Question

Two wires from a battery are tested with a piece of filter paper moistened with \(\mathrm{NaCl}\) solution containing phenolphthalein, an acid-base indicator that is colorless in acid and pink in base. When the wires touch the paper about an inch apart, the rightmost wire produces a pink coloration on the filter paper and the leftmost produces none. Which wire is connected to the positive terminal of the battery? Explain.

Short answer

The rightmost wire is connected to the positive terminal of the battery. This is because the pink coloration on the filter paper indicates a basic solution, which is a result of the oxidation reaction occurring at the anode. Since the anode is always connected to the positive terminal of the battery, the rightmost wire must be the positive terminal.

Step-by-step solution

Understanding Electrolysis Reactions

In the given setup, the moistened filter paper with \(\mathrm{NaCl}\) solution acts as an electrolyte, meaning that the ions in the solution will move under the influence of the electric current from the battery. When the current passes through the solution, several redox reactions might occur, meaning that some ions will be oxidized while others will be reduced. For this particular situation, the possible reduction reaction at the cathode is: \[ \mathrm{2H^+(aq) + 2e^- \longrightarrow H_2(g)} \] The possible oxidation reactions at the anode are: \[ \mathrm{2Cl^-(aq) \longrightarrow Cl_2(g) + 2e^-} \] \[ \mathrm{4OH^-(aq) \longrightarrow O_2(g) + 2H_2O + 4e^-} \] By understanding the possible reactions, we can make sense of the observation made and link this knowledge to which wire is connected to the positive terminal.

Analyzing the Behavior of Phenolphthalein

Phenolphthalein is an acid-base indicator that changes color based on the pH of the solution. In an acidic solution (pH below 7), phenolphthalein remains colorless, while in a basic solution (pH above 7), it turns pink. Since the filter paper turned pink near the rightmost wire, that means the pH in that area has increased due to the formation of a basic solution. By identifying the reactions that lead to the formation of a basic solution, we can determine the terminals of the battery.

Identifying the Formation of a Basic Solution

In the list of possible reactions we compiled in Step 1, only one reaction leads to the formation of a basic solution: \[ \mathrm{4OH^-(aq) \longrightarrow O_2(g) + 2H_2O + 4e^-} \] This reaction occurs at the anode and is responsible for creating a basic solution near the wire. As a result, the phenolphthalein indicator turns pink. The anode is where oxidation takes place, and it is always connected to the positive terminal of the battery.

Conclusion

Based on the observation of the pink coloration on the filter paper, we can conclude that the rightmost wire is connected to the positive terminal of the battery. This is because the pink coloration indicates the presence of a basic solution at the rightmost wire, which is the result of the oxidation reaction occurring at the anode.