An indicator has the reaction Hln(aq) ↔ 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?
When a strong base is poured into a solution of the indicator Hln(aq) ↔ H+(aq) + In-(aq), the following changes occur:
1. Dissociation of the strong base: A strong base, such as sodium hydroxide (NaOH), will completely dissociate into its constituent ions in water. In this case, NaOH will dissociate into Na+ and OH- ions.
2. Effect on the reaction: The presence of OH- ions, which are a strong base, will react with the H+ ions in the reaction to form water. This leads to a decrease in the concentration of H+ ions in the solution.
3. Shifting the reaction: According to Le Chatelier's principle, reducing the concentration of H+ ions will cause the equilibrium to shift to the right, towards the reactant side. As a result, more Hln (the indicator) will be formed.
4. Concentration of HIn: Since the equilibrium shifts towards the reactant side, there will be an increase in the concentration of HIn (undissociated indicator) compared to its dissociated form, In-.
5. Color change: The HIn form of the indicator usually has a different color than its In- form. In this case, the HIn form might have one color (for example, yellow), while the In- form might have a different color (for example, blue). Therefore, as more HIn is formed due to the addition of the strong base, the solution might change from the color associated with In- to the color associated with HIn.
When a strong acid is poured into the solution:
1. Dissociation of the strong acid: A strong acid, such as hydrochloric acid (HCl), will completely dissociate into its constituent ions in water. In this case, HCl will dissociate into H+ and Cl- ions.
2. Effect on the reaction: The presence of H+ ions, which are a strong acid, will react with the In- ions in the reaction to form Hln. This leads to an increase in the concentration of H+ ions in the solution.
3. Shifting the reaction: Increasing the concentration of H+ ions will cause the equilibrium to shift to the left, towards the product side. As a result, more In- will be formed.
4. Concentration of HIn: Since the equilibrium shifts towards the product side, there will be a decrease in the concentration of HIn compared to In-.
5. Color change: As the concentration of In- increases due to the addition of the strong acid, the solution might change from the color associated with HIn to the color associated with In-.
To understand how the indicator changes color when a strong base is poured into a solution, let's break down the reaction and its equilibrium.
The indicator is initially in the form Hln (aq), which is the acidic form. When this indicator is dissolved in water, it undergoes a dissociation reaction:
Hln(aq) ↔ H+(aq) + In-(aq)
In this equation, Hln represents the acidic form of the indicator, H+ is a hydrogen ion, and In- is the conjugate base of the indicator. The symbol (aq) indicates that these species are in the aqueous phase.
Now, when a strong base is poured into the solution, it dissociates completely into its ions. For example, let's consider sodium hydroxide (NaOH). It dissociates to form Na+ and OH- ions:
NaOH(aq) → Na+(aq) + OH-(aq)
As OH- is added to the solution, it reacts with the H+ ions present in the equilibrium to form water:
OH-(aq) + H+(aq) → H2O
This reaction reduces the concentration of H+ ions in the solution. According to Le Chatelier's principle, when we decrease the concentration of one of the reactants (in this case, H+ ions), the equilibrium shifts to favor the formation of more of that reactant. As a result, the reaction shifts to the left, which means more Hln is formed.
Now, the color change in the indicator is due to the difference in the colors of Hln and In-. Generally, Hln is a different color compared to In-. In acidic conditions, where Hln is predominant, the solution may appear one color. In basic conditions, where In- is more abundant, the solution may appear a different color.
When a strong acid is poured into the solution, the opposite reaction occurs. The H+ ions from the acid react with the In- ions, forming more Hln and reducing the concentration of In-. As a result, the equilibrium shifts to the right, favoring the formation of more Hln. This, in turn, may cause a color change in the indicator solution again.
In summary, the color change in the indicator is due to the shifts in the equilibrium between the acidic form (Hln) and the conjugate base (In-) in response to changes in the concentration of H+ ions. The solution will change color when there is a shift in the equilibrium favoring either Hln (acidic conditions) or In- (basic conditions), depending on whether a strong base or a strong acid is added.