Effect of Concentration on the Rate of Reaction between Sodium Thiosulphate and Hydrochloric Acid

Effect of Concentration on Rate of Reaction

In any chemical reaction, several reactants react to form one or more new products. These reactants can be gases, solids or even liquids. The rate of reaction depends on many determinants or factors. The presence of catalysts can also accelerate the speed of any reaction. One of the essential components that determine the rate of reaction is the concentration of the reactants taking part in the chemical reaction. In this article, we are going to study the effect of concentration on reaction rate by observing a chemical reaction. The results obtained from the reaction between Na2S2O3 and HCl helps you to understand the topic more clearly.


The rate of the reaction directly depends on the products of the molar concentration of reactants. In this experiment, we will study the reaction between Sodium thiosulphate (Na2S2O3) with hydrochloric acid (HCl). 

Na2S2O3 (aq) + 2HCL (aq) → H2O (l) + SO2 (g)+ 2 NaCl (aq) + S (s)

We can also write the above reaction in ionic form as:

S2O3-2 (aq) + 2H+ (aq) → H2O (l) + S (s) + SO2 (g)

The solution obtained after the reaction is opaque and has a milky appearance due to the presence of sulphur. If we increase the temperature or concentration of the reactants, then the rate of precipitation of sulphur also increases. It happens because when concentration increases, molecular collisions also increase per unit time, which results in a fast rate of product formation.

Materials Required   

To study this experiment properly, we need materials and apparatus given below.

  • Five conical flasks (100 ml)

  • Two burettes

  • Burette Stand

  • Pipette

  • Stopwatch

  • Sodium Thiosulphate

  • 1M Hydrochloric Acid Solution


  1. First of all, take five conical flasks and rinse them with water to clean any residue. Now, label them as 1, 2, 3, 4 and 5 respectively.

  2. Draw the cross mark on any white tile.

  3. Take a burette and add 10 ml of Na2S2O3 solution in flask 1 using it. Similarly, add 20 ml of Na2S2O3 solution in flask 2, 30 ml in flask 3, 40 ml in flask 4 and 50 ml solution in flask 5.

  4. Now, add distilled water in every flask such that the combined volume of water and Na2S2O3 solution becomes 50 ml. It means we need to add 40ml, 30ml, 20 ml and 10 ml distilled water in flask 1, 2, 3 and 4 respectively.

  5. Take 10 ml of 1M HCl solution in a test tube using the burette. Add it in the flask 1, which contains 40 ml water and 10 ml Na2S2O3. Shake it thoroughly and then start the stopwatch immediately.

  6. Place the flask on the white tile having a cross mark. Observe the cross mark from the top and stop the stopwatch as the cross mark becomes invisible. Note the time taken for the whole process.  

  7. Repeat the same procedure with flask 2, 3, 4 and 5. Note the time when the cross mark becomes invisible in every container.

Observations and Results

Flask Number

The volume of Sodium Thiosulphate present in the flask

The volume of Distilled water present in the flask

The volume of HCl added in the solution



10 ml

40 ml

10 ml 



20 ml

30 ml

10 ml



30 ml 

20 ml 

10 ml



40 ml

10 ml

10 ml



50 ml 

0 ml

10 ml


Image will be uploaded soon

The above image shows the graph between 1/t (on the y-axis) and the concentration of Na2S2O3 (on the x-axis). We will obtain a straight sloping line, as shown in the figure.  

From the above graph, it is clear that 1/t is directly proportional to the concentration of Na2S2O3 solution taken during the experiment. We know that 1/t is the direct measure of the rate of reaction. Hence, the pace of chemical reaction in this case directly depends on the concentration of Na2S2O3. However, it doesn't mean that the speed of any chemical reaction doesn't depend on conc. of HCl. We will see a similar result if we keep the concentration of sodium thiosulphate constant and raise the concentration of hydrochloric acid. This experiment clearly shows the effect of concentration on Rate of Reaction.

FAQ (Frequently Asked Questions)

1. What are the precautions to be taken during the experiment to study the effect of concentration on reaction rate?

Ans. First of all, one must ensure that the apparatus must be thoroughly clean before starting the process. Any impurities can lead to inaccuracy during the experiment. Moreover, it is essential to measure the volumes of the distilled water, sodium thiosulphate and hydrochloric acid accurately. You must use the same tile with the same cross mark for all the observations. The temperature variation can also affect the rate of reaction. Hence, it is essential to complete to prevent any temperature variation. Start the stopwatch immediately as you add the HCl in the solution. Finally, view the cross mark on the tile from the top from the same height during the observations.

2. What are the determinants that can affect the rate of reaction?

Ans. There is a direct effect of concentration on Rate of Reaction, as shown in the above experiment. The physical state of the reactants' surface area can also affect the pace of any reaction. For instance, if any metal reacts with gas, then only the molecules present at the surface of the metal can react with gas molecules. Hence, we can increase the surface area of reactants by cutting them in pieces to increase its rate of reaction. An increase in temperature can also enhance the rate of reaction because it will boost the kinetic energy of reactant particles. A presence of a catalyst can also accelerate the chemical reaction.