Hydrolysis step of weak acid and weak base salt

4.
Hydrolysis of weak acid and weak base salts

The feature of weak acid and weak alkaline salt water hydrolysis is that positive ions and negative ions are hydrolyzed in the water at the same time and promote each other to increase the degree of hydrolysis
.

Add the above three formulas to get the hydrolysis reaction formula of NH ₄ Ac

The NH ₄ Ac hydrolysis equilibrium constant is the product of the equilibrium constants of the three reactions, namely

It can be seen from the expression of Kh ^Θ that the tendency of double hydrolysis is greater than that of single hydrolysis, and the degree of hydrolysis increases
.

The NH2 _.

[H ⁺ ]=[NH ₃ ]-[HAc]

From the hydrolysis reaction of NH ₄ ⁺

The Ac ^- hydrolysis reaction to give

Since Kw ^[Theta] = [H ⁺ ] [OH ^- ], so

Substitute [NH ₃ ] and [HAc] into the formula [H ⁺ ]=[NH ₃ ]-[HAc]

When cO »Kh ^[Theta] time, [NH2 _.

Therefore, the H ⁺ concentration is

When cO »Ka of ^[Theta] when

In the expression, [H ⁺ ] has no direct relationship with the concentration c0 of the salt solution, but it should be noted that the expression of [H ⁺ ] is obtained under approximate conditions related to c0, that is, c0 cannot be too small

Using the [H ⁺ ] expression of weak acid and weak alkaline salt solution , the following conclusions can be obtained:

If Ka ^Θ >Kb ^Θ , the degree of hydrolysis of the positive ion of the weak acid and weak base salt is greater than that of the negative ion, and the solution will be acidic after hydrolysis, pH<7;

If Ka ^Θ <Kb ^Θ , the degree of hydrolysis of the positive ion of the weak acid and weak base salt is smaller than that of the negative ion, and the solution will appear alkaline after hydrolysis, with pH>7;

If Ka ^Θ ≈Kb ^Θ , the degree of hydrolysis of the two ions of the weak acid and weak base salt is close, and the solution becomes neutral after hydrolysis, with pH ≈7