Dissociation balance of weak acid and weak base

Weak acids and weak bases are weak electrolytes, and only a small part of them dissociate in the aqueous solution, and the ions produced by the dissociation maintain a balanced relationship with the undissociated molecules or ions
.

1.
Dissociation of weak acid and weak base

Acetic acid is a weak monobasic acid, often abbreviated as HAc, which is partially dissociated in aqueous solution

Often abbreviated as

When the reaction reached equilibrium dissociation, HAc, H ⁺ and Ac ^- concentration (activity should be exact) satisfy the relationship

Ka ^Θ is the dissociation equilibrium constant of weak acid, also called acid dissociation equilibrium constant
.

Ammonia is a typical monobasic weak base, and there is a dissociation equilibrium in the aqueous solution

When the reaction reached equilibrium dissociation, NH2 ^.

Ka ^{Θ is} called the basic dissociation equilibrium constant, and the Kb ^{Θ of} ammonia water = 1.

In an aqueous solution, when the dissociation of acetic acid reaches equilibrium, it can be known from the dissociation reaction formula that H+ and Ac- of equal mass are produced.

[Example 7-1] · DM calculated 0.
10 mol ^-3 HAc solution of [H ⁺ ], [Ac ^- ] and [HAc]
.

When the dissociation reaches equilibrium, [H ⁺ ] in the solution is x mol·dm ^-3
.

The equilibrium constant expression is

Substituting the concentration of each substance at equilibrium into the expression of Ka ^Θ , there is

From the calculation results, it can be seen that the HAc molecules in the system dissociate very little, and the formation of [H ⁺ ] is also very small.

Since c0»[H ⁺ ], then [HAc]≈c0, so

Using this approximate method to calculate [H ⁺ ] does not need to solve the equation
.
However, this approximate condition is that the Ka ^Θ value of the weak acid is small and the initial HAc concentration c0 is large, otherwise the calculated results will produce large errors
.
Generally speaking, when c0≥400Ka ^Θ , the [H ⁺ ] of the solution can be approximated by the simplest formula
.

The same approximation method can also be used for a weak monobasic base.
When c0≥400Kb ^Θ , there is approximately

The degree of dissociation refers to the percentage of the initial amount of a substance that has been dissociated, and is represented by a
.
Similar to the conversion rate of chemical equilibrium, the dissociation degree of weak acid is

Substituting into the dissociation degree calculation formula, there are

[Example 7-2] Calculation mol · DM 0.
10 ^-3 NH2 _.
3
· H ₂ O solution of [OH ^- ] and NH2 _.
3
· H ₂ O dissociation degree
.

Solution in 0.
10 mol·dm ^-3 NH ₃ ·H ₂ O solution

If the concentration of NH ₃ ·H ₂ O is 1.
0×10 ^-3 mol·dm ^-3 , the calculation shows that its dissociation degree a=13.
4%
.
This shows that the smaller the weak electrolyte concentration, the greater the degree of dissociation a
.

The relationship between the dissociation degree a of the weak electrolyte and the dissociation constant is

Both Ka ^Θ and Kb ^Θ are equilibrium constants.
The magnitude of the equilibrium constant can indicate the dissociation tendency of weak acids and weak bases.
The larger the equilibrium constant, the greater the dissociation tendency of weak acids or weak bases
.
Therefore , the size of Ka ^Θ and Kb ^Θ can indicate the relative strength of weak acids and weak bases
.
Generally , acids with Ka ^Θ greater than 10 ^-1 are called strong acids, acids with Ka ^Θ between 10 ^-1～10 ³ are called medium strong acids, and acids with Ka ^Θ less than 10 ^-3 are called weak acids
.
Similarly, bases can also be classified according to the value of Kb ^Θ
.

Related link: Dissociation balance of water