PHYSICAL cHEMISTRY - Ii

Experiment 2

Aim of the Experiment

To study the reaction rate of the reaction of iodide ions with hydrogen peroxide at different concentrations of iodide ions.

Principle

The branch of chemistry that deals with the study of reaction rates and their mechanisms is called chemical kinetics. Kinetic studies also helps to describe the conditions by which the reaction rate can be altered. Factors such as temperature, concentration, pressure and catalyst affect the rate of a chemical reaction.

Some reactions, such as ionic reactions, occur very quickly. For example, precipitation of silver chloride occurs very quickly when aqueous solutions of silver nitrate and sodium chloride are mixed. On the other hand, some reactions occur very slowly, such as the rusting of iron in the presence of air and moisture.

What does the change in the concentration of a reactant mean?

The speed of a reaction or the rate of a reaction is defined as the change in concentration of a reactant or product in unit time. It can be expressed as, the rate of decrease in concentration of any one of the reactants or the rate of increase in concentration of anyone of the products.

Consider the reaction,

R ---> P

Assuming that the volume of the system remains constant, one mole of the reactant produces one mole of the product. If [R₁] and [P₁] are the concentrations of R and P respectively at time t₁ and [R₂] and [P₂] are the concentrations of R and P respectively at time t₂, then,

Dependence of the reactant concentration on the rate of the chemical reaction

Let us understand the dependence of the reactant concentration on the rate of chemical reaction by studying the reaction between iodide ions and sodium thiosulphate in the presence of hydrogen peroxide.

Hydrogen peroxide is an oxidising agent that oxidises iodide ions to iodine in acidic medium.

The reaction is monitored by adding a known volume of sodium thiosulphate solution and starch solution to the reaction mixture. The liberated iodine reacts with sodium thiosulphate and reduces to iodide ions.

When thiosulphate ions are completely consumed, the liberated iodine reacts with starch solution and gives a blue colour.

The time elapsed before the appearance of blue colour, gives an idea about the rate of the reaction.

Requirements

Chemicals: Starch solution, distilled water, 2.5 M H₂SO₄, 0.1 M Potassium iodide (KI) solution, 3 % Hydrogen peroxide (H₂O₂ solution), 0.05 M Sodium thiosulphate(Na₂S₂O₃) Solution.

Apparatus: Conical flasks, Measuring Cylinders, glass rod, stop watch

Procedure

• Take four 250 ml conical flasks and label them as A, B, C and D.

• Add 5ml, 10 ml, 15 ml and 20 ml of 0.1 M potassium iodide (KI) solution to the flasks A, B, C and D respectively.

• Add 10ml of 2.5 M H₂SO₄ to each flask.

• To make the volume of the solution 100ml in each flask add water to them.

• Add 5 ml of starch solution to each flask.

• Add 10 ml of 0.05 M sodium thiosulphate (Na₂S₂O₃) solution to each flask.

• Add 5 ml of 3% H₂O₂ solution to flask A and start the stop- watch immediately.

• Stir the mixture using a glass rod and wait for the blue colour to appear.

• Note the time when the blue colour just appears.

• Repeat the experiment by adding 5 ml of 3% hydrogen peroxide solution to flasks B, C and D and note the time required in each case for the blue colour first appears.

Observation Table:

Result

Precautions:

• Always use freshly prepared solution of sodium thiosulphate.

• Concentration of KI solution should be higher than the concentration of sodium thiosulphate solution.

• Use freshly prepared starch solution.

Questions

1.What is rate of reaction?

2.Give the factors which affect on rate of reaction.

3.Give the difference between idometry and iodimetry titration.

Reference

• http://epathshala.nic.in/wp-content/doc/book/flipbook/Class%20XII/12085-Chemistry%20Part_I/Unit_4/index.html

Developed by

Ms.Bansri Shah

Teaching Assistant ,Chemistry

bansri.shah@gsfcuniversity.ac.in