Objective

Our objective is to study the reaction rate of the reaction of iodide ions with hydrogen peroxide at different concentrations of iodide ions.

The Theory

What is Chemical Kinetics?

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,

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.

Learning Outcomes

• Students understand the terms chemical kinetics and rate of reaction.
• Students understand the effect of concentration of the reactant on the rate of a chemical reaction.
• Once students understand the concept of the experiment and the different steps involved, they can perform the experiment in the real lab more accurately and quickly.