Chapter 14: Acid-Base Equilibrium

Acid Strength of Binary Acid (HA): Down a group (top to bottom), acid strength of HA increases. What happens to the orbitals?

-Orbitals of similar size form strong bonds - Good orbital overlap. -Orbitals of different size form weak bonds - Poor orbital overlap. -As A changes down a group, its size increases, poorer orbital overlap between H and A, H-A bond weakens, acid strength increases. H-A ⇌ H+ + A-

What is the acid base definition from the Svante Arrhenius definition (in 1884)?

-an acid as a compound that dissolves in water to yield hydrogen cations H+ -a base as a compound that dissolves in water to yield hydroxide anions OH-

Acid/Base Strength being inversely related in regards to Ka and Kb (slide 21)

(Ka)x(Kb)= Kw= 1.0x10-14 pKa= -log Ka pKb= -log Kb At 25 °C, pKa + pKb = 14.00

The autionization of water

- In pure water, water also acts both as an acid and a base. - This process is called autoionization of Water. - H2O(l) + H2O(l) ⇌ H3O+(aq) + OH−(aq) - The equilibrium constant for this reaction is called the ion product constant for water, Kw. - Kw = [H3O+][OH−] = 1.0 × 10-14 at 25 °C

What is a buffer?

-A solution containing appreciable amounts of both a weak acid and its conjugate base (or a weak base and its conjugate acid). Buffer solutions resist changes in pH upon addition of a small amount of strong acid or base. EX: After the addition of 1 mL of a 0.01-M HCl solution, the buffered solution has not detectably changed its pH but the unbuffered solution has become acidic, as indicated by the change in color of the methyl orange, which turns red at a pH of about 4. (credit: modification of work by Mark Ott)

What is buffer capacity?

-Amount of strong acid or base that can be added to a buffer solution before the pH changes significantly. -All buffers have a limited capacity of how much H3O+ or OH- they can "soak up". ▫Eventually, all the HA reacts with the added OH- ▫Eventually, all the A- reacts with the added H3O+

What is the Bronsted-Lowry Acid/Base Theory?

-An acid-base reaction is, thus, the transfer of a proton from a donor (acid) to an acceptor (base). -•The species formed when a proton is removed from an acid is the conjugate base of that acid. -•The species formed when a proton is added to a base is the conjugate acid of that base.

Acid Strength of Binary Acid (HA): Across a period (left to right), acid strength of HA increases. What happens to the orbitals?

-As A changes cross a period from left to right, electronegativity increases, the stability of A- increases, the equilibrium shift to right. -When the equilibrium shift to right, it produces more proton H+, the acid strength increases. H-A ⇌ H+ + A-

What are bases?

1. Ionic compounds that contain OH-, fore example, NaOH 2. Molecules - Primarily amines which contain N, like NH3 3. Anions - With and without H, for example HCO3- , CO32-

What are acids?

1. Molecules that contain H: for example, HCl, CH3COOH 2. Cations that contain H: for example, NH4+ 3. Anions that contain H: for example, HCO3-

What are the two types of definitions that were from scientists for acid and base?

1.) Svante Arrhenius definition (in 1884) 2.)Johannes Brønsted and Thomas Lowry Definition (1923)

Hydrolysis of Salt Solutions: what does a neutral solution contain?

A neutral solution contains equal concentrations of hydronium and hydroxide ions.

10. A student titrates a 25 mL of an unknown concentration of HCl with 35 mL of a 0.890 M solution of KOH to reach the equivalence point. What is the pH of the HCl solution? A. -0.10 B. 0.10 C. 1.2

A. -0.10

14. Which acid has a lower pH? A. 1.0 M acetic acid B. 0.50 M acetic acid C. 0.25 M acetic acid

A. 1.0 M acetic acid

18. What is the pOH of a solution that has a pH of 2? A. 12 B. 7 C. 14

A. 12

23.) (see homework) A. 2.6 x 10-4 B. 2.3 C. 0.00501 D. 0.095

A. 2.6 x 10-4

What is the equation for the equilibrium constant for an acid?

HA(aq) + H2O(l) ⇌ H3O+(aq) + A-(aq) simplified as: HA(aq) ⇌ H+(aq) + A-(aq) Ka = "[H3O+][A-]" /"[HA]" = "[H+][A-]" /"[HA]"

What are the strong acids?

HCl, HBr, HI, HClO4, HNO3, H2SO4

The Compound is Amphiprotic when...

If EN of E is intermediate then the molecule is usually amphiprotic. •In the presence of strong acid the compound acts as a base. •In the presence of strong base the compound acts as an acid. vMetalloids and elements near the metalloids have intermediate EN. Example: Al(H2O)3 (OH)3 •In Base: Al(H2O)3(OH)3(aq) + OH−(aq) ⇌ H2O(l) + [Al(H2O)2(OH)4]−(aq) •In Acid: Al(H2O)3 (OH)3(aq) + H3O+(aq) ⇌ Al(H2O)63+(aq) + 3H2O(l)

In water, what happens to strong acids and strong bases?

In water, these strong acids/bases essentially dissociate into ions completely.

13. Blood is considered a buffered solution. What is the pH range of blood? A. 7.35 - 7.45 B. 6.35 - 6.45 C. 8.35 - 8.45

A. 7.35 - 7.45

1. Rank the following acids in order of increasing acidity: H2O, H2S, H2Se, H2Te. A. H2O, H2S, H2Se, H2Te. B. H2Te, H2Se , H2S, H2O. C. H2O, H2Te, H2Se, H2S.

A. H2O, H2S, H2Se, H2Te.

17. What is the conjugate acid of HSO4 -? A. H2SO4 B. SO4 2- C. H2SO4 -

A. H2SO4

8. Rank the following oxoacids in order of increasing acidity: HClO2, HClO4, HClO, HClO3. A. HClO, HClO2, HClO3, HClO4 B. HClO4, HClO2, HClO, HClO3 C. HClO4, HClO3, HClO2, HClO

A. HClO, HClO2, HClO3, HClO4

25. Determine whether a solution of FeCl3 are acidic, basic, or neutral: A. acidic B. basic C. neutral

A. acidic

7. Rank the following oxyanions in order of increasing basicity SO3 2- , SO4 2- . A. SO3 2- < SO4 2- B. SO4 2- < SO3 2-

B. SO4 2- < SO3 2-

3. When titrating a strong acid with a strong base, approximately where would the pH be observed when reaching the equivalence point? A. at the equivalence point, the pH is greater than 7 B. at the equivalence point, the pH is equal to 7 C. at the equivalence point, the pH is less than 7

B. at the equivalence point, the pH is equal to 7

9. If 3 mL of a 0.5 M HBr solution is added to 20 mL of a 0.5 M NaOH solution, the resulting solution would be _____. A. acidic B. basic C. neutral

B. basic

How buffers work

Consider a buffer: HA/A- . Added OH- or H3O+ are consumed and therefore do not significantly change the pH. ▫Any added OH-, is converted into the weak base, A- HA (aq) + OH- (aq) → A- (aq) + H2O ▫Any added H3O+ , is converted to the weak acid, HA A- (aq) + H3O+ (aq) → HA (aq) + H2O

What is base ionization?

The reaction between a Brønsted-Lowry Base and water is called base ionization.

How can the relative strengths of weak acids be determined?

The relative strengths of weak acids can be determined by measuring their equilibrium constants.

Compounds Containing Oxygen and One or More Hydroxyl Group (OH)

These compounds can be acidic, basic, or amphiprotic depending on the electronegativity (EN) of element, E.

What is titration used for?

To find the concentration of acid or base

What is the equilibrium constant for a base called?

base-ionization constant (Kb).

The relative strengths of weak bases can be determined by what?

measuring their equilibrium constants

Acids that contain one ionizable hydrogen atom (H+) are called what?

monoprotic

Hydrolysis of Salt Solutions: Even when the stoichiometrically equivalent quantities of acid and base are used , is the resulting solution necessarily neutral?

no

what is the pH and pOH of water?

pH= 7 pOH= 7

Acids that contain more than one ionizable hydrogen atom (H+) are called what?

polyprotic Diprotic acid: H2SO4 H2CO3 Triprotic acid: H3PO4

What is the equation for the percent ionization of a weak acid? HA(aq) + H2O(l) ⇌ H3O+(aq) + A-(aq)

see pic

▫The larger Ka is of an acid, the ____________ the Kb is of its conjugate base.

smaller

▫The larger Kb is of a base, the _____________ the Ka is of its conjugate acid.

smaller

The larger the Ka of an acid the __________ the acid

stronger bigger=stronger

The larger the Kb of the base, the _____________ the base

stronger larger=stronger

The pH at which the indicator changes color is called what?

the endpoint

Need an indicator that changes color at a pH near what?

the equivalence point

what is an acid-base indicator useful in determining?

the equivalence point in a titration

What is titration?

the process of adding a known amount of solution of known concentration to determine the concentration of another solution

Ka values are related to what?

the strength of the acid

Kb values are related to what?

the strength of the base

The pH of a buffer system can be calculated if.....

we know the concentration of the weak acid [HA] and its conjugate base [A-] HA(aq) + H2O(l) ⇌ H3O+(aq) + A-(aq)

Are indicators strong or weak acids?

weak acids -the weak acid has a different color than its conjugate base

The smaller the Ka of an acid the _______________ the acid

weaker smaller=weaker

The smaller the Kb of the base, the ____________________ the base

weaker smaller=weaker

Hydrolysis of Salt Solutions: The nature of the salt formed determines what?

whether the solution is acidic, basic or neutral.

What is an amphiprotic (amphoteric) species?

•A species that can either accept or donate a proton is called amphiprotic, or amphoteric.

Does an amphoteric species serve as an acid or a base?

•An amphoteric species can serve as an acid or a base.

Weak Acid-Strong Base Titration Half-Equivalence Point:

•At half equilavlence point, half of HA turns into A-, so [HA] = [A-] •So at half equivalence point, pH = pKa •Determining pKa from a titration curve.

Relative Strengths of Acids and Bases

•Brønsted-Lowry Acids are H+ donors •The strength of an acid is measured by the extent to which it dissociates and forms H3O+ in water. HA(aq) + H2O(l) ⇌ H3O+(aq) + A-(aq) •Strong acids dissociate 100% •Weak acids dissociate much less than 100%

Relative Strength of Bases

•Brønsted-Lowry Bases are H+ acceptors •The strength of a base is measured by the extent to which it forms hydroxide ions in water. B(aq) + H2O(l) ⇌ HB+(aq) + OH-(aq) • •Strong bases yield nearly 100% OH- •Weak bases yield much less than 100% OH-

The Compound is acidic when E is Nonmetal

•If EN of E is high then "Bond a" is nonpolar. •Electrons in "Bond b" are pulled toward E, "Bond b" is polar and weak, H+ ions are readily released. •Non-metals have high EN, The compound is acidic when E is non-metal, e.g. H2SO4 (look up lewis structure of H2SO4, slide 33)

The Compound is basic when E is Metal

•If EN of E is low then "Bond a" is polar or ionic. •OH- ions are readily released. •The compound is basic. •Metals have low EN •Metal hydroxides tend to be basic, e.g. NaOH

pH and pOH CONTINUED

•If [H3O+] = [OH-] = 1.0 x 10-7 then the solution is neutral. pH=7 [H3O+] = [OH-] •If [H3O+] > 1.0 x 10-7, then [OH-] < 1.0 x 10-7 and the solution is acidic. pH<7 [H3O+] > [OH-] •If [OH-] > 1.0 x 10-7, then [H3O+] < 1.0 x 10-7 and the solution is basic. pH>7 [H3O+] < [OH-]

H3O+ and OH- Concentration in Water

•In pure water [H3O+] and [OH-] are equal to each other, it is neutral: [H3O+] = [OH−] = 1.0 × 10-7 M •When an acid or base is dissolved in water, [H3O+] and [OH-] are NOT equal to each other any more. •[H3O+] and [OH-] are inversely related to each other. Kw = [H3O+] × [OH-] = 1.0 × 10-14 ▫If [H3O+] > 1.0 x 10-7, then [OH-] < 1.0 × 10-7 and the solution is acidic. ▫If [OH-] > 1.0 x 10-7, then [H3O+] < 1.0 × 10-7 and the solution is basic.

What are the four types of acid-base neutralizations?

•Strong Acid-Strong Base ▫A neutral solution forms; e.g. KClO4 •Strong Acid-Weak Base ▫A weakly acidic solution forms; e.g. FeCl3 •Weak Acid-Strong Base ▫A weakly basic solution forms; e.g. CaF2 •Weak Acid-Weak Base ▫The solution may be acidic or basic; ▫NH4CN is basic (pKb of NH3 is 4.8; pKa of HCN is 9.3)

pH and pOH

•The acidity and basicity of a solution can be expressed in terms of its pH or pOH. •pH = ̶ log [H3O+] = ̶ log [H+] ([H+] = [H3O+] = 10-pH ) •pOH = ̶ log [OH-] ([OH-] = 10-pOH ) •At 25 °C, pH + pOH = 14.00

HA(aq) + H2O(l) ⇌ H3O+(aq) + A-(aq)

•The extent to which an acid, HA, donates a proton to a water molecule depends upon the strength of its conjugate base, A-. •If A- is a strong base, then any proton donated to a water molecule is recaptured by A-. ▫Little H3O+ would exist in solution. ▫HA is therefore a weak acid. •If A- is a weak base, then it is unlikely for A- to recapture a proton donated to a water molecule. ▫A large amount of H3O+ would exist in solution. ▫HA is therefore a strong acid.

Strong Acid-Strong Base Titration

•The pH is initially very low (only strong acid present). •There is a gradual rise in pH as base is added. •Eventually the pH rises rapidly. •Mid-point of the vertical part of the curve is the equivalence point. •The pH at the equivalence point is 7. •Excess NaOH controls the pH after the equivalence point.

Weak Acid-Strong Base Titration

•The pH starts off low, but not as low as the strong acid titration. •There is a gradual rise in pH as base is added. •Eventually the pH rises rapidly. •Mid-point of the vertical part of the curve is the equivalence point. •The pH at the equivalence point is 8.72 •Excess NaOH controls the pH after the equivalence point.

What is acid ionization?

•The reaction between a Brønsted-Lowry Acid and water is called acid ionization.

Determining Ka or Kb

•There are several ways to determine the Ka or Kb value of a weak acid or weak base. •One approach is to measure the pH of a solution prepared by dissolving a known amount of the weak acid or base. In these problems... ▫Given pH ▫Given some information that can be used to find the initial concentration of the acid or base. ▫Calculate Ka or Kb

Hydrolysis of Salt Solutions: The reaction between an acid and a base is called what?

•an acid-base neutralization reaction.

what is the acid base definition from the Johannes Brønsted and Thomas Lowry Definition (1923)?

▫Brønsted-Lowry Acid - A compound that donates a proton (H+) to another compound. Acids contain proton. ▫Brønsted-Lowry Base - A compound that accepts a proton from another compound.

what kind of indicator do you want to choose?

▫You want to choose an indicator that has an endpoint that coincides with the pH at the equivalence point.

What equation can be used to calculate the pH of a buffer?

The Henderson-Hasselbalch equation

What is the equilibrium constant for an acid called?

acid-ionization constant (Ka)

21. Identify and label the Brønsted-Lowry acid, its conjugate base, the Brønsted-Lowry base, and its conjugate acid in the following equation: (see homework) A: Brønsted-Lowry acid and its conjugate base: H2O and OH- , respectively; Brønsted-Lowry base and its conjugate acid: NO2 - and HNO2, respectively. B: Brønsted-Lowry acid and its conjugate base: NO2 - and HNO2 , respectively; Brønsted-Lowry base and its conjugate acid: H2O and OH- , respectively. C: Brønsted-Lowry acid and its conjugate base: NO2 - and OH- , respectively; Brønsted-Lowry base and its conjugate acid: H2O and HNO2, respectively.

A: Brønsted-Lowry acid and its conjugate base: H2O and OH- , respectively; Brønsted-Lowry base and its conjugate acid: NO2 - and HNO2, respectively.

What is the equation for the equilibrium constant for a base?

B(aq) + H2O(l) ⇌ HB+(aq) + OH-(aq)

20. State which of the following species are amphiprotic: (a) H2O (b) H2PO4- (c) S2- (d) CO3 -2 (e) HSO4 - A. (a), (b) and (c) B. (a), (b) and (e) C. (a), (c) and (e)

B. (a), (b) and (e)

19. Which acid has the lowest hydroxide ion concentration? A. 0.010 M HCl B. 0.050 M HNO3 C. 0.015 M HBr

B. 0.050 M HNO3

11. What is the pH of 0.014 M solution of hydroiodic acid? A. 1.4 B. 1.9 C. 0.85

B. 1.9

24.) (see homework) A. 3.058 x 10-3 B. 2.515 C. 0.698

B. 2.515

5. Which of the following is a diprotic acid? A. H3PO4 B. H2PO4 - C. HPO4 2-

B. H2PO4 -

15. Which acid has a higher hydrogen ion concentration? A. 1.0 M HCl B. 0.050 M HNO3 C. 1.25 M HBr

C. 1.25 M HBr

12. What is the hydroxide ion concentration for a 3.65 M HNO3 solution? A. 10.35 M B. 3.65 M C. 2.74 x 10-15 M

C. 2.74 x 10-15 M

16. What is the base in the following reaction: HCl (aq) + H2O (l) Cl- (aq) + H3O+ (aq)? A. Cl- B. H3O+ C. H2O

C. H2O

6. Which of the following is an amphiprotic substance? A. H3PO4 B. PO4 3- C. HPO4 2-

C. HPO4 2-

27.) (see homework) A. [OH- ] = 3.9 10-3 M B. [OH- ] = 1.9 10-4 M C. [OH- ] = 2.9 10-5 M

C. [OH- ] = 2.9 10-5 M

2. When titrating a weak base with a strong acid, approximately where would the pH be observed when reaching the equivalence point? A. at the equivalence point, the pH is greater than 7 B. at the equivalence point, the pH is equal to 7 C. at the equivalence point, the pH is less than 7

C. at the equivalence point, the pH is less than 7

4. Phenolphthalein can be used as an _____ in acid-base titrations. A. equivalence point B. end-point C. indicator

C. indicator

26. Determine whether a solution of KClO4 are acidic, basic, or neutral: A. acidic B. basic C. neutral

C. neutral

22. Identify and label the Brønsted-Lowry acid, its conjugate base, the Brønsted-Lowry base, and its conjugate acid in the following equation: (see homework) A: Brønsted-Lowry acid and its conjugate base: [Al(H2O)6] 3+ and [Fe(H2O)6] 3+ , respectively; Brønsted-Lowry base and its conjugate acid: [Fe(H2O)5(OH)]2+ and [Al(H2O)5(OH)]2+ , respectively. B: Brønsted-Lowry acid and its conjugate base: [Fe(H2O)5(OH)]2+ and [Fe(H2O)6] 3+ , respectively; Brønsted-Lowry base and its conjugate acid: [Al(H2O)6] 3+ and [Al(H2O)5(OH)]2+ , respectively. C: Brønsted-Lowry acid and its conjugate base: [Al(H2O)6] 3+ and [Al(H2O)5(OH)]2+ , respectively; Brønsted-Lowry base and its conjugate acid: [Fe(H2O)5(OH)]2+ and [Fe(H2O)6] 3+ , respectively

C: Brønsted-Lowry acid and its conjugate base: [Al(H2O)6] 3+ and [Al(H2O)5(OH)]2+ , respectively; Brønsted-Lowry base and its conjugate acid: [Fe(H2O)5(OH)]2+ and [Fe(H2O)6] 3+ , respectively

Example of a Amphoteric (Amphiprotic) Species

Example: Water or anions that contain H like HCO3- In the presence of an acid, water acts as a base. HCO3-(aq) + H2O(l) ® CO32-(aq) + H3O+(aq) In the presence of a base, water acts like an acid. HCO3-(aq) + H2O(l) ® H2CO3(aq) + OH-(aq)

•Polyprotic acids ionize in steps. -What happens to the Acid Equilibrium Constant (Ka)?

The Acid Equilibrium Constant (Ka) becomes smaller with each successive step.

What are the strong bases?

LiOH, NaOH, KOH, RbOH, CsOH, Ca(OH)2, Sr(OH)2, Ba(OH)2

what is usually added to the sample being titrated?

One or two drops of indicator are often added to the sample being titrated.

The more oxygen atoms it contains, the __________ the acidity

higher Weak acids: HNO2, H2SO3 Strong acids: HNO3, H2SO4

Acid Strength of Binary Acid (HA): Down a group (top to bottom) acid strength of HA ________________

increases

Acid Strength of Binary Acid (HA): Across a period (left to right), acid strength of HA ______________

increases

•Ka and Kb are _____________ related

inversely