An indicator has the reaction Hln(aq) ¡ê H+(aq) + In-(aq). Explain how the indicator can change color when a strong base is poured into a solution of this indicator. Will a strong base dissociate? Which way will this cause the reaction to shift? Is there more or less HIn? Do HIn and In- have the same color? Will the solution change color when a strong acid is poured into it? Why?
HIn ==> H^+ + In^-
When a strong acid is added, it shifts the equilibrium to the left. Indicator work because the HIn is one color (or no color) and the In is a different color (or no color).
To understand how an indicator changes color when a strong base is added to its solution, let's break down the given reaction and answer each question sequentially.
1. Will a strong base dissociate?
Yes, a strong base will dissociate completely in water, resulting in the formation of hydroxide ions (OH-) and cations of the base.
2. Which way will this cause the reaction to shift?
When the strong base is added to the solution, it will react with the hydrogen ions (H+) present in the indicator solution. The base will accept the hydrogen ions to form water molecules, effectively reducing the concentration of H+ and shifting the equilibrium towards the right (the products side of the reaction).
3. Is there more or less HIn?
The indicator exists in two forms: HIn (the acidic form) and In- (the basic form). The addition of a strong base will cause the equilibrium to shift towards the products side, resulting in the consumption of H+ ions and an increase in the concentration of In-. Therefore, there will be less HIn and more In- formed in the solution.
4. Do HIn and In- have the same color?
No, HIn and In- have different colors. Typically, the acidic form HIn (protonated form) of the indicator exhibits a different color compared to the basic form In- (deprotonated form). This change in color is what indicates the pH of the solution.
5. Will the solution change color when a strong acid is poured into it? Why?
Yes, the solution will also change color when a strong acid is added. Adding a strong acid will increase the concentration of H+ ions in the solution, shifting the equilibrium of the indicator reaction towards the left (the reactants side of the reaction). This will result in the conversion of In- back to HIn, decreasing the concentration of In- and increasing the concentration of HIn, leading to a change in color.
In summary, the addition of a strong base will shift the equilibrium towards the right, favoring the formation of In- and resulting in a color change in the solution. Conversely, the addition of a strong acid will shift the equilibrium towards the left, favoring the formation of HIn and causing the solution to change color in the opposite direction.