Title: The ‘disappearing cross’ experiment
Aim: To determine the effect of concentration on the rate of reaction between sodium thiosulfate
and sulfuric acid.
Apparatus/Materials:
1. 16.0 g dm-3 sodium thiosulfate (VI) solution
2. 0.5 mol dm-3 sulfuric acid
3. Stopwatch
4. 250 mL beaker
5. Sheet of paper with a cross drawn on it
6. 3, 50 mL measuring cylinders
7. 1, 10 mL measuring cylinder
Method:
1. 50 mL of sodium thiosulfate was measure and poured into a beaker.
2. 50 mL of sulfuric acid was then added to the sodium thiosulfate and the time poured was
noted.
3. The beaker was placed over the cross and the time for the cross to be no longer visible
when viewed from above was recoded.
4. Steps 1-4 were repeated 6 more times using the same volume of sulfuric acid but
different volumes of sodium thiosulfate.
Results:
Table 1. Showing the rate of reaction between sodium thiosulfate and sulfuric acid
Experiment
number
1
2
3
4
5
6
7
Vol. of
acid/mL
50
50
50
50
50
50
50
Vol. of
thiosulfate/mL
50
45
40
35
30
25
20
Vol. of
water/mL
0
5
10
15
20
25
30
Time/s
1/T (s-1)
Discussion:
What visible change occurred during the reaction? Why does the cross “disappear” during the
experiment? What substance is formed that causes this effect?
How does the time taken for the cross to disappear change as the volume of sodium thiosulfate
decreases? What trend do you observe between concentration and reaction time? Why
is 1/T used as a measure of the rate of reaction instead of just time? How does the value
of 1/T change as the solution becomes more diluted?
How does decreasing the concentration of sodium thiosulfate affect the number of particles in
solution? Using Collision Theory, explain why a lower concentration results in a slower reaction.
What happens to the frequency of effective collisions as concentration decreases?
Precautions:
1. Start the stopwatch immediately when the acid is added and stop it at the same visual
endpoint.
2. Use clean measuring cylinders and beakers to avoid contamination.
3. Measure all volumes accurately at eye level.
4. Mix the solutions quickly and consistently after adding the acid.
5. Keep the total volume constant in each trial (thiosulfate + water + acid).
6. Carry out the experiment at a constant temperature.
7. Place the beaker directly over the cross in the same position each time.
Sources of error:
1. Inconsistent mixing of solutions, affecting how quickly the reaction proceeds.
2. Inaccurate volume measurements due to misreading the meniscus.
3. Variations in lighting conditions, affecting visibility of the cross.
4. Temperature fluctuations, which can speed up or slow down the reaction.
5. Residue from previous trials if apparatus is not properly cleaned.
6. Delay between adding acid and starting timing.
Limitations:
1.
2.
3.
4.
5.
6.
7.
The endpoint (cross disappearing) is not precise.
The method only provides an approximate rate (1/time), not the true rate of reaction.
The reaction is monitored indirectly through visibility, not by measuring concentration
changes directly.
Difficult to ensure perfectly consistent mixing in every trial.
The experiment assumes uniform turbidity, but sulfur particles may not form evenly.
Limited accuracy compared to colorimeters.
Small timing differences can lead to large differences in calculated rate (1/T).
Reflection:
What did this experiment help you understand about the relationship between concentration and
rate of reaction? How did this activity improve your understanding of Collision Theory? Which
part of the experiment made the concept clearer to you? Where in real life is the effect of
concentration on reaction rate important? Why is it important for scientists and industries to
control reaction rates?
What difficulties did you encounter while carrying out the experiment? Why was it challenging
to determine the exact point when the cross disappeared? How did you deal with any problems
during the procedure?
How reliable do you think your results were? Why? What steps could you take to improve the
accuracy of your results? How could the method be modified to reduce human error?
Conclusion: