Changes go on about you all the time. Some changes are chemical changes, such as gasoline burning or a nail rusting. But what is happening when a chemical change occurs? What is the nature of a chemical reaction?

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To examine the behavior of matter in a chemical reaction, focusing on the behavior of the individual particles of each substance involved.

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Wear protective glasses and an apron at all times. Avoid skin contact with solids and solutions. Dispose of all solutions in the container designated by your teacher. Wash your hands before leaving the laboratory.

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Part I. Your teacher will demonstrate three reactions. Record your observations. Then, answer these questions in your assigned discussion group:

1. What evidence for a chemical reaction did you observe in each demonstration?
2. Define "precipitate".

Part II. Assemble the following materials at your workstation: petri dish, vials containing crystals of lead(II) nitrate, Pb(NO₃)₂(s), and potassium iodide, KI(s), spot plate, small test tube, and two labeled, thin-stem pipets containing solutions of lead(II) nitrate, Pb(NO₃)₂(aq), and potassium iodide, KI(aq). Record all your observations in and complete sketches when appropriate.

1. Grinding of solids. Place a few crystals of solid lead(II) nitrate and solid potassium iodide in the same well of a spot plate. Using the small test tube, carefully grind the solids using a circular motion. Do not press too hard or you may break the test tube.
2. Prepare petri dish. Cover the bottom of the petri dish with distilled water. Place on a white sheet of paper.
3. Simultaneous addition of solutions. Using the thin-stem pipets, simultaneously add 10 drops of lead(II) nitrate solution and 10 drops of potassium iodide solution on opposite sides of the petri dish as shown in the diagram below. Do not move or shake the dish.

   

4. Delayed addition of potassium iodide. Repeat Steps 2 and 3 but wait five minutes before adding the potassium iodide solution.
5. Dump all solutions into the waste container provided. Using a small amount of water, wash the solid from Step 1 into the waste container.
6. Wash hands thoroughly before leaving the laboratory.

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1. How do you know a chemical reaction occurred?
2. How were the results in Step 1 and Step 3 similar? How were they different?
3. Do you think the chemical reactions were the same in both Step 3 and 4? Why?
4. How is the observation in Step 3 different from that in Step 4? What do you think caused this difference?
5. Why did it take longer in Step 3 for an observable change?

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1. When the chemical substances used in this activity are placed in water, they dissolve and individual particles making up the substance break apart. Since the solution looks just like water, what can you say about the size of the particles that make up each substance?
2. Draw a picture to show what you think these particles look like in water. Use XY to represent a dissolved substance and W to represent water.
3. You started with two different substances at the beginning of Step 1. How do you know a new substance was produced as the result of the grinding? Chemists give this kind of chemical reaction the name "double replacement". It is kind of a "changing partners" behavior. These substances "trade partners" to become lead(II) iodide and potassium nitrate. The lead(II) iodide has a yellow color, which you observed in the reactions. Since lead(II) iodide is not very soluble in water, a precipitate (solid) forms in Steps 3 and 4.
4. Let AB represent one solid and CD represent another solid; let AD represent the colored solid formed. Create a diagram that shows a model of the microscopic behavior of these particles if equal quantities are ground together as in Step 1.
5. Using the same symbols, write an equation for the reaction between AB and CD.
6. What would happen if more AB than CD is combined? Create a diagram to show this.

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Teachers Guide

Preparing for the Laboratory Activity

• Major Chemical Concept
• Level
• Expected Student Background
• Time
• Safety
• Materials
• Advance Preparation

Conducting the Laboratory Activity

• Pre-Lab Discussion
• Teacher/Student Interaction
• Anticipated Student Results
• Answers Data Analysis
• Answers to Imply, Apply
• Post-Lab Discussion

Assessing the Laboratory Learning

• Pictures in the Mind
• Written or Oral Report

Preparing for the Laboratory Activity

Major Chemical Concept

A model of the particulate nature of matter and a kinetic model of matter can be developed in terms of the behavior of particles in chemical reactions.

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General chemistry

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Students should be able to:

• Make and record observations in a notebook or data sheet.

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60 min (including questions and post-lab)

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• The laboratory procedures involve no unusual hazards.
• The lead iodide must not be dumped in sink.
• Be sure excess KI is added to the solutions to ensure all the PbI₂ precipitates.

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Non-Consumables (per lab team)

• 1 Microspatula
• 1 Petri dish bottom or lid, glass or plastic
• 1 Spot depression plate or watch glass and 13 x 100 mm test tube

Consumables (per lab team)

• Thin-stem pipet containing 0.25 M lead(II) nitrate, Pb(NO₃)₂
• Thin-stem pipet containing 0.50 M potassium iodide, KI
• Vial, small, containing crystals of lead(II) nitrate, Pb(NO₃)₂
• Vial, small, containing crystals of potassium iodide, KI

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1. Pre-lab Teacher Demonstration
   • 0.1 M sodium carbonate, Na₂CO₃, 4 g per 500 mL
   • 0.1 M calcium nitrate, Ca(NO₃)₂, 4 g per 250 mL
   • 0.02 M iron(III) chloride, FeCl₃, 0.8 g per 250 mL
   • 0.001 M potassium thiocyanate, KSCN, 0.03 g per 250 mL
   • 3 M hydrochloric acid, HCl, 25 mL; 6 mL concentrated HCl per 25 mL
2. Student solutions
   • 0.25 M lead(II) nitrate, Pb(NO₃)₂, 8.3 g per 100 mL
   • 0.50 M potassium iodide, KI, 8.3 g per 100 mL

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Conducting the Laboratory Activity

Pre-Lab Discussion

To review the criteria that determine when a chemical reaction has occurred, complete the following three demonstrations (or have a student demonstrate). Allow students to record their observations and then have them gather in small groups (2 or 4) to discuss and answer the discussion questions.

Reaction 1: Pour 200 mL of 0.1 M Na₂CO₃ solution into a 500 mL beaker. Now add 200 mL of 0.1 M Ca(NO₃)₂ solution. The students will note the formation of a white precipitate.

Reaction 2: Pour 200 mL of 0.02 M FeCl₃ solution into a 500 mL beaker. Add 200 mL of 0.001 M KSCN solution. Students will note the formation of a red-colored solution. During the discussion, be sure they understand the difference between a clear but colored solution and a colorless solution.

Reaction 3: Pour 200 mL of 0.1 M Na₂CO₃ solution into a 500 mL beaker. Add 25 mL of 3 M HCl. Students will observe the evolution of a gas, which you can identify as CO₂.

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During the laboratory activity observe the micropipeting technique being used. This is a good opportunity to help students review and perfect their use of the thin-stem pipets. During the grinding of solid samples, caution against pressing so hard that the test tube breaks. The grinding should be a slow process so students can see the reaction take place slowly and observe the increasing color intensity.

When students are doing Data Analysis Questions 1-5, guide their thinking by reminding them to focus on the observations. Group work will help students draw conclusions.

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Part 1. Grinding of solids: As the grinding occurs, the white materials begin to turn a yellowish color.

Part 2. Simultaneous solution addition: A line of yellow solid forms in the center of the dish after one to two minutes.

Part 3. Delayed addition of KI: A yellow area forms near the KI immediately after it is added.

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1. A yellow solid forms; change of color.
2. Both systems form a yellow solid. The reaction takes longer to form in the solution. The solid formed is not where the reactants were added.
3. Yes, the color formed was the same.
4. The color appears in a different spot. The lead(II) nitrate has had more time to move around.
5. The materials had to move in the water before they got together.

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1. The particles are very small.
2. 
3. The yellow color formed indicated a different substance.
4. 
5. AB + CD --> AD + CB
6. Extra AB will be left in the container.

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Upon completion of the follow-up questions in the Data Analysis and Imply, Apply sections, have students gather in discussion groups to compare their answers. This could be on the day following the laboratory activity. You may want to have them initially answer the questions as a group depending on the degree of cooperative learning techniques you wish to use. Encourage them to come to a group consensus. Groups of three or four work better in this regard than groups of two. Have groups report their findings to the rest of the class and discuss. If their conclusions are not in agreement, repeat the reactions as a demonstration on the overhead projector and help guide the discussion toward development of an accurate particulate model. Students may especially need help in drawing models of the reactions (i.e., Imply, Apply Questions 2, 4, 6).

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Assessing the Laboratory Learning

Pictures in the Mind

The diagram above represents possible models of a reaction involving two solutions like those in Parts 2 and 3 of the laboratory activity. Have your data sheet available as you complete the evaluation. Recall that chemicals in solution often break up into individual particles. Thus if W represents a water molecule, solute AB might break into A and B particles and be drawn as shown in Fig. 1. In order to keep the grid drawings simple, water molecules will be omitted for clarity, as shown in Fig. 2.

Answer the following questions by selecting the grid drawing(s) above that best illustrate the location of the particles in the petri dish at that time in the reaction.

Use the following symbols to represent the reactants and products. (Answers given in italics.)

Reactants AB = clear solution and

CD = clear solution

Products AD = yellow solid and

CB = remaining solution

1. uniformly distributed solution of one chemical A, D, J
2. Like Part 2 - shortly after addition of both materials H
3. Like Part 2 - 4-5 min after addition of both materials F
4. Like Part 3 - immediately after addition of the AB solution only B
5. Like Part 3 - 5 min later, but before addition of CD solution A
6. Like Part 3 - upon addition of CD solution E
7. Like Part 3 - 1 min after addition of CD solution E
8. Petri dish was moved or bumped with both AB and CD added. G, K

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Name three things you learned from this exercise that will enable you to become more successful in the chemistry laboratory.

Answers will vary; all are acceptable. Look for comments regarding the identification of a chemical reaction and the particulate nature of matter.

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