Problem 137 - Chemistry The Science in Context

Question

When electrodes connected to a lightbulb are inserted into an aqueous solution of acetic acid, the bulb glows dimly. Will the bulb become brighter, remain the same, or turn off after one equivalent of aqueous \(\mathrm{NaOH}\) is added to the solution? Write a balanced net ionic equation that supports your answer.

Step-by-Step Solution

Answer

Answer: The lightbulb will remain the same brightness. Explanation: Upon adding NaOH, the reaction between acetic acid and hydroxide ions forms water and acetate ions, while the number of spectator ions (sodium ions) remains unchanged. As there is no reduction in the overall number of ions in the solution, the conductivity remains the same, and the lightbulb's brightness remains unchanged. The balanced net ionic equation for this reaction is: $$ \mathrm{CH_3COOH} + \mathrm{OH^-} \rightarrow \mathrm{H_2O} + \mathrm{CH_3COO^-} $$

Step 1: 1. Write the reaction between acetic acid and sodium hydroxide

The reaction between acetic acid (\(\mathrm{CH_3COOH}\)) and sodium hydroxide (\(\mathrm{NaOH}\)) is a neutralization reaction (acid-base reaction). The products of this reaction are water (\(\mathrm{H_2O}\)) and sodium acetate (\(\mathrm{CH_3COONa}\)). $$ \mathrm{CH_3COOH} + \mathrm{NaOH} \rightarrow \mathrm{H_2O} + \mathrm{CH_3COONa} $$

Step 2: 2. Write the ionic equation of the reaction

To write the complete ionic equation, we need to split the strong electrolytes into their respective ions. $$ \mathrm{CH_3COOH} + \mathrm{Na^+} + \mathrm{OH^-} \rightarrow \mathrm{H_2O} + \mathrm{Na^+} + \mathrm{CH_3COO^-} $$

Step 3: 3. Write the balanced net ionic equation

In the complete ionic equation, we can cancel out the spectator ions, which are the same on both sides of the equation. In this case, the sodium ion (\(\mathrm{Na^+}\)) is the spectator ion. The balanced net ionic equation is: $$ \mathrm{CH_3COOH} + \mathrm{OH^-} \rightarrow \mathrm{H_2O} + \mathrm{CH_3COO^-} $$

Step 4: 4. Analyze the effect on the lightbulb's brightness

The lightbulb glows dimly in the acetic acid solution, which indicates that there are ions present that can conduct electricity. Upon adding an equivalent amount of \(\mathrm{NaOH}\), we can observe that the net ionic equation shows that hydroxide ions (\(\mathrm{OH^-}\)) react with acetic acid (\(\mathrm{CH_3COOH}\)) to form water and acetate ions (\(\mathrm{CH_3COO^-}\)). The spectator ions in this reaction (sodium ions) do not change, and there is no reduction in the overall number of ions in the solution. Therefore, the conductivity of the solution remains the same, and the lightbulb's brightness remains the same as well. Hence, after adding one equivalent of aqueous \(\mathrm{NaOH}\) to the solution, the lightbulb will remain the same brightness. The balanced net ionic equation that supports this answer is: $$ \mathrm{CH_3COOH} + \mathrm{OH^-} \rightarrow \mathrm{H_2O} + \mathrm{CH_3COO^-} $$

Key Concepts

Acid-Base Reaction

An acid-base reaction is a chemical process where an acid and a base react to form water and a salt. This is one of the most fundamental types of chemical reactions. In the case of the original exercise, acetic acid (a weak acid) reacts with sodium hydroxide (a strong base) in what is known as a neutralization reaction. The products of this kind of reaction typically include water, owing to the combination of hydrogen ions from the acid and hydroxide ions from the base, and a salt, which results from the remaining components of the acid and the base.
This type of reaction is essential because it helps demonstrate basic chemical interaction and neutralization, showing how acids can be neutralized by bases to form less acidic solutions.

Net Ionic Equation

A net ionic equation provides a simplified version of a chemical equation. It highlights only the components that actually participate in the chemical reaction and omits the spectator ions, which do not change during the process. In the given exercise, the reaction between acetic acid and sodium hydroxide can be expressed as a net ionic equation:

• The complete ionic equation first writes out all soluble electrolytes as ions. For instance: \( \mathrm{CH_3COOH} + \mathrm{Na^+} + \mathrm{OH^-} \rightarrow \mathrm{H_2O} + \mathrm{Na^+} + \mathrm{CH_3COO^-} \)
• The spectator ions, like \( \mathrm{Na^+} \), are present in the solution before and after the reaction and can be canceled out.

Thus, the net ionic equation becomes:
\[ \mathrm{CH_3COOH} + \mathrm{OH^-} \rightarrow \mathrm{H_2O} + \mathrm{CH_3COO^-} \] This equation clearly shows the essence of the reaction: acetic acid reacting with hydroxide ions to form water and acetate ions.

Conductivity

Conductivity in aqueous solutions refers to the ability of the solution to conduct an electric current, which depends on the presence of free ions. The original exercise mentions a lightbulb, which glows when electrodes are inserted into the acetic acid solution. This dim glow indicates that the solution conducts electricity, albeit weakly, because acetic acid is a weak electrolyte that partially ionizes in water.
Adding sodium hydroxide, a strong electrolyte, increases the number of ions via the neutralization process, maintaining similar conductivity. Since the total number of ions present in the solution does not decrease significantly when the reaction between acetic acid and sodium hydroxide occurs, the brightness of the bulb remains unchanged.

Neutralization Reaction

Neutralization reactions occur when an acid and a base react to form water and salt, effectively reducing the acid and base's active components. This type of reaction results in the formation of a neutral (or nearly neutral) pH solution. In the supplied exercise, the reaction between acetic acid and sodium hydroxide is a prime example of this type of reaction.

• Water formation: \( \mathrm{H^+} \) from the acid and \( \mathrm{OH^-} \) from the base form water, symbolizing neutralization.
• Salt formation: The remaining components of the acid and base form sodium acetate (salt).

The pronounced effect of a neutralization reaction is to lessen the acidic or basic properties of the reactants, thereby forming a more neutral solution. Although this reaction consumes ions, it generates new ions almost equivalently in the solution, maintaining overall conductivity. Thus, in practical terms, while the reaction alters the chemical composition, the physical property of conductivity—which makes the bulb glow—remains unchanged.