Sodium Thiosulfate

Theory:

Reactions in the liquid phase differ considerably from those in the gas phase. Solvent particles have a greater control in influencing the velocity of the reaction. This is largely due to the solvent interactions with the reactants. This is seen particularly with ionic species in aqueous environments where ionic reactions develop due to the dissociation of the salt into its ionic species. Water facilitates this dissociation because of the energy released in the salvation process. The effect of the solvent and behaviour of ions in aqueous solution are demonstrated in the activity coefficient of the reacting species. The velocity of the reactions is explained by the transition state theory. The rate constant can be written as:-

k = k'T K*γaA γbB

h γM*

k = boltzman constant, T is absolute temperature h is planks constant k* is the equilibrium constant between activated complex and reactants in given in terms of concentrations and the γ's are activity coefficients.

The ionic reaction in aqueous systems can be thought of as from ideal reference stat due solute -solute and solvent-solute interactions can be expressed in of free energy changeΔGni, where ΔGni = k'Tlnγi. Using Debye - Huckel Potentials, the activity coefficient is given as:

k'Tlnγi = zi2e2 - zi2e2

2εa 2ε(1+βa)

Zi's are the ionic valencies of the reacting species, ε is the macroscopic dielectric constant, e is the electronic charge, β is the ionic strength, and a is the closest states.

From both equations the rate constant of the reaction in the actual state is

lnK = nk0 - z1z2e2 + z1z2e2β where k0 is the rate constant in the reference state.

εak'T εk'T(1+βa)

for ionic reactions, it is a hypothetical reference state where ε⇒∞ and β⇒0

For dilute aqueous solutions at 25oC, one may reduce the previous equation to logk/k0 = 1.02z1z2β1/2

Where β = ½ ∑c1z12

the rate constant should increase with increasing β, this being the primary salt effect. A straight line should be observed when log K is plot agains β1/2. However there is deviation with increases in ionic strength.

In the experiment: 2I- + S2O8 ⇒ I2 + 2SO42-was observed.The appearance of iodine is measured, where the time taken for iodine to appear is measured, when an initial amount of persulphate is added. In the reaction, the iodine formed gets complexed with the starch indicator and forms the blue colour. This reaction