Problem 85 - Chemistry and Chemical Reactivity

Question

Describe the effect on the \(\mathrm{pH}\) of the following actions: (a) adding sodium acetate, \(\mathrm{NaCH}_{3} \mathrm{CO}_{2},\) to \(0.100 \mathrm{M}\) \(\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}\) (b) adding \(\mathrm{NaNO}_{3}\) to \(0.100 \mathrm{M} \mathrm{HNO}_{3}\) (c) Explain why there is or is not an effect in each case.

Step-by-Step Solution

Answer

Adding sodium acetate increases pH; NaNO3 doesn’t change pH. Sodium acetate forms a buffer, NaNO3 doesn’t affect acid.

Step 1: Understanding Sodium Acetate in Acetic Acid

When sodium acetate, \(\mathrm{NaCH}_3\mathrm{CO}_2\), is added to a solution of acetic acid, \(\mathrm{CH}_3\mathrm{CO}_2\mathrm{H}\), the acetate ion \(\mathrm{CH}_3\mathrm{CO}_2^-\) is introduced into the solution. This increases the concentration of the conjugate base of acetic acid, establishing a buffer system in the solution. As a result, the \(\mathrm{pH}\) of the solution will increase, becoming less acidic due to the buffering effect.

Step 2: Analyzing Sodium Nitrate in Nitric Acid

Adding \(\mathrm{NaNO}_3\) to a solution of \(0.100\ \mathrm{M}\ \mathrm{HNO}_3\) does not significantly change the \(\mathrm{pH}\). \(\mathrm{NaNO}_3\) is a salt that dissociates into sodium ions and nitrate ions neither of which affect the \(\mathrm{pH}\) because \(\mathrm{Na}^+\) is neutral and \(\mathrm{NO}_3^-\) comes from a strong acid and remains unreactive. Therefore, there is no buffering or additional neutralization to alter the \(\mathrm{pH}\).

Step 3: Explaining Effects

The \(\mathrm{pH}\) change with sodium acetate in acetic acid results from the presence of a common ion, acetate, which leads to a buffering effect and increased \(\mathrm{pH}\) (less acidic). Conversely, \(\mathrm{NaNO}_3\) added to \(\mathrm{HNO}_3\) doesn't affect \(\mathrm{pH}\) since it doesn't alter the acid-base balance; \(\mathrm{HNO}_3\) remains dominant in determining \(\mathrm{pH}\).

Key Concepts

Understanding pH

The concept of pH is essential in chemistry, referring to the acidity or basicity of a solution. Mathematically, it is the negative logarithm of the hydrogen ion concentration: \[ \text{pH} = -\log [H^+] \].
The scale ranges from 0 to 14, with 7 as neutral.

• pH less than 7 indicates an acidic solution.
• pH greater than 7 indicates a basic solution.
• pH of exactly 7 is neutral, like pure water.

Understanding pH helps in determining the chemical nature of a solution.
For instance, adding a substance like sodium acetate to acetic acid changes the pH by introducing additional ions that interact with hydrogen ions, thus affecting acidity.

Conjugate Bases Explained

A conjugate base is what remains after an acid donates a proton (hydrogen ion). In the context of acetic acid (\( \text{CH}_3\text{CO}_2\text{H} \)), the conjugate base is the acetate ion (\( \text{CH}_3\text{CO}_2^- \)).
This ion can accept a hydrogen ion, making it important in forming buffer solutions:

• The addition of conjugate bases like acetate helps stabilize pH changes.
• This equilibrium is crucial for maintaining acidity at desired levels.

When substances like sodium acetate are added to acetic acid, they increase the concentration of the conjugate base, enhancing the solution’s ability to resist pH changes through buffering.

Basics of Acetic Acid

Acetic acid is a weak acid with the formula \( \text{CH}_3\text{CO}_2\text{H} \). Being weak means it doesn't fully dissociate in water:
\[ \text{CH}_3\text{CO}_2\text{H} \leftrightarrow \text{CH}_3\text{CO}_2^- + H^+ \]

• Acetic acid is a common component in vinegar.
• Its weak nature allows for effective buffering when paired with its conjugate base.
• This characteristic makes it prevalent in laboratory buffer solutions.

By understanding acetic acid's behavior, particularly its partial dissociation, we can see how buffer solutions can be created to control the pH of various chemical environments.

Balancing Acid-Base Systems

The balance between acids and bases in a solution is crucial for maintaining a stable pH. This balance is known as acid-base balance.

• A buffer solution, like a mix of acetic acid and sodium acetate, maintains stability despite adding acids or bases.
• The common ion effect plays a key role; adding common ions shifts equilibrium, adjusting the pH change resistance.

When an acid or base is added to a buffered solution, the change in pH is minimal.
This is because the buffer neutralizes added hydrogen or hydroxide ions through equilibrium reactions.
Buffers are vital in biological systems for maintaining cellular function and enzyme activity.