Question

A student dissolves some phosphoric acid (pKa = 2.1, 6.8, 12.4) in water such that the total concentration of all forms of phosphate is 1.16 mmol L-1. Some of the phosphoric acid dissociates into H2PO4–. Which of the following statements is true about this process (select all that apply)? (Hint: consider which of the statements mean the same thing as each other.)

Q < K

K > 1

ΔGrxn < 0

The pH must be decreasing

ΔG° < 0

Answer 1

Ans. Phosphoric acid successively and spontaneously dissociates into respective anion and H⁺ because water is weak acid than all the forms of phosphoric species (pKa of water = 14.0)-

            H₃PO₄ + H₂O ------------> H₂PO4^- + H₃O⁺

            H₂PO4^2- H₂O ------------> HPO4^2- + H₃O⁺

            HPO4^3- + H₃O -----------> PO4^3- + H₃O⁺

#Option 1. Correct.

Phosphoric acid successively and spontaneously dissociates into respective anion and H⁺ to yield a solution of equilibrium concentrations of all the chemical species relative to their pKa values. Let the equilibrium constant for the equilibrate solution be K.

When reaction quotient Q is smaller than equilibrium constant, the reaction goes in forward direction.

Compare Kp and Q

            When Q = K            - the reaction is at equilibrium

                        Q > K             - Reaction goes to the left to attain equilibrium

                        Q < K             - Reaction goes to the right to attain equilibrium.

Since Q < K, the reaction would go to the right, i.e. dissociation of H₃PO₄ into H₂PO4^- to attain equilibrium.

# Option 2. Incorrect. Since the acid is weak, the equilibrium constant (or, in another term, the acid dissociation constant) would always be less than 1.

#Option 3. Correct.

Using the equation dG = dG⁰ + RT lnKeq                      - equation 1

Where, dG = calculated/ experimental free energy change = ?

dG⁰ = standard/ theoretical free energy change

T = temperature in kelvin = (⁰C + 273.15) K

K = equilibrium constant under given condition

Compare dG and dG⁰ :

If dG = dG^0’               - the reaction is at equilibrium

            If dG > dG^0’               – the reaction would go in backward direction

            If dG < dG^0’               - the reaction would go in forward direction

When dG_rxn < 0, i.e. when dG is less than dG⁰ on a relative scale, the reaction would go to the right, i.e. dissociation of H₃PO₄ into H₂PO4^- to attain equilibrium.

# Option 4: Correct. The pH must be decreasing (the solution becomes more acidic) as the weak acid gradually dissociates to form more H⁺. Gradual increase in H⁺ will cause gradual decrease in PH.

Option 5. Correct. dG⁰ < 0 represents negative value of dG⁰. A reaction with –ve value of dG⁰ is spontaneous in nature.

Since the acid dissociates spontaneously to establish equilibrium, the reaction as dG⁰ less than 0.0.

So, correct options are- 1, 3, 4, 5