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 S2O32- reacting 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
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
1. Mathematics is a tool of science.
2. One’s experimental results can lead to more questions than answers.
3. 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 5 250 mL beakers
1 50 mL graduated cylinder 1 25 mL graduated cylinder
Stirring rod Distilled water
Kimwipes (or lens paper) Pipette
0.50M HCl
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.
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. 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?
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.
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)