Question

Identify the stronger acid in each of the following pairs: a. \(\mathrm{NH}_{4}^{+}\) or \(\mathrm{H}_{3} \mathrm{O}^{+}\) b. \(\mathrm{H}_{2} \mathrm{SO}_{4}\) or \(\mathrm{HCN}\) c. \(\mathrm{H}_{2} \mathrm{O}\) or \(\mathrm{H}_{2} \mathrm{CO}_{3}\)

Short answer

a. \( \mathrm{H}_3 \mathrm{O}^+ \) b. \( \mathrm{H}_2 \mathrm{SO}_4 \) c. \( \mathrm{H}_2 \mathrm{CO}_3 \)

Step-by-step solution

Compare \( \mathrm{NH}_4^+ \) and \( \mathrm{H}_3 \mathrm{O}^+ \)

To determine the stronger acid between \( \mathrm{NH}_4^+ \) and \( \mathrm{H}_3 \mathrm{O}^+ \), consider their relative positions in the acid strength hierarchy. \( \mathrm{H}_3 \mathrm{O}^+ \) is the protonated form of water and is known to be a stronger acid than \( \mathrm{NH}_4^+ \), which is the conjugate acid of ammonia.

Compare \( \mathrm{H}_2 \mathrm{SO}_4 \) and \( \mathrm{HCN} \)

First, recall that \( \mathrm{H}_2 \mathrm{SO}_4 \) is a strong acid that completely dissociates in water, while \( \mathrm{HCN} \) is a weak acid that does not fully dissociate. Therefore, \( \mathrm{H}_2 \mathrm{SO}_4 \) is a stronger acid than \( \mathrm{HCN} \).

Compare \( \mathrm{H}_2 \mathrm{O} \) and \( \mathrm{H}_2 \mathrm{CO}_3 \)

Consider the relative acid strength. Water (\( \mathrm{H}_2 \mathrm{O} \)) is typically neutral and not considered acidic in the context of Bronsted acids, while \( \mathrm{H}_2 \mathrm{CO}_3 \) (carbonic acid) is a weak acid. Therefore, \( \mathrm{H}_2 \mathrm{CO}_3 \) is the stronger acid compared to water.

Key concepts

Acid Hierarchy

To compare the strength of acids, we can place them in an acid hierarchy. This is a ranking system where acids are ordered based on their ability to donate protons. Strong acids are at the top because they easily give up a proton. Weak acids are toward the bottom because they hold onto their protons more tightly.
When comparing any two acids, knowing their relative positions in this hierarchy helps us determine which one is stronger. For example, in the given exercise, we determined that \(\text{H}_3 \text{O}^+\) is stronger than \( \text{NH}_4^+\) because it is higher up in the hierarchy.
Always remember: the higher an acid is in the hierarchy, the more it dissociates in water, and the stronger it is.

Strong Acids

Strong acids completely dissociate in water, meaning they release all their protons. Examples include sulfuric acid (\( \text{H}_2 \text{SO}_4 \)), hydrochloric acid (\( \text{HCl} \)), and nitric acid (\( \text{HNO}_3 \)).
In the exercise, we saw that \( \text{H}_2 \text{SO}_4 \) is much stronger than \( \text{HCN} \). This is because \( \text{H}_2 \text{SO}_4 \) completely dissociates in water, releasing more protons and thus making it a stronger acid.
One important thing to note about strong acids is that their strength is not affected by concentration. Even in dilute solutions, they remain fully dissociated.

Weak Acids

Weak acids do not fully dissociate in water. This means that only a small fraction of their molecules release protons. Some common examples are acetic acid (\( \text{CH}_3 \text{COOH} \)), formic acid (\( \text{HCOOH} \)), and hydrocyanic acid (\( \text{HCN} \)).
In our exercise, \( \text{HCN} \) was identified as a weak acid compared to \( \text{H}_2 \text{SO}_4 \). This is because \( \text{HCN} \) does not fully dissociate, making it weaker.
For weak acids, the concentration of the acid can affect its pH, as not all molecules ionize to produce protons.

Bronsted Acids

Bronsted acids are substances that donate protons (\( \text{H}^+ \)) in a reaction. This definition helps us understand why some substances act as acids in different circumstances.
For example, \( \text{H}_2 \text{CO}_3 \) (carbonic acid) was found to be a stronger acid than water (\( \text{H}_2 \text{O} \)) in the exercise because it donates protons more readily.
Understanding Bronsted acids is crucial because it allows us to predict how acids will behave in various chemical reactions. It helps not just in identifying the stronger acid but also in understanding the wider context of acid-base chemistry.