A possible pre-lab for Expt. #13: Understanding the role of sodium thiosulfate
Excel spreadsheet: 120-Ex13 –prelab (Sheet2)
1. Goals
- To show that the reaction studied is very slow
- To show the role of sodium thiosulfate in the reaction system
- To clarify the definition of the rate of reaction
- To show how the sodium thiosulfate is used to measure the rate of reaction
2. Background information
The reaction being studied is: 2 I-(aq) + S2O82-(aq) → I2(aq) + 2 SO42-(aq)(1)
We define the rate of reaction as the rate at which product I2 is being formed. Thus:
Rate = (5)
The rate is not a constant: it depends on the concentrations of the reagents via the rate law:
Rate = k [I-]X [S2O82-]Y(2)
Experimentally, we cannot measure the concentration of I2 directly, so we use sodium thiosulfate, i.e. the S2O32- ion, as an “indicator”: this reacts rapidly with I2:
2 S2O32-(aq) + I2(aq) → S4O62-(aq) + 2 I-(aq)(3)
3. Describing the spreadsheet – select Sheet 2
In the model used here, values of k, X and Y are assumed.
The initial molar concentrations of the reagents, I- and S2O82-, and of the indicator, S2O32-, are entered, by typing (no units) in the orange-brown cells D5, D6 and F5.
The reaction time is varied using the slider. For coarse adjustments, click on the movable bar and drag. For the finest time adjustments, click/tap on the arrows at the top and bottom of the slider. For intermediate adjustments, click/tap on the background region of the slider.
The amounts, in mol/L, of the main actors, the reagents, the indicator and the product I2, are shown in the colored table and in the bar chart. The reaction time is also shown and the rate of reaction, calculated from equation (2).
4. Exploration (I): Goal 1.
Enter initial concentrations as follows: I-: 0.070M; S2O82-: 0.040M; [S2O32-]: 0.010M
Vary the time in coarse fashion. Note that as reaction time increases:
- The concentrations of the reagents decrease and that of I2 increases
- The rate of reaction decreases
- At the longest times (about 9 hours!), reaction is still not complete: the reaction is quite slow!
Question 1. Explain why the rate decreases as time increases: use equation (2). If at long times, the reaction becomes complete, what would the rate be then?
5. Exploration II: Goal 2.
Return the slider to time = 0. Now adjust the time slowly (click on arrow or on slider background). Watch the concentration of S2O32- (in yellow) decrease slowly. Note that no I2 is formed at this stage.
Question 2. Why is no I2 present at this stage?
Question 3. Why is the S2O32- concentration decreasing as time increases?
Continue until no S2O32- remains – and keep going for another minute or so.
Question 4. At what stage of the reaction does I2 first appear?a) Before the last S2O32- disappears OR b) Directly after the last S2O32- disappears OR c) At a somewhat later time?
Question 5. Explain your answer to Question 4.
In lab, at the point where I2 first appears, the solution becomes suddenly blue: that is the reaction time which you will record.
6. Exploration III: Measuring the rate of reaction: Goals 3 and 4.
*** Please note that none of the “measurements” of rate found in the following will be exactly the same, as all rates are an average over time and rate varies with time, as we saw earlier.
a) Set the initial S2O32- indicator concentration to zero.
Note that product I2 now appears as soon as the reaction is started.
Run the reaction for, say, 25 seconds.
Divide the current [I2] by the time, 25s, to get the rate (averaged over the first 25 seconds). Item 6. Report the value you obtain. The value you get should be very similar to that given in cell L9, computed from the rate law.
b) Set the initial S2O32- concentration to 0.00100M (which is a typical value used in the experiment).
Run the reaction until the S2O32- just disappears and I2 first appears.
Item 7.Record the time. Apply equation (4) for the rate:
Rate = (4)
and evaluate the rate using the time just recorded.
Question 8. Is the rate close to that measured in part 6a)?
Question 9. Try to explain why equation (4) is correct. Remember that it must be consistent with the definition of rate in equation (5) above.
c) Finally, note that most of the reagents, I- and S2O82-, are still present. Hardly any reaction has occurred, so we have measured the initial rate of the reaction.
7. Further explorations.
Vary the initial I- and S2O82- concentrations over the range 0.01 to 0.1M and test whether the rate changes significantly. Also vary the S2O32- concentration, if you wish, over the range 0 to 0.002M. Report your findings.
C:\Y0809D\Sum09\Sum09-Ex13-Excel1.doc