Same ion effect and salt effect

1.
Same ion effect

Dissociation equilibrium is also a chemical equilibrium, so when the concentration of a certain substance in the equilibrium system changes, the equilibrium will move and the equilibrium will be re-established under new conditions
.

For example, the dissociation equilibrium of weak electrolyte ammonia

If a small amount of strong electrolyte NH ₄ Cl is added to the ammonia water, the introduction of NH ₄ ⁺ destroys the balance of the original system, and the balance shifts to the direction of generating NH ₃ ·H ₂ O, thereby reducing the solution of NH ₃ ·H ₂ O Off-degree
.

Similarly, adding the strong electrolyte NaAc to the weak acid HAc solution will produce similar results
.

In the weak electrolyte solution, adding a strong electrolyte with the same ion as its dissociation reaction reduces the dissociation degree of the weak electrolyte.

[Example 7-4] mol · DM to 0.
10 ^-3 NH2 a _.
3 · H ₂ was added solid NH O solution _.
4 Cl, making NH2 _.
4 concentration of Cl in mol · DM 0.
10 ^-3 , seeking solution [OH ^- ] and NH2 _.
3 · H ₂ O dissociation of a
.

The concentration of dissociated NH ₃ ·H ₂ O at equilibrium is x mol·dm ^-3 , there is

Substituting the equilibrium concentration of each substance into the equilibrium constant expression, we get

Since c0»400Kb ^Θ and the introduction of NH ₄ CI reduces the dissociation degree, it can be calculated approximately

0.

Substituting the equilibrium constant expression, we get

This result is significantly reduced compared with the dissociation degree of 0.

2.
Salt effect

Adding NH ₄ Cl to the ammonia water , NH ₄ ⁺ has an effect on the balance of weak electrolytes, and the introduction of CI ^{-in the} system also has a certain effect on the balance
.

In the previous examples of weak acids and weak bases, the calculation process is based on the concentration of each substance in the system.
It is approximated as f=1 and a≈c
.

If the concentration of ions in the solution increases, the effect of ionic atmosphere cannot be ignored.

Since the value of f is less than 1, so that a large concentration of ions in the solution, [OH ^- ] concentration increases, i.

In the weak electrolyte solution, adding a strong electrolyte that does not have the same ion with the weak electrolyte dissociation reaction, thereby increasing the degree of dissociation of the weak electrolyte is called the salt effect
.

The salt effect is generally small
.