SOTM LAB: C7
I: TEACHER NOTES & GUIDELINES
TITLE OF LAB: Chemical Kinetics: Concentration vs. Reaction Rate
DEVELOPERS OF LAB: Michael Boms (JD814), Phillip Cameron (JD522), Gail Gold (JD889),
Mark Tretter (JD580); Kathy Poncelet (JD521), Mikki Bieber (JD705), Pamela Iacovella (JD723),
SOTM staff
OVERVIEW OF LAB
DESCRIPTION
A spectrophotometer will be used to study different aspects of the kinetics of a reaction. The effect
of concentration of one of the reactants will be investigated initially to explore the reaction and to
introduce students to the use of the Spectronic-20 spectrophotometer. After completing the initial
experiment, students will propose and design an experiment to investigate another factor which they
think may affect the rate of this reaction.
The reaction to be studied is that between sodium thiosulfate and hydrochloric acid. The thiosulfate
ion decomposes in an acid solution producing colloidal sulfur:
S2O32-(aq) + 2 H+ (aq) 
S(s) + SO2 + H2O(l)
The times measured are those required to produce the same degree of turbidity produced by the
formation of colloidal sulfur. The reaction will be run with different concentrations of S 2O32reacting with excess HCl (aq). The results obtained will be used to develop a graph of the sodium
thiosulfate concentration vs. the length of time it takes for the colloidal sulfur to be formed. Once
the graph is constructed, the rate of the reaction at any given time, the order of the reaction and the
rate constant (k) for the reaction can be determined.
CURRICULUM CONSIDERATIONS
As written, the lab is intended for above average Regents Chemistry or AP Chemistry students who
are studying chemical kinetics including the rate laws and orders of reactions. This lab can be
simplified for use with other students for studying factors that affect rates of reactions and limiting
reactants. Analysis of data can be done using a spreadsheet and graphing software if students are
familiar with the software.
SAFETY CONSIDERATIONS
1. Because of the caustic nature of the HCl and the use of glassware, students should wear goggles
at all times during lab.
2. The handling and disposal of all chemicals should be done in accordance with OSHA
guidelines. Please note that some sulfur dioxide fumes are released during the lab.
3. If HCl gets on your skin or clothing, dilute immediately with water. See teacher to apply
sodium bicarbonate (baking soda) to skin if burning sensation continues.
4. Wipe up any spills immediately.
5. Notify the teacher of any accidents immediately.
BACKGROUND INFORMATION
1
A. SCIENTIFIC VIEWPOINT
How quickly a reaction proceeds is often affected by the concentration of one or more of the
reactants of the system. The rate of any reaction may be expressed by the equation:
Rate = k[A]m[B]n
where [A] and [B] are the initial concentrations of the reactants and the exponents m and n are
the rate order or order of the reaction with respect to each reactant. The constant, k, is the rate
constant for the reaction at that given temperature. In this experiment, [A] represents the
thiosulfate concentration, [S2O32-] and [B] represents the hydrochloric acid concentration,
[HCl]. For the reaction in this experiment, this rate equation or rate law simplifies to:
Rate = k[S2O32-]m
since the hydrochloric acid concentration has no effect on the rate of the reaction. By
graphical analysis of the data, the order of the reaction and the value of the constant, k, can be
determined.
To accomplish this graphical analysis, the students first prepare a graph of concentration of
thiosulfate, which is determined in the first question, vs. time. Students then choose two data
points, point 1 and point 2, on the graph, draw lines tangent to these points and determine the
slope of each line. The slope of the line is the instantaneous rate of the reaction at that point.
By comparing the ratio of the rates at these two points to the ratio of the concentrations of
S2O32- at these two points:
Rate2/Rate1 = k[S2O32-@point 2]m / k[S2O32-@point1]m
The students can determine, m, the order of the reaction. If two ratios are equal, then the order
is a first order reaction because m = 1. If the reaction rate ratio increases fourfold, then order
of the reaction is second order because m =2. The value of the constant (k) can be obtained by
dividing the rate by the [S2O32-]m at that point. In the second graph, students graph
concentration of thiosulfate vs. 1/time. A linear shape of the line will verify that order of the
reaction is first order.
B. COMMON MISCONCEPTIONS
1. Increasing the concentration of any reactant will always increase the reaction rate.
2. The rate of the reaction is constant throughout the course of the reaction.
3. If students have previously studied chemical equilibrium, they may incorrectly
assume the exponents in the rate law are the coefficients in the balanced equation for
the reaction.
OBJECTIVES
CONTENT OBJECTIVES:
1. To determine how a change in concentration affects the rate of a chemical reaction.
2. To calculate the instantaneous rate of a chemical reaction at any given time from a graph
developed using lab generated data.
3. To determine the order of the reaction.
4. To calculate the rate constant (k) for the reaction.
5. To determine and investigate other factors which affect the rate of a chemical reaction.
SKILL OBJECTIVES:
1. Use of technology to collect data in an investigation to minimize human error.
2. Use of mathematical skills in the analysis of laboratory data.
3. Use of computer software to analyze data.
ATTITUDINAL OBJECTIVES
2
1.
2.
3.
Mathematics is a tool of science.
One’s experimental results can lead to more questions than answers.
Science follows a logical thinking process.
EQUIPMENT/MATERIALS
PROVIDED BY SOTM:
Spectronic-20
cuvettes (3 per Spectronic 20)
0.10 M Na2S2O3 (1 L per 10 setups)
IBM laptops (when available for collection and analysis of data)
PROVIDED LOCALLY:
Timer
1 50 mL graduated cylinder
Stirring rod
Kimwipes (or lens paper)
0.50M HCl
5 250 mL beakers
1 25 mL graduated cylinder
Distilled water
Pipette
ADVANCED PREPARATION
15 minutes before using, turn on the Spectronic-20's
One liter of 0.50 M HCl per class of 30:
II: PRELAB
PRELAB EXERCISE FOR ELICITING STUDENTS’ PRIOR KNOWLEDGE AND
MISCONCEPTIONS
The teacher should begin the lesson by performing a series of demonstrations coupled with probing
questions to elicit students’ prior knowledge and misconceptions about reaction rates.
Students should record observations and, in groups, make inferences based on the demonstrations about
what factors affect the rate of this reaction and how. These factors should be listed on the board and
discussed by the class.
(factors: concentration, temperature, particle size, and nature of the reactants)
The students should then write the rate law for the experiment.
DEMONSTRATIONS
a. 20.0 mL of 0.50 M HCl is added to 100.0 mL of 0.050 M Na2S2O3 in a beaker. (A laser pointer
could be used to help students observe the Tyndall effect produced by the colloidal sulfur.) The
students should qualitatively observe the reaction’s progress.
b. Elicit student predictions when repeated with 20.0 mL of 1.0 M HCl is added to 100.0 mL of
0.050 M Na2S2O3.
c. Repeat step b with 20.0 mL of 0.50 M HCl is added to 100.0 mL of 0.10 M Na2S2O3.
3
III: EXPLORATION OF SCIENTIFIC PRINCIPLE &
INTRODUCTION OF EXPERIMENTAL PROTOCOL
Propose the problem to be studied. Elicit students’ responses on how the problem could be tested based
on the demonstrations. A discussion of the reliability and reproducibility of the human response should
be done. The use of the Spectronic-20 introduced as a tool to compensate for shortcomings of human
response. If students have not used the Spectronic-20 before, a brief overview of its operation should be
covered. This discussion should include the need to calibrate and check the Spectronic-20 for each
reaction. Assumptions and limitations of the procedure also need to be included. The mathematical
analysis of the data could be reviewed prior to or after the data collection.
QUESTIONS THAT COULD BE ASKED:
How do you know a reaction is occurring?
How long did the reaction take for this evidence to appear?
How would you measure the length of time of the reaction?
How could you be sure you are stopping at the same point during the reaction each time that the
measurement is done?
Do you think each person in the lab would get the same measurement for reaction time?
How could the human error in the lab be minimized?
PROBLEM
How can the effect of concentration on the rate of a reaction be studied?
SPECIAL NOTES:
USE OF THE SPECTRONIC-20:
1. Turn on the Spectronic-20 fifteen minutes before use to allow it to warm up.
2. With the cell holder empty, set the wavelength control to 500 nm. Make sure that the filter on
the Spectronic-20 is set to the correct wavelength range.
3. Adjust the zero adjustment knob so that the machine indicates 0% Transmission.
4. Place a cuvette of distilled water into the cell holder with the indicator on the cuvette aligned
with the indicator on the cell holder. Set the Spectronic-20 to read 100%Transmission. (Once
this is accomplished, remove the cuvette and set aside for subsequent adjustments. - Before
each reaction is run, you should check to be sure that the Spectronic-20 is set at 100% T).
5. Place a cuvette with the reaction mixture in the cell holder. Align the indicator on the cuvette
with the indicator on the cell holder.
CARE OF THE CUVETTES:
1. Wash cuvettes with detergent and water only. Do Not Use a brush. Remember after each
reaction to remove the cuvette, discard the solution, and wash the cuvette immediately with
water in order to prevent sulfur residue from coating the inside of the cuvette.
2. Use matching cuvettes.
3. Do not confuse cuvettes with small test tubes. Place the cuvettes back into the box they came in
after they have been cleaned and dried.
LAB PROCEDURE:
1. Prepare a data table to record the beaker # and the time required for the reaction to occur.
Record the formula of the hydrate of the Na2S2O3 used to make the solutions.
2. Label 5 250 mL Beakers 1-5.
3. Obtain 15 mL of the 0.1 M Na2S2O3 solution.
4
4. Mix the following dilutions in the appropriate beaker.
#1) Mix 1.0 mL of Na2S2O3 + 9.0 mL distilled water
#2) mix 2.0 mL of Na2S2O3 + 8.0 ml distilled water
#3) mix 3.0 mL of Na2S2O3 + 7.0 ml distilled water
#4) mix 4.0 mL of Na2S2O3 + 6.0 ml distilled water
#5) mix 5.0 mL of Na2S2O3 + 5.0 ml distilled water
5. With a 25 mL graduated cylinder measure 10.0 mL of 0.50 M HCl solution.
6. After making sure that the Spectronic-20 has been properly set to read 0% T and 100%T, have
one member of the lab group start the timer as the another pours the graduated cylinder of acid
into the beaker #1. Stir briefly with a stirring rod to be sure the solution is homogeneous.
7. Pipette the solution into a clean cuvette so that it is about 2/3 full, Wipe the outside of the
cuvette with a Kimwipe or a piece of lens paper to make sure it is clean. Insert the cuvette into
the cell holder of the Spectronic-20 in a firm, but gentle manner. (The remaining solution in the
beaker should be set aside and observed for qualitative evaluation).
8. Continue timing until the Spectronic-20 reads 90% Transmission. Stop the timer and record the
length of time in your data table. (Note: for the first beaker it may take over 5 minutes)
9. Remove the cuvette, discard the solution, and wash the cuvette and beaker immediately with
water in order to prevent the residue from coating the inside of the glassware.
10. Repeat Steps 5 through 9 for each of the remaining solutions. Repeat for any of the solutions, if
the data seems questionable.
POST-LAB CALCULATIONS: QUESTIONS/DISCUSSION:
1. Calculate the molarity of each sodium thiosulfate solution you used.
2. Using the data collected, construct two separate graphs:
Graph A. Plot the concentration of sodium thiosulfate vs. time.
Graph B. Plot the concentration of sodium thiosulfate vs. 1 /time.
3. Using Graph A:
a. Determine the instantaneous rate of the reaction for two separate concentrations by
constructing a line tangent to the curve at those points and determining its slope.
b. Use the above data to calculate the order of the reaction.
c. Calculate the rate constant (k) for the reaction.
4. What does the appearance of Graph B tell you about the order of the reaction?
5
IV: ELABORATION OF SCIENTIFIC PRINCIPLE:
INQUIRY BASED STUDENT INVESTIGATION
TEACHER LEAD DISCUSSION QUESTIONS FOR CREATING INQUIRY PORTION OF LAB
1. How does doubling the concentration of the Na2S203 concentration affect the rate of this
chemical reaction? .
2. Based upon the prelab demonstration that you observed, how does doubling the concentration
of HCl affect the reaction rate?
3. How would reducing the concentration affect the reaction rate?
4. In the reaction between Na2S203 and HCl, which reactant was the limiting reagent? How do you
know?
5. How could you test the following hypothesis: "The rate of a reaction is constant throughout the
course of the reaction." Explain, and give evidence for your statement.
6. What other information could you obtained from the graphs that you developed?
Remind students of the other factors from prelab demonstrations and discussion.
From the above questions, have students develop a problem and design an experiment to test their
hypothesis or prediction.
6
Student’s Name ______________________
PLAN FOR DATA COLLECTION & ANALYSIS
(Teacher’s note: Questions you may want to use to guide students through planning their
inquiry)
What question do you want to answer?
What is your hypothesis?
What do you think will be your outcome?
Are you only testing one variable?
How are you keeping all other conditions unchanged?
Will you be able to get all of the materials that you will need?
Checkpoint (Teacher checks students’ plan for feasibility.)
CONDUCTING THE EXPERIMENT
(Teacher’s note: Questions you may want to use to guide students through conducting their
inquiry)
Do you have all of the materials needed to conduct your experiment?
Are you following acceptable safety protocol?
Are you following your own procedure or have you needed to make revisions?
Have you recorded you data correctly and with the correct units?
Checkpoint (Teacher monitors students’ investigations in progress.
ANALYSIS OF DATA
(Teacher’s note: Questions you may want to use to guide students through analysis of their
inquiry)
What calculations will you need to reach your conclusion?
Are you calculations in the correct units?
Checkpoint (Teacher checks students’ analysis.)
DISCUSSION OF RESULTS
(Teacher’s note: Questions you may want to use to guide students through the conclusion
of their inquiry)
COMPARE
Does your calculations parallel what you had predicted? Explain.
Were there any sources of error in you procedure? Explain.
Does your data support or dispute your hypothesis? Explain.
PERSUADE
Can you use your conclusion to convince a non-science student that your conclusion is correct? Explain.
RELATE
How can you relate your findings to the world outside the lab room?
7
V: EVALUATION
1. Identify and explain one conclusion that can be drawn from the first part of the lab and your
inquiry lab.
(The higherthe concentration of Na2S203 , the faster the reaction rate.)
2. Why was the Spectronic-20 used in the lab? What did the Spectronic-20 measure?
(It reduces human error and provides a measurable stopping point. It measures the cloudiness
(turbidity) of the mixture.)
3. Why should the Spectronic-20 be calibrated before each measurment?
(To insure the accuracy of the measurement and insure that the settings have not changed.)
4. Given the reaction: 2 N2O5 --> O2 + 2 N2O4
a) For the data below, graph the concentration of N2O5 vs. time:
time (seconds)
[N2O5] (M)
0
0.200
300
0.180
600
0.161
900
0.144
1200
0.130
b) Using the tangent method, calculate the instantaneous rate of reaction for time = 900
seconds.
c) Write the rate law for the reaction. (rate = k[N2O5] m)
5. Explain how you could experimentally test the statement: the rate of a reaction decreases as the
reaction proceeds.
(Measure the length of time for several predetermined %Transmittances for one reaction.
Graph the data and determine, using the tangent method, the instantaneous rate of reaction
near the beginning, the middle and end of the measurements.)
This material is based upon work supported by the National Science Foundation under Grant No. ESI 9618936. Any opinions, findings, and
conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National
Science Foundation.
8
Rubric for assessment of Student Inquiry Lab
Student’s Name___________________________
Student activity
Students have proposed a question.
Students have proposed a hypothesis.
Students have stated reason for predicted outcome.
Students have created procedure
Students have considered safety and viability.
Students have identified controls.
Students have identified variables.
Students have created a material list.
Students have successfully run procedure.
Students have recorded data correctly.
Students have completed calculations.
Students have listed % error and sources of error.
Students have proposed ways to improve the
procedure.
Have students tested their hypothesis?
Are the student’s conclusions consistent with their
data?
Are student’s data, calculations and outcome logical
and consistent with known scientific concepts?
What new questions have arisen during this lab?
Date of Completion
Teachers Initials
9