Topic 19: Ionic Equilibria in Aqueous Systems

Question 1

Acid-Base Buffer

Answer 1

Solution that lessens the impact on the pH of the addition of acid/base

Question 2

Components of conjugate acid-base buffer

Answer 2

Conjugate acid-base pair
a) WA + CB
b) WB + CA

Question 3

How does an acid-base buffer neutralize acid/base?

Answer 3

a) A large amount of the acidic component of the buffer consumes small amounts of added OH-
b) A large amount of the basic component A- consumes small amounts of added H3O+

Question 4

Definition of buffer capacity

Answer 4

Measure of the ability to maintain the pH following the addition of strong acid or base

Question 5

Factors affecting buffer capacity

Answer 5

a) Absolute concentration
b) Relative concentration

Question 6

How does absolute concentration affect buffer capacity?

Answer 6

a) The more concentrated the buffer components, the greater the capacity
b) Amounts of the components must be greater than the amount of H3O+ or OH- added

Question 7

How does relative concentration affect buffer capacity?

Answer 7

a) The closer the component concentrations are to each other, the greater the capacity
b) A buffer whose pH is equal to or near the pKa of its acid has the highest capacity for a given concentration

Question 8

Definition of buffer range

Answer 8

pH range over which the buffer is effective

Question 9

What is the usable range of a buffer?

Answer 9

Within ±1 pH unit of the pKa of the acid component

Question 10

How is a buffer prepared?

Answer 10

a) Choose conjugate acid-base pair
b) Calculate the ratio of buffer component concentrations
c) Determine the buffer concentration
d) Mix the solution and correct the pH / Partial neutralization

Question 11

Conditions for common-ion effect

Answer 11

a) An ion is added to an equilibrium mixture that already contains that ion
b) The position of equilibrium shifts away from forming it

Question 12

Consequence of the common ion effect

Answer 12

Common ion A- suppresses the dissociation of HA

Question 13

Henderson-Hasselback Equation

Answer 13

pH=pKa+log⁡([base]/[acid] )

Question 14

Strong Acid + Strong Base Titration Curve (Major species in each step)

Answer 14

a) Initial pH = pH of strong acid
(Cl- / H3O+)
b) pH changes only gradually until equivalence (Cl- / H3O+ / Na+)
c) Very sharp jump in pH at equivalence (6 – 8 units) (Na+ / Cl-)
d) After equivalence the curve flattens out at the pH of the strong base (Na+ / Cl- / OH-)

Question 15

Weak Acid + Strong Base Titration Curve (Major species in each step)

Answer 15

a) Initial pH = pH of weak acid (HPr)
b) pH stays relatively constant until equivalence (Pr- / HPr / Na+)
c) Jump in pH at equivalence (Pr- / Na+)
d) After equivalence, the curve flattens out at the pH of the strong base (Pr- / Na+ / OH-)

Question 16

What is the half-equivalence point?

Answer 16

Position where half of an acid has been neutralized by a base and converted into a salt, while the other half in the flask remains unreacted

Question 17

Strong Acid and Weak Base (Major species in each step)

Answer 17

a) Initial pH = pH of weak base (NH3)
b) pH stays relatively constant through the buffer region to equivalence (NH3 / NH4+ Cl-)
c) Jump in pH at equivalence (Cl- / NH4+)
d) After equivalence the curve flattens out at a pH of weak base (Cl- / NH4+ / H3O+)

Question 18

Strong base and Polyprotic acids (Major species in each step)

Answer 18

a) The same amount of base is required per mole of H+
b) There are two equivalence points and two buffer regions. The pH at the midpoint of each buffer region is equal to the pKa of the acidic species present then
c) The pH of the first equivalence point is below 7.00 due to weak acid HSO3-
d) The pH of the second equivalence point is above 7.00 due to weak base SO3-

Question 19

Definition of an indicator

Answer 19

Weak acid or a weak base where the components of the conjugate acid-base pair have different colors

Question 20

Formula that explain the function of an indicator

Answer 20

Equilibrium response to a change in the pH of the medium
HIn(aq) ⇌ H+(aq) +In-(aq)

Question 21

When does an indicator change color (end point)?

Answer 21

When the pH is equal to their pKa

Question 22

When is an indicator effective at signaling the equivalence point?

Answer 22

When its end-point (pH at which it changes color) coincides with the pH at the equivalence poin

Question 23

What is the pH range of an indicator?

Answer 23

pH values between which the indicator has intermediate colors
a) +- 1 pH on either side of the pKa

Question 24

Solubility-product constant (Ksp)

Answer 24

Equilibrium value between solid solute and aqueous ions at small amounts
a) How far the dissolution occurs?
b) Ksp=[Pb2+][F-]^2

Question 25

Effect of a common ion on solubility

Answer 25

Adding a common ion decreases the solubility of a slightly soluble ionic compound

Question 26

Assumption of Ksp

Answer 26

Small dissolved amount of compound dissociates completely into separate ions

Question 27

Effect of acidity on solubility

Answer 27

If a slightly soluble ionic compounds contains the anion of a weak/strong acid, addition of H3O+ increases/does not change the compound’s solubility

Question 28

What would happen if… ?
a) Qsp = Ksp
b) Qsp < Ksp
c) Qsp > Ksp

Answer 28

a) No change
b) No precipitate will form
c) Precipitate will form

Question 29

Explanation of selective precipitation

Answer 29

A solution of one precipitating ion is added to a solution of two ionic compounds until the Qsp of the more soluble compound is almost equal to its K_sp

Question 30

Definition of complex ions

Answer 30

Adduct of a central metal ion covalently bonded to two or more ligands (anions/molecules)

Question 31

Formation of a complex ions

Answer 31

When a salt dissolves in water, a complex ion forms with water as ligands around the metal ion

Question 32

To what extent complex ions form from its hydrated ion?

Answer 32

Complex ions form readily from the hydrated ion

Question 33

Relationship between complex ion and solubility of precipitates

Answer 33

A ligand increases the solubility of a slightly soluble ionic compound if it forms a complex ion with the compound’s cation.

Question 34

Dissolution of complex ions of amphoteric hydroxides

Answer 34

Dissolve very little in water but to a much greater extent in both acidic and basic solution
a) Reaction of H3O+ with OH- anion
b) Formation of the complex ion with 4OH-. Insoluble hydroxide might form, which is further dissolved by addition of OH-