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In Part III, the phenolphthalein indicator is used to monitor the equilibrium shifts of the ammonia/ammonium ion system. The phenolphthalein equilibrium established with water is Hph(aq)(colorless) + H2O (l) H3O+ (aq) + ph-(aq)(pink or red). You compared the color of the solutions in three test tubes that initially contained 3 mL of 0.1 M ammonium hydroxide and a few drops of phenolphthalein indicator. In the first test tube, you added 1 M NH4Cl dropwise. What color change was observed and what did this color change indicate about the shift in the phenolphthalein equilibrium? a. The solution turned a more intense pink or red color indicating that the phenolphthalein equilibrium shifted to the left, producing more of the pink or red colored Hph.

User Seeingidog
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The pink color in the solution fades. Some of the colored indicator ion converts to the colorless indicator molecule.

Step-by-step explanation

What's the initial color of the solution?


\text{NH}_4\text{Cl} is a salt soluble in water.
\text{NH}_4\text{Cl} dissociates into ions completely when dissolved.


\text{NH}_4\text{Cl} \; (aq)\to {\text{NH}_4}^(+) \; (aq) +{\text{Cl}}^(-) \; (aq).

The first test tube used to contain
\text{NH}_4\text{OH}.
\text{NH}_4\text{OH} is a weak base that dissociates partially in water.


\text{NH}_4\text{OH} \; (aq) \rightleftharpoons {\text{NH}_4}^(+)  \;(aq)+ {\text{OH}}^(-) \; (aq).

There's also an equilibrium between
\text{OH}^(-) and
{\text{H}_3\text{O}}^(+) ions.


{\text{OH}}^(-)\;(aq) + {\text{H}_3\text{O}}^(+) \;(aq) \to 2\; \text{H}_2\text{O} \;(l).


\text{OH}^(-) ions from
\text{NH}_4\text{OH} will shift the equilibrium between
\text{OH}^(-) and
{\text{H}_3\text{O}}^(+) to the right and reduce the amount of
{\text{H}_3\text{O}}^(+) in the solution.

The indicator equilibrium will shift to the right to produce more
{\text{H}_3\text{O}}^(+) ions along with the colored indicator ions. The solution will show a pink color.

What's the color of the solution after adding NH₄Cl?

Adding
\text{NH}_4\text{Cl} will add to the concentration of
{\text{NH}_4}^(+) ions in the solution. Some of the
{\text{NH}_4}^(+) ions will combine with
\text{OH}^(-) ions to produce
\text{NH}_4\text{OH}.

The equilibrium between
\text{OH}^(-) and
{\text{H}_3\text{O}}^(+) ions will shift to the left to produce more of both ions.


{\text{OH}}^(-)\;(aq) + {\text{H}_3\text{O}}^(+) \;(aq) \to 2\; \text{H}_2\text{O} \;(l)

The indicator equilibrium will shift to the left as the concentration of
{\text{H}_3\text{O}}^(+) increases. There will be less colored ions and more colorless molecules in the test tube. The pink color will fade.

User Kenor
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