Objective

Our objective is to study the effect of concentration on the rate of reaction between sodium thiosulphate and hydrochloric acid.

The Theory

What is Chemical Kinetics?

For any chemical reaction, its feasibility and extent of reaction are very important. The feasibility of a reaction can be predicted by thermodynamics and the extent can be determined from chemical equilibrium. Like feasibility and extent, it is important to know the rate and the factors affecting the rate of a chemical reaction. The branch of chemistry that deals with the study of reaction rate and its mechanism is called Chemical Kinetics.

What does the rate of a reaction mean?

In all the chemical reactions the reactants are consumed and new products are formed. So the rate of a reaction is defined as the rate of decrease in concentration of any one of the reactants or the rate of increase in concentration of any one of the product.

Consider the hypothetical reaction, R > P.

[R] and [P] are the concentrations of the reactants and the product respectively.

What are the factors that influence the rate of a reaction?

There are various factors that can affect the rate of a chemical reaction.

• Temperature
• Concentration of the reactant
• Physical state
• Catalyst

The concentration of the reactant plays an important role in the rate of a reaction. As the concentration of the reactant increases, the number of reacting molecules increases. Because of the increase in the number of molecules, the number of collisions also increases as a result the rate of the reaction increases.

Reaction between Sodium thiosulphate and Hydrochloric Acid

The effect of concentration of the reactant on the rate of a reaction can be studied easily by the reaction between sodium thiosulphate and hydrochloric acid.

Sodium thiosulphate reacts with dilute acid to produce sulphur dioxide, sulphur and water. Sulphur dioxide is a soluble gas and dissolves completely in aqueous solution. The sulphur formed however is insoluble and exist in the mixture as a white or pale yellow precipitate or a colloid that gives a milky appearance and makes the solution opaque. Therefore the rate of the reaction can be studied by monitoring the opaqueness of the reaction. This can be easily done by measuring the time taken (t) for forming a certain amount of sulphur.

In this experiment, the reaction is carried out by mixing the reactants in a conical flask, and placing the conical flask on the top of a piece of white paper that has a cross mark on it. Before the reaction starts, the mark is clearly visible from the top of the conical flask through the solution. However, as the reaction proceeds, sulphur precipitates that make the solution more opaque and eventually the mark gets completely masked. The time taken (t) for the mark to totally disappear indicates how fast the reaction has occurred.

We can plot a graph between 1/t (in second) and the concentration of sodium thiosulphate by taking 1/t along ordinate axis and concentration of Na₂S₂O₃ along abscissa. The graph should be a straight slopping line.

From the graph, it is clear that 1/t is directly proportional to the concentration of Na₂S₂O₃. But 1/t is a direct measure of the rate of the reaction. Therefore the rate of the reaction between Na₂S₂O₃ and HCl is directly proportional to the concentration of Na₂S₂O₃ solution.

Learning Outcomes

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