Reaction Rate and Temperature

• Experiment objective: Observe how the reaction rate between Sodium Thiosulphate and Hydrochloric acid changes with temperature.

Reaction Overview

• Sodium thiosulphate reacts with hydrochloric acid.
• The balanced chemical equation is Na2S2O3 (aq) + 2HCl (aq) → 2NaCl (aq) + SO2 (g) + S (s)
• The ionic equation is S2O3 2+ (aq) + 2H + → S (s) + SO2 (g) + H2O (l)
• The reaction produces a Sulphur precipitate, making the solution cloudy.
• The reaction rate is monitored by measuring the time it takes for a fixed amount of Sulphur to form.
• A visual method involves timing how long it takes for a mark under the reaction vessel to disappear due to the Sulphur precipitate.
• Dilute hydrochloric acid is added to Sodium Thiosulphate at different temperatures across multiple experiments.
• Temperatures for the experiment must not exceed 55 °C.

Experimental Setup Temperatures

• Experiment 1: 15°C
• Experiment 2: 25°C
• Experiment 3: 35°C
• Experiment 4: 45°C
• Experiment 5: 55°C
• The actual temperature should be accurately measured, using the mean of the initial and final temperature.

Materials

• Thermometer
• Water bath
• 25 ml measuring cylinder
• 5 test tubes
• 50 ml glass vial or 50 ml beaker
• Plastic graduated pipette (Pasteur pipette)
• Stop watch
• 1 mol.dm-3 HCl solution (or 0.5 mol.dm-3 H2SO4 solution)
• 0.05 mol.dm-3 Sodium Thiosulphate solution
• Small piece of white paper marked with a cross

Procedure

• Collect 10 ml of prepared acid (1.0 mol.dm-3 HCl or 0.5 mol.dm-3 H2SO4) in a labeled plastic beaker.
• Measure 5 ml of 0.05 mol.dm-3 Na2S2O3 solution into each of 5 labeled test tubes.
• Place each tube on white paper with a cross marked on it.
• Insert a thermometer into test tube 1 and record the initial temperature of the Na2S2O3 solution.
• Add 1 ml of acid to the test tube using a Pasteur pipette quickly, and start the timer immediately.
• Monitor the tube from above, and record the time until the cross disappears from view.
• Record the temperature of the reaction mixture during the reaction.
• Use a water bath to maintain reaction temperatures for subsequent experiments, repeating the procedure.

Data Analysis

• The experiment investigates the time taken to produce a fixed amount of Sulphur at different temperatures.
• A graph of Sulphur produced vs. time would initially be a straight line.
• Initial reaction rate = (amount of Sulphur produced) / time.
• The reaction rate is proportional to 1/time (1/t).
• Calculate 1/t to four decimal places for each of the five temperatures, where t is the time for the cross to be obscured.
• Plot a graph with 1/t on the y-axis and temperature on the x-axis.
• Multiplying 1/t values by a common factor of 104 may ease plotting.

Technical Analysis

• Rate constant K varies with temperature T (in Kelvin): K = Ae-Ea/RT
• Since K is the inverse of time (1/t), the formula becomes: [1/t] = [-Ea/R] [1/T] + ln A, which is equivalent to y = ax + b.
• 'John Bird' can be referenced for formula workings, in this experiment the rate constant is proportional to 1/t.
• Calculate the activation energy, expressing the result in kJ/mol, given that R = 8.31 J/K.mol.
• Plot a graph of ln(1/t) on the y-axis against 1/T (temperature in Kelvin).