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.

Answer 1

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.