Teaching the Concept of Rates of Chemical Reactions By: Tatiana Vrabie & Jiwei Li Introduction to Reaction Rates Before studying reaction rates, students may imagine that most reactions occur very quickly, and proceed to completion. However, Some reactions do not occur at all under most conditions , and others are very slow. What makes some reactions occur within a few seconds while others occur over days, or even years? What can you do to speed up or slow down a reaction? Why do factors such as temperature, surface area, and concentration affect reaction rates? Students will answer these questions and others as they learn about the rates of chemical reactions. • Lesson Sequence • Lesson 1: Expressing and Measuring Reaction Rates Lesson 2: The Rate Law and Order of Reaction • Lesson 3: Theories of Reaction Rates • Lesson 4: Reaction Mechanisms and Catalysts • Lesson 5: Lab Investigation • Lesson 6: Project Curriculum Expectations D3.5 explain, using collision theory and potential energy diagrams, how factors such as temperature, the surface area of the reactants, the nature of the reactants, the addition of catalysts, and the Concentration of the solution control the rate of a chemical reaction D2.8 plan and conduct an inquiry to determine how various factors (e.g., change in temperature, addition of a catalyst, increase in surface area of a solid reactant) affect the rate of a chemical reaction [IP, PR, AI] Advance Preparation • Prerequisite for Rate of Reaction unit is knowledge • • • about Enthalpy and Heats of Reaction as well as Hess’s Law and Thermochemical Equations. Many lessons include computer simulations, therefore computers availability should be arranged in advance. For Lab Activities students should be asked to bring lab coats, goggles and gloves. Video demonstration require teacher to check up Internet and projector function in the classroom For the Project or Thought Lab, teacher should come to class prepared with several suggestions of catalysts and enzymes. Therefore, teacher may need to do some initial research. Teaching Approach： Expressing and Measuring Reaction Rates Brainstorm：slow and fast reactions, for example: rusting of metals, combustion. Virtual Lab Demo: Mg and Fe reactions with HCl. http://www.infoplease.com/chemistry/simla b/singlehcl.html Teaching Approach: The Rate Law and Order of Reaction Video: concentration and reaction rates Lab: Iodine-Starch Clock to find empirically RR as function of initial concentration and formulate Rate Law equation: Rate = k [A]m[B]n k –rate constant; m, n – individual orders of reaction Theories of Reaction Rates Animation: Factors affecting rate of reactions Gizmo Worksheet “Collision Theory” and animations Five factors are affecting Rates of Reaction (RR): • Nature of reactant • Concentration • Temperature • Presence of a Catalyst • Surface area WHY? Why? It is explained by Collision Theory: • The average kinetic energy of particle is proportional to temperature of sample • Collisions of particles is a must for reaction to occur • Only collision of particles with sufficient energy leads to successful reaction • RR depends on frequency of collisions and the fraction of successful collisions Everything is because of ENERGY Not all particle have sufficient energy to provide successful reaction That’s why reactions needs Activation Energy to occur Heating provides additional energy and more particle can be successful in collision Teaching Approach: Reaction Mechanisms and Catalysts action Jigsaw Activity on reaction mechanism and rate-determining problems solving Molecularity Elementary step Rate law 1 A -> products rate = k [A] 2 A + A -> products A + B -> products rate = k [A]2 rate = k [A] [B] 3 A + A + A -> products A + 2 B -> products A + B + C -> products rate = k [A]3 rate = k [A] [B]2 rate = k [A] [B] [C] Animation: activity of homogeneous catalysts Teaching Approach: Inquiring Lab In this lab activity students will examine some of the key factors that influence the rate of a reaction: – nature of reactants – particle size – temperature – concentration – catalysts Students are given the lab materials and then asked to design the procedure by themselves and predict the result. Potential Student Difficulties • Students may mix up the stoichiometric coefficients with reaction order • Finding the equation of line in Iodine Clock lab • Understanding that the frequency of collisions is not the function of temperature • Misconception of catalyst and enzyme Solutions to Student Difficulties • Show in the lab that reaction order is determined empirically and may be non-integer number, differentiate with colors. Give some example of non-integers orders. • Give the students general equation of a straight line and review how it may be found from plot. • Contrast and show by animation that frequency of collisions does not increase significantly with temperature, but average energy of particle does, which is more important for reaction to occur. • Asking students to brainstorm a list of what they already know. Address that catalysts and enzymes are regenerated unchanged at the end of a reaction, and they usually increase the rate of a reaction. Applications and Societal Issues/Implications The project of “Bulletin” or “Thought Lab” is designed for applications and societal issues study. Teacher helps students create a list of catalysts that are used in industries and a list of enzymes. Students choose one industrial catalyst and one enzyme and use electronic and print resources to research the substances to make a bulletin or an analysis report. Applications and Societal Issues/Implications • Many reactions that produce useful compounds proceed too slowly to be used in industries. • Chemists and engineers use catalysts to speed up the reactions in order to obtain products at a reasonable rate and under mild conditions. • Biological catalysts or enzymes are necessary to sustain life. • Most drugs and vitamins are involved in enzyme regulation. • For some purposes is beneficial to slow down chemical reactions, especially in food industry (food preservation reactants, storage in the cold conditions, in the liquid nitrogen, inert gases), inhibition of microorganisms growth, some surgical processes with body tissue cooling, etc. • The rate of decay of a radioactive isotopes might also serve as a clock. • The project of “Bulletin” or “Thought Lab” can address some applications and societal issues. Differentiated Assessment • Assessment for learning Anecdotal notes during brainstorm, Q&A before and after Demo, checklist on answers on Anticipation Guide. • Assessments as learning Numerical problems solving, Questions and Answers, Worksheet answers checklist. • Assessment of learning Self –assessment, Laboratory Report, Performance Checklist, peer evaluation in numeric problem solving, short quiz. • The project of “Bulletin” can be alternated as a “thought lab”, a song, an essay, a diary entry, an animation or even a role play performance for multiple intelligence evaluation Accommodations • Consult individual student IEPs for specific accommodation and modifications in teaching methodologies • When grouping students take into account availability of peer help • Help students with creation of graphic organisers • Cooperate with special education providers about tests, assignment, etc. • Permit ESL/ESD students to use translators and extend timelines for them • Record key words on the blackboard (or whiteboard) • Provide appropriate level resources for reading • Use more visual aids Safety Considerations • Teacher review the safety rules before lab and • Students should have their lab coats, goggles and gloves ready for use. As students will use computers to complete virtual labs and research, it is important that teacher ensures students using computer in a appropriate way.