Effect of Concentration and Temperature on Rate of Reaction

Reaction Between Sodium Thiosulphate and HCl – Rate of Reaction Study

Sodium Thiosulphate + HCl Reaction

Effect of Concentration and Temperature on Rate of Reaction

Objective

To investigate the effect of concentration and temperature on the rate of reaction between sodium thiosulphate and hydrochloric acid by measuring the time taken for sulphur precipitate to obscure a marked cross.

Chemical Reaction

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

In this reaction:

  • Sodium thiosulphate reacts with hydrochloric acid to produce sodium chloride, water, sulphur dioxide gas, and solid sulphur
  • The solid sulphur forms a precipitate, making the solution cloudy
  • The rate of reaction is measured by timing how long it takes for enough sulphur to form to obscure a marked cross beneath the reaction vessel
  • The reaction follows first-order kinetics with respect to thiosulphate concentration

Key Concepts

Collision Theory

The rate of a chemical reaction depends on:

  • Frequency of collisions between reactant particles
  • Energy of collisions (must exceed activation energy)
  • Orientation of collisions (proper alignment for reaction)

Effect of Concentration

Increasing reactant concentration increases the number of particles per unit volume, leading to more frequent collisions and a faster reaction rate.

Effect of Temperature

Increasing temperature increases the kinetic energy of particles, resulting in:

  • More frequent collisions
  • Higher proportion of collisions with energy ≥ activation energy
  • Faster reaction rate (typically doubles for every 10°C rise)

Materials Required

Chemicals

  • Sodium thiosulphate solution (0.1 M)
  • Hydrochloric acid (1.0 M)
  • Distilled water
  • Ice (for temperature variation experiments)

Apparatus

  • Conical flasks (250 mL)
  • Measuring cylinders (10 mL, 50 mL)
  • Stopwatch or timer
  • White paper with marked black cross
  • Thermometer
  • Water bath (for temperature control)
  • Safety goggles and gloves

Safety Equipment

  • Lab coat or apron
  • Safety goggles
  • Chemical-resistant gloves
  • Fume hood (for SO2 gas)
  • Spill kit

Experimental Procedure

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Part A: Effect of Concentration

  1. Prepare different concentrations of sodium thiosulphate by diluting 0.1 M solution with distilled water as per the observation table.
  2. Measure 50 mL of the first sodium thiosulphate concentration into a clean conical flask.
  3. Place the flask on a white paper with a clearly marked black cross.
  4. Measure 5 mL of 1.0 M hydrochloric acid into a small measuring cylinder.
  5. Add the hydrochloric acid to the sodium thiosulphate solution, start the stopwatch immediately, and gently swirl to mix.
  6. Look down through the solution at the cross. Stop the timer when the cross is no longer visible due to sulphur precipitate.
  7. Record the time taken for the cross to disappear.
  8. Repeat steps 2-7 for each concentration of sodium thiosulphate.
  9. Clean the conical flask thoroughly between trials.

Part B: Effect of Temperature

  1. Prepare a water bath at a specific temperature (e.g., 30°C, 40°C, 50°C).
  2. Measure 50 mL of 0.1 M sodium thiosulphate into a conical flask and place it in the water bath until it reaches the desired temperature.
  3. Similarly, warm 5 mL of 1.0 M hydrochloric acid to the same temperature.
  4. Place the flask on the marked paper and add the acid, starting the timer immediately.
  5. Record the time for the cross to disappear at each temperature.
  6. Repeat at different temperatures, ensuring all other conditions remain constant.

Safety Precautions

  • Personal Protection: Always wear safety goggles, gloves, and a lab coat throughout the experiment.
  • Acid Handling: Hydrochloric acid is corrosive. Handle with care and avoid skin contact. If spilled, neutralize with sodium bicarbonate.
  • Gas Production: Sulphur dioxide (SO2) gas is produced. Work in a well-ventilated area or fume hood.
  • Glassware: Handle glassware carefully to avoid breakage and injury.
  • Waste Disposal: Dispose of chemical waste according to institutional guidelines. Neutralize acid waste before disposal.
  • First Aid: Know the location of eyewash stations and emergency showers. In case of acid contact, rinse immediately with plenty of water.

Observation Table for Reaction Rate Investigation

Volume of HCl solution added each time = 5 cm3

Sr. No Volume of Na2S2O3 soln. (cm3) Volume of water added (cm3) Conc. of Na2S2O3 soln. (M) Time (t) for cross to disappear (s) 1/t (s-1)
1 50 0.00 0.10 45.2 0.0221
2 40 10 0.08 58.7 0.0170
3 30 20 0.06 78.5 0.0127
4 20 30 0.04 124.3 0.0080
5 10 40 0.02 265.8 0.0038

Note: The rate of reaction is proportional to 1/t (where t is the time for the cross to disappear).

As concentration decreases, reaction time increases and reaction rate (1/t) decreases.

Interactive Reaction Simulation

Adjust the concentration and temperature to see how they affect the reaction rate. Click “Start Reaction” to begin the simulation.

✖
00.0 s
Na2S2O3 Concentration 0.10 M
0.02 M 0.10 M
Temperature 25°C
10°C 50°C

Simulation Parameters

Current Reaction Rate: 0.0221 s⁻¹

Predicted Time: 45.2 seconds

Explanation: Higher concentrations mean more thiosulphate particles available to collide with HCl, leading to faster sulphur formation. Higher temperatures increase particle kinetic energy, making collisions more energetic and frequent.

Exercise: Design an Experiment

Design an experiment to measure the rate of reaction using 0.1 molar sodium thiosulphate solution.

Experimental Design

  1. Objective: Determine the rate of reaction between sodium thiosulphate and hydrochloric acid at constant temperature.
  2. Materials: 0.1 M Na2S2O3, 1.0 M HCl, conical flask, measuring cylinders, stopwatch, white paper with black cross, thermometer.
  3. Procedure:
    • Measure 50 mL of 0.1 M sodium thiosulphate into a conical flask.
    • Place the flask on the marked paper.
    • Measure 5 mL of 1.0 M HCl.
    • Add HCl to the flask, start timer immediately, and swirl gently.
    • Observe from above and stop timer when cross disappears.
    • Record time (t).
    • Repeat 3 times for reliability.
  4. Calculations:
    • Calculate average time: tavg = (t1 + t2 + t3) / 3
    • Calculate reaction rate: Rate = 1 / tavg
    • If tavg = 45.2 s, then Rate = 1/45.2 = 0.0221 s-1
  5. Variables:
    • Independent: Concentration of Na2S2O3 (constant at 0.1 M)
    • Dependent: Time for cross to disappear (t)
    • Controlled: Volume of HCl (5 mL), temperature, same cross marking, same observer
  6. Expected Results: With 0.1 M Na2S2O3 at 25°C, expect t ≈ 45-50 seconds.

Data Analysis and Graphical Interpretation

Interpreting the Graph

The graph shows 1/t (reaction rate) plotted against concentration of sodium thiosulphate.

  • A straight line through the origin indicates the reaction is first order with respect to sodium thiosulphate concentration.
  • The gradient of the line represents the rate constant (k) for the reaction at that temperature.
  • As concentration increases, 1/t increases linearly, showing direct proportionality between concentration and reaction rate.

Calculations from Data

Rate Equation: Rate = k[Na2S2O3]n

From the graph, since it’s a straight line through origin, n = 1 (first order).

Rate constant (k) = gradient of the line = Δ(1/t) / Δ[conc]

Using points (0.10, 0.0221) and (0.02, 0.0038):

k = (0.0221 – 0.0038) / (0.10 – 0.02) = 0.0183 / 0.08 = 0.2288 s-1 M-1

Temperature Effect Analysis

Arrhenius Equation

The relationship between rate constant (k) and temperature (T) is given by:

k = A e-Ea/RT

Where:

  • A = frequency factor (pre-exponential constant)
  • Ea = activation energy (J/mol)
  • R = gas constant (8.314 J/mol·K)
  • T = absolute temperature (K)

Sample Temperature Data

Temperature (°C) Temperature (K) Time (s) 1/t (s-1) ln(1/t) 1/T (K-1)
20 293 68.5 0.0146 -4.23 0.00341
30 303 45.2 0.0221 -3.81 0.00330
40 313 29.8 0.0336 -3.39 0.00319
50 323 19.5 0.0513 -2.97 0.00310

Conclusion: The reaction rate approximately doubles for every 10°C rise in temperature, consistent with typical chemical kinetics.

Sources of Error and Improvements

Potential Errors

  • Human error in timing: Different observers may judge “disappearance of cross” differently.
  • Temperature fluctuations: Room temperature changes during experiments.
  • Inconsistent mixing: Variations in swirling intensity affect reaction rate.
  • Measurement inaccuracies: Errors in measuring volumes of solutions.
  • Cross visibility: Differences in lighting or cross thickness.

Improvements to Experiment

  • Use a light sensor and data logger to objectively determine when cross disappears.
  • Use water baths with thermostats for precise temperature control.
  • Use automated stirrers for consistent mixing.
  • Calibrate all measuring equipment before use.
  • Standardize the cross (printed, not hand-drawn) and lighting conditions.
  • Have the same observer for all trials or use multiple observers and average their timings.

Short Answer Questions

1. Why does the solution become cloudy during the reaction?
Because solid sulphur (S) precipitates out of the solution, forming fine particles that scatter light.
2. What is the relationship between reaction time and reaction rate?
Reaction rate is inversely proportional to reaction time: Rate ∝ 1/t.
3. Why is a black cross used in this experiment?
To provide a clear visual reference point. The cross disappears from view when enough sulphur precipitate forms to obscure it.
4. What gas is produced in this reaction?
Sulphur dioxide (SO2) gas is produced.
5. How does increasing concentration affect collision frequency?
Increasing concentration increases the number of particles per unit volume, leading to more frequent collisions between reactant particles.
6. What would happen if we doubled the concentration of sodium thiosulphate?
The reaction time would approximately halve, and the reaction rate would approximately double (if the reaction is first order with respect to thiosulphate).
7. Why is the reaction faster at higher temperatures?
Higher temperatures increase the kinetic energy of particles, resulting in more frequent collisions and a higher proportion of collisions with energy ≥ activation energy.
8. What safety precaution is particularly important due to gas production?
Work in a well-ventilated area or fume hood to avoid inhaling sulphur dioxide gas.
9. What is the purpose of repeating the experiment with different concentrations?
To establish the relationship between concentration and reaction rate, and determine the order of reaction with respect to sodium thiosulphate.
10. Why should all experiments be conducted at the same temperature when studying concentration effects?
To ensure temperature is a controlled variable, so any changes in reaction rate can be attributed solely to changes in concentration.
11. What does a straight line through the origin on a 1/t vs concentration graph indicate?
It indicates the reaction is first order with respect to that reactant (sodium thiosulphate in this case).
12. How would you calculate the rate constant from your graph?
The rate constant (k) is equal to the gradient of the line on a 1/t vs concentration graph.
13. What is activation energy?
The minimum amount of energy that reactant particles must possess for a successful collision leading to reaction.
14. Why is it important to swirl the flask after adding the acid?
To ensure thorough and rapid mixing of the reactants, providing consistent initial conditions for each trial.
15. What would be the effect of using more concentrated hydrochloric acid?
The reaction would be faster because there would be more H⁺ ions available to react with thiosulphate ions.

Interactive Quiz: Reaction Kinetics

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Real-World Applications

Food Industry

Reaction rates affect food preservation, cooking times, and shelf life. Understanding kinetics helps optimize food processing.

Pharmaceuticals

Drug stability and expiration dates depend on reaction rates. Kinetics studies help determine proper storage conditions.

Environmental Science

Atmospheric reactions (like ozone depletion) and pollutant degradation rates are studied using kinetics principles.

Manufacturing

Chemical production efficiency depends on optimizing reaction rates through temperature and concentration control.

Biological Systems

Enzyme-catalyzed reactions in living organisms follow kinetics principles. Temperature affects metabolic rates.

Materials Science

Corrosion rates, polymer curing times, and material degradation all depend on chemical kinetics.