Problem 95 - Chemistry The Molecular Science

Question

State whether equal molar amounts of these would have a \(\mathrm{pH}\) equal to 7 , less than 7 , or greater than 7 . (a) A weak base and a strong acid react. (b) A strong base and a strong acid react. (c) A strong base and a weak acid react.

Step-by-Step Solution

Answer

(a) \( \text{pH} < 7 \); (b) \( \text{pH} = 7 \); (c) \( \text{pH} > 7 \).

Step 1: Understanding Reaction (a)

A weak base reacts with a strong acid. The strong acid will donate protons easily, while the weak base partially accepts them. The resulting solution will likely have leftover acidic protons, resulting in a solution with \( \text{pH} < 7 \).

Step 2: Analyzing Reaction (b)

A strong base reacts with a strong acid. Both fully dissociate in water and neutralize each other. The resulting solution will be neutral, therefore \( \text{pH} = 7 \).

Step 3: Evaluating Reaction (c)

A strong base reacts with a weak acid. The weak acid does not donate protons as readily as a strong acid. The strong base will likely have excess hydroxide ions, leading to a solution with \( \text{pH} > 7 \).

Key Concepts

Acids and Bases

Acids and bases are fundamental concepts in chemistry that describe how substances interact with water. Acidic substances release hydrogen ions (\(\text{H}^+\)) when dissolved in water, while basic substances release hydroxide ions (\(\text{OH}^-\)).
The acidity or basicity of a solution is measured using the \(\text{pH}\) scale, which ranges from 0 to 14. A \(\text{pH}\) of 7 represents a neutral solution, such as pure water. Solutions with a \(\text{pH}\) less than 7 are acidic, and those with a \(\text{pH}\) greater than 7 are basic. The strength of an acid or base is determined by its ability to donate or accept protons.
It's important to know that acids and bases can affect reactions significantly due to their ability to change the \(\text{pH}\) of the environment, affecting the chemical reactivity of other solutes.

Weak and Strong Acids/Bases

The distinction between weak and strong acids or bases lies in how completely they dissociate in water.

• Strong acids and bases dissociate fully in water, releasing a high concentration of hydrogen or hydroxide ions, respectively. Common examples include hydrochloric acid (\(\text{HCl}\)) and sodium hydroxide (\(\text{NaOH}\)).
• Weak acids and bases, on the other hand, only partially dissociate, meaning they release fewer ions than their strong counterparts. Acetic acid (\(\text{CH}_3\text{COOH}\)) and ammonia (\(\text{NH}_3\)) are typical examples.

The extent to which these substances dissociate influences the \(\text{pH}\) of the resulting solution. For instance, in a reaction involving a strong acid and a weak base, the strong acid will release many more hydrogen ions than the base can neutralize. Hence, the resulting solution will tend to be more acidic with a \(\text{pH} < 7\).
The complete or partial dissociation characteristic is pivotal in predicting the outcome of reactions involving these substances.

Neutralization Reactions

Neutralization reactions occur when an acid and a base react to form water and a salt. This is a type of chemical reaction where the acidic and basic characteristics are nullified.
In cases where a strong acid and a strong base react, both substances fully dissociate, and their ions combine to form water and a neutral salt. The resulting solution is neutral, having a \(\text{pH} = 7\).
If a strong acid reacts with a weak base or vice versa, the stronger substance dominates, resulting in the product being either slightly acidic or basic.

• Strong Acid + Weak Base: Typically results in an acidic solution (\(\text{pH} < 7\)), because the weak base doesn't fully neutralize the acid.
• Weak Acid + Strong Base: Often produces a basic solution (\(\text{pH} > 7\)), as the strong base doesn't get completely neutralized by the weak acid.

Understanding these reactions helps in predicting the \(\text{pH}\) of the resulting mixtures and in various applications, such as buffering or titration, which are crucial in analytical chemistry and everyday chemical processes.