Step-by-Step Experiment: Observing the Effect of Concentration on Reaction Speed
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
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.
Draw the cross mark on any white tile.
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.
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.
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.
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.
Repeat the same procedure with flask 2, 3, 4 and 5. Note the time when the cross mark becomes invisible in every container.
Wash the flasks and add 10ml of sodium thiosulphate in the first flask and add 10 ml more to each subsequent flask.
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.
10ml HCl is to be added to the first flask with the help of a burette and the stopwatch should be started immediately.
Take a white tile and put a cross mark on it distinctly. Put the first flask on the time and observe.
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.
FAQs on How Concentration Influences Reaction Rate: Sodium Thiosulphate and HCl
1. What is the basic principle behind studying the reaction between sodium thiosulphate and hydrochloric acid?
The fundamental principle is to observe a visible change to measure reaction time. In this reaction, a solid precipitate of elemental sulphur is formed, which makes the solution cloudy or opaque. By timing how long it takes for a mark placed under the reaction flask to become obscured, we can determine the rate of reaction. This setup allows for a straightforward investigation of how factors like concentration affect the reaction speed.
2. How does increasing the concentration of sodium thiosulphate affect its reaction rate with HCl?
Increasing the concentration of sodium thiosulphate (Na₂S₂O₃) increases the rate of reaction. According to the collision theory, a higher concentration means there are more thiosulphate ions per unit volume. This leads to more frequent and effective collisions with hydrogen ions from the HCl, resulting in a faster formation of the sulphur precipitate and a shorter time for the solution to turn cloudy.
3. What is the balanced chemical equation for the reaction between sodium thiosulphate and hydrochloric acid?
The balanced chemical equation for the reaction is:
Na₂S₂O₃(aq) + 2HCl(aq) → 2NaCl(aq) + SO₂(g) + S(s) + H₂O(l)
The key product used for observation in this experiment is the solid sulphur (S), which causes the turbidity, while sulphur dioxide (SO₂) is a gas with a distinct smell.
4. Why does the solution turn cloudy during this reaction?
The solution turns cloudy or opaque due to the formation of insoluble elemental sulphur (S) as a solid precipitate. Sodium thiosulphate reacts with hydrochloric acid in a decomposition reaction that produces various products, including solid sulphur. As the tiny particles of sulphur disperse throughout the solution, they scatter light, causing the characteristic milky or cloudy appearance.
5. How is the rate of reaction typically measured in the sodium thiosulphate and HCl experiment?
The rate of reaction is measured by observing the time it takes for a fixed amount of precipitate to form. The common method involves:
- Placing the reaction flask on a piece of paper with a cross 'X' marked on it.
- Starting a stopwatch the moment the reactants are mixed.
- Stopping the watch when the cross 'X' is no longer visible through the cloudy solution.
The rate of reaction is then considered to be inversely proportional to the time taken (Rate ∝ 1/t). A shorter time indicates a faster reaction rate.
6. What is the role of collision theory in explaining why concentration affects this reaction's rate?
Collision theory states that for a reaction to occur, reactant particles must collide with sufficient energy (activation energy) and in the correct orientation. When you increase the concentration of a reactant like sodium thiosulphate, you increase the number of reactant particles in the same volume. This leads to a higher frequency of collisions between thiosulphate ions and hydrogen ions, thereby increasing the chances of successful collisions per second and accelerating the reaction rate.
7. Besides concentration, what are the other major factors that influence the rate of a chemical reaction?
Apart from concentration, several other factors can influence the rate of a chemical reaction:
- Temperature: Increasing the temperature generally increases the reaction rate by providing particles with more kinetic energy.
- Catalyst: A catalyst can increase the rate by providing an alternative reaction pathway with lower activation energy.
- Surface Area: For reactions involving solids, increasing the surface area (e.g., by using a powder instead of a lump) increases the rate.
- Nature of Reactants: The inherent chemical properties and bond strengths of the reactants determine their reactivity.
8. What key precautions should a student take when performing the experiment on the effect of concentration?
To ensure accurate results and safety, several precautions are essential:
- Ensure all apparatus, especially the conical flasks, are clean and dry to avoid contamination.
- Measure the volumes of sodium thiosulphate solution, HCl, and distilled water precisely using a burette or measuring cylinder.
- Use the same cross mark on the paper for all trials and view it from the same vertical position to maintain consistency.
- Conduct the experiment in a well-ventilated area as sulphur dioxide (SO₂) gas, which is an irritant, is produced.
- Start the stopwatch immediately upon adding the final reactant (usually HCl) to the mixture.
