Question

Please answer all questions

2. a) Briefly explain the concepts on which the Debye-Huckel's theory for ionic solutions is based. (7 Marks). b) Briefly explain the relationship between the activity coefficient and the ionic strength of an electrolyte as seen in the context of the Deybe-Huckel limiting law. (5 Marks) ffers from the limiting law. (6 Marks) d) Calculate activity a 0.250 mol kg CaCl2 solution at 35 degree C. (Let A-0.509,7 Marks)

Answer 1

a.) Ionic solutions have interactions between solute-solvent and solute -solute molecules and the interactions are electrostatic in nature. Electrostatic forces are given by the coulomb's law and these forces are directly proportional to 1/r^-2. In case real solutions we use term activity in place of concentration and activity of a solution is given by:

a = $\gamma$c/c^o

value of activity cofficient in case of ideal electrolytic solution will be equal to one. For an an ideal electrolytic solution, the solution should be very dilute.

An electrolyte consists of both positive and negative charged ions and the mean activity coefficient is defined for it, it is denoted by $\gamma _{\pm }$ , For example if we are using NaCl, then

$\gamma _{\pm }$ = $(\gamma _{Na^{+}}\gamma_{cl^{-}})$

Debye- Huckel theory assumed three assumptions of how ions behave in the solution:

1.) Electrolytes will dissociates completely into it's ions in the solution.

2.) Electrolytic solutions are very dilute , of the range of 0.01 M.

3.) Each ion in the solution will be surrounded by the ion of opposite charge (Positive ion will be surrounded by negative ion and vice-versa).

4.) Solvent only provides medium of dielectric constant and it has no other roles.

The diagram of the model is shown below :

Resusentation ed Solution e an ebotvolyte

b.) Debye huckel formula which is used to calculate the activity cofficient is given by :

ET)72 T- 298k, Tun, log"(土 Whi, しon cation buta 243 go uc. Strength. uf given b :- 2 Cǐ → co w.tution eb uon

c.) Extended Debye huckel equation is used for solutions of greater ionic strength.Debye huckel equation is given in the above part and equation of debye huckel extended law is given by :

A 242.VI activity Coutuuent ae etve diam A and B ahs Constantz, A 0 5085 0.3281 at 25。c um waterqi a fcluent

   d.) Calculation of activity 0.250 mol/kg CaCl₂ solution is shown in the picture given below :

HsALWe have Cad2 delation, a Soluion wit disoiate unto up on , 2 i whue Z: haiga on ion and mi dlalty Thas 081 013 五om aquation ③ we have,