Question

Identify the following as a weak or strong acid or base: (a) \(\mathrm{NH}_{3}\), (b) \(\mathrm{H}_{3} \mathrm{PO}_{4}\), (c) \(\mathrm{LiOH}\), (d) \(\mathrm{HCOOH}\) (formic acid), (e) \(\mathrm{H}_{2} \mathrm{SO}_{4},\) (f) \(\mathrm{HF},(\mathrm{g}) \mathrm{Ba}(\mathrm{OH})_{2}\).

Short answer

NH₃: weak base, H₃PO₄: weak acid, LiOH: strong base, HCOOH: weak acid, H₂SO₄: strong acid, HF: weak acid, Ba(OH)₂: strong base.

Step-by-step solution

Understanding Weak vs. Strong

Before identifying whether each substance is a strong or weak acid or base, it's important to understand the definitions: Strong acids or bases dissociate completely in water, meaning they ionize completely. Weak acids or bases only partially dissociate in water.

Identifying Ammonia ( \(\mathrm{NH}_3\) )

Ammonia is a common weak base. It only partially accepts a proton in water to form \(\mathrm{NH}_4^+\). Hence, \(\mathrm{NH}_3\) is a weak base.

Examining Phosphoric Acid ( \(\mathrm{H}_3\mathrm{PO}_4\) )

Phosphoric acid is not one of the seven strong acids commonly recognized (HCl, HBr, HI, HNO₃, HClO₃, HClO₄, and H₂SO₄). It ionizes to some extent but not completely. Therefore, \(\mathrm{H}_3\mathrm{PO}_4\) is a weak acid.

Identifying Lithium Hydroxide ( \(\mathrm{LiOH}\) )

Lithium hydroxide is an alkali metal hydroxide and is considered a strong base because it fully dissociates into \(\mathrm{Li}^+\) and \(\mathrm{OH}^-\) in solution.

Analyzing Formic Acid ( \(\mathrm{HCOOH}\) )

Formic acid, or \(\mathrm{HCOOH}\), is a carboxylic acid and known to be a weak acid as it does not completely dissociate in solution.

Assessing Sulfuric Acid ( \(\mathrm{H}_2\mathrm{SO}_4\) )

Sulfuric acid is one of the seven strong acids. It completely dissociates into \(\mathrm{H}^+\) and \(\mathrm{HSO}_4^-\) in its first dissociation in water, thus it is a strong acid.

Evaluating Hydrofluoric Acid ( \(\mathrm{HF}\) )

Hydrofluoric acid is an outlier among the hydrogen halides since it is a weak acid, dissociating incompletely in water.

Determining Barium Hydroxide ( \(\mathrm{Ba(OH)}_2\) )

Barium hydroxide is considered a strong base because it fully dissociates in water into \(\mathrm{Ba}^{2+}\) and two \(\mathrm{OH}^-\) ions.

Key concepts

Strong Acids

A strong acid is characterized by its ability to completely dissociate in water. This means that when a strong acid is dissolved in water, it fully ionizes, releasing all its hydrogen ions. This leads to a high concentration of hydrogen ions in the solution, making it highly acidic.

Some of the most commonly recognized strong acids include:

• Hydrochloric acid (\[\mathrm{HCl} \])
• Sulfuric acid (\[\mathrm{H_2SO_4} \]), which dissociates completely during its first ionization
• Nitric acid (\[\mathrm{HNO_3} \])

These acids are known for their corrosive nature and high reactivity, particularly in solution.Sulfuric acid is often used as a benchmark in Acid-Base Chemistry due to its widespread industrial applications and strong acidic properties.

Weak Acids

Weak acids, unlike strong acids, do not fully dissociate in water. Instead, they reach an equilibrium state where both the undissociated acid and its ions coexist in the solution. This equilibrium means that weak acids have a higher pH than strong acids, due to lower concentrations of hydrogen ions.

Some familiar examples of weak acids include:

• Acetic acid (\[\mathrm{CH_3COOH} \])
• Formic acid (\[\mathrm{HCOOH} \]), which is commonly found in natural settings
• Phosphoric acid (\[\mathrm{H_3PO_4} \])

These acids are often used in a variety of applications due to their milder nature, which makes them safer to handle compared to strong acids.

Strong Bases

Strong bases fully dissociate in water, meaning they completely separate into their ionic components. This results in a high concentration of hydroxide ions (\[\mathrm{OH^-} \]) in the solution, making it strongly basic or alkaline.

Common strong bases include:

• Sodium hydroxide (\[\mathrm{NaOH} \])
• Potassium hydroxide (\[\mathrm{KOH} \])
• Lithium hydroxide (\[\mathrm{LiOH} \])
• Barium hydroxide (\[\mathrm{Ba(OH)_2} \])

Due to their high reactivity and ability to drastically raise the pH of solutions, strong bases are widely used in industrial processes, cleaning, and as laboratory reagents.

Weak Bases

Weak bases only partially dissociate in water, which means they do not fully ionize to form hydroxide ions. This partial dissociation creates a solution with a moderate pH, which is less basic compared to solutions of strong bases.

Common examples of weak bases include:

• Ammonia (\[\mathrm{NH_3} \]), which accepts protons partially to form \[\mathrm{NH_4^+} \]
• Methylamine (\[\mathrm{CH_3NH_2} \])

Weak bases are prevalent in biological systems and are integral in buffer solutions because they do not drastically alter the pH.

Dissociation in Water

Dissociation in water is a fundamental concept in understanding acid-base chemistry. It refers to the process by which molecules or ionic compounds separate into smaller particles, such as ions, when dissolved in water. This process is crucial for both strong and weak acids and bases.

When a strong acid or base dissociates, it completely separates into its constituent ions. For example:- \[\mathrm{HCl} \] dissociates into \[\mathrm{H^+} \] and \[\mathrm{Cl^-} \].- \[\mathrm{NaOH} \] dissociates into \[\mathrm{Na^+} \] and \[\mathrm{OH^-} \].In contrast, weak acids and bases do not fully dissociate, leading to an equilibrium between the undissociated molecules and their ions. This equilibrium state is essential for buffer solutions and many natural processes. Understanding the dissociation process helps in predicting the behavior and reactivity of various acids and bases in an aqueous environment.