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:
Method 2: Gas Collection. Magnesium reacts with sulfuric acid to produce hydrogen gas, which is collected in a gas syringe to measure volume over time:
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 (2.0 mol/dm3)
- Magnesium ribbon (3 cm strips)
- Dilute sulfuric acid (1.0 mol/dm3 and 1.5 mol/dm3)
- 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 sulfuric 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 1.5 mol/dm3 sulfuric 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 |
Data Processing & Analysis
- 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
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.
View Model Answer & Mark Scheme
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.