Magic Pitcher Demonstration:

Magic Pitcher Demonstration:

Phenolphthalein is an indicator of acids (colorless) and bases (pink). Sodium hydroxide is a base, and it was in the pitcher at the beginning, so when added to the phenolphthalein in beakers 2 and 4, it turned pink (top half of the graphic).

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Explanation:

Equilibrium: HIn --> H⁺ + In^-
colorless red

The equilibrium shifts right, HIn decreases, and In^- increases. As the pH increase between 8.2 to 10.0 the color becomes red because of the equilibrium shifts to form mostly In^- ions.

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The third beaker has only the NaOH but no phenolphthalein, so it remained colorless. The first beaker contain acetic acid and is skipped over at first.

After pouring beakers 2, 3, 4 back into the pitcher it give a pink solution.

Bottom half of the graphic: When the pitcher is then poured back into beakers 2, 3, 4 it is a pink solution.

In the first beaker, a strange thing happens in that the pink solution coming out of the pitcher now changes to colorless. This happens because the first beaker contains some vinegar or acetic acid which neutralizes the NaOH, and changes the solution from basic to acidic. Under acidic conditions, the phenolphthalein indicator is colorless.

Neutralization: HC₂H₃O₂ + NaOH --> Na(C₂H₃O₂) + HOH

Explain the color indicator change:

Use equilibrium principles to explain the color change for phenolphthalein at the end of the demonstration.

Solution:

The simplified reaction is: H⁺ + OH^- --> HOH

As OH^- ions are added, they are consumed by the excess of acid already in the beaker as expressed in the above equation. The hydroxide ions keep decreasing and the hydrogen ions increase, pH decreases.

See lower equation: The indicator equilibrium shifts left, In^- ions decrease. Below pH 8.2 the indicator is colorless. As H⁺ ions are further increased and pH decreases to pH 4-5, the indicator equilibrium is effected and changes to the colorless HIn form.

Equilibrium: HIn --> H⁺ + In^-
colorless red