Problem 74 - Introductory Chemistry Atoms First

Question

True or false? The acidity of a given volume of 1.0 \(\mathrm{M}\) hydrofluoric acid is the same as that of the same volume of \(1.0 \mathrm{M}\) hydrochloric acid. Explain your answer.

Step-by-Step Solution

Answer

False. The acidity of a given volume of 1.0 M hydrofluoric acid is not the same as that of the same volume of 1.0 M hydrochloric acid. This is because HCl is a strong acid and completely dissociates in water, producing a 1.0 M concentration of H+ ions, whereas HF is a weak acid and only partially dissociates, resulting in a lower H+ ion concentration.

Step 1: Understanding the dissociation of acids in water

When an acid dissolves in water, it donates a hydrogen ion (H+) or proton to the water, forming hydronium ions (H3O+). This process is called dissociation. The strength of an acid is determined by its ability to donate protons to water. Strong acids, like HCl, completely dissociate in water, releasing H+ ions. On the other hand, weak acids like HF only partially dissociate, releasing fewer H+ ions.

Step 2: Comparing the dissociation of hydrofluoric acid (HF) and hydrochloric acid (HCl)

Both hydrofluoric acid (HF) and hydrochloric acid (HCl) dissolve in water, donating H+ ions and forming their respective anions (F- and Cl-). The difference lies in their ability to dissociate in water. HCl is a strong acid, meaning it completely dissociates in water: \[HCl_{(aq)} \rightarrow H^+_{(aq)} + Cl^-_{(aq)}\] In contrast, HF is a weak acid, meaning it only partially dissociates in water: \[HF_{(aq)} \rightleftharpoons H^+_{(aq)} + F^-_{(aq)}\] Because HF is a weak acid, the concentration of H+ ions released in the solution will be lower than that for HCl.

Step 3: Comparing the acidity of 1.0 M hydrofluoric acid and 1.0 M hydrochloric acid

Since HCl completely dissociates in water, a 1.0 M solution of HCl would also have a concentration of H+ ions equal to 1.0 M. On the other hand, since HF only partially dissociates in water, a 1.0 M solution of HF would have a lower concentration of H+ ions than 1.0 M.

Step 4: Conclusion

From the comparison of the dissociation of HF and HCl and the concentrations of H+ ions released in the solutions, it can be concluded that the acidity of a given volume of 1.0 M hydrofluoric acid is not the same as that of the same volume of 1.0 M hydrochloric acid. The statement is, therefore, False.

Key Concepts

strong acids

In chemistry, an acid's strength refers to its ability to donate protons to the surrounding water molecules. Strong acids excel at this task. They dissociate fully in aqueous solutions, which means that every molecule of acid releases a proton to the water.

Here are key points to remember about strong acids:

• Complete Dissociation: Strong acids, such as hydrochloric acid (HCl), dissociate completely, resulting in a high concentration of hydrogen ions (H⁺) in the solution.
• pH Levels: Because of the high concentration of hydrogen ions, solutions of strong acids typically have a lower pH.
• Common Examples: Besides hydrochloric acid, sulfuric acid (H₂SO₄) and nitric acid (HNO₃) are other examples of strong acids.

The idea here is quite simple: strong acids are like open-handed donors, eagerly giving away their hydrogen ions to increase the acidity of a solution.

weak acids

Weak acids are a bit more reserved compared to strong acids. They do not completely dissociate in water, meaning they only donate some of their hydrogen ions.

When studying weak acids, it's important to grasp the following:

• Partial Dissociation: Weak acids, such as hydrofluoric acid (HF), only partially dissociate in water. This leads to a lower concentration of hydrogen ions in the solution compared to what an equal concentration of a strong acid would produce.
• Equilibrium: The dissociation of weak acids often involves an equilibrium state. This equilibrium highlights that not all acid molecules donate hydrogen ions.
• Higher pH Level: A solution of weak acid will typically have a higher pH than a strong acid solution at the same concentration.

Think of weak acids as hesitant donors. They hold onto some of their hydrogen ions and thereby result in a less acidic solution compared to their strong counterparts.

hydrochloric acid

Hydrochloric acid, often abbreviated as HCl, is one of the most well-known strong acids. It is commonly found in laboratories and even in the human stomach as gastric acid.

There are several characteristics that define hydrochloric acid:

• Complete Dissociation: HCl completely dissociates in water, which results in a high concentration of hydronium ions (H₃O⁺) and chloride ions (Cl⁻).
• Low pH: Solutions of hydrochloric acid are highly acidic, usually having very low pH values.
• Wide Applications: It's used in numerous applications, ranging from cleaning agents to food production, due to its strong acidic nature.

An easy way to remember the nature of HCl is to associate it with the word 'complete.' Whether it's complete dissociation or complete ionization, HCl leaves no room for half-measures.

hydrofluoric acid

Hydrofluoric acid (HF) is a weak acid, yet it is notoriously aggressive with certain materials. Its partial dissociation is key to understanding its behavior in solutions.

Here are some essential insights about hydrofluoric acid:

• Partial Dissociation: Unlike hydrochloric acid, HF only partially ionizes in water, producing fewer hydrogen ions.
• Unique Reactivity: Hydrofluoric acid can aggressively react with glass and other substances, despite being a weak acid in aqueous solutions.
• Moderate pH Levels: Because of its partial dissociation, HF solutions have a higher pH compared to strong acids such as HCl at the same concentration.

Think of hydrofluoric acid as a paradox: weak in its dissociation but strong in its ability to cause damage to certain materials. This unique combination warrants respectful handling and careful study.