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

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 Law Enthalpy and and Heats of Reaction Thermochemical Equations . as well as Hess

’

s 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

Reaction ( RR ): are affecting Rates of

• • • • •

Nature of reactant Concentration Temperature Presence of a Catalyst Surface area

WHY?

Why?

It is explained by Collision

Theory:

• • • • The average kinetic energy

proportional to temperature

of particle is of sample

Collisions of particles

to occur is a must for reaction Only

collision of particles with

energy

sufficient

leads to successful reaction RR the depends on

fraction of frequency

of collisions and

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 1 2 3 Elementary step

A -> products A + A -> products A + B -> products A + A + A -> products A + 2 B -> products A + B + C -> products

Rate law

rate = k

[A]

rate = k

[A] 2

rate = k

[A] [B]

rate = k rate = k

[A] 3 [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 empirically reaction order and may be non-integer is determined number, differentiate with colors. Give some example of non-integers orders.

• Give the students general equation of a straight line review how it may be found from plot.

and • Contrast and show by animation that frequency of collisions does not increase significantly with temperature, but average energy of particle important for reaction to occur.

does , which is more • Asking students to brainstorm a list of what they already know. Address that increase the rate catalysts and of a reaction.

enzymes are regenerated unchanged at the end of a reaction, and they usually

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 conditions. catalysts to speed up the reactions in order to obtain products at a reasonable rate and under mild • Biological catalysts or enzymes are

necessary

to sustain life. • Most drugs and vitamins are involved in enzyme regulation.

• • • For some purposes 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.

is beneficial to slow down chemical reactions, The rate of decay of a radioactive isotopes might also serve as a clock.

The project of “ Bulletin ” or “ Thought Lab applications and societal issues.

” can address some

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 etc.

Permit ESL/ESD students to use translators them about tests, assignment, and extend timelines for • 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.