Reaction Kinetics – Order of Reaction of Sodium thiosulphate with Hydrochloric acid
Introduction
Sodium thiosulphate and hydrochloric acid react as shown below:
Na2S2O3 (aq) + 2HCl (aq) → 2NaCl (aq) + H2O (l) + SO2 (aq) + S (s)
Rate equation for the reaction is:
Rate = k [Na2S2O3]m [HCl]n
Where k is the rate constant, m is the order of reaction with respect to sodium thiosulphate and n is the
order with respect to hydrochloric acid.
Practical Pre-class Activity
Complete the following.
This will need to be submitted prior to the class used to perform the experiment.
1. Read the procedure on the following pages.
2. Write a list of equipment and chemicals required for this experiment.
3. Write down 3 safety requirements for this experiment. You will need to include the following:
- Identify the Danger (especially the chemicals used and/or produced, or the equipment used)
- Why it is dangerous (i.e. Why the chemical is dangerous and/or why is the equipment dangerous)
- What could get damaged/injured when using the chemical and/or equipment
- What should be done to decrease the danger/injury when using the chemical and/or equipment
Procedure:
A. Determine order with respect to Na2S2O3
1. Using separate measuring cylinders, pour 5 ml of the 0.25 M sodium thiosulphate solution into a
100ml beaker, followed by 20ml distilled water. Take/record the temperature of the solution (why?).
2. Using a 10 ml measuring cylinder, measure out 10 ml of 2.0 M hydrochloric acid.
3. On a piece of white paper, draw a cross, and stand the beaker of thiosulphate solution on the paper,
so the cross can be seen when viewed from above (through the solution).
4. Pour the acid into the sodium thiosulphate solution in the flask, swirl in one circle, and put back on the
cross, and immediately start the timer.
5. Watch (from above), and stop the timer just as the cross disappears from sight.
6. Record the time it takes for the cross to disappear, in the table below.
7. Repeat steps 1-5, according to the volumes quoted in the table below, and fill in the last three
columns.
Na2S2O3(mL)
dist water(mL)
HCl (mL)
5
20
10
10
15
10
15
10
10
20
5
10
25
0
10
[Na2S2O3] mol L-1
time (s)
1/t (1/s)
B. Determine order with respect to HCl
Follow a similar procedure to that in part A, but keep the volume of sodium thiosulphate solution constant
at 10.0 ml and vary the volume of hydrochloric acid as shown in the table below.
HCl (mL)
dist water (mL)
Na2S2O3(mL)
5
20
10
10
15
10
15
10
10
20
5
10
25
0
10
[HCl] mol L-1
time (s)
1/t (1/s)
Laboratory Report write-up
Introduction
The Introduction for the Report must be a MINUMUM of 300 words. The Introduction must include an
explanation of at least two relevant chemical concepts involved with this experiment.
Aim
Write a clear relevant aim for this experiment.
Equipment and Chemicals
Make a reference to this practical sheet. There will be no need to re-write the list of Equipment and
Chemicals as this will be done as part of the pre-class activity.
Safety Considerations
There will be no need to re-write this as this was done as part of the pre-class activity.
Results
Tabulate your results and comment on whether there are any anomalous data or not.
Data Analysis and Discussion
1. Plot a graph of [Na2S2O3] against 1/t.
Fully describe the shape of this graph and use it to determine the order of reaction with respect to
Na2S2O3 at the recorded temperature.
2. Plot a graph of [HCl] against 1/t.
Fully describe the shape of this graph and use it to determine the order of reaction with respect to HCl at
the recorded temperature.
3. Write the complete Rate Expression for this reaction. Include a calculation of the rate constant, k with
appropriate units and state the overall order of the reaction.
4. Describe two possible errors in this experiment.
Experimental Design
1. Describe TWO (2) ways that you could improve the experiment to give more reliable and valid results.
You need to consider the experimental design.
2. Discuss ONE (1) Green Chemistry principle for this experiment. If you think that the experiment
follows (or does not follow) this Green Chemistry principle, explain why (or why not).
Conclusion
Write a conclusion for the experiment that summarizes the findings and responds back to the aim.
References
Include citation of references used for writing this report using the APA referencing system and include
the list of references at the end of this report.