Question

Write the formula of the conjugate acid of each base. (a) \(\mathrm{NH}_{3}\) (b) \(\mathrm{F}^{-}\) (c) \(\mathrm{CO}_{3}{ }^{2-}\)

Short answer

The conjugate acids for the bases \(\mathrm{NH}_{3}\), \(\mathrm{F}^{-}\) and \(\mathrm{CO}_{3}{ }^{2-}\) are \(\mathrm{NH}_{4}^{+}\), \(\mathrm{HF}\) and \(\mathrm{HCO}_{3}^{-}\), respectively.

Step-by-step solution

Find the conjugate acid of \(\mathrm{NH}_{3}\)

Adding \(\mathrm{H}^{+}\) to the base \(\mathrm{NH}_{3}\) results in the conjugate acid \(\mathrm{NH}_{4}^{+}\).

Find the conjugate acid of \(\mathrm{F}^{-}\)

Adding \(\mathrm{H}^{+}\) to the base \(\mathrm{F}^{-}\) results in the conjugate acid \(\mathrm{HF}\).

Find the conjugate acid of \(\mathrm{CO}_{3}{ }^{2-}\)

Adding \(\mathrm{H}^{+}\) to the base \(\mathrm{CO}_{3}{ }^{2-}\) results in the conjugate acid \(\mathrm{HCO}_{3}^{-}\).

Key concepts

Acid-Base Reactions

Understanding acid-base reactions is fundamental in chemistry, as they are prevalent in various chemical processes and biological systems. An acid-base reaction involves the transfer of a proton (hydrogen ion, \( \mathrm{H}^{+} \) from an acid to a base. According to the Arrhenius definition, acids are substances that increase the concentration of \( \mathrm{H}^{+} \) ions in solution, while bases are substances that increase the concentration of hydroxide ions \( \mathrm{OH}^{-} \). However, the Brønsted-Lowry theory provides a broader definition, where acids are proton donors and bases are proton acceptors, regardless of the solvent.

For instance, in the exercise provided, when ammonia \( \mathrm{NH}_{3} \) acts as a base, it accepts a proton to become the conjugate acid \( \mathrm{NH}_{4}^{+} \). The idea of conjugates is crucial as it helps us understand how substances can behave differently under different circumstances, such as \( \mathrm{F}^{-} \) gaining a proton to become hydrofluoric acid \( \mathrm{HF} \) or carbonate ion \( \mathrm{CO}_{3}^{2-} \) accepting a proton to form bicarbonate \( \mathrm{HCO}_{3}^{-} \). These reactions are reversible, and the direction of the equilibrium can be influenced by the acid and base strength and the reaction conditions.

Brønsted-Lowry Acids and Bases

The Brønsted-Lowry theory extends beyond Arrhenius's concept by not limiting acids and bases to aqueous solutions. A Brønsted-Lowry acid is any species capable of donating a proton, while a Brønsted-Lowry base is any species capable of accepting a proton. This theory introduces the concept of conjugate acid-base pairs; when an acid donates a proton, it becomes a conjugate base, and when a base accepts a proton, it becomes a conjugate acid.

The transformation involves a simple exchange, as seen in the textbook exercise. For example, \( \mathrm{NH}_{3} \) is a base that accepts a proton to become \( \mathrm{NH}_{4}^{+} \), its conjugate acid. Conversely, any conjugate base, such as \( \mathrm{HF} \) losing a proton, becomes the conjugate base \( \mathrm{F}^{-} \). The strength of an acid or base is often related to the stability of their conjugate counterparts. Strong acids have weak conjugate bases, and strong bases have weak conjugate acids, implying that strong acids and bases have a higher tendency to donate or accept protons, respectively.

Chemical Formulas

Chemical formulas represent molecules and compounds using symbols and subscripts. These symbols, from the periodic table, denote the types of atoms present, while subscripts indicate the quantity of each type of atom in the molecule or compound. Understanding chemical formulas is essential because they convey substantial information about the composition and potential reactions of substances.

In the context of acid-base chemistry, we use chemical formulas to communicate the structure of acids, bases, and their conjugates. Take \( \mathrm{CO}_{3}^{2-} \) for example; this formula shows that the carbonate ion consists of one carbon atom and three oxygen atoms, carrying an overall charge of -2. When \( \mathrm{CO}_{3}^{2-} \) accepts a proton, the chemical formula changes to \( \mathrm{HCO}_{3}^{-} \), representing the bicarbonate ion. This change in formula indicates a change in both the composition and the charge as a result of the acid-base reaction. Such precise chemical notation allows chemists to predict product formation, reaction conditions, and understand the nature of chemical species in various contexts.