16.2.6 Trends in Acid and Base Strengths

Question 1

Trends in Acid and Base Strengths

Answer 1

• Factors that influence acidity include electronegativity, electron affinity (EA), bond strength, and the
acid-dissociation constant (K a ).
• The entropy of hydration is a measure of the change in disorder when a substance is dissolved in water.
• Adding oxygen stabilizes the negative charge on a conjugate base by inductive effects and resonance effects.
• The oxoacids of the elements in their highest oxidation states demonstrate trends in acidity and basicity.

Question 2

note

Answer 2

• Factors that influence acidity include electronegativity, electron affinity (EA), bond strength, and the acid dissociation constant (K a ).
• Compounds with hydrogen bonded to second-period elements increase in acidity as the electron affinity of the element increases. This trend is not reproduced in the halogen column where chlorine forms the most acidic compound with hydrogen. The trend in the halogens is explained by bond strength and the acid-dissociation constant.
• The acid-dissociation constant reveals a clear discontinuity between the strong acids (with K a&raquo_space;1) and hydrofluoric acid, a weak acid (K a < 1). This discontinuity can be explained by a trend in entropies of hydration.
• The entropy of hydration is a measure of the change in disorder when a substance is dissolved in water.
• The fluoride ion is a more compact ion than chloride ion or the larger halides and is therefore able to better organize the water hydrating it. This accounts for the dramatic change in ionizing ability of hydrofluoric acid compared to the other binary acids.

Question 3

note 2

Answer 3

• Adding oxygen stabilizes the negative charge on a conjugate base by inductive effects and resonance effects.
• The inductive effect is demonstrated by comparing the nitrite and nitrate ions produced from nitrous and nitric acid. The additional, highly electronegative, oxygen atom allows the nitrate ion to better stabilize the negative charge of the ion. This means that the proton in nitric acid is better able to dissociate from the nitrate ion and therefore nitric acid is a stronger acid.
• The resonance effect is demonstrated in a comparison between sulfurous acid and sulfuric acid. The bisulfate ion formed by the dissociation of sulfuric acid has three resonance structures because of its additional oxygen atom. This delocalizes the formal charge on the oxygen atoms, thus stabilizing the ion relative to bisulfite. The more stable anion means that the conjugate acid is a stronger acid.
• The oxoacids of elements in their highest oxidation state demonstrate trends in acidity and basicity.
• Elements to the left of the periodic table are more basic
  because O 2– is a Brønsted-Lowry base (it accepts a proton from water, forming hydroxide ion (OH–)). Elements to the right of the periodic table are more acidic because they react with water like a Lewis acid (accepting an electron pair) forming a Brønsted-Lewis acid (capable of donating a proton). An example of the latter is carbonic acid (H 2 CO 3 ), formed when carbon dioxide (CO 2 ) reacts with water.
• Increasing basicity down the groups is due to decreasing electronegativity of the element, causing the oxygen atoms to function more as bases (accepting a proton).

Question 4

What is the definition of a Lewis acid?

Answer 4

Any substance that can accept an electron pair

Question 5

How does bond polarity affect the relative strength of an acid?

Answer 5

Increasing bond polarity increases relative acid strength

Question 6

Which of the following acids is the strongest?

Answer 6

HI

Question 7

Which of the following is not a Brønsted-Lowry acid?

Answer 7

LiH

Question 8

What is a Brønsted-Lowry acid?

Answer 8

Any substance that can donate an H + ion

Question 9

What is a metal hydride?

Answer 9

Hydrogen bonded to a less electronegative metal

Question 10

Why is HI a stronger acid than HF?

Answer 10

The bond strength of HF is greater than the bond strength of HI.

Question 11

Which of the following acids is the strongest?

Answer 11

HClO4

Question 12

What is a strong acid?

Answer 12

A substance that produces hydronium ions and completely dissociates.