Topic: CHEMICAL DYNAMICS--REACTIONS

Source: A seminar presented in Central Illinois Saturday Science Education Seminars for Elementary Teachers

Presenter: Michael J. Welsh
Chemistry Department
Illinois State University
Normal, IL 61790

Funded by: Scientific Literacy Center
Illinois State Board of Education
1990-1993

Abstract: Chemical Dynamics--Reactions provides examples of safe chemistry for the elementary classroom that can be done with over-the-counter materials. The tasks of the chemist can be summarized into three broad categories--classify materials and reactions, develop chemical syntheses to create new materials, and do chemical analyses to determine the quality of manufactured products. While students need to be exposed to other details in their education, the basic tasks will serve as an outline for seminar activities. Each seminar activity will use demonstrations and hands-on materials to illustrate task. While the activities presented here may be directly applied in grades 5-8, younger children can benefit from demonstrations and a more controlled exposure to these concepts and processes.

CHEMICAL DYNAMICS--REACTIONS (Teacher Copy)

INTRODUCTION

Chemists participate in a variety of different tasks. Those tasks can be divided into three broad categories. Chemists classify materials and reactions, develop chemical syntheses to create new materials, and do various chemical analyses to determine the quality and maintain control of manufacturing the materials they have developed.

Clearly a chemist's job may be subdivided and categorized in much more detail. Students should be exposed to a more detailed list of tasks as they continue their education. However, the divisions provide presented here provide a fundamental understanding of the chemist's job.

These basic tasks will serve as the outline for the seminar activities presented here. Students will learn from both demonstration and hands-on activities to illustrate given concepts.

Activities in this packet can be easily modified for use with grades 1-8. Previous experience is not needed.

CHEMICAL DYNAMICS--REACTIONS (Teacher Copy)

TABLE OF CONTENTS

Part I Introduction to Classifying Reactions.................4

Lesson 1 Decomposition Chemistry of a Hydrate.............5

Lesson 2 Forming the Magnesium Sulfate Hydrate............7

Part II Introduction to Chemical Synthesis....................9

Lesson 3 Preparation of Sterno...........................10

Part III Introduction to Chemical Analysis....................11

Lesson 4 Estimating the Number of Milligrams of..........12
Vitiman C in a Serving of Tang

CHEMICAL DYNAMICS--REACTIONS (Teacher Copy)

PART I INTRODUCTION TO CLASSIFYING REACTIONS

Matter I ÄÄÄÄÄ> Matter II

How does chemistry work?
How do we communicate it?

Is the combustion process a decomposition reaction?

Consider:

Demonstration A: Burning a piece of paper (towel) flames on the surface seem to be carrying something away.

The paper towel ash seems to weigh less, a question elementary students can investigate!

How does a chemist communicate building up and decomposition reactions?

Demonstration B: Chemical Logic

If a strip of magnesium is heated to a high temperature the strip will burn and produce a bright light and you will be left with ash. (Flashcubes)
Does the ash weigh more or less?
Ask your students to decide!

Hindsight: Mg is an element
Ash is a compound
Substance X (Oxygen) is added to the magnesium

Need to decide if this is a build up or decomposition reaction

1. Magnesium + Substance X ÄÄÄ> Ash
-build up reaction
-ash will have greater mass; will be a compound of Mg and mystery X

2. Magnesium ÄÄÄ> Ash + Substance X
-decomposition reaction
-ash will have less mass than magnesium; it might be only one component of magnesium due to the smaller mass

Need to measure mass! Mass measurements provide basic information about the nature of a reaction and allowed chemists to get the burning reaction right.

CHEMICAL DYNAMICS--REACTIONS (Teacher Copy)

LESSON 1 DECOMPOSITION CHEMISTRY OF A HYDRATE--EPSON SALTS

Background Information

Objective: The objective of this activity is to illustrate a decomposition reaction.

Activity Abstract: The chemical compound magnesium sulfate hydrate (MgSO4
7H2O) is heated to high temperature and it decomposes to form the chemical compounds magnesium sulfate (MgSO4) and water (H2O). This entire chemical reaction can be summarized by the following statement called a chemical equation:

MgSO4
7H2O ÄÄÄ> MgSo4 + 7H2O

It shows the chemist exactly how atoms and molecules are being altered during the chemical reaction.

The simple technique of weighing a substance before and after a chemical reaction provides a tremendous amount of information about the nature of the chemical reaction. Understanding the nature of the reactions allows the chemist to categorize the reaction.

Materials and Preparations

Chemicals: Epsom salts (MgSO4
7H2O)

Equipment: Sterno heater wire gauze
aluminum cups tongs
balance Styrofoam cup
thermometer

Procedure

1. Determine the mass of the aluminum dish.

2. Add approximately 5 grams of Epsom salts to the aluminum dish.

3. Determine the mass of the aluminum dish and its contents.

4. Place the aluminum dish on the wire gauze heating stage of the Sterno heater and heat the contents of the dish for ten minutes. Note the evolution of water and the change in the appearance of the white salt as the chemical reaction progresses.

5. Using a pair of tongs, remove the aluminum dish from the Sterno heater and let it cool.

6. After the aluminum dish has cooled, determine the mass of the aluminum dish and its contents.

LESSON 1 Continued

7. Calculate the percentage of water in Epsom salts.

8. Place 30 ml (1 oz.) of water in a Styrofoam cup.

9. Determine the temperature of the water in the cup.

10. Add the magnesium sulfate, MgSO4, produced by the above chemical reaction to the water in the Styrofoam cup.

11. Observe the temperature change produced by the addition of magnesium sulfate to water.

Note: The quantitative aspects of this laboratory activity will be a subject addressed by a group.

CHEMICAL DYNAMICS--REACTIONS (Teacher Copy)

LESSON 2 FORMING THE MAGNESIUM SULFATE HYDRATE--EPSON SALTS

Background Information

Objective: The objective of this activity is to illustrate a building up reaction.

Activity Abstract: The chemical compound magnesium sulfate (MgSo4) can be hydrated to the compound magnesium sulfate hydrate (MgSO4
7H2O). This is an example of a building up reaction. The entire chemical reaction can be summarized by the following statement, called a chemical equation:

MgSO4 + 7H2O ÄÄÄÄ> MgSO4
7H2O

It shows the chemist exactly how atoms and molecules are being altered during the chemical reaction.

Materials and Preparations

Chemicals: MgSO4 anhydrous (from previous activity)

Equipment: aluminum cup eyedropper
balance

Time: If you have time, this will take two days.

Procedure

1. Determine the mass of the aluminum dish.

2. Add approximately 2.5 grams of MgSO4 to the aluminum dish. Or use the MgSO4 from the previous experiment.

3. Determine the mass of the aluminum dish and its contents.

4. Add (drop-wise) enough water to the MgSO4 to create a slurry. Mix and let the excess water evaporate over a few days. (You may want to speed up the process by heating, but be careful, you could drive off the water as in the first activity.)

5. Observe the temperature change produced by the addition of water to magnesium sulfate. The opposite temperature change occurs when the hydrate (MgSO4
7H2O) is dissolved in water!

Continued

LESSON 2 Continued

6. After all the excess water has evaporated off and you have formed the Epsom salt crystals (MgSO4
7H2O), determine the mass of the aluminum dish and its contents.

7. If you used the MgSO4 from the first activity for this activity, was the mass lost in the first part equal to the mass gained in this part. Should it be? If you are very careful with your measurements they should be almost the same.

Note: The beauty of this system is using the cyclic nature of dehydrating and hydrating the MgSO4 to illustrate decomposition and building up reactions. The mass loss of seven waters per molecule is easily observed even with crude balances. But, the best part is that Epsom salts are cheap and easy to get.

CHEMICAL DYNAMICS--REACTIONS (Teacher Copy)

PART II INTRODUCTION TO CHEMICAL SYNTHESIS

Chemists develop chemical transformations in the hope of creating new, useful materials. These materials are designed to have specific desirable properties.

There are countless procedures for synthesizing compounds. However, when you restrict the synthesis to relatively safe, readily available materials that are familiar to the experimenters (elementary students) the task becomes more difficult. The following synthesis uses familiar house hold materials that can be bought in the grocery store to synthesize another familiar substance which can be bought in the grocery store as well.

CHEMICAL DYNAMICS--REACTIONS (Teacher Copy)

LESSON 3 PREPARATION OF STERNO

This lesson and related activity centered around a paper published by Dr. Welsh in Journal of Chemical Education. See the activity cited in the reference below.

Welsh, M.J. (1990). Preparation of Sterno From Blackboard Chalk, A "Safe" Student-Teacher Activity. Journal of Chemical Education, Vol. 67, No. 4, Pg. 337, April.

If there is time beyond the above activity

Place equal amounts of calcium carbonate (approximately 3 teaspoons of crushed chalk) in two 16 oz. clear plastic cups. Add 100 ml of H2O to each cup. Buffer one cup with 20 ml of a saturated sodium acetate solution. Add equal amounts (approximately 40 ml) of acetic acid (vinegar) to each cup. Observe the difference in the rate of reaction. The presence of a common ion (acetate anion) slows the reaction.

CHEMICAL DYNAMICS--REACTIONS (Teacher Copy)

PART III INTRODUCTION TO CHEMICAL ANALYSIS

One of the most important tasks a chemist performs is chemical analysis. The chemist can determine the identity and quantity of a substance, maintain the quality of a substance, and solve a mystery (Forensic Chemistry--"Quincy") with an appropriate chemical analysis.

Demonstration Part 1 - The Reaction

First demonstrate the reaction used in the chemical analysis on a qualitative basis. Starch a piece of white cotton cloth (handkerchief). Drop a small amount of 2% tincture of iodine (drug store) on the cloth. The starch in the cloth will cause the yellow iodine solution to stain the cloth blue. The blue color is formed by a chemical interaction between iodine and starch.

I- + I2 + starch ÄÄÄÄ> I3- -starch complex (blue-black)

This stain can easily be removed by blotting the cloth with a moist vitamin C tablet. The vitamin C and iodine react chemically:

I2 + ascorbic acid ÄÄÄÄ> oxidized ascorbic acid + 2I-
to form water soluble colorless products.

Demonstration Part 2 - Analytical Technique

To illustrate the principle of chemical analysis, dissolve a 500 mg vitamin C tablet in 200 ml of water in a 16 oz. clear plastic cup. Mix the solution thoroughly. The starch binder in the tablet will not dissolve. Pipet about 1 ml of this solution into another 16 oz. clear plastic cup containing another 200 ml of water mixed with 4 ml of starch solution. Into a another 16 oz. clear plastic cup containing 200 ml of water mixed with 4 ml of starch solution, pipet about 2 ml of the vitamin C solution. After preparing these cups in front of the students, ask them which of the two preparations contains more vitamin C. The answer is obvious.

Using the shell game, switch the cups around quickly so that the students lose track of the more concentrated solution. Demonstrate that the cup containing more vitamin C will consume more tincture of iodine. The cups should use approximately 10 to 20 drops (respectively) of iodine solution before turning blue.

CHEMICAL DYNAMICS--REACTIONS (Teacher Copy)

LESSON 4 ESTIMATE THE NUMBER OF MILLIGRAMS OF
VITAMIN C IN A SERVING OF TANG

Background Information

Objective: The objective of this activity is to illustrate the nature of chemical analysis and estimate the amount of vitamin C in Tang.

Activity Abstract: When ascorbic acid (vitamin C) molecules react with iodine molecules, the ascorbic acid is oxidized (looses electrons) and the iodine is reduced (gains electrons). Reduced iodine cannot react with starch. When all the ascorbic acid is gone, added iodine molecules are then able to react with starch. The amount of iodine added before the blue-black complex forms is directly related to the amount of ascorbic acid in the solution.

I2 + ascorbic acid ÄÄÄÄ> oxidized ascorbic acid + 2I-

I- + I2 + starch ÄÄÄÄ> I3- -starch complex (blue-black)

Oxidized ascorbic acid in fruit juice is produced when the juice is in contact with oxygen in the air. Oxidized ascorbic acid is not useful to the body as a vitamin.

The minimum daily requirement (U.S. RDA) for vitamin C is 0.060 g/day (60 mg/day). The formula and structure of ascorbic acid are C6H8O6 and

CH2OH
³
³
HOCH
³ / O \
HC C ÍÍ O
³ ³
³ ³
C ÍÍÍÍÍ C
³ ³
OH OH

A very simple method of counting drops will be used to estimate the amount of vitamin C in a single serving of Tang.

Materials and Preparations

Chemicals: vitamin C tablets 100 mg, 250 mg, and 500 mg
starch solution
tincture of iodine
Tang

LESSON 4 Continued

Equipment: 16 oz. clear plastic cups
eyedroppers
stirring rods (plastic spoons)

Solutions: stable starch solution, tincture of iodine

Prepare 1: Prepare a stabilized starch solution by mixing 2 grams of soluble starch (cornstarch or potato starch) and 10 grams of MnSO4
H2O into 500 ml water. Boil the solution for 1 minute and cool.

Prepare 2: Tincture of iodine can be bought at the store and used as is. However, if you want to make a larger quantity the following recipe can be used. Dissolve 0.6 grams of potassium iodide (KI) in 500 ml of water. Then dissolve 0.6 grams of iodine in 50 ml of ethyl alcohol. Mix the two solutions and dilute to 1.0 L with water.

Procedure

³ Workshop Activity! ³
³ As a group we will work out the procedures, ³
³ results, and draw conclusion in the space ³
³ below. ³
³ ³