Materials Included In Kit

Additional Materials Required

Prelab Preparation

Red Dye No. 3 solution, 1.0 M x 10^–5 M, 500 mL

• Solution 1: Mass 0.350 g of FD&C Red Dye No. 3. Add this to a 100-mL volumetric flask filled with approximately 50 mL of distilled or deionized water. Cap and mix thoroughly. Fill to 100-mL mark with distilled or deionized water, then cap again and mix. [Red No. 3] = 4.0 x 10^–3 mol/L
• Solution 2: Using a 5-mL volumetric pipet, transfer 5 mL of solution 1 to a clean 100-mL volumetric flask. Fill to the 100-mL mark with distilled or deionized water. Cap and mix thoroughly. [Red No. 3] = 2.0 x 10^–4 mol/L
• Solution 3: Using a 25-mL volumetric pipet, transfer 25 mL of solution 2 to a clean 500-mL volumetric flask. Fill to the 500-mL mark with distilled or deionized water. Cap and mix thoroughly. [Red No. 3] = 1.0 x 10^–5 mol/L

Blue Dye No. 1 solution, 0.68 M x 10^–5 M, 500 mL (optional)

• Solution 1: Mass 0.300 g of FD&C Blue Dye No. 1. Add this to a 250-mL volumetric flask filled with approximately 150 mL of distilled or deionized water. Cap and mix thoroughly. Fill to 250-mL mark with distilled or deionized water, then cap again and mix. [Blue No. 1] = 1.4 x 10^–3 mol/L
• Solution 2: Using a 5-mL volumetric pipet, transfer 5 mL of solution 1 to a clean 100-mL volumetric flask. Fill to the 100-mL mark with distilled or deionized water. Cap and mix thoroughly. [Blue No. 1] = 6.8 x 10^–5 mol/L
• Solution 3: Using a 50-mL volumetric pipet, transfer 50 mL of solution 2 to a clean 500-mL volumetric flask. Fill to the 500-mL mark with distilled or deionized water. Cap and mix thoroughly. [Blue No. 1] = 0.68 x 10^–5 mol/L

Safety Precautions

Sodium hypochlorite solution is a corrosive liquid; it causes skin burns. The solution reacts with acid to evolve chlorine gas when heated. Sodium hypochlorite solution is also moderately toxic by ingestion and inhalation. Keep away from skin and clothing. Work in a fume hood or well-ventilated lab only. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Remind students to wash hands thoroughly with soap and water before leaving the laboratory. Please consult current Safety Data Sheets for additional safety information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. All reaction solutions and sodium hypochlorite solution may be disposed of according to Flinn Suggested Disposal Method #26b.

Lab Hints

• Most colorimeters use four similar wavelengths: violet (430 nm), blue (470 nm), green (565 nm) and red (635 nm). If your colorimeter does not give sufficient absorbance for FD&C Red Dye No. 3 samples at the setting nearest 530 nm, use FD&C Blue Dye No. 1. Its 630-nm maximum absorbance is close to the 635-nm window of most colorimeters. If using the Blue Dye No. 1, dilute the bleach solution by a factor of 1:5 to get similar rate times as those of the Red Dye No. 3 samples.

Teacher Tips

• Sensitive balance. A balance capable of reading ±0.001 g or better is needed to get accurate measurements of the mass of the small amount of FD&C Red Dye No. 3 needed to make the standard solution. If only centigram balances are available, change the procedure that calls for measuring 0.35 g of FD&C Red Dye No. 3 to 0.87 g of Red Dye No. 3. Add this to a clean 250-mL volumetric instead of a 100-mL volumetric to make Solution 1 in the Prelab Preparation section.

Answers to Prelab Questions

1. Calculate the molarity of a 5.15% (w/v%) sodium hypochlorite (NaOCl) solution.

   (51.5 g NaOCl/1 L) x (mole NaOCl/74.44 g NaOCl) = 0.69 moles NaOCl/L

2. Calculate the molarity of a solution of FD&C Blue Dye No. 1 (MW 792.8 g/mol) if a 0.5028-g sample is diluted to 100 mL in a volumetric flask and then two serial dilutions are carried out in which 5-mL aliquots are diluted to 100 mL using volumetric equipment.

   Solution No. 1: (0.5028 g Blue No. 1/0.1 L) x (1 mole Blue No. 1/792.8 g Blue No. 1) = 6.3 x 10^–3 M
   Solution No. 2: (5 mL/100 mL) x 6.3 x 10^–3 M = 3.2 x 10^–4 M
   Solution No. 3: (5 mL/100 mL) x 3.2 x 10^–4 M = 1.6 x 10^–5 M

3. In a few brief sentences, explain:
   1. Why it is necessary to mix the solution very rapidly when the bleach is added.

      To prevent the reaction from consuming any significant amount of FD&C Red Dye No. 3 before the reactant concentrations are uniform throughout the solution.

   2. The purpose of the blank solution.

      To establish a zero reading on the spectrophotometer for the red dye No. 3 concentration.

   3. How the shapes of the plots [dye] vs. time, ln[dye] vs. time and 1/[dye] vs. time are used to determine the order of the reaction with respect to the dye concentration.

      The rate of reaction involving two reactants, A and B, can be expressed as an equation:

      Rate = d[A]/dt = k[A]^a[B]^b

      where d[A]/dt represents the disappearance of A over time and where a and b can each equal 0, 1 or 2. If [B]>>>[A], then [B]^b can be considered a constant. This reduces the rate equation to

      d[A]/dt = K[A]^a

      The integration of this equation yields the following straight line equation when:

      1. a = 0; [A] = Kt + [A]₀ and a plot of [A] versus time yield a straight line with K as the slope and [A]₀ as the initial concentration at time 0.
      2. a = 1; ln[A] = –Kt + ln[A]₀ and a plot of ln[A] versus time yield a straight line with K as the slope and ln[A]₀ as the natural log of the initial concentration at time 0.
      3. a = 2; 1/[A] = Kt +1/[A]₀ and a plot of 1/[A] versus time yield a straight line with K as the slope and 1/[A]₀ as the inverse of the initial concentration at time 0.

Sample Data

Standard Solutions Data Table

Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Sample 6

Answers to Questions

Post-Laboratory Review

6. From a visual inspection of these plots, select the best linear relationship to establish a, the order of the reaction with respect to the dye.

   In all the reactions, the relationship of the natural log of red dye No. 3 concentration, ln[red dye No. 3], to time yielded a straight line, indicating a to have a value of 1.

7. Using the plots that give linear relationships, determine the rate constant for each trial.

   Since the [Bleach] is much, much greater than [Red No. 3], [Bleach]^b can be expressed as a constant. The rate equation incorporates this constant into a pseudo-rate constant: Rate = d[A]/dt = K[A]^a. For each trial, the pseudo-rate constant is equal to the slope of the ln[Red No. 3] versus time curve.

   {13820_Answers_Table_11}
8. By comparing the slopes of the lines for trials 1, 5 and 6, determine b, the order of the reaction with respect to hypochlorite. Since the [Bleach] is a constant throughout the reaction, K can be expressed as: K = k[Bleach]^b
   {13820_Answers_Table_12}
   Comparing K for Sample 1 to Sample 5 yields the following equations
   {13820_Answers_Equation_11}
   If b = 0, then both ratios equal.
   If b = 1, then the ratios are 1:2 and 1:3.
   If b = 2, then the ratios are 1:4 and 1:9.
   The ratios are:
   {13820_Answers_Equation_12}
   Therefore, b = 1.

Introduction

In this lab, the rate of the reaction of FD&C Red Dye No. 3 (Red No. 3) with sodium hypochlorite will be studied. The color change occurring in this reaction will be followed using a spectrophotometer to quantitatively measure the rate of disappearance of the colored reagent. The rate data will be used to determine the rate law for this oxidation–reduction reaction.

Concepts

• Rate law
• Reaction rate
• Rate constant

Background

Organic molecules that possess a series of alternating double and single bonds are said to be conjugated and are often highly colored. Extended conjugation within the molecule of Red Dye No. 3 causes the π−π* absorption to occur in the visible region of the spectrum, at 530 nm. When the dye reacts with hypochlorite, the color disappears due to reaction of hypochlorous acid with alkenes to form alcohols. In this case, HOCl can add to any of three sites on the molecule. The resulting product no longer has the extended conjugation system and the absorption of the less conjugated product occurs at a lower wavelength, outside of the visible region of the spectrum.

The rate of a reaction can be represented either by the disappearance of reactants or the appearance of products. Since Red No. 3 is the only colored species in the reaction, we can monitor the rate of the reaction shown above by recording the decrease in the color of the solution with time. That is, where the exponents a and b indicate the order of the reaction with respect to each reagent, and k is the overall rate constant for the reaction at a specific temperature. The objective of this experiment is to determine the values of the exponents a and b and the value of k at room temperature.

If the concentration of the bleach is held constant throughout a reaction by having a large excess present, then the rate law simplifies to where [OCl^–]^b has been absorbed in the pseudo-rate constant k′. Rearranging this rate expression to: gives a form that can be integrated depending on the value of a. If a = 1, the integrated expression becomes: and a plot of ln[Red No. 3] vs. time will give a straight line with a slope of k′. If, however, a = 2, the integrated expression becomes: and a plot of 1/[Red No. 3] vs. time will give a straight line with a slope of k′.

Thus, if one experimentally determines the concentration of the dye at various times during a reaction, the relationship of the concentration with time that gives a linear fit establishes the order of the reaction with respect to the dye (i.e., a value for a).

A second set of rate data, collected for reactions where the concentration of the dye is held constant and the initial excess concentration of the bleach is changed in a simple ratio between trials, can then allow a determination of a value for b. For example, since the pseudo rate constant k′ = k[OCl^–]^b, if [OCl^–] is doubled between trials and the k′ also doubles, then b must = 1. On the other hand, if [OCl^–] is doubled and the observed rate constant increases by a factor of 4, then b = 2. However, if [OCl^–] is doubled between trials and there is no change in the observed rate of the reaction, then b = 0 and it can be concluded that the bleach is not involved in the rate-determining step of the reaction.

Experiment Overview

The purpose of this experiment is to determine the total rate law for the oxidation–reduction reaction between FD&C Red Dye No. 3 and sodium hypochlorite. The experiment involves several steps. First, the order for each of the reactants is found by varying the concentration of each reactant individually. Once the orders are known, the rate constant is calculated.

Materials

Prelab Questions

1. Calculate the molarity of a 5.15% (w/v%) sodium hypochlorite (NaOCl) solution.
2. Calculate the molarity of a solution of FD&C Blue Dye No. 1 (MW 792.8 g/mol) if a 0.5028-g sample is diluted to 100 mL in a volumetric flask and then two serial dilutions are carried out in which 5-mL aliquots are diluted to 100 mL using volumetric glassware.
3. In a few brief sentences, explain:
   1. Why it is necessary to mix the solution very rapidly when the bleach is added.
   2. The purpose of the blank solution.
   3. How the shapes of the plots [dye] vs. time, ln[dye] vs. time and 1/[dye] vs. time are used to determine the order of the reaction with respect to the dye concentration.

Safety Precautions

Sodium hypochlorite solution is a corrosive liquid; it causes skin burns. The solution reacts with acid to evolve chlorine gas when heated. Sodium hypochlorite solution is also moderately toxic by ingestion and inhalation. Keep away from skin and clothing. Work in a fume hood or well-ventilated lab. Wear chemical splash goggles and chemical-resistant gloves and apron. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

Preparing the Red Dye No. 3 Calibration Curve

1. Place five clean, dry test tubes in a test tube rack.
2. Using clean 3-mL and 12-mL syringes, add the standard FD&C Red Dye No. 3 solution and distilled or deionized water to each test tube in the amounts listed in the table below.
   {13820_Procedure_Table_1}
3. Note: For best results in the steps that follow, handle cuvets at the top so no fingerprints are in the light path and polish cuvets with a tissue.
4. Follow the procedure for colorimetric measurements of the solution as directed by the instructor. Generally, spectrophotometers are used as follows: Turn the instrument on and allow it to warm up for 15 minutes. Set the wavelength at 530 nm. With no light passing through the instrument, set the percent transmittance to zero with the “zero” control. Place a cuvet that is about ⅔ full of standard 1 (distilled water) into the sample holder and set the percent transmittance to 100% with the appropriate control (not the zero control). Fill a cuvet about ⅔ full of the standard 2 solution, place it in the spectrophotometer and read the absorbance. Consult the instrument manual for details on its use. Measure and record the absorbance of the solution. The absorbance of this solution should fall between 0.300 to 0.600. (Consult your instructor if the value falls outside of this range.)
5. Measure the absorbance of each of the standard solutions at 530 nm, using distilled water as the zero absorbance reference in the spectrophotometer. Record the absorbance value for each standard solution used in the Standard Solutions Data Table.
6. Clean and rinse the 5 test tubes. Dispose of the red dye solutions as directed by your instructor.

Sample Preparation

1. Label six test tubes with numbers 1–6.
2. Use the 12-mL syringes to add the appropriate amount of standard FD&C Red Dye No. 3 solution, and distilled water to each test tube as specified in the first two columns of the following table. Note: Do not add the bleach to any of the solutions at this point.
   {13820_Procedure_Table_2}

Data Collection

1. Select one team member to be the timer and one to be the sample preparer. The timer will time the reaction, while the second person will prepare the reaction solutions and record their absorbance at timed intervals.
2. If not already on, set up the spectrophotometer as in step 4 of the calibration procedure.
3. Fill a clean, 3-mL syringe with 1 mL of the sodium hypochlorite solution.
4. With test tube 1 in one hand and the 3-mL syringe in the other, inject the 1 mL of sodium hypochlorite solution into the test tube as your partner starts timing.
5. Quickly stopper the test tube, mix the solutions, and then place the test tube in the spectrophotometer.
6. Record the absorbance at the next 20-second interval from the initial mixing start time, and each subsequent 20-second interval.
7. Leaving the test tube in the spectrophotometer, continue recording the absorbance of the solution until the absorbance reaches 0.050.
8. Repeat steps 3–7 for test tubes 2, 3 and 4.
9. For test tube 5, add 2 mL of sodium hypochlorite solution to the 3-mL syringe in step 3. Then repeat steps 4–7.
10. For test tube 6, add 3 mL of sodium hypochlorite solution to the 3-mL syringe in step 3. Repeat steps 4–7. 

Student Worksheet PDF

13820_Student1.pdf