Ch 17: Acids and Bases

Question 1

A strong acid implies ______ ________ between the H⁺ and A^-, resulting in complete ionization.

A weak acid implies a ______ attraction between the H⁺ and A^-, resulting in only partial ionization.

Answer 1

weak attraction

strong

Question 2

What is meant by an “amphoteric species”?

Answer 2

That it can act either as an acid or a base.

p. 740

Question 3

We define pH as…

And pOH as….

Answer 3

…the negative of the logarithm of the hydronium ion concentration:

pH = -log[H₃O⁺]

pOH = -log[OH^-]

p. 742-743

Question 4

Since the pH scale is based on logarithmic function, an increase of 1 on the pH scale corresponds to….

Answer 4

…..a factor of 10 decrease in [H₃O⁺].

p. 743

Question 5

The log of a number is….

Answer 5

…the exponent to which 10 must be raised to obtain that number.

log 10¹ = 1

log 10^-1 = -1

Question 6

It is possible for concentrated acid solutions to have a ________ pH.

Answer 6

negative

Question 7

We can use the dissociation constant for water (K_W; also called the ion product constant for water) to find the [OH^-] concentration when we are given [H₃O⁺] (or vice versa) since this mathematical relationship exists:

Answer 7

K_W = [OH^-] [H₃O⁺] = 1.0 x 10^-14

p. 741

Question 8

What is the mathematical relationship between pH and pOH?

Answer 8

pH + pOH = 14.00

p. 744

Question 9

How can we calculate the pK_a from the K_a value? (or vice versa)

Answer 9

pK_a = -log [K_a]

Question 10

Because strong acids, by definition, completely ionize in solution and because we can usually ignore the contribution of the autoionization of water, the concentration of H₃O⁺ is a strong acid solution is _____ to the ____________ of the strong acid.

For example, a 0.10 M HCl solution has an [H₃O⁺] concentration of 0.10 M and a pH of 1.00

0.10 M HCl → [H₃O⁺] → pH = -log(0.10) = 1.00

Answer 10

equal to the concentration

Question 11

We can define the percent ionization of a weak acid using the equation…

Answer 11

Percent ionization = (concentration of ionized acid/ initial concentration of acid) x 100% = [H₃O⁺] _equil / [HA]_initial x 100%

Question 12

The conjugate acid is a base that has….

Answer 12

….accepted a H⁺ ion.

Question 13

In general, anions tend to form either ______ or neutral solutions, while cations tend to form either ______ or neutral solutions.

Answer 13

basic

acidic

cations - acidic or neutral

anions – basic or neutral

Remember there can be only one a in each pair, so cations must be acidic, while anions must be basic.

Question 14

An anion that is the conjugate base of a weak acid is itself, a ____ ____.

Answer 14

weak base

Question 15

An anion that is the conjugate base of a strong acid, is _________, and forms solutions that are neither acidic nor basic.

Answer 15

pH-neutral

Question 16

In general, the weaker the acid, the stronger the conjugate base. In contrast, the conjugate base of a strong acid, such as Cl−, does not act as a base because this reaction lies far to the _____. The stronger an acid, the weaker its _________ ____.

Answer 16

right

conjugate base

Question 17

Is NO₃^- a weak base or pH neutral?

Answer 17

Since this is the conjugate base of strong acid, (nitric acid), it is therefore, pH-neutral.

Question 18

Is NO₂^- a weak base or pH neutral?

Answer 18

Since Is NO₂^- is the conjugate base of a weak acid (nitrous acid), it is a weak base.

Question 19

What is the mathematical relationship between K_a and K_b?

What about in terms of using the log function?

Answer 19

K_a x K_b = K_w = 1.0 x 10^-14

log (K_a x K_b) = log K_w

log K_a + log K_b = log K_w

log K_a + log K_b = log 10^-14 = -14

-log K_a - log K_b = 14

Since -log K = pK → pK_a + pK_b = 14

Question 20

In general, cations that are the counterions of _____ _____ are themselves pH-neutral (they form solutions that are neither acidic nor basic). For example, Na+, K+, and Ca2+ are the counterions of the strong bases NaOH, KOH, and Ca(OH)2 and are therefore themselves pH-neutral.

Although these counterions interact with water molecules via ion–dipole forces, they do not ionize water and they do not contribute to the acidity or basicity of the solution.

Answer 20

strong bases

Question 21

In general, a cation that is the conjugate acid of a weak base is a ____ ____.

Answer 21

weak acid

Question 22

Small, highly charged metal cations such as Al³⁺ and Fe³⁺ form weakly ______ solutions. For example, when Al³⁺ is dissolved in water, it becomes hydrated according to the equation:

In effect, the binding of a water molecule to a cation like this makes the water more acidic.

Neither the alkali metal cations nor the alkaline earth metal cations ionize water in this way, but the cations of many other metals do. The smaller and more ______ ________ the cation, the more acidic its behavior.

Answer 22

acidic

highly charged

Question 23

Is the cation Cr³⁺ a weak acid, or pH-neutral?

Answer 23

Cr³⁺ is a small, highly charged, metal cation, and is therefore, a weak acid.

Question 24

Salts in which neither the cation nor the anion acts as an acid or a base form pH-neutral solutions. A salt in which the cation is the counterion of a strong base and in which the anion is the conjugate base of strong acid forms a _______ ________.

Answer 24

neutral solution

Examples include NaCl, Ca(NO₃)₂, and KBr.

Question 25

Salts in which the cation does not act as an acid and the anion acts as a base form _____ \_\_\_\_\_\_\_\_. A salt in which the cation is the counterion of a strong base and in which the anion is the conjugate base of a weak acid forms a basic solution.

Answer 25

basic solutions Examples include: NaF, Ca(C₂H₃O₂)₂ (calcium acetate), and KNO₂ (potassium nitrite)

Question 26

**Salts in which the cation acts as an acid and the anion does not act as a base form _____ solutions.** A salt in which the cation is either the conjugate acid of a weak base or a small, highly charged metal ion and in which the anion is the conjugate base of *strong* acid forms an *acidic* solution.

Answer 26

**acidic**

Question 27

**Salts in which the cation acts as an acid and the anion acts as a base form solutions in which the pH depends on the relative strengths of the acid and the base.** A salt in which the cation is either the conjugate acid of a weak base or a small, highly charged metal ion and in which the anion is the conjugate base of a *weak* acid forms a solution in which the pH depends on the relative strengths of the acid and base.

Question 28

Question 29

The neutral cations are the conjugate acids of the…

Answer 29

….strong bases.

Question 30

The neutral anions are the conjugate bases of the…

Answer 30

….7 strong acids.

Question 31

K_a2 is always smaller than K_a1 because…

Answer 31

….the first proton separates more easily from a neutral molecule, but the second one separates from an anion. The negatively charged anion, holds the positively charged proton more tightly, making it difficult to remove, and resulting in a smaller value of K_a.

Question 32

We can treat most polyprotic acid solutions as if the first ionization was the only one that contributes to the H₃O⁺ concentration. A major exception is a dilute solution of ________ acid. See Example 17.18.

Answer 32

sulfuric acid

Question 33

Why is does HF have an “acidic proton”, while LiH does not?

Answer 33

LiH is an ionic compound, with the negative charge on the H atom, therefore, it cannot be acidic. In contrast, the H-F bond is polar with the positive charge in the H atom. This is what makes HF an acid. That partial positive charge makes it easier for the H to be lost as an H+ ion.

Question 34

What is the trend in bond strength and polarity of the hydrides of group 6A and 7A?

Answer 34

2 different reasons: from Left to RIGHT, they become more acidic because the H-Y bond becomes more polar. From top to BOTTOM, they become more acidic because the H-Y bond becomes weaker.

Question 35

Which acid has the largest K_a? HClO₂ HBrO₂ HIO₂ For oxyacids, acid strength __________ with increasing electronegativity of the atom bonded to the oxygen atoms.

Answer 35

HClO₂ INCREASES

Question 36

The weaker the acid (the smaller the K_a), the ________ the conjugate base.

Answer 36

stronger

Question 37

When Mn³⁺ is dissolved in water, it becomes hydrated. The ________ form of the ion then acts as a Brønsted-Lowry acid (i.e., it donates a proton to water): Mn(H₂O)₆³⁺(aq) + H₂O(l) ⇌ Mn(H₂O)₅(OH)₂⁺(aq) + H₃O⁺(aq) You may also show the reaction without ligands as: Mn³⁺(aq) + 2 H₂O (l) ⇌ MnOH²⁺(aq) + H₃O⁺(aq)

Answer 37

hydrated

Question 38

What is the Arrhenius definition of an acid/base?

Answer 38

An acid is a substance that produces H⁺ ions in an aqueous solution. A base is a substance that produces OH⁻ ions in aqueous solution. p. 735

Question 39

What are the Bronsted-Lowry definitions of an acid or a base?

Answer 39

An acid is a proton (H⁺) donor. A base is a proton acceptor.

Question 40

How would you find the pH of a solution that is 4.8×10⁻² M HClO₄, and 4.5×10⁻² M in HCl?

Answer 40

Both are strong acids, which dissociate completely in solution. So, the total concentration of H3O+ is the sum of H3O+ concentration of each acid: pH = −log ([HI]+[HClO₄]) = −log (4.8×10⁻² + 4.5×10⁻²) = 1.03