Question

Of the following acids, which have relatively strong conjugate bases? a. \(\mathrm{HNO}_{2}\) b. HCOOH c. \(\mathrm{HClO}_{4}\) d. \(\mathrm{HNO}_{3}\)

Short answer

The acids with relatively strong conjugate bases are a. \(\mathrm{HNO}_{2}\) (nitrous acid) and b. HCOOH (formic acid).

Step-by-step solution

Determine if the given acids are strong or weak

To determine if the acids are strong or weak, we must check the dissociation constant values (Ka). Typically, strong acids have larger Ka values while weak acids have smaller Ka values. For the given acids, we have: - \(\mathrm{HNO}_{2}\) (nitrous acid): Ka \(\approx 4.5 \times 10^{-4}\) (weak) - HCOOH (formic acid): Ka \(\approx 1.8 \times 10^{-4}\) (weak) - \(\mathrm{HClO}_{4}\) (perchloric acid): Ka \(\approx 10^{10}\) (strong) - \(\mathrm{HNO}_{3}\) (nitric acid): Ka \(\approx 10^{1}\) (strong)

Identify the conjugate bases of the given acids

The conjugate base of an acid is formed when the acid loses a proton (H+ ion). Therefore, the conjugate bases of the given acids are: - \(\mathrm{HNO}_{2}\) (nitrous acid): Conjugate base is \(\mathrm{NO}_{2}^{-}\) - HCOOH (formic acid): Conjugate base is HCOO- - \(\mathrm{HClO}_{4}\) (perchloric acid): Conjugate base is \(\mathrm{ClO}_{4}^{-}\) - \(\mathrm{HNO}_{3}\) (nitric acid): Conjugate base is \(\mathrm{NO}_{3}^{-}\)

Determine the strength of the conjugate bases

Since strong acids have weak conjugate bases and weak acids have strong conjugate bases, we can now determine the strength of the conjugate bases: - \(\mathrm{HNO}_{2}\): Weak acid, strong conjugate base (\(\mathrm{NO}_{2}^{-}\)) - HCOOH: Weak acid, strong conjugate base (HCOO-) - \(\mathrm{HClO}_{4}\): Strong acid, weak conjugate base (\(\mathrm{ClO}_{4}^{-}\)) - \(\mathrm{HNO}_{3}\): Strong acid, weak conjugate base (\(\mathrm{NO}_{3}^{-}\)) Based on this information, the acids with relatively strong conjugate bases are: a. \(\mathrm{HNO}_{2}\) b. HCOOH

Key concepts

Conjugate Bases

In acid-base chemistry, a **conjugate base** is what an acid becomes after it donates a proton (H⁺ ion). When an acid loses a proton, the remaining part is its conjugate base. This concept is crucial for understanding the behavior and strength of acids and bases in chemical reactions.

When thinking about conjugate bases, keep these points in mind:

• Each acid has a corresponding conjugate base. For example, when nitrous acid ( \(\mathrm{HNO}_{2}\)) loses a proton, the conjugate base \(\mathrm{NO}_{2}^{-}\) is formed.
• As a general rule, the weaker the acid, the stronger its conjugate base. This is because if the acid doesn’t fully release its protons, the conjugate base is relatively stronger in pulling them back.
• Conversely, strong acids like perchloric acid (\(\mathrm{HClO}_{4}\)) tend to have weak conjugate bases because the acid fully releases its protons.

Understanding the nature of conjugate bases is essential to predicting how substances will interact chemically.

Acid Strength

The **strength of an acid** refers to its ability to donate protons in a solution. Strong acids release most of their protons, resulting in high acidity. Meanwhile, weak acids only release some of their protons. This is a critical factor in reactions involving acids, as it influences their reactivity and the strength of their conjugate bases.

Here are some essential features of acid strength:

• Acid strength is often indicated by the value of the dissociation constant \((K_a)\). Strong acids have a high \(K_a\), while weak acids have a low \(K_a\).
• An example of a strong acid is nitric acid (\(\mathrm{HNO}_{3}\)) with \(K_a \approx 10^{1}\), which means it dissociates almost entirely in water.
• Weak acids such as formic acid (HCOOH) have a smaller \(K_a \approx 1.8 \times 10^{-4}\), indicating less dissociation.
• Remember, weaker acids produce stronger conjugate bases, just as \(\mathrm{HNO}_{2}\) forms the strong conjugate base \(\mathrm{NO}_{2}^{-}\).

By analyzing the acid strength, chemists can predict how readily theacid will participate in reactions and its general reactivity.

Dissociation Constant (Ka)

The **dissociation constant (\(K_a\))** is a numerical value that expresses how well an acid dissociates in water. It's a key component in determining the strength of an acid.

Here's what you should know about \(K_a\):

• A high \(K_a\) indicates a strong acid. This means that most of the acid molecules release their protons into the solution. Perchloric acid (\(\mathrm{HClO}_{4}\), \(K_a \approx 10^{10}\)) is an example of a strong acid.
• A low \(K_a\) signifies a weak acid. This implies that only a few molecules dissociate to release protons. Nitrous acid (\(\mathrm{HNO}_{2}\), \(K_a \approx 4.5 \times 10^{-4}\)) is a weak acid.
• The value of \(K_a\) helps to compare the relative strengths of different acids. For instance, knowing the \(K_a\) of both nitrous acid and formic acid allows us to understand that both are weak but can compare which one is weaker.
• In chemical equations, \(K_a\) is used to balance reactions and predict equilibrium positions in acid-base systems.

Understanding \(K_a\) enables scientists and students alike to better predict andmeasure the outcomes of various acid-base reactions in different environments.