Activity and Activity Coefficient

Activity and Activity Coefficient

• In a solution, diferent charged ions (cations and anions) have electrostatic interaction. This interaction in the neutral solutions do not have between inons.

• When ions in a solution have high

concentrations, cations tend to be surrounded by

nearby anions and anions tend to be surrounded

by nearby cations. This effect is significant at ion

concentrations of 0.01 M and greater.

Activity and Activity Coefficient

• When ions in a solution have high concentrations, cations tend to be

surrounded by nearby anions and anions tend to be surrounded by

nearby cations. This effect is significant at ion concentrations of 0.01

M and greater.

Activity and Activity Coefficient

where Z

denotes the charge on an ion.

C

is related to the formal concentration of each ion ^.

The charge concentration of a solution is describe

by the ionic strength, which is calculated from the

following expression:

Activity and Activity Coefficient

Activity and Activity Coefficient

Based on these activity coefficients, the

equilibrium constants of ideal solutions can be

used to determine equilibria in non-ideal

solutions. Activity coefficients are unitless

numbers that are computed from the Debye-

Hückel equation:

• Activity coefficients can be computed from the Debye-Hückel equation:

Activity and Activity Coefficient

Effective hydrated diameters of ions in aqueous solution (25 ^oC).

Cations Anions

Rb⁺, Cs⁺, NH₄⁺, Tl⁺, Ag⁺ 0.25

K⁺ Cl^-, Br^-, I^-, NO₃^-, HCOO^- 0.3

OH^-, F^-, HS^-, ClO₄^-, MnO₄^- 0.35 Na⁺ HCO₃^-, H₂PO₄^-, HSO₃^-, CH₃COO^- 0.4-0.45

Hg₂²⁺ HPO₄^2-, SO₄^2-, CrO₄^2- 0.40

Pb²⁺ CO₃^2-, SO₃^2- 0.45

Sr²⁺, Ba²⁺, Cd²⁺, Hg²⁺ S^2- 0.5

Li⁺, Ca²⁺, Cu²⁺, Zn²⁺

Sn²⁺, Mn²⁺, Fe²⁺, Ni²⁺, Co²⁺ Phthalate^2-, C₆H₅COO^- 0.6

Mg²⁺, Be²⁺ 0.8

H⁺, Al³⁺, Cr³⁺, Fe³⁺, La³⁺ 0.9

Activity and Activity Coefficient