chapter 16 acid–base equilibria © 2012 pearson education, inc

Chapter 16

Acid–Base Equilibria

© 2012 Pearson Education, Inc.

Acidsand

Bases

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.


Acidsand

Bases

A. Na+ and OH–

B. H+ and OH–

C Na+ and Cl–

D. H+ and Cl–

Acidsand

Bases

Some Definitions• Brønsted–Lowry

– An acid is a proton donor.– A base is a proton acceptor.

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

A. Hydrogen bondB. Dispersion bondC. Covalent bondD. Ionic bond

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

A. Both substances act as Brønsted-Lowry bases.B. Neither substance acts as a Brønsted-Lowry

base.

C. H2S(aq)

D. CH3NH2(aq)

Acidsand

Bases

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.


Acidsand

Bases

Sample Exercise 16.1 Identifying Conjugate Acids and Bases(a) What is the conjugate base of HClO4, H2S, PH4

+, HCO3?

(b) What is the conjugate acid of CN, SO42, H2O, HCO3

?

Practice ExerciseWrite the formula for the conjugate acid of each of the following: HSO3

, F, PO43, CO.

Acidsand

Bases

Sample Exercise 16.2 Writing Equations for Proton-Transfer ReactionsThe hydrogen sulfite ion (HSO3

) is amphiprotic. Write an equation for the reaction of HSO3 with

water (a) in which the ion acts as an acid and (b) in which the ion acts as a base. In both cases identify the conjugate acid–base pairs.

Practice ExerciseWhen lithium oxide (Li2O) is dissolved in water, the solution turns basic from the reaction of the oxide ion (O2) with water. Write the equation for this reaction and identify the conjugate acid–base pairs.

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 strong

bases.• Substances with negligible acidity do not

dissociate in water.– Their conjugate bases are

exceedingly strong.


Acidsand

Bases

A. No reactionB. O2–(aq) + H+(aq) → HO2–(aq) C. O2–(aq) + 2H+(aq) → H2O2(aq)D. O2–(aq) + H2O(aq) → 2OH–(aq)

Acidsand

Bases

A. Strong baseB. Weak baseC. Negligible basicityD. Never a base

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 that K is not measured (K >> 1).

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

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


Acidsand

Bases

Sample Exercise 16.3 Predicting the Position of a Proton-Transfer Equilibrium

For the following proton-transfer reaction use Figure 16.3 to predict whether the equilibrium lies to the left (Kc < 1) or to the right (Kc > 1):

Practice ExerciseFor each reaction, use Figure 16.3 to predict whether the equilibrium lies to the left or to the right:

(a)

(b)

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.


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 25C, Kw = 1.0 1014


Acidsand

Bases

pH

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

pH = log [H3O+]


Acidsand

Bases

Sample Exercise 16.4 Calculating [H+] for Pure Water

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

Practice ExerciseIndicate whether solutions with each of the following ion concentrations are neutral, acidic, or basic: (a) [H+] = 4 109 M; (b) [OH] = 1 107 M; (c) [OH] = 7 1013 M.

Acidsand

Bases

Sample Exercise 16.5 Calculating [H+] from [OH]

Calculate the concentration of H+ (aq) in (a) a solution in which [OH] is 0.010 M, (b) a solution in which

[OH] is 1.8 109 M. Note: In this problem and all that follow, we assume, unless stated otherwise, that the temperature is 25 C .

Practice ExerciseCalculate the concentration of OH(aq) in a solution in which (a) [H+] = 2 106 M;

(b) [H+] = [OH]; (c) [H+] = 100 [OH].

Acidsand

Bases

pH

• Therefore, in pure water,pH = log (1.0 107) = 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

A. No, the pH range is 1-14.B. No, the definition of pH does not permit it to

have a negative value.C. Yes, for any solution with a concentration of

base greater than 1 M.D. Yes, for any solution with a concentration of

acid greater than 1 M.

Acidsand

Bases

Sample Exercise 16.6 Calculating pH from [H+]Calculate the pH values for the two solutions of Sample Exercise 16.5.

SolutionAnalyze We are asked to determine the pH of aqueous solutions for which we have already calculated [H+].

Plan We can calculate pH using its defining equation, Equation 16.17.

Solve(a) In the first instance we found [H+] to be 1.0 1012 M, so that

pH = log(1.0 1012) = (12.00) = 12.00

Because 1.0 1012 has two significant figures, the pH has two decimal places, 12.00.

(b) For the second solution, [H+] = 5.6 106 M . Before performing the calculation, it is helpful to estimate the pH. To do so, we note that [H+] lies between 1 106 and 1 105. Thus, we expect the pH to lie between 6.0 and 5.0.We use Equation 16.17 to calculate the pH: pH = log(5.6 106) = 5.25

Check After calculating a pH, it is useful to compare it to your estimate. In this case the pH, as we predicted, falls between 6 and 5. Had the calculated pH and the estimate not agreed, we should have reconsidered our calculation or estimate or both.

Practice Exercise(a) In a sample of lemon juice, [H+] = 3.8 104 M. What is the pH? (b) A commonly available window-cleaning solution has [OH] = 1.9 106 M. What is the pH?Answers: (a) 3.42, (b) [H+] = 5.3 109 M, so pH = 8.28

Acidsand

Bases

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

Because

[H3O+] [OH] = Kw = 1.0 1014

we know that

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

or, in other words,

pH + pOH = pKw = 14.00


Acidsand

Bases

A. pH = 17.00; basic because pH > 7B. pH = 11.00; basic because pH > 7C. pH = 3.00; acidic because pH < 7D. The pH cannot be determined without [H+]

information. Solution is basic because pOH < 7.

Acidsand

Bases

Sample Exercise 16.7 Calculating [H+] from pOHA sample of freshly pressed apple juice has a pOH of 10.24. Calculate [H+].

Practice ExerciseA solution formed by dissolving an antacid tablet has a pH of 9.18. Calculate [OH ].

Acidsand

Bases

pH

These are the pH values for several common substances Fig. 16.5.


Acidsand

Bases

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.


Acidsand

Bases

A. pH is near 7.B. pH is <7.C. pH is greater > 10.D. pH is greater > 12.

Acidsand

Bases

A. Thymol blueB. Bromothymol blueC. PhenophthaleinD. None of the above

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

Sample Exercise 16.8 Calculating the pH of a Strong AcidWhat is the pH of a 0.040 M solution of HClO4?

Practice ExerciseAn aqueous solution of HNO3 has a pH of 2.34.What is the concentration of the 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

A. 0.001 M solution of Ba(OH)2 has the higher pH.B. 0.001 M solution of NaOH has the higher pH.C. Both solutions have the same pH as both have

the same concentration.

Acidsand

Bases

Sample Exercise 16.9 Calculating the pH of a Strong BaseWhat is the pH of (a) a 0.028 M solution of NaOH, (b) a 0.0011 M solution of Ca(OH)2?

Practice ExerciseWhat is the concentration of a solution of (a) KOH for which the pH is 11.89, (b) Ca(OH)2 for which the pH is 11.68?

Acidsand

Bases

A. Nothing happens when CH3– is added to water.

B. CH3– loses a proton to form CH2

2– when added to water.

C. CH3– reacts with water to form CH3OH and H+.

D. CH3– removes a proton from water to form CH4 and

OH–.

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 is the acid.


Acidsand

Bases

A. OB. HC. ND. C

Acidsand

Bases

Sample Exercise 16.10 Calculating Ka from Measured pH

A student prepared a 0.10 M solution of formic acid (HCOOH) and found its pH at 25 C to be 2.38. Calculate Ka for formic acid at this temperature.

Practice ExerciseNiacin, one of the B vitamins, has the molecular structure

A 0.020 M solution of niacin has a pH of 3.26. What is the acid-dissociation constant for niacin?

Acidsand

Bases

Calculating Percent Ionization

• Percent ionization = 100

• In this example,

[H3O+]eq = 4.2 103 M

[HCOOH]initial = 0.10 M

[H3O+]eq

[HA]initial

Percent ionization = 1004.2 103

0.10= 4.2%


Acidsand

Bases

Sample Exercise 16.11 Calculating Percent Ionization

As calculated in Sample Exercise 16.10, a 0.10 M solution of formic acid (HCOOH) contains 4.2 103 M H+(aq). Calculate the percentage of the acid that is ionized.

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

Acidsand

Bases

Calculating pH from Ka

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

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

Ka for acetic acid at 25C is 1.8 105.


Acidsand

Bases

A. Because Ka for weak acids slightly less than 1 and therefore, the extent of ionization is very small and can be neglected.

B. Because most weak acids of low to moderate concentration undergo very little ionization, often less than 1% in solution, and thus the extent of ionization can be neglected.

C. Because most weak acids of high or low concentration will form minimal conjugate base and thus the extent of ionization can be neglected.

D Because for each weak acid dissociating in solution, a weak acid molecule is formed with minimal conjugate base forming and thus the extent of ionization can be neglected.

Acidsand

Bases

Sample Exercise 16.12 Using Ka to Calculate pH

Calculate the pH of a 0.20 M solution of HCN. (Refer to Table 16.2 or Appendix D for the value of Ka.)

Practice ExerciseThe Ka for niacin (Practice Exercise 16.10) is 1.5 105. What is the pH of a 0.010 M solution of niacin?

Acidsand

Bases

A. No, as the CH3COOH concentration increases the following reaction should produce more product and thus increase the percent ionized: CH3COOH CH3COO– + H+.B. No, because Ka changes slightly as the concentration of CH3COOH increases and this permits the degree of ionization to change as shown on the graph. C. Yes, changing the concentration of the acid in different solutions of CH3COOH does not change the value of Ka and thus LeChâtelier’s principle is still valid.D. Yes, the percent concentration must decrease as the concentration of CH3COOH increases otherwise the concentration of all three species would increase to ensure that Ka remains constant. The decrease in percent ionization with an increase in acid concentration offsets the fact that the rate of formation of the product is twice the rate of disappearance of the acid.

Acidsand

Bases

Sample Exercise 16.13 Using Ka to Calculate Percent Ionization

Calculate the percentage of HF molecules ionized in (a) a 0.10 M HF solution, (b) a 0.010 M HF solution.

Practice ExerciseIn Practice Exercise 16.11, we found that the percent ionization of niacin (Ka = 1.5 105) 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, (b) 1.0 103 M.

Acidsand

Bases

Polyprotic Acids 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.


Acidsand

Bases

A. A H bonded to a O—C— is more acidic than the H atoms shown in red.

B. A H bonded to a O—C— is only slightly less acidic than the H atoms shown in red.

C. A H bonded (shown in red) to a O—C that is part of the —COOH(carboxylate) group is acidic but the H bonded to in the form of HO—C— (alcohol) group is not acidic.

D. A H bonded (shown in red) to a O—C that is part of the —COOH(carboxylate) group is acidic but the H bonded to in the form of HO—C— (alcohol) group is part of a basic group.

Acidsand

Bases

A. The 3 subscript signifies that the equilibrium constant is reported to 3 significant figures.

B. The 3 subscript signifies that H3PO4 typically forms solutions of pH = 3.00.

C. The 3 subscript in Ka3 signifies that H3PO4 has an acidic proton that causes a very low pH for the acid solution.

D. This is the acid dissociation constant for the 3rd and final proton from H3PO4

Acidsand

Bases

Sample Exercise 16.14 Calculating the pH of a Polyprotic Acid Solution

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

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

Practice Exercise(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], in this solution.

Acidsand

Bases

Weak BasesBases react with water to produce hydroxide ion.

The equilibrium constant expression for this reaction is

where Kb is the base-dissociation constant.


[HB] [OH][B]Kb =

Acidsand

Bases

Weak Bases

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


Acidsand

Bases

A. CarbonB. NitrogenC.HydrogenD. Oxygen

Acidsand

Bases

Sample Exercise 16.15 Using Kb to Calculate OH

Calculate the concentration of OH in a 0.15 M solution of NH3.

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

Acidsand

Bases

Sample Exercise 16.16 Using pH to Determine the Concentration of a Salt

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. Using the information in Equation 16.37, calculate the number of moles of NaClO added to the water.

Practice ExerciseWhat is the molarity of an aqueous NH3 solution that has a pH of 11.17?

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.


Acidsand

Bases

pKa(HF) pKb(F–)A. 3.17 10.83B. 10.83 3.17C. 9.60 4.40D. 4.40 9.60

Acidsand

Bases

Sample Exercise 16.17 Calculating Ka or Kb for a Conjugate Acid–Base Pair

Calculate (a) Kb for the fluoride ion, (b) Ka for the ammonium ion.

Practice Exercise(a) Which of these anions has the largest base-dissociation constant: NO2

, PO43, or N3

?(b) The base quinoline has the structure

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

Acidsand

Bases

Sample Exercise 16.17 Calculating Ka or Kb for a Conjugate Acid–Base Pair

Continued

(b) For NH3, Table 16.4 and in Appendix D give Kb = 1.8 105, and this value in Equation 16.40 gives us Ka for the conjugate acid, NH4

+:

Acidsand

Bases

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)


Acidsand

Bases

NO3– CO3

2–

A. Increase IncreaseB. No change DecreaseC. Decrease DecreaseD. No change Increase

Acidsand

Bases

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.• 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.© 2012 Pearson Education, Inc.

Acidsand

Bases

Reactions of Cations with Water

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


Acidsand

Bases

Effect of Cations and Anions1. 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.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 a weak acid

and a weak base, the affect on pH depends on the Ka and Kb values.


Acidsand

Bases

Sample Exercise 16.18 Determining Whether Salt Solutions areAcidic, Basic, or Neutral

Determine whether aqueous solutions of each of these salts are acidic, basic, or neutral: (a) Ba(CH3COO)2, (b) NH4Cl, (c) CH3NH3Br, (d) KNO3, (e) Al(ClO4)3.

Practice ExerciseIndicate which salt in each of the following pairs forms the more acidic (or less basic) 0.010 M solution:

(a) NaNO3 or Fe(NO3)3, (b) KBr or KBrO, (c) CH3NH3Cl or BaCl2, (d) NH4NO2 or NH4NO3.

Acidsand

Bases

Sample Exercise 16.19 Predicting Whether the Solution of anAmphiprotic Anion Is Acidic or Basic

Predict whether the salt Na2HPO4 forms an acidic solution or a basic solution when dissolved in water.

Practice ExercisePredict whether the dipotassium salt of citric acid (K2HC6H5O7) forms an acidic or basic solution in water.

Acidsand

Bases

Factors Affecting Acid Strength

• 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.


Acidsand

Bases

A. Moving down a column, H-X bond strength increases; moving left-to-right across a period, electronegativity increases.

B. Moving down a column, H-X bond strength increases; moving left-to-right across a period, electronegativity decreases.

C. Moving down a column, H-X bond strength decreases; moving left-to-right across a period, electronegativity increases.

D. Moving down a column, H-X bond strength decreases; moving left-to-right across a period, electronegativity decreases.

Acidsand

Bases

Factors Affecting Acid StrengthIn 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.


Acidsand

Bases

A. HIO2

B. HBrO3

Acidsand

Bases

Factors Affecting Acid Strength

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


Acidsand

Bases

A. –CH3

B. –OH2

C. –NH3

D. –COOH

Acidsand

Bases

Lewis Acids

• Lewis acids are defined as electron-pair acceptors.• Atoms with an empty valence orbital can be Lewis acids.• 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.


Acidsand

Bases

A. One or more atoms with an unshared pair of electrons

B. One or more atoms with an electron deficiencyC. Must have a positive chargeD. Must have a negative charge

chapter 16 acid–base equilibria © 2012 pearson education, inc

Documents

conjugate bases

conjugate base of hclo4

pearson education

stronger base

brnstedlowry base

conjugate acid of cn

water acts

proton h