Dissociation of multiple weak acids

Weak acids such as H ₂ S, H ₂ CO ₃ and H ₃ PO ₄ that can dissociate two or more H ⁺ are called polybasic weak acids
.

The dissociation of multiple weak acids is completed step by step, and there is a dissociation equilibrium in each step of dissociation

For example, H ₂ S is a weak dibasic acid that dissociates in two steps

The total dissociation reaction formula of H ₂ S is

The equilibrium constant of the total dissociation reaction is

It should be noted that the total dissociation equilibrium relationship expresses the relationship between the concentrations of [H ⁺ ], [S ^2- ], and [H ₂ S] in the equilibrium system.

As long as the three coexist in the equilibrium system, each substance The equilibrium concentration must satisfy the relationship represented by the above equilibrium constant expression.
S- is the dissociation product in the second step, so [H ⁺ ]≠2[S ^2- ]

Under normal temperature and pressure, c0=0.

1 mol·dm ^-3 in saturated H ₂ S aqueous solution , so the concentration of S ^2- and HS ^- in the solution can be controlled by adjusting the acidity .

For multiple weak acids, generally there are

Kal ^Θ》Ka2 ^Θ》Ka3 ^Θ

This can be understood from the perspective of electrostatic attraction and balance between ions
.

The charge of the ions dissociated in the first step is low, and the electrostatic attraction between the ions is small, so it is easy to dissociate; at the same time, the H+ dissociated in the first step has an inhibitory effect on the dissociation balance of the second step

Since Ka1 ^Θ > Ka2 ^Θ , in the dissociation of multiple weak acids, the [H ⁺ ] in the solution is generally determined by the first-order dissociation equilibrium; the concentration of -1 valent acid ion is approximately equal to the [H ⁺ ] in the solution ; -2 The concentration of valence acid ions is approximately equal to Ka2 ^Θ
.

It must be noted that for mixed acid solutions, the above conclusions are generally not applicable

[Example 7-3] mol · DM seek 0.

010 ^-3 H ₂ CO.

Ka1 ^Θ =4.
5×10 ^-7 , Ka2 ^Θ =4.
7×10 ^-11
.

Solution Because of the Ka1 ^Θ》Ka2 ^{Θ of} H ₂ CO ₃ , the concentration of H ⁺ in the system is mainly determined by the first dissociation

The calculation results show that the concentration of H+ dissociated in the second step (equal to the concentration of CO ₃ ^2- ) is far less than the concentration of H+ dissociated in the first step.

The conclusion that H+] is generally determined by the first-order dissociation equilibrium is correct, and this conclusion can be used directly in the calculation

For multiple weak acids such as H ₃ PO ₄ solutions, the calculation of ion concentration is similar to that of dibasic weak acids, that is, [H ⁺ ] in the solution is determined by the first step of dissociation; the concentration of -1 valent acid ion is equal to [H ⁺ in the system ]; The concentration of -2 valent acid radical ions is equal to the second-order dissociation constant Ka2 ^Θ
.

From the [H ⁺ ] of the tribasic acid system and the initial concentration of tribasic acid c0, the concentration of various acid radical ions can be obtained by using various equilibrium constants, the relationship between [H ⁺ ] and c0