Expt 056 -- Thiosulfate in Acid Solution
Description
The acidification of thiosulfate solutions leads to the formation of colloidal sulfur. The rate of this reaction is studied by measuring the time required for the reaction mixture to become so turbid that is ceases to transmit light.
Background
- A description of the sequence of steps by which a chemical reaction takes place is called a reaction mechanism. Many studies go into trying to determine possible mechanisms. The rates at which reactions take place provide important insights.
- The rate law for a chemical reaction is a quantitative expression involving constants related to the nature of the chemical reaction and the concentrations of reactants. In order for reactants to react, they must come in contact with one another (or at least come nearby). The probability of a collision is related to a function of the concentration. So, for the general rate law:
- rate = k [A]a[B]b[C]c
- [A], [B], and [C] represent concentrations of reactants and or catalysts, and a, b, and c represent exponents that may or may not be related to the coefficients of the corresponding balanced chemical equation.
- The quantities in brackets are read as moles/liter and are raised to an appropriate power. Multiplied together with the constant (k), they give the rate of the reaction.
- The numerical values of a, b, and c must be determined by experimentation. These numbers determine the order of the reaction. Added together they give the over-all order of the reaction. It is the purpose of this experiment to determine the order of S2O32-, a reactant.
- The reaction to be studied in this experiment is the acid catalyzed production of colloidal sulfur from thiosulfate:
- S2O32- --> S + SO32-
- The rate of this reaction at various concentrations of S2O32- will be studied.
Safety
Wear goggles and aprons. Handle the dilute hydrochloric acid with caution. Wash spills with water. Wash hands after the experiment.
Procedure
- Practice the 'shake-down' technique. Fill the first 5 wells of each of 2 12-well strips with 3 drops of distilled water. The instructor will demonstrate the "shake-down" method of mixing. Stack one strip atop the other. When one strip is inverted and stacked on the second strip, capillary action keeps the liquid in the upper wells.
- !!!Click here to See Picture.
- Holding the plates firmly together shake them once vigorously in a downward motion. This is done by flicking hands downward as fast as possible and then stopping abruptly. There is no upward motion; this is a 'shake-down' method.
- !!!Click here to See Movie.
- !!!Click here to See Picture.
- Prepare two 12-well strips as follows:
|
Strip A
|
Strip
|
B |
|
drops 0.15 M
Na2S2O3
|
drops 1 M
HCl
|
drops
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
|
|
Well 8
|
3
|
2
|
7
|
- Add the components dropwise using plastic storage pipets.
- Mark the numbers 1 through 8 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.)
- !!!Click here to See Picture.
- Work with a partner. One partner mixes and observes while the other notes and records times. Use the 'shake-down' technique to mix the chemicals. Note the time to the nearest second. Place the wells on the numbers on the paper that you prepared. Place well 1 over number 1.
- !!!Click here to See Movie.
- Record the time required for the well's number to become too obscured by sulfur to read. Both partners should try reading the last points.
- Clean the strip immediately after the last well changes or the experiment is ended. Scrub the wells with a cotton swab.
- !!!Click here to See Movie.
- 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.
- !!!Click here to See Picture.
- 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 by using a large excess. We may write the rate:
- Rate = k'[S2O32-]a
- Where [H+]b has been absorbed into the pseudo-rate constant, k'.
- This experiment has been set up in a clever fashion. The endpoint color appears after a fixed amount of colloidal sulfur has been produced. Because the amounts of reactant used up in causing the production of this amount of sulfur are small, the reactant concentrations remain essentially constant throughout the time of reaction.
- The expression for rate is:
- Δ[S2O32-]/Δt
- But at the endpoint, the moment just after enough sulfur to obscure reading a number has been produced, the concentrations of the reacting species have hardly changed at all. Therefore, the rate is related to a constant divided by the time it takes to reach the endpoint. Plotting 1/Æt 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.
- Theoretical plots for a=0, a=1, and a= 2 are plotted. See Figure below.
- If a = 0, a horizontal line is expected; if a = 1, a straight line through the origin is expect; and if a = 2, a parabola is expected.
- Plot a graph of the results. Use the x-axis for number of drops and the y-axis for the reciprocal of the reaction time. Draw the best fitting curve to this plot.
- Based upon the graphs, determine the order of the reaction with respect to S2O32-.
Questions
- Identify the role of the HCl in this reaction.
- The following data are synthetic, not real. Suppose the following data had been obtained:
|
drops S2O32-
|
time
|
|
10
|
32
|
|
9
|
43
|
|
8
|
52
|
|
7
|
71
|
|
6
|
89
|
|
5
|
146
|
|
4
|
202
|
|
3
|
390
|
- Determine the order for S2O32- from this imaginary data set.
Handout
Name ___________________________ Class ________
Teacher__________________________
SmallScale 56 Thiosulfate in Acid Solution
Makeup students should time the movie of the reaction to collect data.
Plot your data as described in the Data Analysis section.
Based upon the graphs, determine the order of the reaction with respect to S2O32-.
Curriculum-
This is a 2nd year, advanced placement experiment. Considerable data analysis is involved, and students without experience in algebra are likely to be intimidated. This activity provides excellent preparation for AP chemistry, however. This experiment also can fit in if and when radioactive decay is discussed. The mathematical analyses are similar.
Safety-
Wear goggles and aprons. Handle the dilute hydrochloric acid with caution. Wash spills with water. Wash hands after the experiment.
Time-
Teacher Preparation: 15 minutes
Class Time: 30 minutes
Materials-
- 3 mL of 0.15 M Na2S2O3 (Dissolve 3.72 g sodium thiosulfate pentahydrate (Na2S2O3•5H2O) in enough water to make 100 mL solution.)
- 1 mL of 1.0 M HCl (Add 33 mL of 3 M HCl to enough distilled water to make 100 mL of solution.)
- distilled water
- 2 12-well strips
- cotton swabs
Disposal-
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.
Data Table-
|
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
|
|
Well 8
|
3
|
102
|
Data Analysis-
A plot of the concentration of S2O32- against time is linear. See Figure below.
Under the circumstances of this experiment, first order in [S2O32-] is implied.
Answers-
- Q1. Identify the role of the HCl in this reaction.
- A1. HCl is serving as a catalyst.
- Q2. The following data are synthetic, not real. Suppose the following data had been obtained:
|
drops S2O32-
|
time
|
|
10
|
32
|
|
9
|
43
|
|
8
|
52
|
|
7
|
71
|
|
6
|
89
|
|
5
|
146
|
|
4
|
202
|
|
3
|
390
|
- Determine the order for S2O32- from this imaginary data set.
- A2. A parabolic curves fits this data better than does a linear curve; the order is 2.
Computer Use-
The graphical analysis of the data from the experiment may be accomplished using a computer. The curves shown were obtained using DeltaGraph® Professional.
CoopLearn-
The experiment is best done in pairs. Data can be pooled, or a wider range of concentrations studied.
Reference-
A macro scale version of this experiment was suggested to us by Dr. Darrell Beach, Culver, IN.
Key Words 1-
colloidal sulfur, kinetics, oxidation, rate, rate law, redox, reduction, time, turbidity, mechanism, order of reaction, reaction rate
Elements-
S H