chapter 10 acids and bases

1Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings.

Chapter 10 Acids and Bases

10.1 Acids and Bases10.2 Conjugate Acid-Base Pairs10.3 Strengths of Acids and Bases10.4 Dissociation Constants10.5 Ionization of Water10.6 The pH Scale10.7 Reactions of Acids and Bases

(just neutralization reactions)10.9 Buffers


Arrhenius acids Produce H+ ions in water.

H2OHCl H+(aq) + Cl– (aq)

Are electrolytes. Have a sour taste. Corrode metals. React with bases to form salts

and water.

10.1 Acids

Examples:

Vinegar (5% acetic acid)

Citric acid (fruits)

Ascorbic acid (vitamin C)

Lactic acid (in muscles)

Phosphoric acid (Coke)


Arrhenius bases Produce OH– ions in

water. Taste bitter or chalky. Are electrolytes. Feel soapy and slippery. React with acids to form

salts and water.

Bases

Examples:

Draino (NaOH)

Baking Soda (NaHCO3)

Bleach (NaOCl)

Ammonia (NH3)

Antacid


Comparing Acids and Bases


Identify each as a characteristic of an

A) acid or B) base

____1. Has a sour taste.

____2. Produces OH- in aqueous solutions.

____3. Has a chalky taste.

____4. Is an electrolyte.

____5. Produces H+ in aqueous solutions.

Learning Check


Identify each as a characteristic of an

A) acid or B) base

A 1. Has a sour taste.

B 2. Produces OH– in aqueous solutions.

B 3. Has a chalky taste.

A, B 4. Is an electrolyte.

A 5. Produces H+ in aqueous solutions.

Solution


Names of Acids

Acids with H and one nonmetal are named with the prefix hydro- and end with -ic acid.HCl hydrochloric acid

Acids with H and a polyatomic ion are named by changing the end of an –ate ion to -ic acid and an –ite ion to -ous acid.

HClO3 chloric acid

HClO2 chlorous acid


Some Acids and Their Anions


Name each of the following as acids: A. HBr 1. bromic acid

2. bromous acid 3. hydrobromic acid

B. H2CO3 1. carbonic acid

2. hydrocarbonic acid3. carbonous acid

Learning Check


A. HBr 3. hydrobromic acid

The name of an acid with H and one nonmetal begins with the prefix hydro- and ends with -ic acid.

B. H2CO3 1. carbonic acid

An acid with H and a polyatomic ion is named by changing the end of an –ate ion to -ic acid.

Solution


Bases with OH- ions are named as the hydroxide of the metal in the formula.

NaOH sodium hydroxide

KOH potassium hydroxide

Ba(OH)2 barium hydroxide

Al(OH)3 aluminum hydroxide

Fe(OH)3 iron (III) hydroxide

Some Common Bases


Match the formulas with the names:

A. ___ HNO2 1) hydrochloric acid

B. ___Ca(OH)2 2) sulfuric acid

C. ___H2SO4 3) sodium hydroxide

D. ___HCl 4) nitrous acid

E. ___NaOH 5) calcium hydroxide

Learning Check


Match the formulas with the names:A. 4 HNO2 4) nitrous acid

B. 5 Ca(OH)2 5) calcium hydroxide

C. 2 H2SO4 2) sulfuric acid

D. 1 HCl 1) hydrochloric acid

E. 3 NaOH 3) sodium hydroxide

Solution


According to the Brønsted-Lowry theory, Acids are hydrogen ion (H+) donors. Bases are hydrogen ion (H+) acceptors.

donor acceptor hydronium ion

HCl + H2O H3O+ + Cl-

+ -

+ +

Johannes BrØnsted and Thomas Lowry (1923)


When NH3 dissolves in water, a few NH3 molecules react with water to form ammonium ion NH4

+ and a hydroxide ion.

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

acceptor donor

+ - + +

NH3, A Bronsted-Lowry Base


10.2 Conjugate Acids and Bases An acid that donates H+ forms a conjugate base. A base that accepts a H+ forms a conjugate acid. In an acid-base reaction, there are two conjugate

acid-base pairs.

acid 1 conjugate base 1 + +

base 2 conjugate acid 2

HF H2O H3O+ F-


Conjugate Acid-Base Pairs

A conjugate acid-base pair is two substances related by a loss or gain of H+.

+ +

acid 1– conjugate base 1

base 2– conjugate acid 2

HF H2O H3O+ F-

HF, F-

H2O, H3O+


Learning Check

A. Write the conjugate base of the following:

1. HBr

2. H2S

3. H2CO3

B. Write the conjugate acid of the following:

1. NO2–

2. NH3

3. OH–


Solution

A. Write the conjugate base of the following:

1. HBr Br-

2. H2S HS–

3. H2CO3 HCO3–

B. Write the conjugate acid of the following:

1. NO2– HNO2

2. NH3 NH4+

3. OH– H2O


Learning Check

Identify the following that are acid-base conjugate pairs.

1. HNO2, NO2–

2. H2CO3, CO32 –

3. HCl, ClO4 –

4. HS–, H2S

5. NH3, NH4+


Solution

Identify the following that are acid-base conjugate pairs.

1. HNO2, NO2–

4. HS–, H2S

5. NH3, NH4+


Learning Check

A. The conjugate base of HCO3– is

1. CO32– 2. HCO3

– 3. H2CO3

B. The conjugate acid of HCO3– is

1. CO32– 2. HCO3

– 3. H2CO3

C. The conjugate base of H2O is

1. OH– 2. H2O 3. H3O+

D. The conjugate acid of H2O is

1. OH– 2. H2O 3. H3O+


Solution

A. The conjugate base of HCO3 – is

1. CO32–

B. The conjugate acid of HCO3– is

3. H2CO3

C. The conjugate base of H2O is

1. OH–

D. The conjugate acid of H2O is 3. H3O+


Strong acids completely ionize (100%) in aqueous solutions.HCl + H2O H3O+ (aq) + Cl– (aq)

(100 % ions) Strong bases completely (100%) dissociate

into ions in aqueous solutions. H2O

NaOH Na+ (aq) + OH–(aq) (100 % ions)

10.3 Strengths of Acids and Bases


Strong and Weak Acids

In an HCl solution, the strong acid HCl dissociates 100%.

A solution of the weak acid CH3COOH contains mostly molecules and a few ions.


Only a few acids are strong acids. The conjugate bases of strong acids are weak

bases.

Strong Acids


Most acids are weak acids. The conjugate bases of weak acids are strong bases.

Weak Acids


Most bases in Groups 1A and 2A are strong bases. They include

LiOH, NaOH, KOH, and Ca(OH)2

Most other bases are weak bases.

Strong Bases


Learning Check

Identify each of the following as a strong or weak acid or base.

A. HBr

B. HNO2

C. NaOH

D. H2SO4

E. Cu(OH)2


Solution

Identify each of the following as a strong or weak acid or base.

A. HBr strong acid

B. HNO2 weak acid

C. NaOH strong base

D. H2SO4 strong acid

E. Cu(OH)2 weak base


Learning Check

A. Identify the stronger acid in each pair.

1. HNO2 or H2S

2. HCO3– or HBr

3. H3PO4 or H3O+

B. Identify the strong base in each pair.

1. NO3– or F-

2. CO32– or NO2

–

3. OH– or H2O


Solution

A. Identify the stronger acid in each pair.

1. HNO2 2. HBr

3. H3O+

B. Identify the stronger base in each pair. 1. F-

2. CO32–

3. OH–


Equilibrium

H2SO4 + H2O H3O+ + HSO4-

stronger stronger weaker weaker acid base acid base

CO3- + H2O HCO3

- + OH-

weaker weaker stronger stronger base acid acid base


10.4 Strong Acids

In water, the dissolved molecules of a strong acid are essentially all separated into ions.

The concentrations of H3O+ and the anion (A–) are large.


Weak Acids

In weak acids, The equilibrium

favors the undissociated (molecular) form of the acid.

The concentrations of the H3O+ and the anion (A–) are small.


Dissociation Constants In a weak acid, the rate of the dissociation of

the acid is equal to the rate of the association.

HA + H2O H3O+ + A–

The equilibrium expression for a weak acid is

Keq = [H3O+][A-]

[HA][H2O]


Acid Dissociation Constants (Ka)

The [H2O] is large and remains unchanged. It is combined with the Keq to write the acid dissociation constant, Ka.

Ka = Keq[H2O] = [H3O+][A-]

[HA]


Writing Ka for a Weak Acid

Write the Ka for H2S.

1. Write the equation for the dissociation of H2S.

H2S + H2O H3O+ + HS–

2. Set up the Ka expression

Ka = [H3O+][HS-]

[H2S]


Learning Check

Write the Ka for HCN.


Solution

Write the Ka for HCN.

Write the equation for the dissociation of HCN.

HCN + H2O H3O+ + CN–

Set up the Ka expression

Ka = [H3O+][CN–]

[HCN]

Note: Ka = Keq [H2O]


Acid dissociation constants

Larger Ka (stronger acid)

Larger Kb (stronger base)

Smaller Ka (weaker acid)

Smaller Kb (weaker base)


In water, H+ is transferred from one H2O molecule to another.

One water molecule acts as an acid, while another acts as a base (amphiprotic).H2O + H2O H3O+ + OH –

.. .. .. .. :O: H + :O:H H:O:H + + :O:H–

.. .. .. .. H H H water molecules hydronium hydroxide

ion (+) ion (–)

10.5 Ionization of Water


In pure water, the ionization of molecules produces small, but equal quantities of H3O+ and OH-

ions.H2O + H2O H3O+ + OH-

Molar concentrations are indicated as [H3O+] and [OH-].[H3O+] = 1.0 x 10-7 M[OH-] = 1.0 x 10-7 M

Pure Water is Neutral


Adding an acid to pure water increases the [H3O+].

In acids, [H3O+] exceeds 1.0 x 10–7 M.

As [H3O+] increases, [OH–] decreases.

Acidic Solutions


Adding a base to pure water increases the [OH–].

In bases, [OH–] exceeds 1.0 x 10–7M.

As [OH–] increases, [H3O+] decreases.

Basic Solutions


Comparison of [H3O+] and [OH–]


The ion product constant, Kw, for water is the product of the concentrations of the hydronium and hydroxide ions.

Kw = [H3O+][OH–] We can obtain the value of Kw using the

concentrations in pure water.

Kw = [1.0 x 10–7][1.0 x 10–7]

= 1.0 x 10–14

Ion Product of Water, Kw


Kw in Acids and Bases

In neutral, acidic, and basic solutions, the Kw is equal to 1.0 x 10–14.


What is the [H3O+] of a solution if [OH–] is 1.0 x 10-8

M?

Rearrange the Kw expression for [H3O+ ].

Kw = [H3O+][OH–] = 1.0 x 10-14

[H3O+] = 1.0 x 10-14

[OH–]

[H3O+] = 1.0 x 10-14 = 1.0 x 10-6 M 1.0 x 10- 8

Calculating [H3O+]


The [H3O+] of lemon juice is 1.0 x 10–3 M.

What is the [OH–] of the solution?

1) 1.0 x 103 M

2) 1.0 x 10–11 M

3) 1.0 x 1011 M

Learning Check


The [H3O+] of lemon juice is 1.0 x 10–3 M. What is the [OH–] of the solution?

2) 1.0 x 10–11 M

Rearrange the Kw to solve for [OH–]

Kw = [H3O+ ][OH–] = 1.0 x 10–14

[OH- ] = 1.0 x 10 -14 = 1.0 x 10–11 M 1.0 x 10 - 3

Solution


The [OH–] of a solution is 5 x 10–2 M. What is the [H3O+ ] of the solution?

1) 2 x 10–2 M

2) 1 x 102 M

3) 2 x 10–13 M

Learning Check


The [OH–] of a water solution is 5 x 10–2 M.

What is the [H3O+] of the solution?

3) 2 x 10–13 M

[ H3O+] = 1.0 x 10 –14

5 x 10–2

Solution


A. What is the [OH–] if the [H3O+ ] is 1 x 10–4 M? Identify the solution as acidic, basic, or neutral.

1) 1 x 10–6 M 2) 1 x 10–8 M 3) 1 x 10–10 M

B. What is [H3O+] if the [OH–] is 5 x 10–9 M? Identify the solution as acidic, basic, or neutral.

1) 1 x 10–6 M 2) 2 x 10–6 M 3) 2 x 10–7 M

Learning Check


Kw = [H3O+][OH–] = 1.0 x 10 14

A. 3) [OH–] = 1.0 x 10–14 = 1.0 x 10–10

1.0 x 10–4 acidic

B. 2) [H3O+] = 1.0 x 10–14 = 2 x 10–6

5 x 10–9 acidic

Solution


10.6 pH Scale

The pH scale: Is used to indicate the acidity of a solution. Has values that usually range from 0 to 14. Indicates an acidic solution when the values

are less than 7. Indicates a neutral solution with a pH of 7. Indicates a basic solution when the values

are greater than 7.


0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Neutral

[H3O+]>[OH-] [H3O+] = [OH-] [H3O+]<[OH-]

Acidic Basic

pH Range


Identify each solution as1. acidic 2. basic 3. neutralA. ___ HCl with a pH = 1.5 B. ___ Pancreatic fluid [H3O+] = 1 x 10–8 M

C. ___ Sprite soft drink pH = 3.0D. ___ pH = 7.0E. ___ [OH– ] = 3 x 10–10 MF. ___ [H3O+ ] = 5 x 10–12

Learning Check


Identify each solution as

1. acidic 2. basic 3. neutral

A. 1 HCl with a pH = 1.5

B. 2 Pancreatic fluid [H3O+] = 1 x 10–8 M

C. 1 Sprite soft drink pH = 3.0

D. 3 pH = 7.0

E. 1 [OH–] = 3 x 10–10 M

F. 2 [H3O+] = 5 x 10–12

Solution


Testing the pH of Solutions

The pH of solutions can be determined using a a) pH meter, b) pH paper, and c) indicators that have specific colors at different pHs.


mx = n, then logm n = x, m = 10

Ordinarily log10 (log in calculator)

log10 100 = 2.000 (102 = 100)

log10 1 = 0.000 (100 = 1)

log10 0.001 = -3.000 (10-3 = 0.001)

Logarithms (log)


1. Count the significant digits in the number (n) (log n = x )

2. Add that same number of decimal points to the log of the number (x).

Example: log10 2.00 = 0.301

log10 2.0 = 0.30

log10 2 = 0.3

log10 2.0 x 103 = 3.30

Logarithms (log) Sig Figs


pH is: Defined as the negative log of the hydrogen ion

concentration.

pH = - log [H3O+]

The value of the negative exponent of [H3O+] for concentrations with a coefficient of 1.

[H3O+] = 1 x 10-4 pH = 4.0

[H3O+] = 1 x 10-11 pH = 11.0

pH Scale


A calculator can be used to find the pH of a

solution with a [H3O+] of 1 x 10-3 as follows. 1. Enter 1 x 10-3 by pressing 1 (EE) 3 (+/–). The EE key gives an exponent of 10 and the +/– key

changes the sign.2. Press the log key to obtain log 1 x 10-3. log (1 x 10-3) = –33. Press the +/– key to multiply by –1. (-3) +/– = 3

Calculations of pH


Significant Figures in pH

When expressing log values, the number of decimal places in the pH is equal to the number of significant figures in the coefficient of [H3O+].

[H3O+] = 1 x 10-4pH = 4.0

[H3O+] = 1.0 x 10-6 pH = 6.00

[H3O+] = 2.4 x 10-8 pH = 7.62


A. The [H3O+] of tomato juice is 2 x 10-4 M.

What is the pH of the solution?

1) 4.0 2) 3.7 3) 10.3

B. The [OH-] of a solution is 1.0 x 10-3 M.

What is the pH of the solution?

1) 3.00 2) 11.00 3) –11.00

Learning Check


A. 2) 3.7 pH = – log [ 2 x 10-4] = 3.7 2 (EE) 4 (+/–) log (+/–)

B. 2) 11.00 Use the Kw to obtain [H3O+] = 1.0 x 10-11

pH = – log [1.0 x 10- 11] = – 11.00 (+/–) = 11.00

Solution


Calculating [H3O+] from pH

The [H3O+] can be expressed with the pH as the negative power of 10.

[H3O+] = 1 x 10-pH

If the pH is 3.0, the [H3O+] = 1 x 10-3

On a calculator

1. Enter the pH value 3.0

2. Use (+/–) to change sign 3.0 (+/–) = –3.0

3. Use the inverse log key (or 10x) to obtain

the [H30+]. = 1 x 10-3 M


A. What is the [H3O+] of a solution with a pH of 10.0?

1) 1 x 10 -4 M 2) 1 x 1010 M

3) 1 x 10 -10 MB. What is the [H3O+] of a solution with a

pH of 5.7?

1) 2 x 10- 6 M 2) 5 x 105 M

3) 1 x 10-5 M

Learning Check


A. What is the [H3O+] of a solution with a pH of 10.0? 3) 1 x 10-10 M 1 x 10-pH

B. What is the [H3O+] of a solution with a pH of 5.7? 1) 2 x 10- 6 M

5.7 (+/–) inv log (or 10x) = 2 x 10-6

Solution


pOH

The pOH of a solution: Is related to the [OH-]. Is similar to pH.

pOH = – log [OH-] Added to the pH value is equal to 14.0.

pH + pOH = 14.0


pH and pOH

The sum of the pH and pOH values is equal to 14.0 for any aqueous solution.


Learning Check

What is the pH and the pOH of coffee if the [H3O+] is 1 x 10-5 M?

1) pH = 5.0 pOH =7.0

2) pH = 7.0 pOH = 9.0

3) pH = 5.0 pOH = 9.0


Solution

What is the pH and the pOH of coffee if the [H3O+] is 1 x 10-5 M?

3) pH = 5.0 pOH = 9.0

pH = – log [1 x 10-5] = –(– 5.0) = 5.0

pH + pOH = 14.0

pOH = 14.0 – pH = 14.0 – 5.0 = 9.0

or [OH-] = 1 x 10-9

pOH = – log [1 x 10-9] = –(– 9.0) = 9.0


An acid such as HCl produces H3O+ and a base such as NaOH provides OH-.HCl + H2O H3O+ + Cl-

NaOH Na+ + OH-

In a neutralization reaction, the H3O+ and the OH- combine to form water.

H3O+ + OH- 2 H2O The positive ion (metal) and the negative ion

(nonmetal) are the ions of a salt.

10.7 Neutralization Reactions


In the equation for neutralization, an acid and a base produce a salt and water.

NaOH + HCl NaCl + H2O

base acid salt water

Ca(OH)2 + 2 HCl CaCl2 + 2H2O

base acid salt water

Neutralization Equations


Write the balanced equation for the neutralization of magnesium hydroxide and nitric acid.

1. Write the formulas of the acid and base.Mg(OH)2 + HNO3

2. Balance to give equal OH- and H+.

Mg(OH)2 + 2 HNO3

3. Write the products (a salt and water) and balance: Mg(OH)2 + 2HNO3 Mg(NO3)2 + 2H2O

Balancing Neutralization Reactions


Write strong acids, bases, and salts as ions.

H+ + Cl- + Na+ + OH- Na+ + Cl- + H2O

Cross out matched ions.

H+ + Cl- + Na+ + OH- Na+ + Cl- + H2O

Write a net ionic reaction.

H+ + OH- H2O

Ionic Equations for Neutralization


Select the correct group of coefficients for the

following neutralization equations.

A. __ HCl + __ Al(OH)3 __AlCl3 + __ H2O

1) 1, 3, 3, 1 2) 3, 1, 1, 1 3) 3, 1, 1 3

B. __ Ba(OH)2 + __H3PO4 __Ba3(PO4)2 + __ H2O

1) 3, 2, 2, 2 2) 3, 2, 1, 6 3) 2, 3, 1,

Learning Check


A. 3) 3, 1, 1 3

3HCl + 1Al(OH)3 1AlCl3 + 3H2O

B. 2) 3, 2, 1, 6

3Ba(OH)2 + 2H3PO4 1Ba3(PO4)2 + 6H2O

Solution


Antacids neutralize stomach acid (HCl). Alka-Seltzer NaHCO3, citric acid, aspirin Chooz, Tums CaCO3

Di-gel CaCO3 and Mg(OH)2

Gelusil, Maalox, Al(OH)3 and Mg(OH)2

Mylanta Milk of Magnesia Mg(OH)2

Rolaids AlNa(OH)2CO3

Antacids


Learning Check

Write the neutralization reactions for stomach acid HCl and Mylanta.


Solution

Write the neutralization reactions for stomach acid HCl and Mylanta.

Mylanta: Al(OH)3 and Mg(OH)2

3HCl + Al(OH)3 AlCl3 + 3H2O

2HCl + Mg(OH)2 MgCl2 + 2H2O


When an acid or base is added to water, the pH changes drastically.

A buffer solution resists a change in pH when an acid or base is added.

10.9 Buffers


Buffers Absorb H3O+ or OH- from foods and

cellular processes to maintain pH. Are important in the proper functioning of

cells and blood. In blood maintain a pH close to 7.4. A

change in the pH of the blood affects the uptake of oxygen and cellular processes.

Buffers


A buffer solution Contains a combination of acid-base conjugate

pairs. Contains a weak acid and a salt of the

conjugate base of that acid. Typically has equal concentrations of a weak

acid and its salt. May also contain a weak base and a salt with

the conjugate acid.

Components of a Buffer


The acetic acid/acetate buffer contains acetic acid (CH3COOH) and sodium acetate (CH3COONa).

The salt produces sodium and acetate ions.

CH3COONa CH3COO- + Na+

The salt provides a higher concentration of the conjugate base CH3COO- than the weak acid.

CH3COOH + H2O CH3COO- + H3O+ Large amount Large amount

Buffer Action


The function of the weak acid is to neutralize a base. The acetate ion in the product adds to the available acetate.

CH3COOH + OH– CH3COO– + H2O

Function of the Weak Acid


The function of the acetate ion CH3COO- (conjugate base) is to neutralize H3O+ from acids. The weak acid product adds to the weak acid available.

CH3COO- + H3O+ CH3COOH + H2O

Function of the Conjugate Base


The weak acid in a buffer neutralizes base. The conjugate base in the buffer neutralizes acid. The pH of the solution is maintained.

Summary of Buffer Action


Learning Check

Does each combination make a buffer solution? 1) yes 2) no Explain.

A. HCl and KCl

B. H2CO3 and NaHCO3

C. H3PO4 and NaCl

D. CH3COOH and CH3COOK


Solution

Does each combination make a buffer solution?

1) yes 2) no Explain.

A. HCl + KCl no; HCl is a strong acid.

B. H2CO3 + NaHCO3 yes; weak acid and its salt.

C. H3PO4 + NaCl no; NaCl does not contain a conjugate base of H3PO4.

D. CH3COOH + CH3COOK

yes; weak acid and its salt.


pH of a Buffer

The [H3O+] in the Ka expression is used to determine the pH of a buffer.

Weak acid + H2O H3O+ + Conjugate base

Ka = [H3O+][conjugate base]

[weak acid][H3O+] = Ka x [weak acid]

[conjugate base]

pH = –log [H3O+]


Calculation of Buffer pH

The weak acid H2PO4- in a blood buffer

H2PO4-/HPO4

2- has Ka = 6.2 x 10-8. What is the pH of the buffer if it is 0.20 M in both H2PO4

- and HPO42-?

[H3O+] = Ka x [H2PO4-]

[HPO42-]

[H3O+] = 6.2 x 10-8 x [0.20 M] = 6.2 x 10-8

[0.20 M]pH = –log [6.2 x 10-8] = 7.17


Learning Check

What is the pH of a H2CO3 buffer that is 0.20 M H2CO3 and 0.10 M HCO3

-? Ka(H2CO3) = 4.3 x 10-7


Solution

What is the pH of a H2CO3 buffer that is 0.20 M H2CO3 and 0.10 M HCO3

- ?

Ka (H2CO3) = 4.3 x 10-7

[H3O+] = Ka x [H2CO3] [HCO3

-]

[H3O+] = 4.3 x 10-7 x [0.20 M] = 8.6 x 10-7

[0.10 M]

pH = –log [8.6 x 10-7] = 6.07

chapter 10 acids and bases

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