Acids and Bases © 2009, Prentice-Hall, Inc. Chapter 16 Acids and Bases Chemistry, The Central...

AcidsandBases

© 2009, Prentice-Hall, Inc.

Chapter 16Acids and Bases

Chemistry, The Central Science, 11th editionTheodore L. Brown, H. Eugene LeMay, Jr.,

and Bruce E. Bursten

AcidsandBases


Some Definitions

• Arrhenius– An acid is a substance that, when

dissolved in water, increases the concentration of hydrogen ions.

– A base is a substance that, when dissolved in water, increases the concentration of hydroxide ions.

AcidsandBases


Some Definitions

• Brønsted-Lowry– An acid is a proton donor.– A base is a proton acceptor.

AcidsandBases


A Brønsted-Lowry acid…

…must have a removable (acidic) proton.

A Brønsted-Lowry base…

…must have a pair of nonbonding electrons.

AcidsandBases


If it can be either……it is amphiprotic.

HCO3-

HSO4-

H2O

AcidsandBases


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.

AcidsandBases


Conjugate Acids and Bases

• The term conjugate comes from the Latin word “conjugare,” meaning “to join together.”

• Reactions between acids and bases always yield their conjugate bases and acids.

AcidsandBases


Sample Exercise 16.1 (p. 671)a) What is the conjugate base of each of the following

acids: HClO4 H2S PH4

+ HCO3

-?

b) What is the conjugate acid of each of the following bases?CN-

SO42-

H2OHCO3

-

AcidsandBases


Practice Exercise 16.1

Write the formula for the conjugate acid of each of the following:

HSO3-

F-

PO43-

CO

AcidsandBases


Sample Exercise 16.2 (p. 671)

The hydrogen sulfite ion (HSO3-) is amphoteric.

a) Write an equation for the reaction of HSO3- with

water, in which the ion acts as an acid.

b) Write an equation for the reaction of HSO3- with

water, in which the ion acts as a base.

In both cases identify the conjugate acid-base pairs.

AcidsandBases



When lithium oxide (Li2O) is dissolved in water, the solution turns basic from the reaction of the oxide ion (O2-) with water.

Write the reaction that occurs, and identify the conjugate acid-base pairs.

AcidsandBases


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.

AcidsandBases



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

exceedingly strong.

AcidsandBases



For the following proton-transfer reaction, use the above figure to predict whether the equilibrium lies predominantly to the left or to the right

HSO4-(aq) + CO3

2-(aq) SO4

2-(aq) + HCO3

-(aq)

AcidsandBases



For each of the following reactions, use the above figure to predict whether the equilibrium lies predominantly to the left or to the right:

a) HPO42-

(aq)+ H2O(l) H2PO4-(aq)+ OH-

(aq)

b) NH4+

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

AcidsandBases



• 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 that K is not measured (K>>1).

AcidsandBases



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

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

CH3CO2H (aq) + H2O (l) H3O+ (aq) + CH3CO2- (aq)

AcidsandBases


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)

AcidsandBases


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 25C, Kw = 1.0 10-14

AcidsandBases



Calculate the values of [H+] and [OH-] in a neutral solution at 25oC.

AcidsandBases



Indicate whether solutions with each of the following ion concentrations is neutral, acidic, or basic:

a) [H+] = 4 x 10-9 M

b) [OH-] = 1 x 10-7 M

c) [OH-] = 7 x 10-13 M

AcidsandBases



Calculate the concentration of H+(aq) in

a) a solution in which [OH-] is 0.010 M (1.0 x 10-12 M)

b) a solution in which [OH-] is 1.8 x 10-9 M (5.0 x 10-6 M)

Assume T = 25oC.

AcidsandBases



Calculate the concentration of OH-(aq) in

a solution in which

a) [H+] = 2 x 10-6 M

b) [H+] = [OH-]

c) [H+] = 100 x [OH-]

AcidsandBases


pH

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

pH = -log [H3O+]

AcidsandBases


pH

• In pure water,

Kw = [H3O+] [OH-] = 1.0 10-14

• Since in pure water [H3O+] = [OH-],

[H3O+] = 1.0 10-14 = 1.0 10-7

AcidsandBases


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.

AcidsandBases


pH

These are the pH values for several common substances.

AcidsandBases



Calculate the pH values for the two solutions described in Sample Exercise 16.5.

(a) 12.00 b) 5.25)

AcidsandBases



a) In a sample of lemon juice [H+] is 3.8 x 10-4 M. What is the pH? (3.42)

b) A commonly available window-cleaning solution has a [OH-] of 1.9 x 10-6 M. What is the pH?(8.28)

AcidsandBases



A sample of freshly pressed apple juice has a pH of 3.76. Calculate [H+].

(1.7 x 10-4 M)

AcidsandBases



A solution formed by dissolving an antacid tablet has a pH of 9.18. Calculate [H+].

(6.6 x 10-10 M)

AcidsandBases


Other “p” Scales

• The “p” in pH tells us to take the negative base-10 logarithm of the quantity (in this case, hydronium ions).

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

– pKw: -log Kw

AcidsandBases


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

AcidsandBases


How Do We Measure pH?

• For less accurate measurements, one can use– Litmus paper

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

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

– Or an indicator.

AcidsandBases


How Do We Measure pH?

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

AcidsandBases


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].

AcidsandBases



What is the pH of a 0.040 M solution of HClO4?

(1.40)

AcidsandBases



An aqueous solution of HNO3 has a pH of 2.34. What is the concentration of the acid?

(0.0046 M)

AcidsandBases


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.

AcidsandBases



What is the pH of

a) a 0.028 M solution of NaOH? (12.45)

b) a 0.0011 M solution of Ca(OH)2? (11.34)

AcidsandBases



What is the concentration of a solution of

a) KOH for which the pH is 11.89?

(7.8 x 10-3 M)

b) Ca(OH)2 for which the pH is 11.68? (2.4 x 10-3 M)

AcidsandBases


Overview of Chapter 16

On your own:• Arrhenius acids and bases• Brønsted-Lowry acids and

bases • Conjugate acid-base pairs• Relative strengths of acids

and bases• Autoionization of water and

Kw

• [H+] > pH• [OH-] > pOH• pH + pOH = 14.00• Determine pH and/or pOH

of strong acids and bases

All of above will be on Thursday’s test

New material:• Application of equilibrium to weak acids

and bases: Ka, Kb

• Calculate Ka from pH and vice versa

• Calculate percent ionization of weak acids

• Polyprotic acids and their successive Ka’s

• Types of weak bases• Calculate concentrations of all species of

weak acids and weak bases at equilibrium, using Ka, Kb, pH or pOH

• Relationship between Ka and Kb, derived from Kw

• Application of Ka/Kb: acid-base properties of salt solutions

• Factors that affect acid strength – binary acids and oxyacids

• Carboxylic acids• Lewis acids and bases

AcidsandBases


Application of Equilibrium to Weak Acids

and Weak Bases

AcidsandBases


Dissociation Constants

• For a generalized acid dissociation,

the equilibrium expression would be

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

• Note that H2O is omitted from Ka because it is a pure liquid

[H3O+] [A-][HA]

Kc =

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

AcidsandBases


Dissociation Constants

The larger the Ka, the stronger the acid

If Ka >> 1, then the acid is completely ionized strong acid

AcidsandBases


Dissociation ConstantsThe greater the value of Ka, the stronger is the acid.

AcidsandBases


Ionization Constants and Equations WS

AcidsandBases


Calculating Ka from the pH

• pH [H+] at equilibrium

• Equilibrium [H+] RICE table

• Use RICE table to find the [other species]

AcidsandBases


Calculating Ka from the pHSample Exercise 16.10

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

We know that

[H3O+] [HCOO-][HCOOH]

Ka =

AcidsandBases



The pH of a 0.10 M solution of formic acid, HCOOH, at 25C 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.

AcidsandBases



pH = -log [H3O+]

2.38 = -log [H3O+]

-2.38 = log [H3O+]

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

4.2 10-3 = [H3O+] = [HCOO-]

AcidsandBases


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 = 0.10

4.2 10-3 4.2 10-3

AcidsandBases



Note that the [HCOOH] remains at 0.10 M at equilibrium– sig figs. This will become very important later in this chapter.

AcidsandBases



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

Ka =

= 1.8 10-4

AcidsandBases



Niacin, one of the B vitamins, has the following molecular structure:

A 0.020 M solution of niacin has a pH of 3.26. What is the acid-dissociation constant, Ka, for niacin?(1.5 x 10-5)

AcidsandBases


Calculating Percent IonizationSample Exercise 16.11

A 0.10 M solution of formic acid (HCOOH) contains 4.2 x 10-3 M H+(aq). Calculate the percentage of the acid that is ionized.

(4.2%)

AcidsandBases


Calculating Percent IonizationSample Exercise 16.11

• Percent Ionization = 100

• In this example

[H3O+]eq = 4.2 10-3 M

[HCOOH]initial = 0.10 M

[H3O+]eq

[HA]initial

Percent Ionization = 1004.2 10-3

0.10= 4.2%

AcidsandBases



A 0.020 M solution of niacin has a pH of 3.26. Calculate the percent ionization of the niacin.

(2.8%)

AcidsandBases


Using Ka to Calculate pH

• Using Ka, we can calculate the concentration of H+ (and hence the pH).

Example: Calculate the pH of a 0.30 M solution of acetic acid, HC2H3O2, at 25C.

Ka for acetic acid at 25C is 1.8 10-5.

AcidsandBases



1. Write the balanced chemical equation clearly showing the equilibrium.

CH3COOH(aq) H+(aq) + CH3COO-

(aq)

AcidsandBases



2. Write the equilibrium expression.

Look up the value for Ka (in a table).

Ka = [H+][ CH3COO-] = 1.8 x 10-5

[CH3COOH]

AcidsandBases



3. Write down the initial and equilibrium concentrations for everything except pure

water.

We usually assume that the equilibrium

concentration of H+ is x.

AcidsandBases



3. Write down the initial and equilibrium

concentrations for everything except pure

water.

AcidsandBases



4. Substitute into the equilibrium constant

expression and solve.

Ka = 1.8 x 10-5 = [H+][C2H3O2-] = (x)(x)

[HC2H3O2] 0.30 – x

Note that Ka is very small (1.8 x 10-5) relative to [HC2H3O2] (0.30 M).

AcidsandBases



4. Substitute into the equilibrium constant expression and solve.

Keep this x

x2 = 1.8 x 10-5

0.30 – x

Neglect x in the denominator since it is extremely small relative to 0.30.(Use ballpark figure of 103 X difference for neglecting x in the denominator )

AcidsandBases




x2 = (1.8 x 10-5 )(0.30) = 5.4 x 10-6

x2 = (5.4 x 10-6)

x = 2.3 x 10-3 M = [H+]

AcidsandBases




Check: Compare x with original [HC2H3O2] of 0.30 M:

2.3 x 10-3 M x 100% = 0.77%, which is < 5%

0.30 M

AcidsandBases



5. Convert x ([H+]) to pH.

pH = - log (2.3 x 10-3) = 2.64

AcidsandBases


What do we do if we are faced with having to solve a quadratic equation in order to determine the value of x?

Often this cannot be avoided.

• However, if the Ka value is quite small, we find that we can make a simplifying assumption. • Assume that x is negligible compared to the initial concentration

of the acid.• This will simplify the calculation.

• It is always necessary to check the validity of any assumption.• Once we have the value of x, check to see how large it is compared to the

initial concentration.

• If x is <5% of the initial concentration, the assumption is probably a good

one. • If x>5% of the initial concentration, then it may be best to solve the

quadratic equation or use successive approximations.

AcidsandBases



Calculate the pH of a 0.20 M solution of HCN. Refer to Table 16.2 for Ka.

(5.00)

AcidsandBases



The Ka for niacin is 1.6 x 10-5. What is the pH of a 0.010 M solution of niacin?

(3.41)

AcidsandBases



Calculate the percentage of HF molecules ionized in

a) a 0.10 M HF solution (7.9%)

b) a 0.010 M HF solution (23%)

AcidsandBases



In Practice Exercise 16.11, we found that the percent ionization of niacin (Ka = 1.5 x 10-5) in a 0.020 M solution is 2.7%. Calculate the percentage of niacin molecules ionized in a solution that is

a)0.010 M (3.9%)

b)a 1.0 x 10-3 M (12%)

AcidsandBases


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.

AcidsandBases


Sample Exercise 16.14 (p. 689)HW

The solubility of CO2 in pure water at 25oC and 0.1 atm pressure is 0.0037 M. the common practice is to assume that all of the dissolved CO2 is in the form of carbonic acid (H2CO3), which is produced by reaction between the CO2 and H2O:

CO2(aq) + H2O(l) H2CO3(aq)

What is the pH of a 0.0037 M solution of H2CO3?

(4.40)

AcidsandBases


Practice Exercise 16.14 (p. 690)optional – requires quadratic

a) Calculate the pH of a 0.020 M solution

of oxalic acid (H2C2O4).

(see Table 16.3 for Ka1 and Ka2).

b) Calculate the concentration of oxalate ion [C2O4

2-].

(pH = 1.80; [C2O42-] = 6.4 x 10-5 M)

AcidsandBases


Weak Bases

Weak bases remove protons from substances. They react with water to produce hydroxide ion.

AcidsandBases


Weak Bases

The equilibrium constant expression for this reaction is

[HB] [OH-][B-]

Kb =

where Kb is the base-dissociation constant.The larger Kb, the stronger the base.

AcidsandBases


Weak Bases

The equilibrium constant expression for this reaction is

[NH4+] [OH-]

[NH3]Kb =

where Kb is the base-dissociation constant.

AcidsandBases


Weak Bases

Kb can be used to find [OH-] and, through it, pH.

AcidsandBases


pH of Basic Solutions Sample Exercise 16.15 (p. 691)

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

[NH4+] [OH-]

[NH3]Kb = = 1.8 10-5

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

AcidsandBases


pH of Basic Solutions

Tabulate the data.

[NH3], M [NH4+], M [OH-], M

Initially 0.15 0 0

At Equilibrium 0.15 - x 0.15 x x

AcidsandBases



(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 =

AcidsandBases



Check the assumption:

1.6 10-3 M x 100% = 1.1%, < 5%

0.15 M

AcidsandBases



Therefore,

[OH-] = 1.6 10-3 M

pOH = -log (1.6 10-3)

pOH = 2.80

pH = 14.00 - 2.80

pH = 11.20

AcidsandBases


Practice Problem 16.15

Which of the following compounds should produce the highest pH as a 0.05 M solution: pyridine, methylamine, or nitrous acid?

(methylamine)

AcidsandBases


Types of Weak Bases

Weak bases generally fall into one of two categories. 1. Neutral substances with a lone pair of electrons

that can accept 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).

AcidsandBases


Types of Weak Bases

Like NH3, amines can abstract a proton from a water molecule by forming an additional N-H bond, as shown in this figure for methylamine.

AcidsandBases


Types of Weak Bases

2. Anions of weak acids are also weak bases.

e.g.: ClO– is the conjugate base of HClO (weak acid):

ClO–(aq) + H2O(l) HClO(aq) + OH–

(aq) Kb = 3.3 x 10–7

AcidsandBases



A solution made by adding solid sodium hypochlorite (NaClO) to enough water to make 2.00 L of solution has a pH of 10.50. How many moles of NaClO were added to the water? (See info immediately above.)(0.60 mol)

AcidsandBases



A solution of NH3 in water has a pH of 11.17. What is the molarity of the solution?

(0.12 M)

AcidsandBases


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.

Alternatively, pKa + pKb = pKw = 14.00 (at 25oC)

AcidsandBases



Calculate

a) the base-dissociation constant, Kb, for the fluoride ion (F-); (1.5 x 10-11)

b) the acid-dissociation constant, Ka, for the ammonium ion (NH4

+). (5.6 x 10-10)

AcidsandBases


Practice Exercise 16.17(Use Appendix D for Ka)

a) Which of the following anions has the largest base-dissociation constant: NO2

-, PO43-, or N3

-?(PO4

3-, Kb = 2.4 x 10-2)

b) The base quinoline has the following structure:

Its conjugate acid is listed in handbooks as having a pKa of 4.90. What is the base-dissociation constant for quinoline?(7.9 x 10-10)

AcidsandBases

Acid/Base Properties of Salt Solutions


AcidsandBases


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)

AcidsandBases


Reactions of Cations with Water

• Cations with acidic protons (like NH4

+) will lower the pH of a solution.

• Most metal cations that are hydrated in solution also lower the pH of the solution.

AcidsandBases


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.

AcidsandBases


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.

AcidsandBases


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 effect on pH depends on the Ka and Kb values.

AcidsandBases


Sample Exercise 16.18 (p. 698)Determine whether aqueous solutions of each of the following salts will be acidic, basic, or neutral:

Hint: Analyze each compound – derived from strong or weak acids and bases?

a) Ba(CH3COO)2,

b) NH4Cl

c) CH3NH3Br

d) KNO3

e) Al(ClO4)3

AcidsandBases



Determine whether aqueous solutions of each of the following salts will be acidic, basic, or neutral:

a) Ba(CH3COO)2,anion of weak acid basic

b) NH4Clcation of weak base acidic

c) CH3NH3Brcation of weak base acidic

d) KNO3

neutral – derived from strong acid + strong basee) Al(ClO4)3

cation of weak base (Al(OH)3) acidic,Al3+ - increase the polarity of O-H in H2O

AcidsandBases



In each of the following, indicate which salt in each of the following pairs will form the more acidic (or less basic) 0.010 M solution:

a) NaNO3 or Fe(NO3)3

b) KBr, or KBrO

c) CH3NH3Cl or BaCl2d) NH4NO2 or NH4NO3

AcidsandBases



Predict whether the salt Na2HPO4 will form an acidic or a basic solution on dissolving in water.

(basic)

AcidsandBases



Predict whether the dipotassium salt of citric acid (K2HC6H5O7) will form an acidic or basic solution in water.

(see Table 16.3 for data)

(acidic)

AcidsandBases


Factors Affecting Acid Strength (Sec. 16.10-16.11)

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

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

AcidsandBases


Factors Affecting Acid Strength

In oxyacids, in which an -OH is bonded to another atom, Y, the more electronegative Y is, the more acidic the acid.

AcidsandBases



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

AcidsandBases



Arrange the compounds in each of the following series in order of increasing acid strength:

a) AsH3, HI, NaH, H2O;

b) H2SO4, H2SeO3, H2SeO4.

AcidsandBases



Arrange the compounds in each of the following series in order of increasing acid strength:

a) AsH3, HI, NaH, H2O

NaH < AsH3 < H2O < HI

b) H2SO4, H2SeO3, H2SeO4

H2SeO3 < H2SeO4 < H2SO4

AcidsandBases


Practice 16.20

In each of the following pairs choose the compound that leads to the more acidic (or less basic) solution:

a) HBr, HF;

b) PH3, H2S;

c) HNO2, HNO3;

d) H2SO3, H2SeO3.

AcidsandBases



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

AcidsandBases


Comparison of Different Types of Acids and Bases

AcidsandBases


Arrhenius (traditional) acids and bases (C19th)

• Acid: compound containing H that ionizes to yield H+ in solution

• Base: compound containing OH that ionizes to yield OH- in solution

(Note: does not describe acid/base behavior in solvents other than water)

Note: Every Arrhenius acid/base is also a Brønsted-Lowry acid/base.

AcidsandBases


Bronsted-Lowry Acids and Bases (1923)

• Acid: H+ (proton) donor• Base: H+ (proton) acceptor

NH3 + H2O NH4+ + OH-

ammonia water ammonium ion hydroxide ion

(B-L base) (B-L acid) (B-L acid) (B-L base)

Note: Every Brønsted-Lowry acid/base is also a Lewis acid/base.

AcidsandBases


Lewis Acids and Bases (1920’s)

• Acid: accepts pair of e-‘s

• Base: donates pair of e-‘s

H+ + [:O:H]- H:O:H

Lewis acid Lewis base

.. ..

.. ..

AcidsandBases


Lewis Acids

• Lewis acids are defined as electron-pair acceptors.

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

AcidsandBases


Lewis Bases

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

a Lewis base.• Lewis bases can interact with things other than

protons, however.

AcidsandBases


Hydrolysis of Metal Ions

• The Lewis concept may be used to explain the acid properties of many metal ions.

• Metal ions are positively charged and attract water molecules (via the lone pairs on the oxygen atom of water).

• Hydrated metal ions act as acids.

• For example:Fe(H2O)6

3+(aq) Fe(H2O)5(OH)2+

(aq) + H+(aq) Ka = 2 x 10–3

AcidsandBases


Hydrolysis of Metal Ions

In general:• The higher the charge, the stronger the M–OH2

interaction.• Ka values generally increase with increasing charge

• The smaller the metal ion, the more acidic the ion. • Ka values generally decrease with decreasing ionic radius • Thus the pH of an aqueous solution increases as the size of the ion increases (e.g., Ca2+ vs. Zn2+) and as the charge increases (e.g., Na+ vs. Ca2+ and Zn2+ vs. Al3+).

AcidsandBases


Sample Integrative Exercise 16 (p. 706)

Phosphorous acid (H3PO3) has the following Lewis structure:

AcidsandBases


Sample Integrative Exercise 16 (p. 706)

a) Explain why phosphorous acid is diprotic and not triprotic.

b) A 25.0 mL sample of a solution of H3PO3 is titrated with 0.102 M NaOH. It requires 23.3 mL of NaOH to neutralize both acidic protons. What is the molarity of the H3PO3 solution?

c) This solution has a pH of 1.59. Calculate the percent ionization and Ka1 for H3PO3, assuming that Ka1 >>Ka2.

d) How does the osmotic pressure of a 0.050 M solution of HCl compare qualitatively with that of a 0.050 M solution of H3PO3? Explain.

Acids and Bases © 2009, Prentice-Hall, Inc. Chapter 16 Acids and Bases Chemistry, The Central...

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Transcript of Acids and Bases © 2009, Prentice-Hall, Inc. Chapter 16 Acids and Bases Chemistry, The Central...