Question

Use Le Châtelier's Principle to fully explain what happens when excess HCl is added to a blue coloured solution of bromothymol blue. Refer to the appropriate equation(s)

Use Le Chatalier's Principle to fully explain what happens when excess NaOH is added to the previous solution of bromothymol blue. Refer to the appropriate equation(s).

Does the value of the equilibrium constant change when hydrochloric acid is added to the test tube? Explain why or why not?

Answer 1

The equilibrium reaction is-----

Thymol blue (blue) + HCl ================= Thymol Blue (yellow)

As you know, equilibrium reactions are governed by Le Chatelier's Principle, which states that the system will shift in such a way as to reduce a stress placed on its position.

A. When you adding hydrochloric acid in excess, i.e. increasing the concentration of hydrogen ions present in solution, the system must shift in such a way as to decrease the concentration of hydrogen ions.

Since the hydrogen ions are present on the reactants' side of the equation, decreasing their concentration means shifting the equilibrium to the right. So the system will move in the forward direction and yellow color solution will form. i.e,

Thymol Blue (blue) + HCl (aq) ---------------- Thymol Blue (yellow)

B when the excess of NaOH is added, then it will react with the HCl and the HCl concentration on the left side starts to decrease. So, the equilibrium will shift to the left side and the reverse reaction will take place to give the blue color of the solution (blue color of Thymol blue)

Thymol blue ( yellow) ------------------------ Thymol Blue (blue) + HCl

C No, the value of equilibrium constant will not change when the excess of HCl is added. The value of equilibrium constant do not change when we change the concentration of things present in equilibrium. It is because when we change the concentration of any thing, the concentration of other thing will automatically change according to le chatelier's principle, and thus have no effect on equilibrium constant. The only thing which can effect the equilibrium constant is the temperature.