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Effect of Concentration on Rate of Reaction Experiment

 

A lab report on study of the effect of concentration on rate of chemical reaction experiment between Sodium Thiosulphate and Hydrochloric Acid (HCl)

Introduction

 

According to the law of mass action, the rate of a reaction is proportional to the active mass or the molar concentration of the reactant at a given temperature.


Aim

 

The aim of this project is to determine the effect of concentration on the rate of a reaction.

 

Principle

 

The reaction which is used is that between sodium thiosulfate and hydrochloric acid

 

Na2S2O3 + 2HCI --> 2NaCI + SO2 + H2O + S

 

Since this reaction produces a precipitate of sulphur from two colourless solutions, the intensity of the precipitate at any given time represents the extent of the reaction. The way in which the intensity of the precipitate is measured is by carrying out the entire reaction in a beaker and by placing the beaker and its contents on a white paper with a cross marked on it. The time for the disappearance of the cross when the contents of the beaker are viewed from above will give a measure of the time taken for a certain fraction of the reaction to occur.

 

Requirements

 

1. 100 mL beakers

2. Sodium thiosulphate

3. HCI acid

4. Stop watch

 

Procedure

 

Place 50cm3 of 0.2M sodium thiosulphate solution in a 100cm3 beaker. Add 5cm3 2M HCl to it and start a stop-watch at the same time. Swirl the beaker carefully a couple of times and place it on a white card with a cross marked on it. Observe the cross by looking down through the solution from above the beaker and stop the stop-watch at the moment the cross disappears and note the time and record.


The concentration of the thiosulphate solution is varied by taking 40, 30, 20 and 10 cm3 of 0.2M thiosulphate solution and making the total volume up to 50cm3 each time by adding water. Add 5cm3 of 0.2M HCI to each as above and note the time taken for the disappearance of the cross in each case and record.


No:

Volume of thiosulfate (0.2M) in cm3

Volume of water in cm3

Volume of HCl

Time taken for disappearance of the cross

1

50

0

5

-

2

40

10

5

-

3

30

20

5

-

4

20

30

5

-

5

10

40

5

-

 

It is found from the table that as the concentration of the thiosulphate solution increases, the time for the cross to disappear from view decreases. 


 Discussion and Conclusion

 

Since the rate of the reaction is inversely proportional to time (a fast reaction takes a short time and a slow reaction takes long lime), we can conclude that as the concentration of the sodium thiosulphate solution increases, the rate of the reaction also increases.

 

VIVA QUESTIONS WITH ANSWERS

 

1.  What is law of mass action?

 

Ans: Rate of a reaction is directly proportional to the product of active masses of the reactants.

 

2. What will you observe when dil.HCI is added to sodium thiosulphate solution?

 

Ans: Formation of a yellow precipitate.

 

3. What is the yellow substance produced?

 

Ans: Sulphur

 

4. Write the equation for the reaction between sodium thiosulphate and HCl

 

Ans: Na2S2O3 + 2HCl --> 2NaCl + SO2 + H2O + S

 

5. What is the effect of concentration on the rate of a reaction?

 

Ans: As the concentration of the reactant increases the rate of reaction also increases.


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