The acidification of thiosulfate solutions leads to the formation of colloidal sulfur.
Practice mixing technique with water:
Select 2 12-well strips. Place 3 drops of water in each of the first 6 wells of each strip. When one strip is inverted and stacked on the second strip, capillary action keeps the liquid in the upper wells. To mix the chemicals, hold the stacked strips in an elevated position and quickly flick them in a downward direction. When you are confident of the mixing technique, completely empty the strips to use in the experiment.
Begin the experiment:
Prepare two 12-well strips as follows:
| Strip A | Strip B | ||
| drops 0.15 M | drops 1 M | drops | |
| Na2S2O3 | HCl | water | |
| Well 1 | 10 | 2 | 0 |
| Well 2 | 9 | 2 | 1 |
| Well 3 | 8 | 2 | 2 |
| Well 4 | 7 | 2 | 3 |
| Well 5 | 6 | 2 | 4 |
| Well 6 | 5 | 2 | 5 |
| Well 7 | 4 | 2 | 6 |
When one strip is inverted and stacked on the second strip, capillary action keeps the liquid in the upper wells. Mark the numbers 1 through 10 on a piece of paper such that they are spaced at the same spacings as the wells in the strip and can be viewed through the strip. (Alternatively, place the strips on a page of printed material such as this page.)
Work with a partner. One partner mixes and observes while the other notes and records times. Mix the chemicals and note the time to the nearest second.
Note the time at which the printed materials disappear (i.e., are no longer visible) through the well. Record for all wells.
Clean the strip immediately after the last well changes or the experiment is ended. Scrub the wells with a cotton swab.
Prepare a graph plotting time as the y-axis and number of drops of thiosulfate as the x-axis.
On the same graph plot the reciprocal of the time as the y-axis. If the second graph has zero slope, the order is zero. If the graph is linear with slope of 1 and passes through the origin, the order is one. If the curve is parabolic, the order is two.
Determine the order of thiosulfate in the reaction.
The rate of reaction can be represented by the following equation:
Rate = k[S2O32-]a[H+]b
The concentration of H+ is held constant in the procedure; all wells in strip B were filled with the same amount of acid. We may write the rate:
Rate = k'[S2O32-]a
Where [H+]b has been absorbed into the pseudo-rate constant, k'.
In each reaction well we wait until the character is no longer visible. Presumably this requires that the same amount of sulfur be produced. The amounts of reactant used up in causing this to take place is small, so the reactant concentrations remain essentially constant throughout the time of reaction.
The expression for rate is:
Δ[S2O32-]/Δt)
But the amount of thiosulfate used up at the time of the endpoint is a constant because the amount (moles) of sulfur is constant for each well. Therefore, the rate is related to a constant divided by the time it takes to reach the end point. Plotting 1/Δt) is the same as plotting a constant times the reaction rate.
But the rate is equal to k'[S2O32-]a. Therefore, a plot of the rate versus [S2O32-] gives an indication of the exponent, a. If the slope is zero, a = 0. If there is a straight line through the origin, a = 1. If there is a parabola, a = 2.
| drops Na2S2O3 | time (sec) | |
| Well 1 | 10 | 30 |
| Well 2 | 9 | 32 |
| Well 3 | 8 | 37 |
| Well 4 | 7 | 42 |
| Well 5 | 6 | 50 |
| Well 6 | 5 | 60 |
| Well 7 | 4 | 78 |
All of the solutions used may be safely discarded at the sink. The test tube and one 12-well strip may need to be brushed to remove sulfur deposits.
The dilute hydrochloric presents a minimal chemical hazard.
Handle the dilute hydrochloric acid with caution. Employ cautions to protect computers when using them in the laboratory.