Problem 110 - Chemistry: The Central Science

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.

Step-by-Step Solution

Answer

The rightmost wire, which produces a pink coloration on the filter paper, is connected to the negative terminal (cathode) of the battery. This is because the cathode induces a redox reaction that increases pH and creates a basic environment, causing the phenolphthalein to turn pink. Meanwhile, the leftmost wire, producing no color change, is connected to the positive terminal (anode) of the battery, where an acidic environment is created.

Step 1: Understanding Phenolphthalein behavior

Phenolphthalein is an acid-base indicator. This means that it changes its color depending on the pH of the solution it is in. In an acidic solution (pH < 7), phenolphthalein is colorless. In a basic solution (pH > 7), phenolphthalein turns pink.

Step 2: Knowing the reactions in the moistened filter paper

When the battery is connected to the moistened filter paper containing NaCl solution, an electrochemical reaction occurs. In this reaction, the neutral salt (NaCl) will get dissociated into its positive (Na+) and negative (Cl-) ions. The positive terminal of the battery, called the anode, will produce a redox reaction that will lead to chloride (Cl-) ions at the paper surface to oxidize to form chlorine (Cl2) gas and the electrons from chloride ions will flow to the anode. This reaction, in turn, will decrease the pH and create an acidic environment around the anode. The negative terminal of the battery, called the cathode, will induce a redox reaction that will reduce H+ ions in the solution to form hydrogen gas (H2) and water. This reaction consumes H+ ions, resulting in a more basic environment around the cathode, which has an increase in pH.

Step 3: Connecting the given information

As we know that phenolphthalein turns pink in basic solutions, the pink coloration on the filter paper occurs where the cathode of the battery is connected, which is the rightmost wire. Meanwhile, the leftmost wire does not produce any visible color change, suggesting that it is the anode where an acidic environment is created.

Step 4: Concluding the answer

Consequently, based on the given information and electrochemical reactions, the rightmost wire, which produces a pink coloration on the filter paper, is connected to the negative terminal (cathode) of the battery. The leftmost wire, which produces no color change, is connected to the positive terminal (anode) of the battery.

Key Concepts

Acid-Base Indicators

Acid-base indicators are special substances that can show changes in the environment's acidity or basicity. They do this by changing color depending on the pH of the solution they are in. One common indicator is phenolphthalein, which behaves distinctly in different pH environments:

• In acidic solutions (pH less than 7), phenolphthalein is colorless.
• In basic solutions (pH greater than 7), it turns a vibrant pink color.

This unique property makes phenolphthalein very useful for detecting changes from an acidic environment to a basic one just by observing the color shift.
In the exercise given, phenolphthalein helps us identify where the solution becomes basic when placed on the filter paper. Hence, it forms part of the visual clue system in many scientific exercises to understand electrochemical environments.

Redox Reactions

Redox reactions are a combination of reduction and oxidation processes. These reactions occur simultaneously, where one substance gains electrons (reduction) and another loses electrons (oxidation). In electrochemistry, redox reactions are fundamental as they form the core of how batteries and galvanic cells work.
When the moistened filter paper with NaCl around the battery wires undergoes redox reactions, you see:

• At the positive terminal (anode), oxidation happens. Chloride ions ( ext{Cl}^-) lose electrons to form chlorine gas ( ext{Cl}_2), darkening the ions and creating an acidic pocket.
• At the negative terminal (cathode), reduction occurs. Hydrogen ions ( ext{H}^+) gain electrons to form hydrogen gas ( ext{H}_2), consuming acidity and making the solution more basic.

This dynamic change in ion activity is evident in multiple reactions in everyday life, from rusting of metals to biological processes involving cellular respiration.

pH and Color Change

pH is a measure of how acidic or basic a solution is. It can range from 0, which is highly acidic, to 14, which is highly basic, with 7 being neutral. pH changes can be visually detected using indicators like phenolphthalein, which show clear color transformations at specific pH ranges.
In the context of the given exercise, when the environment around the cathode becomes basic due to the redox reactions, the pH increases above 7, causing phenolphthalein to turn pink. This color change is central to identifying which part of the setup becomes basic, thereby indicating which wire connects to the negative terminal.
Understanding pH and its impact on color change helps in various scientific and industrial applications, such as water treatment, food processing, and understanding biochemical pathways in living organisms.

Electrode Reactions

Electrode reactions are key parts of any electrochemical process. An electrode is a conductor through which electricity enters or leaves an object, substance, or region. In batteries and electrochemistry setups, electrodes play a pivotal role in facilitating redox reactions.
In the scenario given, each wire from the battery acts like an electrode:

• The anode (positive terminal) facilitates the oxidation reaction and decreases local pH.
• The cathode (negative terminal) promotes a reduction reaction and increases local pH, evident by the pink color from phenolphthalein.

The reactions at electrodes underpin how electrical energy is stored and released. Comprehending these reactions helps in designing more efficient batteries and understanding corrosion, electroplating, and production of chemicals like chlorine and hydrogen.