Question

Consider the equilibrium reaction below. At high concentrations of HA the solution will be yellow; but at low concentrations of HA the solution will be red. Initially the equilibrium mixture below is orange (combination of yellow & red). What color is expected after adding some colorless A–(aq) to the equilibrium solution? A-(aq) + H2O(ℓ) ⇋ HA(aq) + OH-(aq)

Answer 1

Answer:

The correct answer is: Yellow

Explanation:

The given reaction at equilibrium: A⁻(aq) + H₂O(ℓ) ⇋ HA(aq) + OH⁻(aq)

Given: Solution color at high product (HA) concentration = yellow

Solution color at low product (HA) concentration = red

Initial color of solution = orange

According to the Le Châtelier's principle, addition of reactant to the reaction at equilibrium, shifts the equilibrium towards the right side of the reaction or towards the forward direction. Therefore, favoring the product formation.

So, if the reactant A⁻(aq) is added to the reaction mixture, more product (HA) is produced. Therefore, the concentration of the product HA increases.

Since the solution is yellow in color at high product (HA) concentration.

Therefore, when the concentration of the product HA increases upon the addition of reactant A⁻(aq), the color of the solution changes from orange to yellow.

Answer 2

Explanation:

Methyl orange is the indicator which shows color transitions from red to orange and from orange to yellow.

This change in color of methyl orange is due to the change occurring in the pH of the solution.

• When the solution is acidic (pH less than 7) in nature then on addition of methyl orange the color of solution will become red.

• When solution is neutral (pH = 7) then addition of methyl orange will change the color of solution into orange.

• When solution is basic (pH more than 7) in nature then addition of methyl orange will change the color of solution into yellow.

Also, according to Le Chatelier's principle any disturbance caused in an equilibrium reaction will tend to shift the equilibrium in a direction away from the disturbance.

So, when $A^{-}$ is added into the solution then it means that we are increasing the amount of reactant.

Therefore, the equilibrium will shift in the forward direction that is, formation of products is favored. As product side contains HA so, this shift in equilibrium will change the color of solution into yellow.

Thus, we can conclude that yellow color is expected after adding some colorless $A^{-}(aq)$ to the equilibrium solution.