In Part III, the phenolphthalein indicator is used to monitor the equilibrium shifts of the ammonia/ammonium ion system. The phenolphthalein

Question

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.

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Margaret 1 week 2022-11-24T16:01:44+00:00 1 Answer 0 views 0

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    2022-11-24T16:03:39+00:00

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

    Explanation

    What’s the initial color of the solution?

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

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

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

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

    There’s also an equilibrium between [tex]\text{OH}^{-}[/tex] and [tex]{\text{H}_3\text{O}}^{+}[/tex] ions.

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

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

    The indicator equilibrium will shift to the right to produce more [tex]{\text{H}_3\text{O}}^{+}[/tex] 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 [tex]\text{NH}_4\text{Cl}[/tex] will add to the concentration of [tex]{\text{NH}_4}^{+}[/tex] ions in the solution. Some of the [tex]{\text{NH}_4}^{+}[/tex] ions will combine with [tex]\text{OH}^{-}[/tex] ions to produce [tex]\text{NH}_4\text{OH}[/tex].

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

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

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

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