Experiment 09: Rate of Reaction — Sodium Thiosulfate and Acid
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Topic: Rates of Reaction
Level: Ordinary & Higher Level
Exam Focus: Section A Practical + Graph Skills
1. Experiment Snapshot
Feature
Details
Experiment
To investigate the effect of concentration on the rate of reaction between sodium thiosulfate and hydrochloric acid.
Main observation
A yellow sulfur precipitate forms and makes a black cross disappear.
Main measurement
Time taken for the cross to disappear.
Difficulty
⭐⭐⭐ Medium
Exam importance
⭐⭐⭐⭐⭐ Very high for rate, graph and variable questions.
Typical marks
8–16 marks depending on method, graph and evaluation.
2. Aim
To investigate how changing the concentration of sodium thiosulfate solution affects the rate of its reaction with hydrochloric acid.
3. Quick Theory
Sodium thiosulfate reacts with hydrochloric acid to produce a yellow precipitate of sulfur. As more sulfur forms, the mixture becomes cloudy and the black cross underneath the flask disappears.
Safety: Wear goggles. Hydrochloric acid is irritant/corrosive. Sulfur dioxide gas may be produced, so perform in a well-ventilated area and avoid inhaling fumes.
5. Labelled Experiment Diagram
Disappearing Cross Method
6. Variables
Variable Type
In this Experiment
Independent variable
Concentration of sodium thiosulfate solution.
Dependent variable
Time taken for the cross to disappear / calculated rate.
Controlled variables
Volume of acid, concentration of acid, total volume of mixture, temperature, same cross, same observer, same flask.
Examiner Trap: If changing concentration by dilution, keep the total volume constant by adding water.
7. Procedure
Draw a clear black cross on a piece of white paper.
Place a conical flask on top of the cross.
Measure a fixed volume of sodium thiosulfate solution into the flask.
If testing different concentrations, add measured amounts of water to dilute the sodium thiosulfate while keeping total volume constant.
Measure a fixed volume of hydrochloric acid.
Add the acid to the flask and immediately start the stopwatch.
Look down through the mixture at the cross.
Stop the stopwatch when the cross can no longer be seen.
Record the time.
Repeat with different concentrations of sodium thiosulfate.
Calculate the rate using 1/time if required.
Examiner Tip: Start timing immediately when the acid is added, because the reaction begins at once.
8. Results Table Template
Volume Na₂S₂O₃ / cm³
Volume Water / cm³
Volume HCl / cm³
Time / s
Rate = 1/time / s⁻¹
50
0
10
25
0.0400
40
10
10
32
0.0313
30
20
10
45
0.0222
20
30
10
70
0.0143
10
40
10
130
0.0077
Conclusion from example table: As sodium thiosulfate concentration decreases, the time taken increases and the rate decreases.
9. Graph Skills
Students may be asked to plot a graph of rate against concentration, or time against concentration.
Graph
Expected Pattern
Meaning
Rate against concentration
Rate increases as concentration increases.
More particles per unit volume cause more frequent successful collisions.
Time against concentration
Time decreases as concentration increases.
Faster reaction means the cross disappears more quickly.
Higher concentration → more frequent collisions → faster reaction → shorter time
Common Mistake: Students sometimes say “higher concentration gives longer time”. It is the opposite: higher concentration gives a shorter time because the reaction is faster.
10. Scientific Explanation
Increasing concentration means there are more reactant particles in the same volume. This causes more frequent collisions between particles. More successful collisions per second means the reaction rate increases.
Exam answer sentence:
Increasing the concentration of sodium thiosulfate increases the number of particles per unit volume, so collisions occur more frequently and the reaction is faster.
11. Sources of Error and Improvements
Source of Error
Effect
Improvement
Judging when the cross disappears is subjective
Different students may stop timing at different moments.
Use the same observer each time or use a light sensor/colorimeter.
Delay in starting stopwatch
Time measured too short or inconsistent.
Start stopwatch immediately when acid is added.
Temperature changes during experiment
Rate changes because temperature affects collision energy.
Use a water bath or keep room temperature constant.
Volumes measured inaccurately
Concentrations not as planned.
Use accurate measuring cylinders or pipettes.
Flask not cleaned between trials
Contamination affects cloudiness or rate.
Rinse and dry flask between trials.
Different cross thickness or lighting
Cross disappears at a different apparent time.
Use the same cross and same lighting each time.
12. Student Common Mistakes
Forgetting to start the stopwatch immediately after adding acid.
Changing total volume when diluting sodium thiosulfate.
Using final time as rate instead of calculating 1/time.
Forgetting the unit s⁻¹ for 1/time.
Not keeping temperature constant.
Saying the cross disappears because it dissolves.
Forgetting that sulfur precipitate causes the cloudiness.
Drawing a graph with wrong axes or missing units.
13. Examiner Tips
Tip 1: Write “time for cross to disappear”, not just “time taken”.
Tip 2: Rate is often estimated as 1/time.
Tip 3: For fair test questions, mention same total volume, same acid volume and same temperature.
Tip 4: The yellow precipitate is sulfur.
14. Examiner Traps
Trap: The cross does not disappear chemically; it is hidden by the cloudy sulfur precipitate.
Trap: If concentration increases, time decreases but rate increases.
Trap: Rate and time are not the same. Rate is proportional to 1/time.
Trap: If water is added to dilute sodium thiosulfate, the total volume should be kept constant.
A reaction takes 40 seconds. Calculate the rate using 1/time. [2 marks]
Give one source of error and one improvement. [2 marks]
Describe how you would dilute sodium thiosulfate but keep the test fair. [3 marks]
17. Mark Scheme Answers
Q1. To investigate the effect of concentration [1] on the rate of reaction between sodium thiosulfate and hydrochloric acid [1].
Q2. Sulfur precipitate forms [1], making the mixture cloudy and hiding the cross [1].
Q3. Sulfur [1].
Q4. Temperature / volume of acid / concentration of acid / total volume / same cross / same flask. Any two [2].
Q5. Higher concentration means more particles per unit volume [1]; collisions occur more frequently [1]; more successful collisions per second increases rate [1].
Q6. Rate = 1/time [1] = 1/40 = 0.025 s⁻¹ [1].
Q7. Subjective judgement of cross disappearing [1]; use same observer or light sensor/colorimeter [1]. Other valid answers accepted.
Q8. Use less sodium thiosulfate [1]; add water to make the same total volume [1]; keep acid volume/concentration and temperature constant [1].
18. Quick Revision Card
Key Point
Memory Hook
Main observation
Black cross disappears.
Reason cross disappears
Cloudy sulfur precipitate forms.
Rate estimate
Rate = 1/time.
Higher concentration
Shorter time, faster rate.
Fair test
Keep temperature, acid volume and total volume constant.
Main improvement
Use same observer or light sensor/colorimeter.
19. Mastery Checklist
I can state the aim of the experiment.
I can describe the disappearing cross method.
I know sulfur precipitate causes cloudiness.
I can identify independent, dependent and controlled variables.
I can explain the effect of concentration using collision theory.
I can calculate rate using 1/time.
I can describe sources of error and improvements.
I can explain how to keep total volume constant when diluting.