The Sulfur Clock: Using “Time to Turbidity” to Determine a Rate Law Academic Essay

A common environmental parameter that scientists use to establish the
cleanliness or overall health of a body of water is turbidity. A body of water is
said to be turbid if it appears clouded or opaque to the human eye; the
cloudiness is caused by solids (living and non-living) suspended in the water.
Waters with low concentrations of suspended solids are clearer and less turbid
than those with high concentrations of suspended solids. If you’ve ever gone
swimming in a natural body of water and couldn’t readily see the bottom in
shallow sections, you have observed turbidity.
Turbidity can be caused by high concentrations of biota such as phytoplankton,
or by loading of abiotic matter such as sediments. Turbidity is important in
aquatic systems as it can alter light intensities through the water column, thus
potentially affecting rates of photosynthesis. Lowered rates of photosynthesis
may in turn affect the levels of dissolved oxygen available in a given body of
water, thus affecting larger populations such as fish.1 Additionally, higher
turbidity increases water temperatures because suspended particles absorb
more heat. Increased water temperatures also decrease levels of dissolved
oxygen, again impacting fish populations.
Now that you understand the definition of turbidity, we will use the concept of
“time to turbidity” as a way of relating the concentration of suspended solids to
time. The area of study focused on how particulate matter changes as a function
of time is called chemical kinetics. Thus, The Sulfur Clock experiment
illustrates how the rate law of a chemical reaction can be determined
experimentally simply by recording the time to turbidity at various reactant
concentrations. Remember, the rate law for any chemical reaction MUST be
determined experimentally; it cannot be inferred from the coefficients of the
balanced chemical reaction. Thus, chemists are always trying to find innovative,
clever, and simple ways to determine the concentration of chemical substances
as a function of time in order to calculate the kinetic parameters of their chemical
reaction(s) of interest. The time to turbidity determination is a very simple and
easy way to establish the rate of a reaction.
Purpose:
As stated, turbidity is caused by the suspension of solid material in a solution.
The source of the solid material will vary depending on the specific matter under
investigation, but one route is through a chemical reaction. More specifically,
dissolved substances may chemically react to form a product that is insoluble.
As the concentration of the product increases with time, the solution appears to
become turbid.
at which the “+” at the bottom of the reaction vessel can no longer be seen
through the solution in which the chemical reaction is occurring.
Of course, the time to turbidity determination only works for chemical reactions in
which the reactants are fully dissolved in solution, thus appearing “clear” to a
human eye before any reaction occurs. The second caveat necessary for the
time to turbidity determination to be applied to chemical kinetics is that the
product must be insoluble and thus become a suspended solid upon formation.
One chemical reaction that meets these two requirements is the reaction
between hydrochloric acid and sodium thiosulfate (Na2S2O3) to give solid sulfur.
The balanced molecular equation for this reaction is shown below:
2 HCl (aq) + Na2S2O3 (aq) → S (s) + SO2 (aq) + H2O (l) + 2 NaCl (aq)
The order of the reaction with respect to sodium thiosulfate can be determined
experimentally by holding [HCl] constant at various [Na2S2O3] and determining
the time to turbidity. The inverse of time to turbidity gives the rate of the reaction
in units of sec-1. The easiest way to think about this as a determination of rate is
to view the number of molecules of sulfur required for the “+” to disappear as
constant in each reaction, thus the [S] at the end of the reaction is always the
same, but the time required to get to this end [S] will be dictated by the rate law
for the chemical reaction. Thus the rate can be expressed as [S] per unit of time
(sec). By comparing the experimental data for each trial, one can determine how
the change in [Na2S2O3] impacts the rate of the reaction.
The order of the reaction with respect to hydrochloric acid can be determined
similarly. By holding [Na2S2O3] constant at various [HCl] and determining the
time to turbidity, the coefficient for [HCl] in the rate law expression is found
through data analysis. Thus, through the time to turbidity determinations, the
rate law for the formation of solid sulfur from hydrochloric acid can be
determined.
Cautions: The hydrochloric acid (HCl) solution (1.0 M) is moderately toxic by
ingestion and inhalation. It is corrosive to eyes and skin. Wear disposable
gloves when handling this solution. The sodium thiosulfate (Na2S2O3) solution
(0.30 M) is a body tissue irritant. The sulfur (S) produced in this reaction has low
toxicity, but may be a skin and mucous membrane irritant. The reaction
generates aqueous sulfur dioxide (SO2), which is a skin and eye irritant. As
always, wear safety goggles at all times in the laboratory.
Procedure:
Part 1: Formation of Groups
You will work in groups of two or three today. When you come to lab, you will
pro
When the six-well plate is placed on top of the piece of the paper with the outline
of the six-well plate to scale, you will be able to clearly see the “+” through the
plastic. Each well holds at least 5 mL of liquid, so as a group, you should discuss
and plan out how you want to execute your experiments. Because you are
determining time to turbidity, the final total volume must be the same throughout.
The initial concentration of the HCl solution is 1.0 M and the initial concentration
of the Na2S2O3 solution is 0.30 M. You will vary [Na2S2O3] in Part II and vary
[HCl] in Part III; because you cannot make the solutions more concentrated,
water will be used to dilute the reagent that is being varied. You will have a
stopwatch and you will also have 3 mL plastic syringes.
Part II: Vary [Na2S2O3] at constant [HCl] and determine time to turbidity
Use three wells of the six-well plate for this part. The total final volume in the well
must be constant (5 mL) and the reaction will begin as soon as you mix the
reactants. In the first well, combine 3.0 mL 0.30 M Na2S2O3 and 2.0 mL 1.0 M
HCl and start the stopwatch. Determine the time to turbidity. For the second run,
combine 1.5 mL 0.30 M Na2S2O3 and 1.5 mL water. Initiate the reaction by the
addition of 2.0 mL 1.0 M HCl and determine the time to turbidity. Finally, in the
third well, combine 1.0 mL 0.30 M Na2S2O3 and 2.0 mL water. Initiate the
reaction by the addition of 2.0 mL 1.0 M HCl and determine the time to turbidity.
Part III: Vary [HCl] at constant [Na2S2O3] and determine time to turbidity
Use the other three wells of the six-well plate for this part. In the first well,
combine 3.0 mL 1.0 M HCl and 2.0 mL 0.30 M Na2S2O3 and start the stopwatch.
Determine the time to turbidity. For the second run, combine 1.5 mL 1.0 M HCl
and 1.5 mL water. Initiate the reaction by the addition of 2.0 mL 0.30 M Na2S2O3
and determine the time to turbidity. Finally, in the third well, combine 1.0 mL 1.0
M HCl and 2.0 mL water. Initiate the reaction by the addition of 2.0 mL 0.30 M
Na2S2O3 and determine the time to turbidity.
As soon as the time to turbidity determinations have been completed, empty the
contents of the six-well reaction plate into the appropriately labeled waste
container. Rinse and dry each of the wells with soap and water. Use a cotton
Data sheet:
well [Na2S2O3]f* [HCl]f* time to
turbidity (sec)
rate (sec-1)
Part II
1
2
3
Part III
4
5
6
*These are the final reactant concentrations in solution immediately after mixing
and before any reaction has occurred. You completed calculations exactly like
these for your pre-lab quiz.
1. Compare [Na2S2O3] and the rate in Part II of your data table. Calculate
the reaction order for Na2S2O3.
2. Compare [HCl] and the rate in Part III of your data table. Calculate the
reaction order for HCl.
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3. Based on your experimental data write the rate law expression for this
reaction.
4. Based on your rate law expression, if you used 0.60 M Na2S2O3 in this
experiment instead of 0.30 M Na2S2O3, how would this affect
a. The rate of the reaction?
b. The rate law expression?
5. Based on your rate law expression, if you used 0.50 M HCl in this
experiment instead of 1.0 M HCl, how would this affect
a. The rate of the reaction?
b. The rate law expression?
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6. Does the rate of sulfur