Question

1. The determination of the amount of hypoclorite in bleach is accomplished indirectly:

OCl^- + 3 I^- + 2 H⁺ ---> I₃^-  + Cl^- + H₂O

I₃^- + 2 S₂O₃^2- ---> 3 I^- + S₄O₆^2-

1.1 Identify the oxidizing and the reducing agents in each reaction

1.2 What is the net overall reaction between hypochlorite and thiosulfate? Identify the oxidizing and the reducing agents in the overall reaction.

1.3 Why can't the titration of hypochlorite be done directly with thiosulfate?

1.4 What is the purpose of each step and how do these two reactions permit the determination of the amount of hypochlorite?

1.5 What indicator is used and what change indicates the end point of the reaction?

1.6 Based on the volume of a known concentration of thiosulfate, write the stoichiometric expression that yields the moles of hypochlorite at the end point.

Answer 1

1.

1.1 The reaction of hypochlorite with iodide ion is as shown below:

OCl^- + 3 I^- + 2 H⁺ ---> I₃^-  + Cl^- + H₂O

In this reaction, the oxidation number of chlorine atom in OCl^- is +1 where as in Cl^- it becomes -1.

Hence, there is a decrease in oxidation number which means hypochlorite is reduced and oxidising the other reactant.

Hence, OCl^- is the oxidising agent.

The other reactant, I^- oxidation state is -1. In I₃^- the oxidation state of iodine becomes -1/3.

Hence, there is an increase in oxidation number.So iodide is oxidised and reducing the other reactant .

Hence, I^- is the reducing agent.

I₃^- + 2 S₂O₃^2- ---> 3 I^- + S₄O₆^2-

In I₃^- the oxidation state of iodine becomes -1/3. In I^- oxidation state is -1.

Hence, there is a decrease in oxidation number.So I₃^- is reduced and oxidising the other reactant.

Hence, I₃^-  is the oxidising agent.

In a redox reaction if one reactant is oxidising agent means the other reactant will be the reducing agent.

The other reactant S₂O₃^2- is the reducing agent.

1.2

The net overall reaction can be obtained by adding both the given equations.

OCl^- + 2 S₂O₃^2-+ 2 H⁺ --->S₄O₆^2-+ Cl^- + H₂O

In this reaction, the oxidation number of chlorine atom in OCl^- is +1 where as in Cl^- it becomes -1.

Hence, there is a decrease in oxidation number which means hypochlorite is reduced and oxidising the other reactant.

Hence, OCl^- is the oxidising agent.

In a redox reaction if one reactant is oxidising agent means the other reactant will be the reducing agent.

The other reactant S₂O₃^2- is the reducing agent.

1.3:the titration of hypochlorite be done indirectly with thiosulfate.

first hypochlorite is allowed to react with excess iodide in prsence of an acid then hypochlorite convetrs into chloride ions and iodide becomes I₃^-.

These I₃^- ions are allowed to raect with thiosulphate then it converts back into iodide ions.

This is due to the reason that if hypochlorite reacts directly with thiosuphate then the end point of the titration cannot be  detected easily.

1.5. Starch is used as the indicator in the reaction.

I₃^- ion forms blue black complex with starch.

At the end of the reaction, the blue black color of starch disappears.

1.6.

The overall balanced chemical reaction is as shown below:

OCl^- + 2 S₂O₃^2-+ 2 H⁺ --->S₄O₆^2-+ Cl^- + H₂O

1 mole of hypochlorite reacts with 2mol of thiosulphate .

If the concentration and volume of thiosulpahte is known then the no.of moles of hypo reacts can be calculated as shown:

no. of moles of thiosulphate = concentration of thiosulphate solution x volume of thiosulpahte

no.of moles of hypochlorite = 1/2 ( no.of moles of thio sulphate)

no.of moles of hypochlorite = 1/2 ( concentration of thiosulphate solution x volume of thiosulpahte)

1.4.

The hypochlorite in bleach solution is allowed to react with excess iodide in acidic conditions.Then I₃^- forms.The no.of moles of I₃^- formed is equal to the moles of hypochlorite in bleach solution.

We can determine the no.of moles of I₃^- in solution with the known concentration of thiosulphate solution.

Then the concentration of I₃^- decreases and the blue- black color of the solution (due to addition of starch becomes colurless).

no.of moles of hypochlorite = no.of moles of I₃^-

No.of moles of I₃^- = 1/2 (no.of moles of thiosulphate)

hence the amount( no.of moles) of hypochlorite = 1/2 (concentration of thiosulphate solution x volume of thiosulpahte)

After getting the number of moles, if we divide with the volume of hypochlorite solution, we can calculate the amount of OCl^- in the given bleach solution.