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).
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.
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
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