Study of reaction rates of the clock reaction between potassium iodate and sodium sulphite using starch as an indicator

Introduction

The clock reaction experiment is a fascinating demonstration of chemical kinetics that allows students to visually observe and measure reaction rates. This experiment specifically focuses on the reaction between potassium iodate (KIO₃) and sodium sulphite (Na₂SO₃) using starch as an indicator. The dramatic color change from colorless to deep blue-black provides a clear visual marker for timing the reaction, making it an excellent tool for understanding reaction kinetics in a laboratory setting.

Aim

To study the rate of reaction between potassium iodate (KIO₃) and sodium sulphite (Na₂SO₃) using starch solution as an indicator and determine the reaction rate experimentally.

Apparatus Required

• Burette (50 mL) - 2 nos.
• Conical flask (250 mL) - 2 nos.
• Measuring cylinder (100 mL) - 1 no.
• Stopwatch or digital timer
• Glass rod for stirring
• Wash bottle
• White tile or white background sheet

Chemicals Required:

• Potassium iodate (KIO₃) solution (0.02 M)
• Sodium sulphite (Na₂SO₃) solution (0.06 M)
• Starch solution (1%)
• Distilled water

Theory

Chemical Background

The iodine clock reaction involves two simultaneous reactions:

Reaction 1: IO₃⁻ + 3SO₃²⁻ → I⁻ + 3SO₄²⁻

Reaction 2: IO₃⁻ + 5I⁻ + 6H⁺ → 3I₂ + 3H₂O

Overall Reaction: IO₃⁻ + 5SO₃²⁻ + 6H⁺ → 3I₂ + 5SO₄²⁻ + 3H₂O

Mechanism

1. Initially, sodium sulphite reduces potassium iodate to iodide ions
2. Excess iodate ions then react with iodide ions to form iodine
3. Starch forms a complex with iodine, producing the characteristic blue-black color
4. The time taken for color change indicates the reaction rate

Rate Law

The rate of reaction can be expressed as: Rate = k[IO₃⁻]^m[SO₃²⁻]^n

Where:

• k = rate constant
• m and n = order of reaction with respect to respective reactants

Procedure

1. Preparation:

   • Clean all apparatus thoroughly
   • Prepare solutions as per required concentrations
   • Set up stopwatch and ensure proper lighting conditions

2. Experiment Steps:

   • Take 100 mL of 0.02 M KIO₃ solution in a conical flask
   • Add 2-3 drops of starch solution to the KIO₃ solution
   • Mix 50 mL of 0.06 M Na₂SO₃ solution in a separate measuring cylinder
   • Pour the Na₂SO₃ solution into the KIO₃ solution rapidly
   • Start the stopwatch immediately upon mixing
   • Stir the solution gently with a glass rod
   • Observe the solution carefully for color change
   • Stop the stopwatch when blue-black color appears
   • Record the time taken for color change
   • Repeat the experiment at least three times for accuracy

3. Variation Studies (Optional):

   • Change concentration of KIO₃ while keeping Na₂SO₃ constant
   • Change concentration of Na₂SO₃ while keeping KIO₃ constant
   • Record time for each variation

Observation Table

Sample Calculations:

• Average time = (45.2 + 44.8 + 45.0)/3 = 45.0 seconds
• Rate of reaction = 1/45.0 = 0.0222 sec⁻¹

Result

The clock reaction between potassium iodate and sodium sulphite was successfully studied. The average time taken for the appearance of blue-black color with starch indicator was 45.0 seconds. The rate of reaction was calculated to be 0.0222 sec⁻¹.

The experiment demonstrates that:

• Reaction rate is inversely proportional to time
• Color change provides a clear endpoint for timing
• The reaction follows expected stoichiometric principles

Precautions

1. Chemical Handling:

   • Handle potassium iodate and sodium sulphite carefully
   • Avoid skin contact with chemicals
   • Use appropriate safety equipment

2. Measurement Accuracy:

   • Use clean and dry apparatus
   • Read measurements at eye level
   • Ensure proper mixing of solutions

3. Timing Precision:

   • Start stopwatch immediately upon mixing
   • Stop timing precisely at color change
   • Maintain consistent observation conditions

4. Environmental Control:

   • Conduct experiment at constant temperature
   • Ensure uniform stirring technique
   • Use white background for better color observation

Viva Questions and Answers

Q1: Why is starch used as an indicator in this experiment?

A: Starch forms a deep blue-black complex with iodine, providing a sharp, easily observable color change that marks the endpoint of the reaction.

Q2: What is meant by "clock reaction"?

A: A clock reaction is one where there's a sudden, easily detectable change (like color change) after a predictable time interval, allowing precise measurement of reaction rates.

Q3: Why should the solutions be mixed quickly?

A: Quick mixing ensures uniform reaction conditions and prevents premature reaction before timing begins.

Q4: How does concentration affect reaction rate in this experiment?

A: Higher concentrations generally increase reaction rate, decreasing the time for color change to occur.

Q5: What factors can affect the accuracy of this experiment?

A: Temperature variations, improper mixing, inaccurate measurements, and delayed timing can affect accuracy.

Conclusion

The clock reaction experiment between potassium iodate and sodium sulphite successfully demonstrated the principles of chemical kinetics. The consistent timing of color change (45.0 seconds average) indicates reliable experimental technique. This experiment provides valuable insights into reaction rates and serves as an excellent model for understanding chemical kinetics in educational laboratory settings.

The use of starch indicator creates a dramatic visual demonstration that enhances understanding of reaction mechanisms and rate concepts. Proper execution of this experiment builds fundamental skills in laboratory technique, data collection, and scientific observation.