Question

Complete the following sentence: "The lower the \(\mathrm{p} K_{\mathrm{a}}\), the (stronger/weaker) the acid." \(^{\prime \prime}\) How does this parallel pH?

Short answer

"The lower the \(pK_a\), the stronger the acid." This parallels pH because both lower \(pH\) and lower \(pK_a\) values indicate a greater degree of acidity.

Step-by-step solution

1. Define the pKa and pH Scales

To understand the exercise, one must comprehend the meaning of pKa and pH. The pKa value is used to express the acidity of a substance. A lower pKa signifies a stronger acid. On the other hand, the pH scale is a logarithmic scale used to measure the acidity or basicity of a solution. Lower pH values are found in more acidic solutions.

2. Understand the Relationship between pKa and Acid Strength

The pKa of an acid is the negative logarithm of its Ka value (acid dissociation constant). A lower pKa value indicates a larger Ka, meaning the acid dissociates more completely and hence, is stronger.

3. Fill in the Correct Word in the Statement

From the previous explanation, we understand that a lower pKa indicates a stronger acid. So the correct statement is: "The lower the pKa, the stronger the acid."

4. Draw a Parallel to pH

Just like how a lower pKa value represents a stronger acid, a lower pH value also reflects a more acidic solution. This is because pH is also defined as the negative logarithm of the concentration of hydrated hydrogen ions in a solution. A lower pH means a higher concentration of hydrogen ions, and thus a more acidic solution. Therefore, the parallel in pH and pKa is that both lower pH and lower pKa values indicate a greater degree of acidity.

Key concepts

Acid Strength

Acid strength is a way to describe how completely an acid can dissociate into ions. When an acid dissolves in water, it releases hydrogen ions (H⁺), which are responsible for the acidic behavior. A strong acid fully dissociates in solution, meaning it releases all possible hydrogen ions. As a result, it produces a large concentration of hydrogen ions.
In contrast, a weak acid only partially dissociates, leaving many of its molecules intact. This means fewer hydrogen ions are released, resulting in a less acidic solution. Therefore, the strength of an acid depends on its ability to donate protons (H⁺ ions).

• More dissociation = Stronger acid
• Less dissociation = Weaker acid

pH Scale

The pH scale is a measurement tool used to describe how acidic or basic a solution is. It ranges from 0 to 14, where 7 is neutral. Anything below 7 is considered acidic, while anything above 7 is seen as basic (or alkaline).
The pH scale is logarithmic, which means each whole number change represents a tenfold change in acidity or basicity. For instance, a solution with a pH of 4 is ten times more acidic than a solution with a pH of 5.

• pH < 7: Acidic
• pH = 7: Neutral
• pH > 7: Basic

Acid Dissociation Constant (Ka)

The acid dissociation constant, known as Ka, measures the extent to which an acid can dissociate in water. It shows the equilibrium concentration of the acid and its ionized form.
The larger the value of Ka, the more the acid dissociates, indicating a stronger acid. Conversely, a small Ka shows that the acid does not dissociate much and is, therefore, weaker.
To simplify the understanding of Ka values, we use the pKa, which is derived as the negative logarithm of the Ka value:\[ \mathrm{pK_a} = -\log(\mathrm{K_a}) \]This means that a lower pKa corresponds to a higher Ka, and therefore, a stronger acid.

Acidity

Acidity is a term used to describe the concentration of hydrogen ions (H⁺) in a solution. The more hydrogen ions present, the more acidic the solution is.
Acidity is often linked to both pH and pKa values. Lower pH and pKa values indicate higher acidity due to greater hydrogen ion concentration or a stronger tendency of the acid to release these ions.

• High [H⁺] = High acidity
• Low [H⁺] = Low acidity

Understanding acidity is crucial because it affects chemical reactions, biological systems, and even environmental conditions.