Question

5. Two wires from a battery are tested with a piece of filter paper moistened with aqueous NaCl solution. The solution also contained 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 produced a pink color on the filter paper and the leftmost produced no color. Write balanced half-reactions for what is happening at each electrode. Be careful! NaCl (ag) is only there as a supporting electrolyte.

Answer 1

It is because of the phenolphthalein only that shows color due to acid-base reactions. Phenolphthalein is a weak acid and appeared as a colorless powder. therefore, in acidic solution it doesn't show any color change. But in basic medium, it loses H+ to produce phenophthalein ion, which is pink in color.

Since, in the left wire there is no color change, which indicates acidic medium and thus no chemical change in the phenophthalein molecule will be observed in the left electrode. Therefore, half cell reaction in the left electrode

No Change

Phenophthalein (colorless)

In the right wire the color changes to pink. This indicates basic medium and thus phenophthalein molecule loses one proton. Therefore, half cell reaction in the right electrode

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Colorless Pink