Keywords: iodine clock reaction, concentration, reaction rate, potassium iodide, hydrogen peroxide, sulfuric acid, color change, starch, kinetics, experiment

Introduction Chemical kinetics is the study of reaction rates and the factors affecting them. One key factor that influences reaction rates is the concentration of reactants. In this practical investigation, we will explore the iodine clock reaction, a classic experiment demonstrating the effect of concentration on the reaction rate. The iodine clock reaction involves the reaction between potassium iodide (KI) and hydrogen peroxide (H₂O₂) in the presence of an acid, such as sulfuric acid (H₂SO₄). The reaction produces a color change from clear to blue-black when iodine (I₂) forms a complex with starch. The time it takes for the color change to occur is used to determine the reaction rate.

Equipment Needed

1. Safety goggles, gloves, and lab coat
2. 0.1 M potassium iodide (KI) solution
3. 0.02 M hydrogen peroxide (H₂O₂) solution
4. 0.5 M sulfuric acid (H₂SO₄) solution
5. 1% starch solution
6. Five 50 mL beakers or graduated cylinders
7. Five 10 mL graduated cylinders
8. Stopwatch or timer
9. Pipettes or droppers
10. Stirring rod

Step by Step Method

1. Put on your safety equipment, including goggles, gloves, and a lab coat.
2. Label five beakers or graduated cylinders with the corresponding concentrations of KI you will be testing (e.g., 0.1 M, 0.08 M, 0.06 M, 0.04 M, and 0.02 M).
3. Prepare the five different concentrations of KI solution by adding the appropriate volumes of 0.1 M KI and distilled water to each labeled beaker. For example, to make 10 mL of 0.08 M KI, add 8 mL of 0.1 M KI and 2 mL of distilled water.
4. In a separate beaker, mix 5 mL of 0.02 M H₂O₂ and 5 mL of 0.5 M H₂SO₄. This solution will be used for all trials.
5. Add 1 mL of 1% starch solution to each KI solution. Mix well using a stirring rod.
6. For each trial, quickly add 10 mL of the H₂O₂-H₂SO₄ mixture to the corresponding KI-starch solution. Start the stopwatch immediately.
7. Observe the reaction and record the time it takes for the solution to change color from clear to blue-black. This is the reaction time.
8. Repeat steps 6 and 7 for each KI concentration.
9. Calculate the reaction rate for each trial using the formula: Reaction rate = 1 / Reaction time
10. Plot the reaction rates against the KI concentrations on a graph.

Expected Findings and Calculations

You should observe that as the concentration of KI increases, the time it takes for the color change to occur decreases, indicating a faster reaction rate. This demonstrates that higher concentrations of reactants lead to faster reaction rates due to an increased frequency of effective collisions between particles.

Conclusion

The iodine clock reaction experiment effectively demonstrates the relationship between reactant concentration and reaction rate. As the concentration of potassium iodide increases, the reaction rate also increases. This supports the collision theory, which states that higher concentrations of reactants result in more frequent effective collisions

between particles, leading to faster reaction rates. Understanding the factors that affect reaction rates, such as concentration, is essential in various applications, including chemical engineering, pharmaceutical development, and environmental science.

Questions:

1. What is the purpose of the sulfuric acid in the iodine clock reaction?
2. Why does the solution change color from clear to blue-black in the iodine clock reaction?
3. How is the reaction rate calculated in this experiment?
4. What is the collision theory and its significance in this experiment?
5. How can the effect of concentration on reaction rate be explained in terms of the frequency of effective collisions between particles?

Answers:

1. The sulfuric acid (H₂SO₄) serves as an acid catalyst in the iodine clock reaction, increasing the reaction rate by lowering the activation energy.
2. The solution changes color from clear to blue-black due to the formation of a complex between iodine (I₂) and starch. As the reaction progresses, iodine is produced, which reacts with the starch to form the blue-black complex.
3. The reaction rate in this experiment is calculated using the formula: Reaction rate = 1 / Reaction time, where the reaction time is the time it takes for the solution to change color from clear to blue-black.
4. The collision theory states that for a reaction to occur, particles must collide with sufficient energy and proper orientation. This theory is significant in this experiment because it helps explain why the reaction rate increases with the concentration of potassium iodide (KI) – higher concentrations result in more frequent effective collisions between particles.
5. The effect of concentration on reaction rate can be explained by the increased frequency of effective collisions between particles. When the concentration of reactants is higher, there are more particles in a given volume, which increases the probability of effective collisions and thus the reaction rate.

• Practical Chemistry