Question

Classify each of the following as a strong acid, weak acid, strong base, or weak base in aqueous solution. a. \(\mathrm{HNO}_{2}\) b. \(\mathrm{HNO}_{3}\) c. \(\mathrm{CH}_{3} \mathrm{NH}_{2}\) d. \(\mathrm{NaOH}\) e. \(\mathrm{NH}_{3}\) f. \(\mathrm{HF}\) g. h. \(\mathrm{Ca}(\mathrm{OH})_{2}\) i. \(\mathrm{H}_{2} \mathrm{SO}_{4}\)

Short answer

a. \(\mathrm{HNO}_{2}\): Weak acid b. \(\mathrm{HNO}_{3}\): Strong acid c. \(\mathrm{CH}_{3}\mathrm{NH}_{2}\): Weak base d. \(\mathrm{NaOH}\): Strong base e. \(\mathrm{NH}_{3}\): Weak base f. \(\mathrm{HF}\): Weak acid g. No substance given h. \(\mathrm{Ca(OH)}_{2}\): Strong base i. \(\mathrm{H}_{2}\mathrm{SO}_{4}\): Strong acid (first dissociation)

Step-by-step solution

Identify strong acids

Strong acids are acids that completely dissociate in water. There are seven strong acids to remember which are: 1. \(\mathrm{HCl}\) - Hydrochloric Acid 2. \(\mathrm{HBr}\) - Hydrobromic Acid 3. \(\mathrm{HI}\) - Hydroiodic Acid 4. \(\mathrm{HNO}_{3}\) - Nitric Acid 5. \(\mathrm{HClO}_{4}\) - Perchloric Acid 6. \(\mathrm{HClO}_{3}\) - Chloric Acid 7. \(\mathrm{H}_{2}\mathrm{SO}_{4}\) - Sulfuric Acid (first dissociation is strong)

Identify strong bases

Strong bases are bases that dissociate completely into hydroxide ions (\(\mathrm{OH}^{-}\)) in water. Common strong bases include group 1 and 2 hydroxides. Examples include: 1. \(\mathrm{LiOH}\) - Lithium hydroxide 2. \(\mathrm{NaOH}\) - Sodium hydroxide 3. \(\mathrm{KOH}\) - Potassium hydroxide 4. \(\mathrm{Ca(OH)}_{2}\) - Calcium hydroxide 5. \(\mathrm{Sr(OH)}_{2}\) - Strontium hydroxide 6. \(\mathrm{Ba(OH)}_{2}\) - Barium hydroxide

Classify the substances

Now we will classify each of the given substances as a strong acid, weak acid, strong base, or weak base: a. \(\mathrm{HNO}_{2}\): Nitrous acid is not one of the seven strong acids listed. Therefore, it is a weak acid. b. \(\mathrm{HNO}_{3}\): Nitric acid is one of the seven strong acids listed above. Therefore, it is a strong acid. c. \(\mathrm{CH}_{3}\mathrm{NH}_{2}\): This is a derivative of ammonia (\(\mathrm{NH}_{3}\)), which is a weak base. This substance can accept a proton (H+) to become \(\mathrm{CH}_{3}\mathrm{NH}_{3}^{+}\), making it a weak base. d. \(\mathrm{NaOH}\): Sodium hydroxide is one of the strong bases listed above. Therefore, it is a strong base. e. \(\mathrm{NH}_{3}\): Ammonia is not one of the strong bases listed, but can accept a proton (H+) to become \(\mathrm{NH}_{4}^{+}\). Therefore, it is a weak base. f. \(\mathrm{HF}\): Hydrofluoric acid is not one of the seven strong acids listed. Instead, it is known as a weak acid due to its low dissociation in water. g. It seems that there is no substance g in the list. h. \(\mathrm{Ca(OH)}_{2}\): Calcium hydroxide is one of the strong bases listed above. Therefore, it is a strong base. i. \(\mathrm{H}_{2}\mathrm{SO}_{4}\): Sulfuric acid is one of the seven strong acids listed. However, only its first dissociation in water is strong. Therefore, for the given exercise, it is considered a strong acid.

Key concepts

Strong and Weak Acids

Acids are substances that release hydrogen ions ( H^+ ) when dissolved in water. This ability to dissociate distinguishes strong acids from weak acids.

• Strong Acids: These acids completely dissociate in water, releasing all their H^+ ions. Common examples include HCl , HBr , and HNO_3 . Complete dissociation means they have a high conductivity and are very reactive.
• Weak Acids: These dissociate only partially in water, thus releasing fewer H^+ ions. Example: HF and HNO_2 are weak because they don't release all their H^+ ions and therefore have lower conductivity and reactivity compared to strong acids.

Understanding the dissociation level helps predict the behavior and reactivity of the acid in various conditions.

Strong and Weak Bases

Bases are substances that accept hydrogen ions ( H^+ ) or release hydroxide ions ( OH^- ) in water. Their strength depends on their dissociation.

• Strong Bases: These substances dissociate completely in water, producing a large concentration of OH^- ions. Common examples include NaOH and Ca(OH)_2 . They are highly reactive and can conduct electricity efficiently.
• Weak Bases: These only partially dissociate, leading to a lower concentration of OH^- ions in water. Examples are NH_3 and CH_3NH_2 , where not all molecules interact with water.

Recognizing the dissociation extent helps in determining the base's ability to neutralize acids and participate in chemical reactions.

Aqueous Solutions

An aqueous solution is a solution where water acts as the solvent. This is a crucial concept in chemistry because the behavior of acids and bases in water determines their strength.

• Role of Water: Water facilitates dissociation, acting as a medium that allows acids and bases to release hydrogen or hydroxide ions.
• Conductivity: The presence of more ions in an aqueous solution increases conductivity. Strong acids and bases with higher ion dissociation have greater conductivity.

Understanding aqueous solutions helps in analyzing reactions and predicting the movement of ions.

Chemical Dissociation

Dissociation is the process by which molecules split into smaller particles, such as ions, in a solvent like water. Understanding this process is key to distinguishing between strong and weak acids and bases.

• Complete Dissociation: In strong acids and bases, molecules split into ions entirely. This results in more hydrogen ( H^+ ) or hydroxide ( OH^- ) ions in solution.
• Partial Dissociation: In weak acids and bases, only a fraction of molecules dissociate. This leads to a smaller ion concentration, affecting the solution's overall reactivity and conductivity.

Learning about dissociation helps predict solution behaviors, such as pH levels and reactivity.