Bases

Acidsand

Bases

Some Definitions

• ArrheniusAcid: Substance that, when dissolved

in water, increases the concentration of hydrogen ions.

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

Acidsand

Bases

Some Definitions

• Brønsted–LowryAcid: Proton donorBase: Proton acceptor

Acidsand

Bases

A Brønsted–Lowry acid…

…must have a removable (acidic) proton.

A Brønsted–Lowry base…

…must have a pair of nonbonding electrons.

Acidsand

Bases

If it can be either…

...it is amphiprotic.

HCO3−

HSO4−

H2O

Acidsand

Bases

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.

Acidsand

Bases

Acid Dissociation (Ionization) Reactions• Write the simple dissociation (ionization) reaction (omitting

water) for each of the following acids: a. Hydrochloric acid

b. Acetic acid

c. Ammonium ion

d. Anilinium ion (C6H5NH3)

e. Hydrated aluminum (III) ion [Al(H2O)6]3+

Acidsand

Bases

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.

Acidsand

Bases

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.

Acidsand

Bases

Acid and Base Strength

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

exceedingly strong.

Acidsand

Bases

Acid and Base Strength

In any acid-base reaction, the equilibrium will favor the reaction that moves the proton to the stronger base.

HCl(aq) + H2O(l) H3O+(aq) + Cl−(aq)

H2O is a much stronger base than Cl−, so the equilibrium lies so far to the right K is not measured (K>>1).

Acidsand

Bases

Acid and Base Strength

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

C2H3O2(aq) + H2O(l) H3O+(aq) + C2H3O2−(aq)

Acidsand

Bases

Relative Base StrengthUsing the following Ka values, arrange the following species according to their strength as bases:

H2O

F- Ka for HF = 7.2 x 10-4

Cl- N/A

NO2 - Ka for HF = 4.0 x 10-4

CN- Ka for HF = 6.2 x 10-10

Acidsand

Bases

Autoionization of Water

• As we have seen, water is amphoteric.• In pure water, a few molecules act as

bases and a few act as acids.

• This is referred to as autoionization.

H2O(l) + H2O(l) H3O+(aq) + OH−(aq)

Acidsand

Bases

Ion-Product Constant

• The equilibrium expression for this process is

Kc = [H3O+] [OH−]

• This special equilibrium constant is referred to as the ion-product constant for water, Kw.

• At 25°C, Kw = 1.0 10−14

Acidsand

Bases

Calculating [H+] & [OH-]• Calculate [H+] & [OH-] as required for each of the following

solutions at 250C, & state whether the solution is neutral, acidic, or basic.a. = 1.0 x 10-5 M OH-

b. a. = 1.0 x 10-7 M OH-

c. 10.0 M H+

Acidsand

Bases

pH

pH is defined as the negative base-10 logarithm of the hydronium ion concentration.

pH = −log [H3O+]

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 (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.

Acidsand

Bases

pH

These are the pH values for several common substances.

Acidsand

Bases

Other “p” Scales

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

• Some similar examples arepOH −log [OH−]pKw −log Kw

Acidsand

Bases

Watch This!

Because

[H3O+] [OH−] = Kw = 1.0 10−14,

we know that

−log [H3O+] + −log [OH−] = −log Kw = 14.00

or, in other words,

pH + pOH = pKw = 14.00

Acidsand

Bases

Calculating pH, pOH

pH = -log10(H3O+)

pOH = -log10(OH-)

Relationship between pH and pOH

pH + pOH = 14

Finding [H3O+], [OH-] from pH, pOH

[H3O+] = 10-pH

[OH-] = 10-pOH

Acidsand

Bases

Calculating pH & pOH• Calculate pH & pOH for each of the following solutions at 250C.

a. = 1.0 x 10-3 M OH-

b. a. = 1.0 M H+

Acidsand

Bases

Calculating pH • The pH a sample of human blood was measured to be 7.41 at

250C. Calculate pOH, [H+], & [OH-] for the sample.

Acidsand

Bases

How Do We Measure pH?

• For less accurate measurements, one can useLitmus paper

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

• “Blue” paper turns red below ~pH = 5

An indicator

Acidsand

Bases

How Do We Measure pH?

For more accurate measurements, one uses a pH meter, which measures the voltage in the solution.

Acidsand

Bases

Strong Acids

• You will recall that the seven strong acids are HCl, HBr, HI, HNO3, H2SO4, HClO3, and HClO4.

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

• For the monoprotic strong acids,

[H3O+] = [acid].

Acidsand

Bases

pH of Strong Acids• Calculate pH of 0.10 M HNO3.

• Calculate pH of 1.0 x 10-10 M HCl.

Acidsand

Bases

Dissociation Constants

• For a generalized acid dissociation,

the equilibrium expression would be

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

[H3O+] [A−][HA]

Kc =

HA(aq) + H2O(l) A−(aq) + H3O+(aq)

Acidsand

Bases

Dissociation Constants

The greater the value of Ka, the stronger the acid.

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.

• We know that

[H3O+] [COO−][HCOOH]

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 the equilibrium concentrations of all three things.

• 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+]

−2.38 = log [H3O+]

10−2.38 = 10log [H3O+] = [H3O+]

4.2 10−3 = [H3O+] = [HCOO−]

Acidsand

Bases

Calculating Ka from pH

Now we can set up a table…

[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 = 0.104.2 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

Acidsand

Bases

Solving Weak Acid Equilibrium Problems• 1. List the major species in the solution.• 2. Choose the species that can produce H+ and write balanced

equations for the reactions producing H+ .• 3. Using the values of the equilibrium constants for the

reactions you have written, decide which equilibrium will dominate in producing H+ .

• 4. Write the equilibrium expression for the dominant equilibrium• 5. ICE the problem• 6. Substitute the equilibrium [ ] into the equilibrium expression• 7. Solve for x the “easy” way; that is, by assuming [HA]0-x

[HA]0

• 8. Use the 5% rule to verify whether the approximation is valid• 9. Calculate [H+] and pH

Acidsand

Bases

The pH of Weak Acids• The hypochlorite ion (OCl-) is a strong oxidizing agent often found in

household bleaches & disinfectants. It is also the active ingredient that forms when swimming pool water is treated with chlorine. In addition to its oxidizing abilities, the hypochlorite ion has a relatively high affinity for protons (it is a much stronger base than Cl- for example) & forms the weakly acidic hypochlorous acid (HOCl, Ka = 3.5 x 10-8 ). Calculate pH of 0.100 M aqueous solution of hypochlorous acid.

Acidsand

Bases

The pH of Weak Acids continued• HOCl, Ka = 3.5 x 10-8 Calculate pH of 0.100 M aqueous solution of

hypochlorous acid.

Acidsand

Bases

The pH of Weak Acid Mixtures• Calculate the pH of a solution that contains 1.00 M HCN (Ka = 6.2 x

10-10) and 5.00 M HNO2 (Ka = 4.0 x 10-4) . Also calculate the concentration of cyanide ion in this solution at equilibrium.

Acidsand

Bases

Percent Dissociation

100 x ion(mol/L)concentrat initial

d(mol/L)dissociateamount on dissociatiPercent

• In general, the more dilute the weak acid solution, the greater the percent dissociation of the weak acid.

Percent Dissociat

ion

Acid Concentr

ation

H+ Concentr

ation

Acidsand

Bases

Calculate Percent Dissociation• Calculate the percent dissociation of acetic acid (Ka = 1.8x 10-5) in

each of the following solutions.• a. 1.00 M HC2H3O2

• b. 0.100 M HC2H3O2

Acidsand

Bases

Calculation Ka from Percent Dissociation• Lactic acid (HC3H5O3) is a waste product that accumulates in muscle

tissue during exertion, leading to pain & feeling of fatigue. In a 0.100 M aqueous solution, lactic acid is 3.7% dissociated. Calculate the value of Ka for this acid.

Acidsand

Bases

Strong Bases

• Strong bases are the soluble hydroxides, which are the alkali metal and heavier alkaline earth metal hydroxides (Ca2+, Sr2+, and Ba2+).

• Again, these substances dissociate completely in aqueous solution.

Acidsand

Bases

The pH of strong bases• Calculate the pH of a 5.0x 10-2 M NaOH solution.

Acidsand

Bases

Weak Bases• Many types of proton acceptors (bases) do not contain

hydroxide ions. When dissolved in water, they increase the concentration of hydroxide ions because of their reaction with water.

• Ex.

• Bases such as ammonia typically have at least one unshared pair of electrons that is capable of forming a bond with a proton.

OH NH OH NH -(aq)4(aq)(l)23(aq)

Acidsand

Bases

Weak Bases

• We will solve weak base problems in the same manner we solved weak acid problems (look back over the steps you were given

• We will use Kb instead of Ka and will find [OH-] instead of [H+]Remember the process for “switching” from

pOH to pH &/or from [OH-] [H+]

Acidsand

Bases

pH of a Weak Base (I)• Calculate the pH for a 15.0 M NH3 solution (Kb = 1.8x 10-5).

Acidsand

Bases

pH of a Weak Base (II)• Calculate the pH for a 1.0 M methylamine solution (Kb = 4.38x 10-4).

Acidsand

Bases

Polyprotic Acids

• Some acids furnish more than one acidic proton such as H2SO4, H3PO4.Ex. H2CO3

11--3

-23

a223(aq)(aq)(aq)

-3

7-

32

-3

a13(aq)(aq)3(aq)2

5.6x10 ][HCO

]CO][H[K CO H HCO

4.3x10 ]CO[H

]HCO][H[K HCO H COH

Acidsand

Bases

pH of a polyprotic acid• Calculate the pH of a 5.0 M H3PO4 solution and the equilibrium

concentrations of the species H3PO4, H2PO4-, HPO4

2-, & PO43-

.

Acidsand

Bases

pH of sulfuric acid• Calculate the pH of a 1.0 M H2SO4 solution.

• Calculate the pH of a .0100 M H2SO4 solution.

Acidsand

Bases

Acid-Base Properties of Salts

Acidsand

Bases

Acid-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)

Acidsand

Bases

Acid-Base Properties of Salts

Type of Salt Examples

Comment pH of solution

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:

Acidsand

Bases

Acid-Base Properties of Salts

Type of Salt Examples

Comment pH of solution

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:

Acidsand

Bases

Acid-Base Properties of Salts

Type of Salt Examples

Comment pH of solution

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

Acidsand

Bases

Acid-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

Acidsand

Bases

Salts as Weak BasesCalculate the pH of a 0.30 M NaF solution. The Ka value for HF is 7.2 x 10-4

Acidsand

Bases

Salts as Weak Acids ICalculate the pH of a 0.10 M NH4Cl solution. The Kb value for NH3 is 1.8 x 10-5

Acidsand

Bases

Salts as Weak Acids IICalculate the pH of a 0.010 M AlCl3 solution. The Kavalue for Al(H2O)6

3+ is 1.4 x 10-5

Acidsand

Bases

The Acid/Base Properties of Salts Predict whether an aqueous solution of each of the following salts will be acidic, basic, or neutral.

a. NH4C2H3O2

b. NH4CN

c. Al2(SO4)3

Acidsand

Bases

Effect of Structure on Acid-Base Properties

Increasing Acidity

Hypochlorousacid

Chlorousacid

Chloricacid

Perchloricacid

Acidsand

Bases

The Lewis Acid-Base Model

• Lewis Acid: An electron pair acceptor• Has an empty orbital to accept a pair of electrons

• Lewis Base: An electron pair donor• Has a lone pair of electrons

Acidsand

Bases

Lewis Acids & BasesFor each reaction, identify the Lewis acid & base.

a. Ni2+(aq) + 6NH3(aq) Ni(NH3)6

2+ (aq)

b. H+(aq) + H2O(l) H3O+

(aq)