Problem 84 - Chemistry and Chemical Reactivity

Question

Arrange the following \(0.10 \mathrm{M}\) solutions in order of increasing pH. (a) \(\mathrm{NaCl}\) (b) \(\mathrm{NH}_{4} \mathrm{Cl}\) (c) \(\mathrm{HCl}\) (d) \(\mathrm{NaCH}_{3} \mathrm{CO}_{2}\) (e) \(\mathrm{KOH}\)

Step-by-Step Solution

Answer

The order is (c) HCl, (b) NH₄Cl, (a) NaCl, (d) NaCH₃CO₂, (e) KOH.

Step 1: Understanding pH

The pH scale is a measure of how acidic or basic a solution is. A pH less than 7 indicates an acidic solution, pH equal to 7 is neutral, and a pH greater than 7 indicates a basic solution.

Step 2: Analyze Each Compound

- **NaCl**: A neutral salt formed from a strong acid (HCl) and strong base (NaOH), resulting in a pH around 7. - **NH₄Cl**: An acidic salt from a strong acid (HCl) and a weak base (NH₃), resulting in a pH less than 7. - **HCl**: A strong acid, giving a very low pH. - **NaCH₃CO₂**: A basic salt formed from a weak acid (CH₃COOH) and a strong base (NaOH), resulting in a pH greater than 7. - **KOH**: A strong base with a very high pH.

Step 3: Order by Increasing pH

Arrange the solutions from lowest to highest pH: (c) HCl, (b) NH₄Cl, (a) NaCl, (d) NaCH₃CO₂, (e) KOH. This order is based on the strength and nature of the acids and bases each compound forms.

Key Concepts

Acidic Solutions

Acidic solutions are those with a pH less than 7. These solutions arise due to the presence of compounds that release hydrogen ions \( H^+ \) into the water. For example, hydrochloric acid (HCl) is a strong acid that completely dissociates in water, thus releasing a large amount of \( H^+ \), resulting in a very low pH.

Here are some key characteristics of acidic solutions:

• They taste sour and can turn blue litmus paper red.
• Acidic solutions can conduct electricity because of the ions in the solution.
• They react with metals to produce hydrogen gas.

Understanding these properties helps us recognize substances that may behave as acids in different environments.

Basic Solutions

Basic solutions have a pH greater than 7. Such solutions are formed when substances release hydroxide ions \( OH^- \) into the solution. An example is potassium hydroxide (KOH), a strong base that dissociates completely in water to provide \( OH^- \), thereby increasing the pH significantly.

Key traits of basic solutions include:

• A bitter taste and slippery feel.
• The ability to turn red litmus paper blue, indicating their basic nature.
• They can neutralize acids, forming water and salt.

These properties allow bases to be used in cleaning products, soaps, and as antacids to neutralize excess stomach acid.

Neutral Solutions

Neutral solutions have a pH that equals exactly 7. This occurs when the concentration of hydrogen ions \( H^+ \) is equal to the concentration of hydroxide ions \( OH^- \). A common example is pure water, where this balance exists naturally.

Some key points about neutral solutions:

• They do not change the color of litmus paper, as they are neither acidic nor basic.
• In neutralization reactions, an acid and base combine to form such solutions.
• Neutral solutions are generally non-corrosive and safe for many uses.

A vital concept in chemistry, neutrality implies a balance, making these solutions central in various chemical reactions and daily processes.

Strong Acids and Bases

Strong acids and bases dissociate completely in water, resulting in maximum production of \( H^+ \) for acids and \( OH^- \) for bases. HCl is a strong acid, contributing to an extremely low pH, while KOH is a strong base with a very high pH in solutions.

Characteristics of strong acids and bases:

• They typically have extreme pH values, either very low or very high.
• Strong acids and bases conduct electricity very well due to the high ion concentration.
• They are highly reactive and must be handled with care.

These substances are pivotal in industrial processes, where high reactivity is essential, such as in cleaning, food processing, and research applications.

Weak Acids and Bases

Weak acids and bases do not fully dissociate in water, meaning only a fraction of their molecules release \( H^+ \) or \( OH^- \). Ammonium chloride (NH₄Cl) is an example of a compound that, when dissolved, contributes to a solution being weakly acidic. Similarly, sodium acetate (NaCH₃CO₂) forms a weakly basic solution.

Some highlights about weak acids and bases include:

• They have pH values close to 7, either slightly below or above.
• Weak acids and bases conduct electricity poorly compared to their strong counterparts.
• They react more gradually and are less aggressive in nature.

Due to their mild nature, weak acids and bases find use in biological systems and food products, such as maintaining the pH of blood and adding flavor to culinary dishes.