Question

HELP!!! The reversible reaction for the pH indicator bromophenol blue is (C19H8Br4O5S)H– ↔ (C19H8Br4O5S)2– + H+. Write the equilibrium expression for this reaction.

Answer 1

Answer:

• $Keq=\frac{[(C_{19}H_8Br_4O_5S)^{2-}].[H^+]}{[(C_{19}H_8Br_4O_5S)H^-]}$

Explanation:

In an equilibrium reaction two reactions are occurring: the direct or forward reaction and the reverse reaction.

The equilibrium is attained when both rates, the direct's and the reverse's, are equal.

The equilibrium expression shows the ratio of the product of the molar concentrations of the aqueous species of the right side to the product of the molar concentrations of the left side, each species raised to a power. Each power is equal to the corresponding stoichiometrical coefficient of the balanced equation.

In this case:

• Forward reaction: (C₁₉H₈Br₄O₅S)H⁻ → (C₁₉H₈Br₄O₅S)²⁻ + H⁺

                                                                      the arrow goes from left to right

• Reverse reaction: (C₁₉H₈Br₄O₅S)H⁻ ← (C₁₉H₈Br₄O₅S)²⁻ + H⁺

                                                                      the arrow goes from right to left

• Equilibrium reaction: (C₁₉H₈Br₄O₅S)H⁻ ⇄ (C₁₉H₈Br₄O₅S)²⁻ + H⁺

                                                                        double arrow

Equilibrium expression:

• $Keq=\frac{[(C_{19}H_8Br_4O_5S)^{2-}].[H^+]}{[(C_{19}H_8Br_4O_5S)H^-]}$

All the coefficients are equal to 1, so all the powers are equal to 1.

Note that the use of [ ] means molarity (molar concentration).