Materials Included In Kit

Additional Materials Required

Prelab Preparation

“Blue Bottle” Solution: Prepare 200 mL of blue bottle solution by mixing 100 mL of 0.1 M dextrose solution, 20 mL of 3 M potassium hydroxide solution, 80 mL of distilled or deionized water and 10 drops of 0.1% methylene blue. Note: The blue bottle solution should be prepared fresh at the beginning of the class period.

Safety Precautions

Potassium hydroxide solution is a corrosive liquid and is toxic by ingestion; it is particularly dangerous to eyes and may blister and burn skin. Avoid contact with eyes and skin and clean up all spills immediately. Keep citric acid on hand to neutralize any spills. Methylene blue is slightly toxic by ingestion. Wear chemical splash goggles and chemical-resistant gloves and apron. The dextrose (sugar) solution will attract ants. Remind students to rinse off all work areas with water. Wash hands thoroughly with soap and water before leaving the laboratory. Please consult current Safety Data Sheets for additional safety, handling and disposal information. 

Disposal

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. The waste solutions from Parts A and B may be flushed down the drain with excess water according to Flinn Suggested Disposal Method 26b.

Lab Hints

• The laboratory work for this experiment can reasonably be completed in one 50-minute class period. The Prelab Questions may be assigned separately as preparation for lab, or they may be used as the basis of a cooperative class discussion. If time is short, consider doing Part B as a demonstration.
• Many students will think that the blue–colorless and colorless–blue reactions are the reverse of each other. This is not the case. There are two separate reactions occuring—oxidation of the colorless MB_rd form to the blue MB_ox form by reaction with oxygen and reduction of the blue MB_ox back to the colorless MB_rd by reaction with dextrose.
• Reaction of dextrose with methylene blue in the presence of base results in oxidation of the sugar molecule. The aldehyde or hemiacetal functional group in dextrose is oxidized to a carboxylic acid derivative (gluconic acid or gluconolactone). Oxidation of dextrose in this reaction represents an application of the concept of “reducing sugars” that students may be familiar with from prior biology classes. Dextrose is called a reducing sugar because it acts as a reducing agent in reactions with Cu²⁺ or Ag⁺ ions (recall the Benedict’s test and Tollen’s test from carbohydrate chemistry). See the Supplementary Information in the Further Extensions section for the mechanism of oxidation of dextrose.
• The reaction times depend on the number of times the pipet is shaken. Convenient reaction times are obtained if the pipet is shaken about five times. Shaking the pipets 10 or more times gives longer reaction times. The calculated reaction rates are probably not accurate, therefore, in terms of the actual concentration of methylene blue that undergoes reaction. The calculations are used mainly to illustrate how reaction times and reaction rates are related. Although individual rates may not be accurate, the trends in reaction rate as a function of temperature and concentration are reproducible.
• Hot and cold running water should be suitable for preparing water baths in the 10–40 °C temperature range. Have students try to keep the temperature of the baths constant within ±1 °C by adding more hot or cold water, as needed.
• The potassium hydroxide solutions in Part B are very slippery. Students who get some base on their skin may be fooled by the slippery feel and may not realize that the base is actually burning their skin. Wear gloves!
• It may be helpful to review beforehand the idea that when the rate of reaction increases, the reaction time decreases. Using car travel as an analogy usually clarifies the relationship quite effectively.
• Students should fill the pipets so the solution reaches the 2-cm line when the pipet is bulb-down. This will require some trial and error.

Teacher Tips

• The “blue bottle” reaction is a classic chemistry demonstration. It is used in general science classes to introduce the roles of observation and hypothesis in the scientific method and in chemistry classes to illustrate oxidation and reduction reactions. It is also a perfect demonstration to talk about the mechanisms or pathways of chemical reactions, which are difficult to study otherwise. Call or write us at Flinn Scientific to obtain a free, complimentary copy of the blue bottle demonstration.
• Other redox indicators may be used instead of methylene blue in this reaction. Indigo carmine is green in its oxidized form, yellow in its reduced form. It gives a green–red–yellow color transition with dextrose. (See the Stop-n-Go Light—Demonstration Kit, Flinn Catalog No. AP2083.) Resazurin undergoes a reversible red–colorless reaction in the presence of dextrose. (See the Vanishing Valentine Demonstration Kit, Flinn Catalog No. AP5929.)

Further Extensions

Supplementary Information

Oxidation of dextrose (glucose) in the presence of potassium hydroxide involves an initial acid–base reaction to form the glucoside anion, followed by 2e^– oxidation to gluconolactone.

Answers to Prelab Questions

1. Define the terms oxidation and reduction.

   Oxidation and reduction reactions result from the transfer of electrons from one substance to another. Oxidation refers to the process of losing electrons, reduction to the process of gaining electrons.

2. When methylene blue changes from blue to colorless, is it being oxidized or reduced? In this experiment, what reactant is causing this change? Is this reactant acting as an oxidizing agent or a reducing agent?

   Methylene blue is reduced when it changes from blue to colorless. Dextrose is the reactant that causes this change—it is acting as a reducing agent.

3. Collision theory offers a simple explanation for how reactions occur—reacting molecules must first collide. In order for colliding molecules to be converted into products, they must collide with enough energy and with a suitable orientation to break existing bonds (in the reactants) and form new bonds (in the products). Any factor that changes either the total number of collisions or the average energy of the colliding molecules should affect the reaction rate. .
   1. Using collision theory, predict how increasing the temperature should affect the rate of a chemical reaction. State the prediction in the form of an if/then hypothesis and give a reason for your hypothesis.

      If the temperature of a reaction increases, then the rate of the reaction should also increase. This hypothesis is based on the idea that increasing the temperature increases the average speed of molecules, which should in turn increase both the number of collisions and, more importantly, the average energy of the collisions.

   2. Using collision theory, predict how increasing the concentration of a reactant should affect the rate of a chemical reaction. State the prediction in the form of an if/then hypothesis and give a reason for your hypothesis.

      If the concentration of reactants increases, then the rate of the reaction should also increase. This hypothesis is based on the idea that increasing the number of molecules present in solution should increase the rate of collisions between molecules.

Sample Data

Part A. Effect of Temperature

Answers to Questions

1. How did the reaction time change as the temperature was changed in Part A?

   Using the 21 °C reaction as a “control” for comparison, the reaction time increased when the pipet was placed in a colder water bath (9 °C), decreased when the pipet was placed in warmer water baths (32 °C or 39 °C).

2. How is the rate of a reaction related to the time of reaction?

   The rate of a reaction is inversely related to the time needed for the reaction to occur. The faster the rate of a reaction, the less time that is required for reactants to be converted to products.

3. What effect does temperature have on the rate of the “blue bottle” reaction?

   The rate of the “blue bottle reaction” appears to be very sensitive to temperature. The reaction rate increased dramatically when the temperature was increased by only 10 °C, and also decreased substantially when the temperature was decreased by 10 °C.

4. According to a general “rule of thumb” for chemical reactions, the rate of a reaction will roughly double for every 10 °C increase in temperature. Do the kinetics of the “blue bottle” reaction fit this general rule?

   The kinetics of the “blue bottle” reaction do not seem to fit this general rule. Every 10 °C temperature rise reduced the reaction time by a factor of 3–4. The rate more than tripled! Note: The rule is general enough that student may interpret the threefold increase in reaction rate as “roughly double.”

5. On a separate piece of paper, make a graph of the results in Part A by plotting the reaction time in seconds on the y-axis versus the temperature in kelvins on the x-axis.
   {13895_Answers_Figure_2}
6. Using the graph, estimate how long it would take for the reaction to occur at 275 K and at 325 K. Discuss ways the graph could be improved to give better estimates.

   A typical student graph is shown. Notice that it is almost impossible to predict reaction times at either 275 or 325 K using this graph. In order to estimate the reaction time at 275 K, the y-axis scale would need to be extended beyond 1200 sec (20 min). It is amost impossible to estimate a reaction time at 325 K. The reaction would probably occur so fast that it would be impossible to measure. Note: Given the current popularity of graphing calculators in math and science classes, many of your students will know how to fit a curve to the data and use the tracer function on their calculators to extrapolate the data.

7. Use the “dilution” equation (M₁V₁ = M₂V₂) to calculate the concentration of potassium hydroxide in each test tube 1–3 in Part B.

   M₁ = concentration of KOH before mixing
   M₂ = concentration of KOH after mixing
   V₁ = volume of KOH before mixing
   V₂ = volume of KOH after mixing

   Sample calculation for test tube 1:
   {13895_Answers_Equation_1}

   For test tube 2: M₂ = (3.0 M)(2.0 mL)/(6.0 mL) = 1.0 M
   For test tube 3: M₂ = (3.0 M)(3.0 mL)/(6.0 mL) = 1.5 M

8. The concentration of methylene blue in Part B is approximately 2.0 x 10^–4 M. Divide the concentration of methylene blue by the reaction time in seconds to calculate the average rate of the reaction in units of M/sec for each test tube 1–3.
   {13895_Answers_Equation_2}
9. Does the rate of the “blue bottle” reaction depend on the concentration of potassium hydroxide? Discuss in general terms the effect of reactant concentration on the rate of a chemical reaction.

   Yes, the rate of the “blue bottle” reaction depends on the concentration of potassium hydroxide. In general, the rate of a chemical reaction increases when the concentrations of reactants increase.

10. How much did the rate of the reaction change when the concentration of KOH was doubled (test tubes 1 and 2) or tripled (test tubes 1 and 3)?

    The rate increased by a factor of two when the concentration of KOH was doubled, by a factor of three when the concentration was tripled. Note: The rate appears to be first order with respect to potassium hydroxide, in agreement with the literature.

Introduction

How fast will a chemical reaction occur? If a reaction is too slow, it may not be useful. If the reaction is too fast, it may be harmful or explosive. Measuring and controlling reaction rates makes it possible for chemists and engineers to create a variety of products, everything from antibiotics to fertilizers, in a safe and economical manner. The purpose of this experiment is to investigate how the rate of a reaction can be measured and how varying conditions can affect reaction rates.

Concepts

• Kinetics
• Reaction rate
• Collision theory
• Oxidation–reduction

Background

Kinetics is the study of the rates of chemical reactions. As reactants are transformed into products in a chemical reaction, the amount of reactants will decrease and the amount of products will increase. The rate of the reaction can be determined by measuring the concentration of reactants or products as a function of time. In some cases, it is possible to use a simple visual clue to determine a reaction rate. Thus, if one of the reactants is colored but the products are colorless, the rate of the reaction can be followed by measuring the time it takes for the color to disappear. The average rate of the reaction is then calculated by dividing the molar concentration (M) of the colored reactant by the time needed for the color to disappear. Depending on how fast the reaction occurs, the rate would be reported in units of either M/sec or M/min.

Reactions involving the organic dye methylene blue provide a convenient example to study reaction rates. Methylene blue (abbreviated MB) exists in two forms, a reduced form and an oxidized form. The reduced form of methylene blue (MB_rd) is colorless while the oxidized form (MB_ox) is blue. The reduced form is easily converted to the oxidized form by mixing it with oxygen in the air (Reaction 1). The oxidized form, in turn, can be converted back to the reduced form by treatment with a reducing agent, such as dextrose, which is a reducing sugar.

Experiment Overview

The purpose of this experiment is to investigate how changing the temperature of the reactants or how changing the concentration of potassium hydroxide will affect the rate of reaction of methylene blue. The basic process is always the same—when a colorless solution containing MB_rd is shaken, it turns blue (Reaction 1 in the Background section). The time needed for the solution to turn colorless (Reaction 2 in the Background section) will be measured and then used to determine the average rate of reaction.

Materials

Prelab Questions

1. Define the terms oxidation and reduction. Note: Consult your textbook, if necessary, for definitions and examples.
2. When methylene blue changes from blue to colorless, is it being oxidized or reduced? In this experiment, what reactant is causing this change? Is this reactant acting as an oxidizing agent or a reducing agent?
3. Collision theory offers a simple explanation for how reactions occur—reacting molecules must first collide. In order for colliding molecules to be converted into products, they must collide with enough energy and with a suitable orientation to break existing bonds (in the reactants) and form new bonds (in the products). Any factor that changes either the total number of collisions or the average energy of the colliding molecules should affect the reaction rate.
   1. Using collision theory, predict how increasing the temperature should affect the rate of a chemical reaction. State the prediction in the form of an if/then hypothesis and give a reason for your hypothesis.
   2. Using collision theory, predict how increasing the concentration of a reactant should affect the rate of a chemical reaction. State the prediction in the form of an if/then hypothesis and give a reason for your hypothesis.

Safety Precautions

Potassium hydroxide solution is a corrosive liquid; it is particularly dangerous to eyes and may blister and burn skin. Avoid contact with eyes and skin and clean up all spills immediately. Methylene blue is slightly toxic by ingestion. Wear chemical splash goggles and chemical-resistant gloves and apron. Rinse off all work areas with water. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

Part A. Effect of Temperature

1. Obtain four 100- or 150-mL beakers and make water baths at approximately the following temperatures: 10 °C, 20 °C, 30 °C and 40 °C. In order to obtain easily measured reaction times, avoid temperatures above 40 °C or below 10 °C.
2. Obtain four thin-stem pipets and place a mark 2 cm from the bottom on each pipet bulb.
3. Fill each pipet bulb to the 2-cm mark with the “blue bottle” solution. Tie a knot in the stem of each pipet to seal it (see Figure 1).
   {13895_Procedure_Figure_1}
4. Place one pipet into each of the four water baths prepared in step 1. Let the pipets stand in the bath for 3–5 minutes. Record the temperature of each water bath in the data table.
5. Remove the pipet from the 20 °C water bath, start timing, then quickly shake the pipet five times and immediately return it to the water bath.
6. Stop timing when the blue color fades completely and the solution turns colorless. Record the elapsed time in seconds in the data table.
7. Repeat steps 5 and 6 with the other three pipets. Record all time and temperature readings in the data table. Note: Try to shake the pipets the same way each time. After shaking, return the pipets to their respective water baths.
8. Dispose of the pipets as directed by your instructor.

9. Obtain three test tubes and cork stoppers and label the tubes 1–3.
10. Using a graduated cylinder, add 3.0 mL of dextrose solution to each of the three labeled test tubes.
11. Add one drop of methylene blue solution to each test tube.
12. Measure 1.0 mL of 3.0 M potassium hydroxide solution into a clean graduated cylinder, then add 2.0 mL of distilled water to get a final volume of 3.0 mL.
13. Pour the contents of the graduated cylinder into test tube 1. Stopper the test tube and shake gently to mix the solutions.
14. Measure 2.0 mL of 3.0 M potassium hydroxide solution into a clean graduated cylinder, then add 1.0 mL of distilled water to get a final volume of 3.0 mL.
15. Pour the contents of the graduated cylinder into test tube 2. Stopper the test tube and shake gently to mix the solutions.
16. Measure 3.0 mL of 3.0 M potassium hydroxide solution into a clean graduated cylinder.
17. Pour the contents of the graduated cylinder into test tube 3. Stopper the test tube and shake gently to mix the solutions.
18. Allow the test tubes to sit undisturbed at room temperature until the blue color fades. Note: This may take a few minutes.
19. Check the temperature of the solutions to be sure they are all about the same temperature. Record the temperature in the data table.
20. With your finger firmly on the stopper, shake test tube 1 vigorously five times and immediately start timing.
21. Stop timing when the blue color fades completely and the solution turns colorless. Record the elapsed time in seconds in the data table.
22. Repeat steps 20 and 21 using test tube 2 and then again using test tube 3.
23. Dispose of the contents of the test tubes as directed by your instructor.

Student Worksheet PDF

13895_Student1.pdf