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Speed me up, Scotty!
by Sally Lang
University

Grade: 10-12
Time Allotment: Two fifty-minute class periods.
Subject Matter: Chemistry, Mathematics, Computer Technology.

Overview:
          In chemical reactions, bonds are being broken and new bonds are being formed. Whether or not a reaction proceeds depends upon the energy of the reactants and the energy of the products. A reaction will proceed if the energy of the products is lower than the energy of the reactants. In this case, the reaction is said to be going "downhill". A reaction may proceed going "uphill" if the products will achieve enough entropy (randomness) relative to the products to overcome the increase in energy required for the reaction to go. Even when the products of a reaction are at a lower energy than the reactants (the reaction is "downhill") the reactants still must go "up a small hill" before beginning its "big descent". This small increase in energy required to begin a reaction is called its activation energy.

         Since chemical reactions are a consequence of atoms colliding with one another, there are three main ways to increase the rate of a reaction. One way is to increase the concentration of one of the reactants, increasing the likelihood of atomic collision. Another way to speed up the rate of a reaction is to heat the reactants, thus increasing the amount of collisions likely to occur. This effectively lowers the activation energy. The last way to increase the rate of a chemical reaction is to provide a catalyst, a substance that serves to substantially lower the activation energy required to get a reaction to proceed. This lesson explores the energy and entropy changes in chemical reactions and the effects of concentration and catalysts on the rate of reactions.

Learning Objectives:
Students will be able to:

  • Describe the driving forces behind chemical reactions.
  • Define energy, entropy, exothermic reaction, endothermic reaction, activation energy, and catalyst.
  • Draw energy diagrams for "uphill", "downhill", heated, and catalyzed reactions.
  • List the three ways in which chemical reactions can be accelerated.
  • Design an appropriate laboratory experiment, collect and analyze data, and draw conclusions.
  • Employ Microsoft Excel to calculate and analyze data and to create graphs and perform graphical analyses of that data.

Standards:
Taken from www.idahoachieves.com/standards.html

649. 01 Concepts of Scientific Inquiry: Understand scientific inquiry and develop critical thinking skills
65002 Concepts of Physical Science: Understand the structure and function of matter and molecules and their interactions.
65003. Understand chemical reactions
65501. Technology: Understand the relationship between science and technology and develop the abilities of technological design and application.

Media Components:
Video:
The Driving Force (The World of Chemistry) Program 13, Produced by the University of Maryland and the Educational Film Center. Videocassette Dist By 1990

Molecules in Action (The World of Chemistry) Program 14, Produced by the University of Maryland and the Educational Film Center. Videocassette Dist By 1990

Web Sites:
http://members.aol.com/ChangChem3/CALbasicROR1.html
This web site explains the forces behind chemical reactions, The Collision Theory of Atoms, factors affecting the rate of reactions, and real-world examples of catalyz
ed reactions.

Materials:
  • 3 10-mL graduated cylinders
  • 100-mL beaker
  • stopwatch
  • dropping pipet
  • stirring rod
  • apron
  • safety goggles

Prep For Teachers
Prior to teaching this lesson, bookmark the Web Site on each computer in the classroom, and have the Video set to the proper starting point. On the second day of the lesson, when the laboratory will be performed by the students, have all of the materials cleaned and set out at the laboratory stations. Have at least 4 L of all three of the solutions prepared. Have a copy of each of the attached sheets available for each student.

Introductory Activities
Setting the Stage
Ask the students "Why do we keep certain foods in the refrigerator?" At least one of your students will probably answer that the reason some food is kept in the refrigerator is to keep if from going bad.
Ask your students, "What is happening to food when it goes bad?" You will most likely get such answers as "it is breaking down", "it is rotting", "it is a chemical reaction."

Ask the students how long they think it would take some foods to break down and what would happen if it took their bodies that long to break down food to use it for energy? Tell them that all these questions deal with the rates of chemical reactions.

Pass out a Video Skeletal Notes (Focus for Media Interaction) worksheet to each student, and instruct them to complete it while watching the video segments. Begin the Video, The Driving Forces. Focus for Media Interaction: While watching the video Define - Exothermic Reaction; Endothermic Reaction and Entropy.

Start when you see the fire, just before the narrator says, "In most chemical reactions, energy of the system decreases". PLAY the video until you see the man in the lab coat with a bowl of lettuce. FASTFORWARD until just after you see a wrecked building, just before the man in the lab coat reappears. PLAY until you see wine glasses. When you see the wine glasses, press STOP.

START the Video 14, Molecules in Action at the beginning of the video. PLAY until after you see HBr formed in animation and then STOP. Ask the students how they think increasing the concentration of a reactant will affect the amount of collisions taking place. Ask them how they think this will affect the rate at which HBr will be formed. Press PLAY and continue the video. STOP after you see the diagram showing that heat effectively lowers the activation energy. FASTFORWARD until just before you see a big thumb and forefinger removing a class stopper from some laboratory equipment. PLAY until just after the next hill diagram. Then STOP. FASTFORWARD until after you see a man holding in his hand a big ball and stick model of a molecule. Before pressing PLAY, preface by telling the students that they are in the middle of discussing a rhodium catalyst. Then press PLAY. STOP after you hear "very little rhodium is lost." To review the introductory part of the lesson, ask the students what are the three things they learned that will cause the rate of a reaction to increase.

Learning Activities
Distribute the Web Site worksheet (FOCUS FOR MEDIA INTERACTION) to each of the students. Explain to them that each student is required to complete one, even though they will be working in groups of 4 or 5 on the computer. Tell the students that when they have sufficiently accessed the site for the provided URL, they need to click on the green icon next to the word "Catalysts" under the Menu bar at the top of the Web page. Release the students to their computers to complete their assignment. Should take about 15 minutes. When the class has reconvened, lead the students in a 5 minutes discussion on their findings, asking the following guiding questions.

  1. What did you discover to be the reason why a catalyst speeds up a reaction? (After you receive their answers, explain again that the catalyst merely forces the atoms in the proper position to react, and that it does this by aligning them on its surface so that they are right next to each other and can trade/share electrons.)
  2. What is the term for a biological catalyst? (After they have answered, remind them of the question at the beginning of class, and explain that it is because our bodies have enzymes that we are able to break down food very quickly.
  3. Enzymes are put to use in several different industries? What are some of those industries and how are enzymes utilized?

When the class is over, tell the students that they will be performing reactions in the laboratory the following day and that they will need to wear long pants and closed-toed shoes. Distribute the laboratory sheets at this time and instruct students to read them for homework so that they will understand the laboratory upon entering the lab the next day.

Culminating Activity
Looking at the Procedure for the laboratory (see attached), talk through the process with the students following along. Ask them if there are any questions about what they will be doing in the laboratory, and review any steps necessary. Release the students into the laboratory. Circulate the room during the experiment, answering any questions and monitoring the students' safety. Allow 25-30 minutes for the students to complete the laboratory.

When students have completed gathering data, have them clean up their lab stations and return to the classroom.

Have the students go to a computer and following the laboratory sheet instructions for Data Analysis, enter their data and perform the necessary functions on the Microsoft Excel program. As there will be 4-5 students at the computer in most classrooms, and not all students will be able to be with their lab partner, each group of 4-5 at the computer should pick one set of data to work from. (All data should be within the same ballpark, and exhibiting the same trends, anyway.) If you are lucky enough to have access to as many computers as you have lab groups, you should require each lab group to do their own Excel data analysis. Each student should turn in a laboratory sheet, but only one Excel Workbook and Graph are required per computer group.

At the end of class, review the class on their findings with the following questions:

  1. Do your graphs look as you expected them to? Why or why not?
  2. What do you think a T vs. time graph would look like? (falling from left to right).
  3. How much effect did the catalyst have on the rate of the reaction?
Extensions
English-Students write a detailed laboratory report including conclusion and sources of error.
Math-Students calculate the concentrations of the solutions by hand instead of relying on the Excel program.
Math-Students study the rate of change of chemical reactions using rudimentary differential calculus (if applicable for that grade level)
Health-Students make a flow chart demonstrating the specific enzymes required to break down food during the digestive process.

Community Connections:

  • Take a fieldtrip to your local dairy farm, cheese factory, or brewery. What enzymes are used in these processes? Under what conditions must the reactions take place to keep the enzyme active? Write a report or prepare a PowerPoint presentation about the industry you chose and its reliance on enzymes.
  • Visit a cement company. Find out the amount of time required for cement to form up. What would be some of the consequences of not allowing it enough time? What would be some of the consequences if the cement hardened to quickly? Why are the cement drums on the trucks always spinning? Write a report about the Chemistry behind cement.
  • Go to your local library and research enzymes in the body. Consult a physician. How many enzymes are in the human body? What are some examples of diseases that occur as a result of a person lacking that enzyme? While there are too many to list, prepare a report of a disease about which you learned. Consult a physician or the Internet, and provide examples of symptoms and prognoses.

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