Required Practical 5: Rates of Reaction

Scientific Principles & Theory

Scientific Background: The rate of reaction is the change in concentration of reactants or products per unit time. According to collision theory, reacting particles must collide with sufficient energy (activation energy) and correct orientation to react. Increasing concentration increases the number of particles per unit volume, which increases collision frequency.

Method 1: Turbidity (Disappearing Cross). Sodium thiosulfate reacts with hydrochloric acid, forming a yellow precipitate of sulfur which makes the solution cloudy:

Na₂S₂O₃(aq) + 2HCl(aq) → 2NaCl(aq) + S(s) + SO₂(g) + H₂O(l)

Method 2: Gas Collection. Magnesium reacts with hydrochloric acid to produce hydrogen gas, which is collected in a gas syringe to measure volume over time:

Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)

Experimental Variables

Independent Variable

The concentration of sodium thiosulfate (Method 1) or concentration of acid (Method 2).

Dependent Variable

The time taken for the cross to disappear (Method 1) or the volume of hydrogen gas produced over time (Method 2).

Control Variables

Method 1: volume of acid, concentration of acid, total volume of solution, temperature. Method 2: volume of acid, length/surface area of magnesium, temperature.

Lab Risk Assessment

Hazard	Associated Risk	Control Measure
Sulfur dioxide gas (Method 1)	Toxic and can trigger asthma attacks	Perform the experiment in a well-ventilated room; dispose of reaction mixtures immediately into a container of sodium carbonate solution; do not inhale gas.
Hydrogen gas (Method 2)	Highly flammable	Keep away from naked flames and spark sources.
Dilute acids	Skin and eye irritation	Wear safety goggles; wash spills off skin immediately.

Apparatus & Procedure

Required Apparatus

Sodium thiosulfate solution (40 g/dm³)
Dilute hydrochloric acid (1.0 mol/dm³ and 2.0 mol/dm³)
Magnesium ribbon (3 cm strips)
Conical flask (100 cm³)
Measuring cylinders (10 cm³, 50 cm³, 100 cm³)
White paper with a black cross
Gas syringe and stand with clamps
Stopwatch
Distilled water

Step-by-Step Procedure

Method 1 (Turbidity): Pour 50 cm³ of 40 g/dm³ sodium thiosulfate solution into a conical flask. Place the flask on paper marked with a black cross.
Add 10 cm³ of dilute hydrochloric acid, swirl the flask once, and start the stopwatch immediately.
Look straight down through the flask and stop the stopwatch when the cross is no longer visible. Record the time.
Repeat the steps using diluted thiosulfate concentrations: mix 40 cm³ thiosulfate + 10 cm³ water, then 30 cm³ + 20 cm³, then 20 cm³ + 30 cm³, and 10 cm³ + 40 cm³. Keep total volume at 50 cm³.
Method 2 (Gas Collection): Measure 50 cm³ of 1.0 mol/dm³ hydrochloric acid and pour it into a conical flask. Set up the gas syringe and clamp it in place.
Add a 3 cm magnesium ribbon to the flask, quickly insert the rubber bung connected to the gas syringe, and start the stopwatch.
Record the volume of gas in the syringe every 10 seconds for 60 seconds.
Repeat the procedure using 2.0 mol/dm³ hydrochloric acid.

Fig 1. Laboratory experimental setup for Required Practical 5.

Sample Data & Calculations

This representative dataset illustrates the values typically obtained when carrying out this experiment in the laboratory:

Time (seconds)	Gas volume at 1.0 mol/dm³ (cm³)	Gas volume at 1.5 mol/dm³ (cm³)
0	0	0
10	15	22
20	26	38
30	35	48
40	41	54
50	45	57
60	48	58

Average rate of reaction = Volume of gas produced / Time elapsed.
To find the reaction rate at a specific time (e.g., at t = 20 s):
Plot a graph of time (x-axis) vs. volume of gas (y-axis) and draw a curve of best fit.
Draw a straight line tangent to the curve at the 20-second point.
Calculate the gradient of the tangent line: Gradient = Change in y / Change in x.
Worked Example: Tangent at 20 s crosses points (5 s, 10 cm³) and (35 s, 40 cm³). Gradient = (40 - 10) / (35 - 5) = 30 / 30 = 1.0 cm³/s.

Conclusion & Evaluation

Chemical Explanation: Evaluating the experimental outcomes against known values ensures validity. Understanding the source of systematic and random deviations allows for better experimental designs in future trials.

Experimental Error Analysis

Error Type & Source	Effect on Final Result	Mitigation Strategy
Systematic Error Gas escapes when inserting the rubber bung	The volume of gas recorded is lower than the true volume, underestimating the initial rate of reaction.	Use a flask with a side-arm and a divided boiling tube, or drop the magnesium from a suspended thread inside the sealed flask to start the reaction without opening it.
Random Error Subjective sight check in turbidity method	Human variations in deciding when the cross disappears can skew individual times.	Ensure the same person observes the cross disappearance for all concentrations, looking from the exact same height and angle.

Exam Practice

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[6 marks]

A student investigates the effect of sodium thiosulfate concentration on the rate of reaction with hydrochloric acid. Plan an experiment to carry out this investigation, detailing how to obtain different concentrations and how to process the results.

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Marking guidance: award 1 mark for each correct, clearly explained step in the model answer below, up to 6 marks.

Model Answer (6/6 Marks):

Apparatus: Set up a conical flask, measuring cylinders, a stopwatch, and a sheet of white paper marked with a clear black cross.
Baseline run: Add 50 cm³ of 40 g/dm³ sodium thiosulfate solution to the flask. Place the flask directly over the black cross. Add 10 cm³ of hydrochloric acid, swirl, and start the stopwatch.
Observation: Look vertically down through the mouth of the flask. Stop the stopwatch the exact moment the yellow precipitate of sulfur obscures the black cross. Record the time.
Concentration variations: Repeat the procedure, diluting the sodium thiosulfate with distilled water to vary concentration while keeping the total volume constant at 50 cm³ (e.g., 40 cm³ thiosulfate + 10 cm³ water, etc.).
Controls: Keep the volume and concentration of acid, the total liquid volume, and the temperature constant. Repeat each run to calculate a mean.
Data analysis: The rate can be represented as 1/time (s⁻¹). Plot a graph of sodium thiosulfate concentration (x-axis) against 1/time (y-axis). A straight line through the origin indicates that rate is directly proportional to concentration.

Examiner Tip:

Always specify that the total volume of the thiosulfate and water mixture must remain constant (50 cm³) so that the concentration is the only variable being altered.

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