Question

Classify each of these as a strong acid, weak acid, strong base, weak base, amphiprotic substance, or neither acid nor base. (a) \(\mathrm{HCl}\) (b) \(\mathrm{NH}_{4}^{+}\) (c) \(\mathrm{H}_{2} \mathrm{O}\) (d) \(\mathrm{CH}_{3} \mathrm{COO}^{-}\) (e) \(\mathrm{CH}_{4}\) (f) \(\mathrm{CO}_{3}^{2-}\)

Short answer

(a) Strong acid, (b) Weak acid, (c) Amphiprotic, (d) Weak base, (e) Neither, (f) Weak base.

Step-by-step solution

Classify HCl

Hydrochloric acid (\(\mathrm{HCl}\)) is a well-known strong acid. It completely dissociates in water, producing \(\mathrm{H^+}\) ions.

Classify NH4+

The ammonium ion (\(\mathrm{NH_4^+}\)) is a weak acid. It can donate a proton to form \(\mathrm{NH_3}\).

Classify H2O

Water (\(\mathrm{H_2O}\)) is an amphiprotic substance. It can act as either an acid or a base, donating or accepting a proton.

Classify CH3COO−

The acetate ion (\(\mathrm{CH_3COO^-}\)) is a weak base. It can accept a proton to form \(\mathrm{CH_3COOH}\).

Classify CH4

Methane (\(\mathrm{CH_4}\)) is neither an acid nor a base. It does not donate or accept protons.

Classify CO3^2−

The carbonate ion (\(\mathrm{CO_3^{2-}}\)) is a weak base. It can accept a proton to form \(\mathrm{HCO_3^-}\).

Key concepts

Strong Acids

Strong acids are substances that completely dissociate into ions when dissolved in water. This means they break apart completely, contributing to the concentration of hydrogen ions, resulting in a low pH. Because they dissociate completely, they are very efficient at conducting electricity in their aqueous solutions.

Common examples of strong acids include:

• Hydrochloric acid (\( \mathrm{HCl} \)), known for its role in stomach acid and industrial applications.
• Sulfuric acid (\( \mathrm{H_2SO_4} \)), frequently used in battery acid.
• Nitric acid (\( \mathrm{HNO_3} \)), often used in fertilizers and explosives.

Key Point: Remember that strong acids dissociate completely, unlike weak acids, and have a very low pH level.

Weak Acids

Weak acids only partially dissociate in water. This partial dissociation means they release fewer hydrogen ions compared to strong acids, leading to a higher pH than strong acids.

Examples of weak acids are:

• Acetic acid (\( \mathrm{CH_3COOH} \)), commonly found in vinegar.
• Formic acid (\( \mathrm{HCOOH} \)), which you might find in some ant venoms.
• Ammonium ion (\( \mathrm{NH_4^+} \)), which is part of the buffer system in biological organisms.

Since weak acids do not fully dissociate, they are poor conductors of electricity in their solutions.
Tip: Weak acids often establish an equilibrium between the undissociated and dissociated forms in water.

Strong Bases

Strong bases are characterized by their ability to completely dissociate in solution, releasing hydroxide ions (\( \mathrm{OH^-} \)). As with strong acids, their complete dissociation provides solutions that are good conductors of electricity, with a high pH value.

Typical examples include:

• Sodium hydroxide (\( \mathrm{NaOH} \)), commonly known as lye or caustic soda.
• Potassium hydroxide (\( \mathrm{KOH} \)), often used in soap making.

Focus Point: If a base completely dissociates and offers a high concentration of \( \mathrm{OH^-} \) ions, it's strong. This makes strong bases effective for neutralizing acids and for roles in industrial processes.

Weak Bases

Weak bases are substances that only partially accept protons or release hydroxide ions in solution. This partial ionization results in lower concentrations of hydroxide ions compared to strong bases, hence their solutions are less conductive.

Some common weak bases are:

• Ammonia (\( \mathrm{NH_3} \)), often used in cleaning agents and fertilizers.
• Acetate ion (\( \mathrm{CH_3COO^-} \)), a product of vinegar neutralization.
• Carbonate ion (\( \mathrm{CO_3^{2-}} \)), found in limestone and marble.

Remember: Always consider the equilibria involved in a solution containing weak bases, as this affects their behavior in chemical reactions.

Amphiprotic Substances

Amphiprotic substances have the unique ability to act either as an acid or a base, depending on the circumstances. This dual characteristic is quite special and allows these substances to participate in a variety of chemical reactions.

A classic example is water (\( \mathrm{H_2O} \)):

• As an acid, it can donate a proton to become hydroxide (\( \mathrm{OH^-} \)).
• As a base, it can accept a proton to become hydronium (\( \mathrm{H_3O^+} \)).

Other amphiprotic substances include bicarbonate (\( \mathrm{HCO_3^-} \)) and dihydrogen phosphate (\( \mathrm{H_2PO_4^-} \)).
Helpful Hint: The amphiprotic nature allows these substances to help maintain pH balance in solutions, acting buffer roles in biological systems.