Question

Consider a solution prepared by mixing a weak acid HA, HCl, and NaA. Which of the following statements best describes what happens? a. The \(\mathrm{H}^{+}\) from the HCl reacts completely with the \(\mathrm{A}^{-}\) from the NaA. Then the HA dissociates somewhat. b. The \(\mathrm{H}^{+}\) from the HCl reacts somewhat with the A- from the NaA to make HA, while the HA is dissociating. Eventually you have equal amounts of everything. c. The \(\mathrm{H}^{+}\) from the HCl reacts somewhat with the \(\mathrm{A}^{-}\) from the NaA to make HA while the HA is dissociating. Eventually all the reactions have equal rates. d. The \(\mathrm{H}^{+}\) from the \(\mathrm{HCl}\) reacts completely with the \(\mathrm{A}^{-}\) from the NaA. Then the HA dissociates somewhat until "too much "H' and \(\mathrm{A}^{-}\) are formed, so the \(\mathrm{H}^{+}\) and \(\mathrm{A}^{-}\) react to form HA, and so on. Eventually equilibrium is reached. Justify your choice, and for choices you did not pick, explain what is wrong with them.

Short answer

The correct statement is (a): The H+ from the HCl reacts completely with the A- from the NaA. Then the HA dissociates somewhat. This statement takes into account that HCl is a strong acid and dissociates completely, while HA is a weak acid that dissociates partially. The other statements do not properly account for the properties of strong and weak acids.

Step-by-step solution

Recall the properties of strong and weak acids

Strong acids, like HCl, dissociate completely in water, meaning they release all their hydrogen ions (H+) into the solution. Weak acids, like HA, only partially dissociate in water and establish an equilibrium between the acid and its conjugate base (A-).

Analyze the statements

The four given statements are all related to the behavior of H+, A-, HA, and HCl in the mixture. The correct statement will consider that HCl is a strong acid and dissociates completely in water. It will also consider that HA is a weak acid and dissociates partially, reaching an equilibrium with A-.

Choose the correct statement and justify the choice

The correct statement is (a), which says: The H+ from the HCl reacts completely with the A- from the NaA. Then the HA dissociates somewhat. This statement aligns with the properties of strong acids like HCl, which dissociate completely to release H+ ions into the solution. It also considers that weak acids like HA only partially dissociate.

Explain what's wrong with the other statements

• Statement (b) is wrong because it says the H+ from the HCl reacts somewhat with the A- from the NaA to make HA, while the HA is dissociating. This suggests equal amounts of all species, which isn't true because the HCl dissociates completely. • Statement (c) is wrong for the same reason as statement (b); it claims that H+ from the HCl reacts somewhat with the A- from the NaA while the HA is dissociating. Strong acids like HCl don't just react "somewhat," they dissociate completely. • Statement (d) is incorrect because it says the HA dissociates somewhat until "too much" H+ and A- are formed and then react to form HA. This suggests a dynamic between H+ and A- that doesn't accurately represent the behavior of strong and weak acids. In conclusion, statement (a) correctly describes the behavior of the mixture, considering that HCl is a strong acid and dissociates completely and that HA is a weak acid that only dissociates partially. The other statements fail to properly account for the properties of strong and weak acids.

Key concepts

strong acid

When we talk about a strong acid, we're referring to an acid like hydrochloric acid (HCl) which fully dissociates in water. This dissociation means that almost every molecule of HCl breaks apart, releasing hydrogen ions (\(\mathrm{H}^{+}\)) into the solution.
This complete dissociation allows strong acids to effectively increase the concentration of \(\mathrm{H}^{+}\) ions in the solution, lowering the pH significantly.
Because of their ability to release all their hydrogen ions, strong acids react completely with available bases. This characteristic is crucial for predicting their behavior in acid-base reactions.

• Complete dissociation into ions is a key feature.
• Releases a high concentration of \(\mathrm{H}^{+}\) ions.
• Significantly impacts the solution's pH.

weak acid

Weak acids, unlike strong acids, only partially dissociate in water. A classic example is acetic acid (CH₃COOH) or a generic weak acid HA.
In a solution, only a small fraction of HA molecules release their \(\mathrm{H}^{+}\) ions. As a result, there is an equilibrium between the HA molecules and the ions it dissociates into, notably its conjugate base \(\mathrm{A}^{-}\).
This equilibrium reflects a balance, meaning not all acid molecules split into ions. This partial dissociation means weak acids have a much lesser effect on the pH compared to strong acids.

• Doesn't completely dissociate in solution.
• Exists in an equilibrium state between undissociated and dissociated form.
• Less impact on pH than strong acids.

dissociation

The term 'dissociation' describes the process by which acids split into ions in a solution. For acids, this typically means breaking into hydrogen ions (\(\mathrm{H}^{+}\)) and anions (negatively charged ions).
In the context of strong acids, dissociation is complete, meaning every acid molecule breaks apart to release ions.
Weak acids, on the other hand, only dissociate partially, resulting in an equilibrium of ions and undissociated molecules in solution.
This behavior is central to understanding acid-base reactions and acid strength.

• Describes ionization of acids in a solution.
• Strong acids dissociate completely, weak acids partially.
• Key factor in determining the strength of an acid.

conjugate base

A conjugate base forms when an acid donates a \(\mathrm{H}^{+}\) ion in a solution. It's what's left over after the acid's \(\mathrm{H}^{+}\) ion has been released.
For a weak acid like HA, once it releases a \(\mathrm{H}^{+}\), it forms the conjugate base \(\mathrm{A}^{-}\).
This conjugate base can potentially recombine with \(\mathrm{H}^{+}\) ions to form the original acid, maintaining equilibrium in the solution.
Understanding conjugate bases help explain the dynamic nature of acid-base equilibrium.

• Formed when an acid loses its \(\mathrm{H}^{+}\) ion.
• Can recombine with \(\mathrm{H}^{+}\) to reform the acid.
• Plays a role in the reversible nature of weak acid dissociation.