Question

Classify each of these species as a weak or strong acid: (a) \(\mathrm{HNO}_{3}\) (b) HF, (c) \(\mathrm{H}_{2} \mathrm{SO}_{4}\) (d) \(\mathrm{HSO}_{4}^{-}\) (e) \(\mathrm{H}_{2} \mathrm{CO}_{3},\) (f) \(\mathrm{HCO}_{3}^{-},(\mathrm{g}) \mathrm{HCl}\) (h) HCN, (i) \(\mathrm{HNO}_{2}\)

Short answer

(a) \(\mathrm{HNO}_{3}\) is a strong acid. (b) HF is a weak acid. (c) \(\mathrm{H}_{2} \mathrm{SO}_{4}\) is a strong acid. (d) \(\mathrm{HSO}_{4}^{-}\) is a weak acid. (e) \(\mathrm{H}_{2} \mathrm{CO}_{3}\) is a weak acid. (f) \(\mathrm{HCO}_{3}^{-}\) is a weak acid. (g) \(\mathrm{HCl}\) is a strong acid. (h) HCN is a weak acid. (i) \(\mathrm{HNO}_{2}\) is a weak acid.

Step-by-step solution

Classify HNO3

Consider \(\mathrm{HNO}_{3}\), nitric acid. Nitric acid is listed as one of the most common strong acids. Therefore, \(\mathrm{HNO}_{3}\) is a strong acid.

Classify HF

Hydrofluoric acid (HF) is an exception to the general idea that all halogen acids are strong. It is known as a weak acid as it does not entirely dissociate in solution.

Classify H2SO4

Consider \(\mathrm{H}_{2} \mathrm{SO}_{4}\), sulfuric acid. Sulfuric acid is also listed as one of the most common strong acids. So, \(\mathrm{H}_{2} \mathrm{SO}_{4}\) is a strong acid.

Classify HSO4-

Now consider \(\mathrm{HSO}_{4}^{-}\). Sulfuric acid (\(\mathrm{H}_{2} \mathrm{SO}_{4}\)) is a strong acid. However, its conjugate base \(\mathrm{HSO}_{4}^{-}\) is a weak acid.

Classify H2CO3

The compound \(\mathrm{H}_{2} \mathrm{CO}_{3}\), carbonic acid, is not listed as one of the common strong acids, indicating that it is a weak acid.

Classify HCO3-

Next, the ion \(\mathrm{HCO}_{3}^{-}\) is the conjugate base of the weak acid \(\mathrm{H}_{2} \mathrm{CO}_{3}\), so \(\mathrm{HCO}_{3}^{-}\) is a weak acid itself.

Classify HCl

\(\mathrm{HCl}\), hydrochloric acid, is one of the most common strong acids. Hence, \(\mathrm{HCl}\) is a strong acid.

Classify HCN

Hydrogen cyanide (HCN) is weak acid as it does not completely dissociate in solution.

Classify HNO2

The compound \(\mathrm{HNO}_{2}\), nitrous acid, is not listed in the list of common strong acids, so it is a weak acid.

Key concepts

Weak Acids

Weak acids are characterized by their partial ionization in water. Unlike strong acids which disassociate completely, weak acids remain largely intact with only a small percentage of their molecules releasing hydrogen ions (H+) into solution. An example of a weak acid from the exercise is hydrofluoric acid (HF). Its weak acidic property is attributed to the strong bond between hydrogen and fluorine, which does not break easily. Thus, in aqueous solution, HF does not release many H+ ions, resulting in a relatively higher pH compared to strong acids. Other weak acids include carbonic acid (H2CO3) and hydrogen cyanide (HCN), both of which ionize incompletely, leaving many undissociated molecules in solution.

In educational terms, envision weak acids as reluctant participants in a dance, releasing their hydrogen partners into the 'solution dance floor' only sparingly.

Strong Acids

In contrast to weak acids, strong acids such as (HCl) and sulfuric acid (H2SO4) ionize completely when dissolved in water. The complete disassociation means that every molecule of a strong acid contributes one or more H+ ions to the solution. This complete dissociation makes the solutions of strong acids highly conductive and results in a low pH.

A useful analogy for students is to think of strong acids as eager confetti throwers at a parade, tossing every bit of confetti (Hydrogen ions) they have into the crowd (the solution). This maximizes the concentration of H+ ions, which is indicative of the acid's strength.

Acid Dissociation

The process of acid dissociation is the splitting of an acid in water to form H+ and its corresponding conjugate base. The degree to which an acid dissociates is measured by its acid dissociation constant (Ka), which varies significantly among different acids. Generally, a large (Ka) value points to a strong acid, as it shows a greater propensity for releasing hydrogen ions.

During instruction, educators can liken the acid dissociation process to a play where characters (acid molecules) take off their 'hydrogen costumes' to reveal their underlying personas (conjugate bases). A play where nearly all actors shed their costumes relates to a strong acid, while a performance with few changes correlates to a weak acid.

Conjugate Bases

A conjugate base is what remains of an acid molecule after it has donated a H+ ion. Importantly, the strength of a conjugate base is inversely related to the strength of its parent acid. Strong acids have weak conjugate bases because the parent acid gives up its hydrogen ion so readily that the base has little tendency to re-claim the H+ ion. On the other hand, the conjugate bases of weak acids are stronger because their parent acids hold onto their hydrogen ions more tightly, and the bases are more inclined to accept H+ when available.

In our metaphorical classroom, conjugate bases are the remnants of the 'dance of dissociation,' left on the sidelines after the hydrogen ions have joined the solution's dance. Strong acids leave behind wallflower conjugate bases, while weak acids have bases that are more likely to join the dance if given a chance.