Effect of Concentration on Rate of Reaction

In a reaction, many elements or compounds react together to form one or more new products based on the basic elements of the compounds involved in the chemical reaction. The reaction may occur between solids, liquids, and gases, and the rate of each reaction is different from another which depends on a number of factors. Some chemical compounds called catalysts or enzymes help in accelerating the rate of reaction. This rate of reaction is also dependent on the amount or concentration of substances involved in the reaction.

In this article, the effect of concentration on the rate of the reaction between sodium thiosulphate and hydrochloric acid has been discussed in the form of an experiment.

About the 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.

Aim

To understand the effect of concentration on the rate of reaction between sodium thiosulphate (Na₂S₂O₃) and hydrochloric acid (HCl).

Theory

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.

The reaction is between the aqueous solutions of Na₂S₂O₃ and HCl, the rate of which directly depends on the product of the molar concentration of each component of the reaction.

The chemical reaction formula between HCl and Na₂S₂O₃ is as follows:

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

Here, we can see the products of the reaction are water, sulphur dioxide (SO₂ ), sodium chloride (NaCl) and a sulphur atom. Where SO₂  is formed in the gaseous state, NaCl is formed in the aqueous state and sulphur collects in the solid-state.

With the increase in the temperature of the system, the precipitation of sulphur increases. With the increase in concentration, a collision between molecules also increases which increases the chances of getting the products of the reaction, and an increase in temperature provides more kinetic energy to the reaction which in turn increases the rate of reaction, thereby, resulting in faster production of products.

Materials Required

The materials and apparatus required for conducting the reaction are as follows:

• Five flasks of 100ml each 

•  Two burettes of volume 50 ml each

• Pipette

• Burette stand

• Stopwatch

• Sodium thiosulphate

• 1M Hydrochloric acid

• Five conical flasks (100 ml)

• Two burettes

• Burette Stand

• Pipette

• Stopwatch

• Sodium Thiosulphate

• 1M Hydrochloric Acid Solution

Procedure

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 flasks 1, 2, 3 and 4 respectively.

5. Take 10 ml of 1M HCl solution in a test tube using the burette. Add it in 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.

8. Wash the flasks and add 10ml of sodium thiosulphate in the first flask and add 10 ml more to each subsequent flask.

9. Add 40ml of distilled water in the first flask, 30ml in the second, 20ml in the third, 10ml in the fourth and none in the fifth flask.

10. 10ml HCl is to be added to the first flask with the help of a burette and the stopwatch should be started immediately.

11. Take a white tile and put a cross mark on it distinctly. Put the first flask on the time and observe.

12. Observe till the solution is milky and the mark on the tile is visible and note the time right there.

Observations and Results

(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.

The product formed from the reaction is milky in appearance due to the presence of sulphur. Increasing the temperature increases the rate of precipitation. 

Noting the time and plotting a graph with 1/t on the y-axis and the concentration of Na₂S₂O₃  on the x-axis where t is the time taken to form products at different concentrations. It was observed that the graph shows a straight sloping line which means that 1/t is directly proportional to the concentration of Na₂S₂O₃ which means increasing the concentration increased the rate of reaction as well. 

If the concentration of sodium thiosulphate was kept constant and the concentration of hydrochloric acid was increased, the same trend would be observed.