Effect of concentration and temperature on the reaction rate between sodium thiosulphate and hydrochloric acid

Introduction

Understanding reaction rates is fundamental to chemistry education and industrial processes. The effect of concentration and temperature on reaction rate between sodium thiosulphate and hydrochloric acid provides an excellent demonstration of chemical kinetics principles. This classic pitch lab experiment offers students hands-on experience with measuring reaction rates while exploring important factors that influence chemical reactions.

Aim

To investigate how concentration and temperature affect the rate of reaction between sodium thiosulphate and hydrochloric acid and to verify the collision theory through experimental observations.

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Apparatus Required

Basic Equipment:

• Conical flask (100 mL) - 5 pieces
• Measuring cylinder (50 mL) - 2 pieces
• Measuring cylinder (10 mL) - 2 pieces
• Beaker (250 mL) - 2 pieces
• Stopwatch or digital timer
• Thermometer (0-100°C)
• White tile or white paper
• Black cross mark (permanent marker)
• Hot water bath setup
• Ice cubes for cooling
• Safety goggles and gloves

Chemicals:

• Sodium thiosulphate solution (0.1 M)
• Hydrochloric acid solution (1.0 M, 2.0 M, 3.0 M)
• Distilled water

Theory

The reaction between sodium thiosulphate (Na₂S₂O₃) and hydrochloric acid (HCl) produces a precipitate of sulphur, which makes the solution cloudy. The reaction is:

Na₂S₂O₃ + 2HCl → 2NaCl + SO₂ + S↓ + H₂O

Key Principles:

1. Effect of Concentration: According to the collision theory, increasing concentration increases the number of reactant particles per unit volume, leading to more frequent effective collisions and higher reaction rate.

2. Effect of Temperature: Raising temperature increases the kinetic energy of particles, resulting in:

• More frequent collisions
• Higher proportion of particles with activation energy
• Increased reaction rate (approximately doubles for every 10°C rise)

3. Rate Measurement: Reaction rate is measured by timing how long it takes for the black cross to become invisible due to sulphur precipitation.

Procedure

Part A: Effect of Concentration

1. Take a white tile and draw a clear black cross with a permanent marker
2. Place a conical flask over the cross
3. Measure 10 mL of 1.0 M HCl using a measuring cylinder
4. Measure 40 mL of 0.1 M sodium thiosulphate solution
5. Pour the sodium thiosulphate into the conical flask
6. Add the HCl solution quickly and start the stopwatch immediately
7. Observe from above through the solution until the black cross disappears
8. Record the time taken
9. Repeat with different concentrations:
   • 20 mL Na₂S₂O₃ + 30 mL water + 10 mL HCl
   • 30 mL Na₂S₂O₃ + 20 mL water + 10 mL HCl
   • 40 mL Na₂S₂O₃ + 10 mL water + 10 mL HCl
   • 50 mL Na₂S₂O₃ + 0 mL water + 10 mL HCl

Part B: Effect of Temperature

1. Prepare the same volume and concentration mixture as in Part A
2. Heat the sodium thiosulphate solution to 35°C using water bath
3. Cool the HCl to the same temperature
4. Mix and observe as before
5. Repeat at temperatures: 40°C, 45°C, 50°C, 55°C
6. Record time for each temperature

Observation Table

Table 1: Effect of Concentration

Table 2: Effect of Temperature

Result

Effect of Concentration:

• Increased concentration leads to decreased reaction time
• Rate of reaction is directly proportional to concentration of reactants
• Graph of concentration vs. 1/time shows a straight line (direct proportionality)

Effect of Temperature:

• Increased temperature leads to decreased reaction time
• Rate approximately doubles for every 10°C rise in temperature
• Graph of temperature vs. 1/time shows exponential increase

Verification:

Both observations confirm the collision theory and Arrhenius equation, demonstrating that:

• More concentrated solutions have more particles per unit volume
• Higher temperatures increase collision frequency and effectiveness

Precautions

1. Safety First: Always wear safety goggles and gloves when handling hydrochloric acid
2. Temperature Control: Ensure both solutions are at the same temperature before mixing
3. Consistent Observation: Always observe from the same height and lighting conditions
4. Proper Mixing: Add HCl quickly and swirl the flask gently to ensure complete mixing
5. Clean Apparatus: Thoroughly clean all apparatus between experiments
6. Accurate Measurement: Use appropriate measuring cylinders for different volumes
7. Timing Precision: Start timing exactly when solutions are mixed
8. Environmental Control: Conduct experiments away from drafts that might affect temperature

Viva Voce Questions and Answers

Q1: Why is sodium thiosulphate used in this experiment?

A: Sodium thiosulphate is used because its reaction with HCl produces a visible precipitate (sulphur) that makes the solution cloudy, allowing easy measurement of reaction completion.

Q2: How does the cross visibility method work?

A: As the reaction proceeds, sulphur precipitates and makes the solution cloudy. When sufficient precipitate forms, the black cross becomes invisible from above, indicating a fixed point in the reaction.

Q3: What is the significance of 1/time?

A: 1/time represents the rate of reaction. As time decreases for completion, the rate increases, providing a quantitative measure of reaction speed.

Q4: Why must both solutions be at the same temperature?

A: Temperature affects reaction rate significantly. If solutions are at different temperatures, the initial mixing will involve temperature change, affecting the accuracy of the experiment.

Q5: What is the role of water in concentration experiments?

A: Water is used to dilute the sodium thiosulphate solution to achieve different concentrations while maintaining the same total volume for consistent conditions.

Q6: State the collision theory.

A: Collision theory states that for a reaction to occur, reactant particles must collide with proper orientation and sufficient energy (activation energy).

Q7: How does temperature affect effective collisions?

A: Higher temperature increases kinetic energy of particles, resulting in more frequent collisions and higher proportion of collisions with activation energy.

Q8: Why is the volume of HCl kept constant?

A: Keeping HCl volume constant ensures that concentration changes are due to sodium thiosulphate concentration variations only, making the experiment a fair test.

Conclusion

This experiment successfully demonstrates that both concentration and temperature significantly affect chemical reaction rates. The results validate collision theory and provide quantitative data supporting:

• Direct proportionality between concentration and reaction rate
• Exponential relationship between temperature and reaction rate
• Importance of activation energy in chemical reactions

The sodium thiosulphate-HCl experiment remains an excellent tool for understanding chemical kinetics in educational settings, combining theoretical concepts with practical observations to enhance student comprehension of reaction mechanisms.