Topic: Solutions of strong and weak electrolytes

 

In aqueous solutions strong electrolytes are completely dissociated into ions. An effective conditional molar concentration of an ion is called activity (a) of the ion (i). It equals: a_i = fC_i,where f – the activity coefficient. The activity coefficient for different ions changes as the concentration of the solution changes, so in concentrated solutions f< 1, and in deluted solutions it is moving towards 1.

Electrolyte solution is characterized by the ionic strength of the solution(I), which can be calculated as the following:

                                   I (of the solution) = ½ ∑C_i∙Z_i²,

where C ₁ – is the molarity of the i-ion.

Z_i –is the charge of i-ion in the units of the charge of the electron.

The activity coefficient depends on the ionic strength of the solution on the following way lg f =-0,5           Z_i². I 

Alternatively, in aqueous solutions weak electrolytes are not completely dissociated into ions, but only a small part of them. The quantitative characteristic of this process is the degree of electrolytic dissociation (α), which depends on the strength of electrolyte and thus can be as: 0≤ α ≤ 1; for the strong electrolytes

α >0,3 for the weak ones α < 0,03, and for the electrolytes of the middle strength 0,03< α <0,3. The degree of electrolytic dissociation equals

 

                                          N_i        C_i      ν_i

                                  α = —— = —— = ——

                                           N    C        ν

 

N_i- the number of the molecules of the electrolyte, dissociated into ions.

N- the common number of the molecules of the electrolyte in the solution

(the number of the molecules of electrolyte, put into the solvent)

C_i – molarity of dissociated electrolyte.

C- total molarity of the electrolyte.

ν_i – the amount of the electrolytic substance, dissociated into the ions

ν - the common amount of the electrolytic substance in the solution.

 The degree of electrolytic dissociation depends on the number of parameters including the concentration. It makes the degree of electrolytic dissociation an unconvenient value for the quantitative description of dissociation process. The dissociation constant doesn’t depend on the concentration of the weak electrolyte in the solution. In the weak electrolyte solutions (acid HAn or base KtOH) a chemical equilibrium is established:

HAn ↔ H⁺ + An^- (1)

KtOH ↔ Kt⁺ + OH^- (2)

 

The equilibrium is quantitatively characterized by the dissociation constant:

K_a – for weak acids HAn, K_b for weak bases KtOH.

K and α are connected with each other according to the Ostvald’s law, for weak acid

                                             α²·C

                                    K_a = ———, if 1-α ≈ 1, K_a = α²·C;

                                              1-α

For weak base

                                        α²·C

                              K_b = ———, If 1-α ≈ 1, K_b= α²·C;

                                         1-α

     Therefore for a one-base weak acid

 

                              [H⁺]² = K_a∙ C, [H⁺] = C∙α, [H⁺] =K_a/α

 

          For one-acid weak base  

                              [OH^-]² = K_b·C, [OH^-] = C·α, [OH^-] = K_b/α