Question

Which compound, \(\mathrm{NaHCO}_{3}\) or \(\mathrm{NaNO}_{3}\), is expected to behave as a weak base? Why?

Short answer

The compound \(\mathrm{NaNO}_{3}\) is expected to behave as a weak base. This is because its conjugate acid, \(\mathrm{HNO}_{3}\), is a strong acid.

Step-by-step solution

Identify the Conjugate Acids

First of all, remove a proton (\(H^+\)) from each compound to determine their conjugate acids. The conjugate acid of \(\mathrm{NaHCO}_{3}\) is \(\mathrm{H}_{2}\mathrm{CO}_{3}\) (carbonic acid), and the conjugate acid of \(\mathrm{NaNO}_{3}\) is \(\mathrm{HNO}_{3}\) (nitric acid).

Analyze the Strengths of the Conjugate Acids

Bases and their conjugate acids have a reciprocal relationship in terms of their strength. A strong conjugate acid corresponds to a weak base, while a weak conjugate acid corresponds to a strong base. Through referencing standard tables for acid strength (often given by \(pK_a\)) or using prior knowledge, it is known that \(\mathrm{HNO}_{3}\) is a strong acid while \(\mathrm{H}_{2}\mathrm{CO}_{3}\) is a weak acid.

Determine the Strength of the Bases from Their Conjugate Acids

Using the fact that a strong acid gives a weak base and a weak acid gives a strong base, it can be seen that \(\mathrm{NaHCO}_{3}\) is a stronger base than \(\mathrm{NaNO}_{3}\) because its conjugate acid (\(\mathrm{H}_{2}\mathrm{CO}_{3}\)) is weaker than that of \(\mathrm{NaNO}_{3}\). Thus, \(\mathrm{NaNO}_{3}\) is expected to behave as a weak base.

Key concepts

Conjugate Acid

When a base accepts a proton (H^+), it forms a conjugate acid. This means the conjugate acid of a base contains one more proton. For example, when \(\text{NaHCO}_3\) loses a proton, it forms \(H_2CO_3\), which is carbonic acid. Similarly, when \(\text{NaNO}_3\) loses a proton, it forms \(HNO_3\), nitric acid. Understanding conjugate acids helps us evaluate the behavior of a base. By examining the strength of a conjugate acid, we can determine how strong or weak the original base is. A stronger conjugate acid suggests a weaker corresponding base, and vice versa.

Acid Strength

Acid strength refers to the tendency of an acid to donate a proton. This is commonly measured through the \(pK_a\) value, where a lower \(pK_a\) indicates a stronger acid. \(\text{HNO}_3\), with its low \(pK_a\), is classified as a strong acid. It readily donates protons in solution. On the other hand, \(\text{H}_2CO_3\), has a higher \(pK_a\), making it a weaker acid.

• Strong acids dissociate completely in water.
• Weak acids only partially dissociate.

By identifying the acid strength, we gain insights into the properties and potential reactivity of related bases.

Base Strength

Base strength is the tendency of a base to accept protons. This is influenced by the strength of its conjugate acid. The weaker the conjugate acid, the stronger the base, because it has more capacity to accept protons. Consider \(\text{NaHCO}_3\) and \(\text{NaNO}_3\). Since \(\text{H}_2CO_3\) is a weak acid, \(\text{NaHCO}_3\) acts as a comparatively stronger base. Conversely, strong acids like \(\text{HNO}_3\) have weak conjugate bases, making \(\text{NaNO}_3\) a weak base.

• Strong base: readily accepts protons.
• Weak base: does not readily accept protons.

Understanding base strength helps predict how likely a base is to engage in chemical reactions.