Question

Classify each of the following as a strong acid, weak acid, strong base, or weak base in aqueous solution. a. \(\mathrm{HNO}_{2}\) b. \(\mathrm{HNO}_{3}\) c. \(\mathrm{CH}_{3} \mathrm{NH}_{2}\) d. \(\mathrm{NaOH}\) e. \(\mathrm{NH}_{3}\) f. HF g. h. \(\mathrm{Ca}(\mathrm{OH})_{2}\) i. \(\mathrm{H}_{2} \mathrm{SO}_{4}\)

Short answer

a. \(\mathrm{HNO}_{2}\): Weak acid b. \(\mathrm{HNO}_{3}\): Strong acid c. \(\mathrm{CH}_{3} \mathrm{NH}_{2}\): Weak base d. \(\mathrm{NaOH}\): Strong base e. \(\mathrm{NH}_{3}\): Weak base f. HF: Weak acid h. \(\mathrm{Ca}(\mathrm{OH})_{2}\): Strong base i. \(\mathrm{H}_{2} \mathrm{SO}_{4}\): Strong acid

Step-by-step solution

a. Classify \(\mathrm{HNO}_{2}\)

Nitrous acid \(\mathrm{(HNO}_{2})\) is a weak acid since it partially dissociates in water. So, we classify it as a weak acid.

b. Classify \(\mathrm{HNO}_{3}\)

Nitric acid \(\mathrm{(HNO}_{3})\) is a strong acid because it completely dissociates in water. We classify it as a strong acid.

c. Classify \(\mathrm{CH}_{3} \mathrm{NH}_{2}\)

Methylamine, \(\mathrm{CH}_{3} \mathrm{NH}_{2}\), is a weak base since it accepts protons from water, doing so to a lesser extent. So, we classify it as a weak base.

d. Classify \(\mathrm{NaOH}\)

Sodium hydroxide \(\mathrm{(NaOH)}\) is a strong base as it completely dissociates in water to form \(\mathrm{Na}^{+}\) and \(\mathrm{OH}^{-}\) ions. We classify it as a strong base.

e. Classify \(\mathrm{NH}_{3}\)

Ammonia, \(\mathrm{NH}_{3}\), is a weak base since it reacts with water to form ammonium ions (\(\mathrm{NH_{4}^{+}}\)) and hydroxide ions (\(\mathrm{OH^{-}}\)) to a limited extent. We classify it as a weak base.

f. Classify HF

Hydrofluoric acid (HF) is a weak acid, as it dissociates partially in water to produce protons (\(\mathrm{H^{+}}\)) and fluoride ions (\(\mathrm{F^{-}}\)). We classify it as a weak acid.

h. Classify \(\mathrm{Ca}(\mathrm{OH})_{2}\)

Calcium hydroxide, \(\mathrm{Ca}(\mathrm{OH})_{2}\), is a strong base because it dissociates completely in water to form \(\mathrm{Ca}^{2+}\) and \(\mathrm{OH}^{-}\) ions. We classify it as a strong base.

i. Classify \(\mathrm{H}_{2} \mathrm{SO}_{4}\)

Sulfuric acid, \(\mathrm{H}_{2} \mathrm{SO}_{4}\), is a strong acid as its first proton dissociates completely in water, forming \(\mathrm{HSO_{4}^{-}}\) and \(\mathrm{H^{+}}\) ions. We classify it as a strong acid.

Key concepts

Strong Acid

When we talk about a substance being a strong acid, we mean it's an acid that completely dissociates in water, releasing all its hydrogen ions into the solution. An excellent example is hydrochloric acid (HCl), which, in water, fully separates into hydrogen (H+) and chloride ions (Cl-).

You can imagine this as if the acid gives up its protons very readily, and it's this trait that makes the solution very acidic. Strong acids are often used in chemical laboratories for titrations, cleaning glasswares, and preparing samples for reactions because of how readily they provide hydrogen ions.

Weak Acid

In contrast, a weak acid, such as acetic acid (CH3COOH), doesn't dissociate completely in water. Instead, it establishes an equilibrium where most of the acid molecules remain intact, and only a small fraction releases protons (H+).

This incomplete release means the solution won't become as acidic compared to a strong acid of the same concentration. Weak acids have gentler reactivity, which can be advantageous in processes where a delicate pH balance is crucial, like fermentation in food production or buffering systems in biology.

Strong Base

A strong base is quite the opposite of an acid. We define it as a substance that completely dissociates in water to release hydroxide ions (OH-), like sodium hydroxide (NaOH).

Because these bases provide a high concentration of hydroxide ions, they drastically increase the pH of the solution, making it very basic or alkaline. Strong bases are often used in soap making, as cleaning agents, and in industries that require the neutralization of acids.

Weak Base

Then we have weak bases like ammonia (NH3), which don't fully dissociate in water. They only partially release their hydroxide ions through the reaction with water, forming ammonium (NH4+) and hydroxide (OH-) ions to a limited degree.

Weak bases are commonly found in household products like cleaning solutions where a less aggressive pH is desired, or in organic synthesis where selectivity and controlled reactivity are required.

Acids and Bases in Aqueous Solution

When we dissolve acids or bases in water, they undergo a process called dissociation, which is crucial for understanding their strength. In water, strong acids and bases split apart completely, while weak ones do not. It's this very property that defines their behavior in chemical reactions, and how they interact with other substances.

Understanding the behavior of acids and bases in aqueous solutions is essential in chemistry, as it helps predict the outcomes of reactions, including those involved in titrations, buffering systems, and chemical equilibrium.

Dissociation in Water

When substances dissociate in water, they break down into ions. For instance, when strong acids dissociate, they release all their hydrogen ions (H+), whereas strong bases release all their hydroxide ions (OH-).

Weak acids and bases, on the other hand, only partially dissociate, setting up a reversible reaction where the undissociated molecules remain in balance with the dissociated ions. The degree of dissociation in water determines many properties of the solution, including its conductivity, pH, and reactivity.