CHE-5062-2 CHEMISTRY: KINETICS and EQUILIBRIUM · PDF fileChemistry: Kinetics and Equilibrium This learning guide has been produced by the Société de formation à distance des commissions

CHE-5062-2

CHEMISTRY:KINETICS and EQUILIBRIUM

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Ch em i st r y : K i n e t i c s an d Eq u i l i b r i u m

CH E - 5 0 6 2 - 2

L e a r n i n g G u i d e

Chemistry: Kinetics and Equilibrium

This learning guide has been produced by the Société de formation à distance des commissions

scolaires du Québec (SOFAD).

Production Team

Project Manager: Alain Pednault (SOFAD)

Author: Debby Correia Ledo

Illustrations: Marc Tellier

Content Editors: Josée Locas

Gilles St-Louis

Translator: Claudia de Fulviis

Layout and Computer Graphics: Daniel Rémy (I. D. Graphique inc.)

Proofreader: Johanne St-Martin

First Printing: February 2017

© SOFAD, 2016

This work is funded in part by the Ministère de l’Éducation, de l’Enseignement supérieur et de la Recherche du

Québec and by the Canada-Quebec Agreement on Minority Language Education and Second Language Instruction.

All rights for translation and adaptation, in whole or in part, reserved for all countries. Any reproduction by

mechanical or electronic means is forbidden without the express written consent of a duly authorized

representative of SOFAD.

Despite the above statement, the scored activities may be reproduced only by users of the corresponding SOFAD

guide.

Legal Deposit - 2017

Bibliothèque et Archives nationales du Québec

Library and Archives Canada

ISBN: 978-2-89493-624-5 (print)

ISBN: 978-2-89493-625-2 (digital) February 2017

i i i

TABLE OF CONTENTS

Table des matières

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

Sequence 1 – Ever Faster . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2En route to Scored Activity 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Exploration Activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Activity 1.1 Molar Flow Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7Activity 1.2 Speed: A Question of Taste. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28Activity 1.3 How Do You Like Them Apples? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47Integration Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49Concept Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

Sequence 2 – When Speed Equals Progress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64En route to Scored Activity 1 (cont.) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66Exploration Activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68Activity 2.1 Remission and Chemical Kinetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70Activity 2.2 A Magnesium Deficiency. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91Activity 2.3 Chemical Chaos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94Integration Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104Concept Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111Instructions for completing Scored Activity 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112

Sequence 3 – The Quest for Equilibrium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114En route to Scored Activity 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116Exploration Activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118Activity 3.1 Left to Right or Right to Left? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119Activity 3.2 And Then There Was Colour! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135Activity 3.3 Equilibrium and Art . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138Integration Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150Concept Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154

Sequence 4 – Action – Reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156En route to Scored Activity 2 (cont.) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158Exploration Activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160Activity 4.1 An Unbalanced Equilibrium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161Activity 4.2 Fake Blood. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189Integration Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192Concept Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200Instructions for completing Scored Activity 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200

Sequence 5 – Measurable Equilibrium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202En route to Scored Activity 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204Exploration Activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206Activity 5.1 The Equilibrium Constant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208Activity 5.2 A Painful Calculus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237Activity 5.3 A Solution to a Basic Problem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252Integration Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255Concept Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262Instructions for completing Scored Activity 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264

This is a preview of:

- the introduction; and

- the first learning

situation.

CHE-5062-2 – CHEMISTRY: KINETICS AND EQUILIBRIUM

iv

Sequence 6 – Acids and Bases. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266En route to Scored Activity 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 268Exploration Activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270Activity 6.1 Acid–Base Equilibrium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271Activity 6.2 The Beckman pH Meter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 289Activity 6.3 Operation Peanut Butter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292Integration Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319Concept Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324Instructions for completing Scored Activity 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 326

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 327

Self-Evaluation Activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329Section A: Explicit Evaluation of Knowledge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 330Section B: Evaluation of Competencies: The Haber Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343Self-Evaluation Chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 352Competency Evaluation Chart: The Haber Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 354

Answer Key . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3571 – Ever Faster. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3582 – When Speed Equals Progress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3773 – The Quest for Equilibrium. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3924 – Action – Reaction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4035 – Measurable Equilibrium. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4176 – Acids and Bases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 436Self-Evaluation Activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 453

Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 463

Appendices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 471Appendix A: Mathematical Prerequisites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 472Appendix B: Quantities, Units, Equations and Constants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 477Appendix C: Product Solubility Constants (Ksp) at 25°C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 480Appendix D: Acidity Constants (Ka) and Basicity Constants (Kb) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 483Appendix E: Charges of Selected Common Ions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 485Appendix F: Significant Digits and Rounding Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 486

Feedback Sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 489

Tdm2

v

INTRODUCTION©

SO

FAD

– A

ll Ri

ghts

Res

erve

d.

Welcome to the Chemistry: Kinetics and Equilibrium course, the

second course in the Secondary 5 chemistry program. Chemistry

is the branch of science that deals with the way in which elements

interact, combine and form new substances. This learning guide begins with

a study of the rate of reaction of chemical processes and the equilibrium state of reversible reactions.

The rate law, equilibrium constants, the factors that influence the rate and equilibrium of reactions,

Le Châtelier’s principle, and the relationship between pH and the molar concentration of hydronium

and hydroxide ions in aqueous solutions will also be studied.

In addition to acquiring knowledge, in this course, you will develop and strengthen the following three

subject-specific competencies (C):

C3Communicates ideas relating

to questions involving chemistry, using the languages associated

with science and technology

C1 Seeks answers or solutions to problems involving chemistry

C2Makes the most of his/her knowledge of chemistry

C1 will be required to carry out the tasks included in the experimental activities, such as studying

the chemical equilibrium of the reaction used to make fake blood for the special effects in a television

show. C2 will be required to complete tasks in realistic and complex situations, such as describing the

equilibrium behaviour involved in producing a coloured pigment to be used to restore a painting by

Renoir. For both of these comptencies, you will be applying C3 when you explain results or provide

supporting arguments for your choices. For example, you will make recommendations to ensure

that the pieces of beef to be used in a photo shoot for a food magazine are nicely browned. These

competencies will also come into play in the evaluation activities, namely the four scored activities and

the self-evaluation activity.

Introduction CHE-5062-2CHEMISTRY:KINETICS and EQUILIBRIUM


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Knowledge and Techniques

The new knowledge you will acquire that is related to the material world is divided into three core

concepts. First, the rate of reaction is key to understanding dynamic equilibrium. When formulated as

a rate law, it is used to derive the mathematical expression of equilibrium constants. Second, chemical

equilibrium and the factors that influence this equilibrium are crucial to a sound understanding of

chemical processes. Lastly, Le Châtelier’s principle is used to explain, and even predict, changes

in chemical processes whose conditions are modified. The techniques you will learn are those

needed to apply theoretical knowledge in the laboratory, such as using laboratory materials safely;

collecting samples; preparing solutions; verifying the precision, accuracy and sensitivity of measuring

instruments; and interpreting the results of measurements.

The following table shows the distribution of new knowledge and techniques across the six learning

sequences in this learning guide.

Distribution of new knowleDge anD prescribeD techniques

Learning sequence KnowLedge Techniques

1

Ever Faster

• Factors that influence the reaction rate:

nature of the reactants, concentration,

surface area, temperature, catalyst

• Using laboratory materials safely

• Preparing solutions

• Interpreting measurement results

2

When Speed Equals

Progress

• Rate law • Using laboratory materials safely

• Collecting samples


• Verifying the precision, accuracy and

sensitivity of measuring instruments


3

The Quest for

Equilibrium

• Factors that influence the equilibrium

state: concentration




4

Action – Reaction

• Factors that influence the equilibrium

state: concentration, temperature,

pressure

• Le Châtelier’s principle





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Learning sequence KnowLedge Techniques

5

Measurable Equilibrium

• Equilbrium constant: solubility product

constant (Ksp)





6

Acids and Bases

• Equilbrium constant: water ionization

constant (Kw), acidity constant (Ka),

basicity constant (Kb)





Structure of the Learning Guide

This learning guide is organized according to the main characteristics of individualized learning and the principles of

learning through concrete and realistic situations. It can be used by both distance education students and students in

the classroom.

This approach is designed to:

• make you as active a participant as possible;

• make you responsible for your own learning;

• accommodate your personal work pace;

• allow you to make the most of your experience and knowledge.

As you work through the course, you will be able to recognize your successes and failures, determine the reasons for

them and identify what you can do to continue learning. If you are enrolled in an adult education centre, your teacher

or tutor will be available throughout the course to support you and answer any questions you may have. If you find a

particular topic especially difficult, don’t hesitate to ask for help.


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Learning sequences (LS)

In all, there are six learning sequences (LS) which will help you acquire new knowledge and apply it

competently. Each LS is organized in the same way and consists of an introduction setting out the topic

under study; an exploration activity that tests your knowledge of concepts that will come in handy in

the LS; and a series of three types of activities: knowledge-acquisition activities, experimental activities

and expertise-based activities.

knowledge-acquisition

activity

experimental activity

expertise-based activity

Knowledge-acquisition activities promote active discovery of new knowledge, while the other two

types of activities promote competency development by having you carry out more complex tasks.

Experimental activities will help you to develop primarily competency C1, whereas the expertise-based

activities will help you to develop primarily competency C2. Competency C3 will be developed in all the

activities.

As you work through the activities, you will be asked to answer questions that will help you acquire

new knowledge and develop your competencies. While at first you may not be able to answer all of

these questions, you should nonetheless try to find satisfactory answers. The answers and related

explanations are given immediately after. It is important that you try to understand all of the new

concepts that are explained to you.

At the end of each LS is a series of integration exercises dealing with all of the concepts studied in the

sequence as well as a summary of new knowledge, which will help you test your understanding of the

subject matter.

Self-evaluation

Like the certification examination (i.e. the final exam), the self-evaluation activity consists of two

sections to help you better prepare for the exam. Before you do this activity, take the time to read

the list of new knowledge found at the end of each learning sequence, then refer to the table of new

knowledge by sequence presented earlier in this introduction. Complete the self-evaluation activity

without referring to the learning guide or the answer key. Then, compare your answers with those in

the answer key for the self-evaluation activity. Follow the indications on the concepts to be reviewed

that are given in the self-evaluation chart and, if necessary, review parts of the course.

Answer key

The answer key for the exercises in the guide is found after the self-evaluation activity. Refer to it after

you have completed each set of exercises to make sure you have fully understood all of the concepts,

before continuing the activity or going on to the next learning sequence. The answer key also includes the

answers to the questions in the self-evaluation activity.

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Glossary and appendices

The glossary at the end of the guide gives the definitions of the terms with dotted underlining that are

found in the learning sequences. These terms are listed in alphabetical order. Don’t hesitate to refer to

the glossary to help you better understand the terms and expressions you encounter in the guide.

The appendices contain useful information and review prerequisite knowledge.

Scored Activities

This learning guide is accompanied by four scored activities in separate booklets, one of which consists

of an experimental activity. You will have to complete a scored activity after LS 2, LS 4, LS 5 and

LS 6. A reminder to this effect is included at the end of each of these learning sequences. The scored

activities serve as an aid to learning; in addition to explicitly evaluating knowledge acquisition, each one

includes a complex and meaningful learning situation designed to assess your ability to deal with these

situations.

The scored activities are an integral part of the learning sequences and are not optional; you are

required to do them. They will be corrected by your tutor if you are a distance education student or by

your instructor if you attend an education centre. Send them to him or her once you have completed

them as only education centre staff have access to the answer key for these activities.

If you do not have a paper copy of the scored activities, go to the “Diversified Basic Education” of the

SOFAD webside, at http://cours1.sofad.qc.ca/ressources.

Evaluation for Certification Purposes

In order to earn the two credits for this course, you must obtain a mark of at least 60% on the final

examination that will be held in an adult education centre. To be able to write this examination, you

should have an average of at least 60% on the scored activities. Some centres require this result in order

to admit you to the final examination.

The final examination for the Chemistry: Kinetics and Equilibrium course consists of two sections, one

practical and the other theoretical, which are administered in two different sessions. The practical part

is based on a realistic application situation and consists of tasks to be carried out in the laboratory. The

theoretical part consists of the following two sections: “Explicit Evaluation of Knowledge” and “Evaluation

of Competencies.”


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Additional Materials

Have all of the following materials handy.

• A calculator, a lead pencil to write your answers and notes in your guide, a coloured pen to correct

your answers, a highlighter to underline key ideas, an eraser, a ruler, a protractor, etc.

• The experimental activity booklet and the experimental kit. You will have to complete this kit by

adding certain items to it.

Additional Information Regarding Distance Education

Here are some tips on how to organize your time. This course involves approximately 50 hours of work.

• Draw up a study schedule, taking into account your availability and needs, as well as your family,

work and other obligations.

• Try to devote a few hours a week to your studies, preferably setting aside two hours at a time.

• Stick to your schedule as much as possible.

Your tutor will guide you throughout the learning process and provide you with advice, constructive

criticism and feedback that will help you succeed in your studies. Do not hesitate to consult your tutor if

you are having difficulty with the theory or the exercises, or if you simply need encouragment to continue

your studies. Make a note of any questions in writing and contact your tutor during his or her available

hours by telephone and, if necessary, in writing. If his or her availability and contact information were

not provided with this learning guide, ask for them at the education centre where you registered for this

course.

We wish you every success in your studies!

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Overview of a Learning Sequence

3

1

2

Ever Faster

1.1 Molar Flow Rate

Goals •• To use a graph to represent the progress of a chemical reaction

To de�ne the rate of reaction and to express it appropriately in a given situation

1.2 Speed: A Question of Taste

Goals • name these factors

• To understand that, depending on the situation and type of reaction, other factors may affect

To determine that the rate of a chemical reaction may be in�uenced by different factors and to

the reaction rate

Your task

In a situation involving the Maillard reaction, you will:• • understand how these factors affect the reaction kinetics;

de�ne the factors that in�uence the rate of the browning reaction when meat is cooked;

• make recommendations.

1.3 How Do You Like Them Apples?

Goal • To study the different factors that affect the rate of reaction

Your task

• As a participant in a baking contest, you will prepare a comparative table to help you choose the best conditions for keeping the apple wedges for a pie from turning brown.

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Rexample, in industry, the rate of various processes is controlled in order to reduce

e of reaction is a very important concept in a number of different �elds. For

production costs. In the agri-food industry, for example, it is necessary to control

the growth rate of certain microorganisms in order to make foods such as cheese. And, in

the biomedical industry, how fast a disease spreads and how quickly a drug takes effect is

crucial information that needs to be taken into account. In this learning sequence, you will

be asked to de�ne the concept of rate (or speed) of reaction and to identify the various

factors that affect rates of reaction.

Title of the LS

This is the list of activities (with the speci�c goal(s) of each one) and task(s) related to theexperimental activity and the expertise-based activity.

A description of the topic of the LS is given here.

The guide includes 6 learning sequences (LS).This is the number of the LS.

The answers to thenumbered questions arefound in the answer keyat the end of the guide.

The explorationactivity tests certain

concepts that areuseful for carrying out

the LS.

Exploration Activity

The following questions will enable you to test your knowledge of concepts that will be useful in this

learning sequence.

Explain what happens to water molecules when water boils. In your answer, try to include at least one

reference to chemical kinetics or the speed of molecular collisions.

Give an example of speed with respect to the following.

a) Computers:

b) Food:

c) A fairground:

1.1

1.2


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This box contains the task to be carried out.

This box containsthe goal(s) of the

activity.

The type of activity isidenti�ed by the

darkened section inthe circle.

Goal

•

Like several other fruits and vegetables, apples turn brown (or oxidize) when

exposed to air once they have been pared and cut. Apples oxidize upon

contact with the oxygen (O2) in air, but also under the action of an enzyme.

When we peel or cut an apple into pieces, the cellular membrane tears,

In the previous activity, you studied the different factors that affect the rate of reaction. You have

To study the different factors that affect the rate of reaction

entered a baking contest competition, and your task is to make a winning apple pie. Your challenge is to

make sure that your apple wedges do not brown.

releasing and dispersing the enzyme that reacts with the proteins in the �esh

of the apple, which turns brown.

You do some research on the Internet and �nd out that there are several

ways to prevent the apple wedges from turning brown. These methods are all different and involve the

various factors that affect reaction rates, which were covered in the previous activity. You want to test

some of these methods.

Your task

• As a participant in a baking contest, you will prepare a comparative table to help you choose

the best conditions for keeping the apple wedges for a pie from turning brown.

To do this activity, refer to the experimental activity booklet that came with this guide. When you have

completed the activity, answer the following questions.

Activity 1.3 How Do You Like Them Apples?

© Valery121283/Shutterstock.com

The integration exercises, found after the last activity, deal

with the subject matter covered in

the LS.

Integration Exercises

Among the following expressions, choose the ones that apply to a system in a state of equilibrium.

a) An unchanging amount of matter

b) Forward reaction and reverse reaction

c) Equal quantities of reactants and products

d) Reversible reaction

e) Steady state

f) Constant concentrations

g) Continuous in�ux of reactants

h) Open system

i) Dynamic equilibrium

State whether each of the following systems is at equilibrium.

© Tim Masters/Shutterstock.com

Sodium bicarbonate reacting violentlyt

with vinegar

© nattawut thammasak/Shutterstock.com

A gas and liquid mixture in a cylinder

a) b)

3.44

3.45

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The LS concludes with a summary of the key

concepts studied.

Concept Summary

Activity 2.1 – Remission and Kinetics

The reaction rate is proportional to the concentration of the reactants. However, it is not necessarily

directly proportional. In other words, the concentration of a reactant may double without the reaction

rate also doubling. In chemical kinetics a rate constant (k), or proportionality constant, quanti�es the

rate of a chemical reaction. The rate law or rate equation for a chemical reaction is an equation that

links the reaction rate with the concentrations of the reactants and the rate constant.

Consider the following general reaction:

aA + bB ® cC + dD

v = k[A]x[B]y

where r is the overall reaction rate in mol/L•s (or another

time unit),

k is the rate constant,

[A] and [B] are the reactant concentrations in mol/L

and x and y represent the order of the reaction with

respect to each reactant.

It should be noted that the concentration of the reactants is always given in mol/L when this mathematical

expression is used to describe the reaction rate.

Activity 2.3 – Chemical Chaos

For a reaction to occur, the reactant molecules must collide. The collision theory therefore explains

the reaction rate, since the speed of a chemical reaction depends not only on the number of collisions

between the reactant molecules, but also on the effectiveness of these collisions. According to the

therefore produce a chemical reaction:

collision theory, the following criteria must be met in order for effective collisions to occur and

• molecules must collide with the proper orientation;

• the reactant molecules must collide with suf�cient kinetic energy, known as the activation energy, to

make and break the appropriate chemical bonds.

Features of a Learning Sequence

Terms andexpressions with

dotted underliningare de�ned in the

glossary at the endof the guide.

The numbered�gures provide

importantinformation;

pay closeattentionto them.

The blue boxescontain key

conceptsto remember,

generally in the form

of de�nitions andequations.

To maintain your swimming pool, you have to measure the acidity of the water on a regular basis and

add acid as needed. Without the right balance of H+ and OH– ions, you may get heavy algae growth

which will make the pool unusable until the correct concentrations are re-established. Human blood

is slightly basic and has a stable pH that remains in the 7.3 to 7.5 range thanks to complex chemical

equilibria. The blood continually re-adjusts its equilibrium in response to external disturbances

associated with eating and respiration.

The last two learning sequences provided a qualitative and quantitative description of chemical

equilibrium. We will now look at acid–base equilibrium, by placing the emphasis on the ionization

of acids and bases, and on neutralization reactions, which are governed by the laws of chemical

equilibrium.

According to Arrhenius' theory:

Acids produce H+ ions when dissolved in water:

Acid ® H+(aq)

+ Anion

Bases produce OH- ions when dissolved in water:

Base ® OH-(aq)

+ Cation

OH-

H3O+

Bat

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Lem

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Milk

of

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0.0

2.0

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2.2

Tom

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4.2

App

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.1

Milk

6.6

Hum

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7.4

Bak

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8.2

10.5

14.0

1 10-1 10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 10-11 10-12 10-13 10-14

10-14 10-13 10-12 10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 1

Am

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.1

Normal waterin watercourses 6 to 8

INCREASING ALKALINITYINCREASING ACIDITY

1 2 3 4 5 6 7 8 9 10 11 12 130 14

Normal rain 5 to 6.3Acid rain 1 to 5

Figure 6.1 The pH scale.


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These sectionsprovide additional

information, which isnot, strictly speaking,

part of the coursematerial. None of thequestions on the �nalexamination will dealwith the information

found in them.

cousin of the Red Delicious used in this experimental activity, and the green Granny

Smith apple. It is only a question of time before other varieties of apples, such as the

Red Delicious, are also modi�ed in this way.

Soon we will see at the grocer's the �rst varieties of genetically modi�ed apples that

Apples that don't brown: �ction or soon-to-be fact?

don't brown when we cut or or bite into them. They are the Golden Delicious, a

© emprize/Shutterstock.com © stevemart/Shutterstock.com

Did youknow?

The exclamation markbeside a paragraphindicates important

content.

Solid or liquid phase compounds and their effect on chemical equilibrium

Certain reactions, such as those in the Mond process, contain a reactant or product

in the solid or liquid phase. An increase or decrease in the quantity of such a

compound in no way affects the chemical equilibrium of the system, since the rate of

reaction (forward or reverse) is not altered by this change.

In addition, the concentration of the compound remains the same even if its quantity

changes. Since a solid or liquid compound is a pure substance and not a mixture,

it is considered as having no concentration. The term concentration is generally

associated with mixtures and solutions.

Note

These boxes contain tips to make your

work easier.

Tip

As a rule of thumb, reaction rates for many reactions double for every 10°C increase in

temperature.

These boxes containreminders of concepts

covered in previouscourses.

Reminder

Charges of common ions and balanced equations

With Ksp

values, the concentrations are determined by the exponent corresponding to their

it is therefore essential to ensure that the equation is balanced, that is, it should contain the

respective coef�cients in the chemical equation. Before writing the expression for the constant,

same number of atoms of each species on both sides. When ions are involved, we must also

make sure that the sum of the charges is the same on both sides of the equation. For example,

the dissolution reaction of calcium nitrate is represented by:

Charges :

Total charge: 0

Ca(NO3)

2(s) Ca2+(aq)

+ 2 NO3

–(aq)

0 1 ´ (+2) 2 ´ (-1)

= +2 = -2

When the sum of the charges is not the same on both sides, the charge of at least one ion is

incorrect or the equation is not properly balanced. In the dissolution equation for a salt, the

total charge equals zero, because the starting solid is neutral. ions. Remember that alkali metals

have a positive charge (1+) and that alkaline earth metals have a double positive charge (2+).

Halogens have a negative charge (1–), and oxygen carries a double negative charge (2–). Some

complex ions are often encountered as OH–, NO3–, SO4

2–, CO3

2–, PO4

3–.

A table of the charges of some common ions is found in Appendix E.

This section indicatesthat you must

complete a scoredactivity and submit it

for correction.

You must now do Scored Activity 1.

When you �nish, hand it in to your instructor, or send it to your tutor in keeping with thearrangements made when you enrolled.Note: If you do not have the scored activities, you can download them from: http://cours1.sofad.qc.ca/ressources under “Diversi�ed Basic Education.”

2

1

3

4

5

6

Ever Faster

Measurable Equilibrium

Acids and Bases

When Speed Equals Progress

The Quest for Equilibrium

Action – Reaction

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Ever Faster

1.1 Molar Flow Rate

Goals • To define the rate of reaction and to express it appropriately in a given situation• To use a graph to represent the progress of a chemical reaction

1.2 Speed: A Question of Taste

Goals • To determine that the rate of a chemical reaction may be influenced by different factors and to name these factors

• To understand that, depending on the situation and type of reaction, other factors may affect the reaction rate

Your task

In a situation involving the Maillard reaction, you will:• define the factors that influence the rate of the browning reaction when meat is cooked;• understand how these factors affect the reaction kinetics;• make recommendations.

1.3 How Do You Like Them Apples?

Goal • To study the different factors that affect the rate of reaction

Your task

• As a participant in a baking contest, you will prepare a comparative table to help you choose the best conditions for keeping the apple wedges for a pie from turning brown.

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Rate of reaction is a very important concept in a number of different fields. For

example, in industry, the rate of various processes is controlled in order to reduce

production costs. In the agri-food industry, for example, it is necessary to control

the growth rate of certain microorganisms in order to make foods such as cheese. And, in

the biomedical industry, how fast a disease spreads and how quickly a drug takes effect is

crucial information that needs to be taken into account. In this learning sequence, you will

be asked to define the concept of rate (or speed) of reaction and to identify the various

factors that affect rates of reaction.

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En route to Scored Activity 1

You will be required to do Scored Activity 1 once you have completed learning sequences 1 and 2. This

scored activity includes a learning and evaluation situation, which involves completing the following

task.

Get the Disinfectant!

Goal

• To apply problem-solving skills and knowledge of the rate law

Unfortunately, hospitals are great breeding grounds

for bacteria and pathogens of all kinds. Disinfection

and sterilization in healthcare facilities are therefore

of the utmost importance.

A number of companies specialize in the production

of hospital-grade disinfectants, which are used

for general maintenance in healthcare facilities,

the sterilization of surgical instruments and hand

disinfection.

Chlorine and iodine are widely used in hospitals

for their excellent disinfectant and sterilizing

properties. These elements combine with the organic molecules present in all living things by replacing

hydrogen molecules, making it impossible for pathogens to survive.

You work for a company that makes disinfectants and sterilizers for hospitals. Sodium hypochlorite

(NaClO), more commonly known as Javel water, is one of the products that the company makes.

This compound reacts rapidly with hydrogen peroxide (H2O

2), a substance produced by the body

to combat anaerobic (in the absence of oxygen) bacterial infections. In addition, chlorine inhibits

bacterial growth.

Similarly, the disinfecting power of iodine could be exploited by replacing sodium hypochlorite

with hydroiodic acid (HI). Because HI is a strong acid and relatively unstable, it is not used in the

manufacture of disinfectants and sterilizers. All the same, it is interesting to compare the chemical

kinetics of this reaction with the kinetics of the reaction involving Javel water.

© Samrith Na Lumpoon/Shutterstock.com

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Your task

You will:

• study the kinetics of the reaction between hydrogen peroxide and hydroiodic acid;

• compare the kinetics of this reaction with the kinetics of the reaction between hydrogen

peroxide and sodium hypochlorite.

Take the time to familiarize yourself with the content of Section B of Scored Activity 1 now. Then, answer

the following questions as best you can. If you do not have a paper copy of the scored activity, go to the

“Diversified Basic Education” section of the SOFAD website, at http://cours1.sofad.qc.ca/ressources.

In this learning and evaluation situation, you will determine the factors that affect chemical reactions.

You will also derive the expression of the rate law, its rate constant and the order of reaction with

respect to each reactant.

How would you define the rate of a chemical reaction?

Given that the starting concentration of the reactant(s) greatly affects the rate of a chemical reaction,

how would you go about studying its influence on the reaction rate?

Two factors that affect the rate of reaction will be examined in Scored Activity 1. In your opinion,

besides the starting concentration of the reactants, what is the second factor?

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Exploration Activity

The following questions will enable you to test your knowledge of concepts that will be useful in this

learning sequence.

Explain what happens to water molecules when water boils. In your answer, try to include at least one

reference to chemical kinetics or the speed of molecular collisions.

Give an example of speed with respect to the following.

a) Computers:

b) Food:

c) A fairground:

Give an everyday example of a reaction rate that is:

a) slow:

b) fast:

Name a factor which, in your opinion, could affect the speed at which a chemical reaction occurs.

Given that the speed of a car or train is expressed in km/h and that the knot (or nautical mile per hour)

is the unit of speed used in maritime navigation, give an example of a unit that denotes the rate of a

chemical reaction.

Given the combustion reaction of carbon (C(s)

+ O2(g)

CO2(g)

) and given that carbon burns at a rate

of 85 g/min, answer the following questions:

a) How long will it take for 500 g of carbon to burn?

b) Convert the rate of combustion of carbon to mol/h.

c) What is the speed of formation of CO2?

1.1

1.2

1.3

1.4

1.5

1.6

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Goals

• To define the rate of reaction and to express it appropriately in a given situation

• To use a graph to represent the progress of a chemical reaction

The transformation of matter involves a rearrangement of atoms. In other words, the molecules of the

reactants decompose and rearrange themselves to form new compounds, or products. In general, in

a chemical reaction the characteristics and properties of the products are different from those of the

original reactants.

A chemical reaction is a process that involves the rearrangement of the atoms in

a substance. The original molecules, called the reactants, break apart to form new

chemical species, called the products, with properties that are specific to them.

There are several types of chemical reactions. Here are a few: precipitation, oxidation (or combustion),

decomposition, synthesis and neutralization.

Give an example of a balanced chemical equation for the following chemical reactions:

Type of chemicaL reacTion exampLe of a baLanced chemicaL equaTion

Precipitation

Oxidation

Decomposition

Synthesis

Neutralization

Activity 1.1 Molar Flow Rate

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During a chemical reaction, certain physical and chemical changes are visible to the naked eye.

For example, when yeast reacts the result is a cake or bread that rises in the oven. However, not all the

changes that occur in a chemical reaction are visible. Just because we do not see anything happening

does not mean that no reaction is taking place at the microscopic scale. The formation of greenhouse

gases in our atmosphere is an excellent example of this.

Give an example of a chemical reaction in which a change is visible to the naked eye and another

example where the opposite is true (i.e. nothing seems to be happening at the macroscopic scale, but a

reaction is occurring at the microscopic scale).

A burning candle is an example of a combustion reaction. Several signs tell us that a reaction is

occurring. For example, the amount of wax decreases (the candle becomes shorter) and the reaction

produces a flame (light and heat). Depending on the type of wax used, we can sometimes see

black smoke, or soot, rising above the flame. The signs that a reaction is in progress are therefore

numerous, and the candle’s rate of combustion can be measured by monitoring how these signs

change over time. For example, if we measured the height of the candle every minute, we would

be able to determine how quickly the wax was disappearing. We could also measure the amount

of soot formed, or calculate the amount of energy produced as a function of time. All of these

changes indicate that a reaction is occurring and that its progress over time can be tracked and

even quantified.

Expression of the reaction rate

The reaction rate (r) of a chemical process is determined by monitoring the change in a reactant or

product as a function of time. The physical state of the reactant or product tells us which parameter

should be measured in order to determine the reaction rate. Thus, if the compound is:

• a solid, we measure the change in mass (m) or number of particles (n).

• a liquid, we measure the change in mass (m), volume (V) or number of particles (n).

• a gas, we measure the change in mass (m), volume (V), concentration (c) or pressure (P).

• dissolved in aqueous solution, we measure the change in concentration (c).

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Studying the rate of a chemical reaction therefore involves studying a change. This can be done only if

the initial and final conditions of a system are known.

In science, the symbol for change is the Greek letter delta (D). For example, a

change in mass is represented by Dm, where m is the mass. Also, a change is

always calculated by subtracting the initial value from the final value. For instance,

a change in mass is calculated as follows:

Dm = mfinal

- minitial

or mf - m

i

Note

Do the following experiment, and then answer the questions below. Do your best to answer the

questions even if you cannot carry out the experiment. Pour a tablespoon of white vinegar (acetic

acid) into a transparent glass. Then add a quarter teaspoon of sodium bicarbonate (baking soda) to the

vinegar. You will immediately observe vigorous bubbling in the glass, which dies down gradually. After

a few minutes, there is no more activity in the glass. Gently swirl the glass to make sure there are no

more bubbles.

a) Did a chemical reaction take place? Explain.

b) Besides the bubbling in the glass, is there any other sign that a reaction took place? If so, what is it?

c) How long did the reaction last?

d) Suggest a way of measuring the rate of reaction.

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e) Did the reaction proceed at the same rate throughout the experiment? Explain.

In kinematics, speed is defined as the change in the position of a moving object per unit of time. In

chemical kinetics, the concept of speed is the same, that is, it involves a change over time of the

observable characteristics of a chemical reaction. At the beginning of this section, you saw that the

rate of a reaction can be characterized in terms of the observable quantities of either the reactants or

products. The following general definition takes this into account.

Reaction rate is a measure of the change in the quantity of reactants that are

consumed (in terms of mass, number of particles, volume, pressure or concentration)

or the quantity of products that form per unit of time.

Reaction rate may be expressed mathematically as follows:

In terms of the reactants that are consumed: r = -DreactantDtime

In terms of the products that form: r = DproductDtime

Note the negative sign when the reaction rate is defined in terms of the reactants disappearing, since

rate, or speed, can never have a negative value. Given that the reactants are used up in a chemical

reaction, the numerator is negative. However, we know that speed cannot have a negative value.

Carry out the following test: the next time you are a passenger in a car, observe the needle on the

speedometer when the driver backs up the vehicle. Does it point to a negative value? Does it go below

zero? Of course not! The same is therefore true of the rate of reaction. Simply multiply the rate by

-1 to obtain a positive value.

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Now that you are familiar with the definition of reaction rate, can you suggest a mathematical

expression for a given measure of quantity of matter? Find out by doing the following exercise.

a) First, indicate the appropriate unit for each quantity of matter given below.

number of parTicLes

mass VoLume pressure concenTraTion

Units

b) Suggest a mathematical relationship for the reaction rate as a function of the measured quantity of

matter. Assume that the time is in seconds. The first row of the table has been completed for you.

measured quanTiTy of maTTer

maThemaTicaL expression uniT

Number of particles r = Dnumber of particlesDtime

or r = n

t

DD

mol

s

Mass

Volume

Pressure

Concentration

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Reminder

Concentration and related units

Concentration (c) is a measure of the amount of solute contained in a given amount of solution.

It is a ratio comparing the quantity of solute to the quantity of solvent.

Concentration = Quantity of solute

Quantity of solution

The concentration of a solution varies with the relative amounts of solute and solvent present.

For example, if solute is added, the concentration will increase, and if solvent is added,

the concentration will decrease. This is the principle of dilution. Since there are different

ways of expressing the “amount” of a given substance, there’s more than one way to write a

concentration. Here are the most common ways.

• Molar concentration or molarity (M): The number of moles of solute (n) contained in 1 L

(or 1000 mL) of solution (mol/L).

Concentration = Number of moles of soluteVolume of solution

or c = n

V

• Concentration expressed in terms of mass and volume: Generally in g/L, but sometimes units

such as g/mL, mg/L or g/cm3 are also used. Remember that 1 mL = 1 cm3.

Concentration = Mass of soluteVolume of solute

or c = m

V

• Concentration expressed in parts per million (ppm): Number of units of solute found in

one million units of solution. The number of milligrams of solute found in 1 L of solution.

1 ppm = 1 g

1 000 000 mLx1000 mL

1 Lx1000 mg

1 g =

1 mg

1 L

• Concentration expressed as percent composition (%): The number of parts (V or m)

of solute found in one hundred parts of solution (V or m). In this case, concentration is

expressed as % m/V, % V/V or % m/m.

% m/V = Mass of solute100 mL solution

% V/V = Volume of solute100 mL solution

% m/m = Mass of solute100 g solution

Note: You will often see a compound name written in square brackets, such as [HCl]. The

square brackets indicate the molar concentration (M) of whatever is inside the brackets.

This example would be read as “The concentration of hydrochloric acid.”

We will now look at how these concepts are applied to problem solving. Let’s go back to Exercise 1.9.

Using water displacement, at constant temperature and pressure, it is possible to measure the amount

of carbon dioxide (CO2) that is formed in the reaction between acetic acid (CH

3COOH) and sodium

bicarbonate (NaHCO3).

CH3COOH

(aq) + NaHCO

3(aq) ® CH

3COONa

(aq) + CO

2(g) + H

2O

(l)

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After 2 minutes and 35 seconds, 37.9 mL of CO2 are collected. What is the rate of this reaction in mL/s?

r = V

t

DD

= f i

f i

V V

t t

--

= 37.9 mL-0.0 mL155 s-0 s

r = 0.245 mL/s

In determining the rate of the reaction between vinegar and sodium bicarbonate, what other factors

could you monitor besides the volume of CO2 produced? Name two.

To sum up, the rate of a chemical reaction can be determined by monitoring either the disappearance

of the reactants or the appearance of the products. Although various characteristics can be used to

track the progress of a chemical reaction, in general we observe changes in mass, pressure, volume or

concentration per unit of time.

Before continuing our study of reaction rate, let’s review some basic concepts we have seen so far.

Consider the combustion reaction of methanol (CH3OH).

a) What is the balanced chemical equation for this reaction?

Before you continue the exercise, make sure your chemical equation is correct.

b) The initial concentration of methanol is 0.25 mol/L. After 3.5 hours, the concentration of CH3OH is

no more than 0.070 mol/L. What is the rate of the combustion reaction of methanol in mol/L•h?

c) What would this rate be in g/L•s?

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d) The change in this reaction can also be tracked by monitoring the pressure of the CO2. Determine

the reaction rate given that the pressure of CO2 is 2.37 atm after 3.5 hours.

So far, we have focused mainly on chemical reactions with unitary stoichiometric coefficients (i.e. all

the reactants and products have coefficients of 1).

A + B ® C + D

What happens when the stoichiometric coefficients are not all equal to one, as in the previous exercise?

The reaction rate is, in fact, a proportion that is based on the stoichiometric coefficients.

For example, consider the following reaction:

H2(g)

+ Br2(g)

® 2 HBr(g)

The graph below shows the concentration of the reactants and products in this reaction as a function

of time. Note that the concentration of the products changes more rapidly than the concentration of

the reactants. This is explained by the fact that for each hydrogen molecule (or bromine molecule)

used up, two molecules of hydrogen bromide are produced. As you can see, the importance of correctly

balancing chemical equations cannot be overstated.

graph 1.1 – concentration of reactants anD proDucts versus time

Co

nce

ntr

atio

n (

mo

l/L

)

Time (s)

Concentrationof reactants

Concentrationof products

0 10 20 30 40 50

4.0

3.0

2.0

1.0

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The stoichiometric coefficients are very useful for determining the general reaction rate (rg); consider the

following general balanced equation:

aA + bB ® cC + dD

where rg is the general reaction rate in mol/L•s or any other

unit of concentration per unit of time,

rA, r

B, r

C and r

D are the reaction rates of reactants A and

B and products C and D respectively,

and a, b, c and d are the stoichiometric coefficients of the

reactants and products.

rg =

rAa

=

rBb

=

rCc

=

rDd

Assume that the previous general reaction rate is 0.25 mol/L•h. The reaction rate can be determined for

each reactant and product.

H2(g) + Br

2(g) ® 2 HBr(g)

rH2 = 1 ´ r

g rBr2 = 1 ´ r

gr

HBr = 2 ´ r

g

rH2 = 0.25 mol/L•h rBr2

= 0.25 mol/L•h rHBr

= 0.50 mol/L•h

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Reminder

Stoichiometry and stoichiometric calculations

Stoichiometry is the study of the quantities of reactants and products reacting together in a

chemical reaction. It tells us whether all the reactants have been used up in the reaction or

if there are some left over. The example below is a review of some basic concepts and how

stoichiometric calculations are carried out.

We want to determine how many grams of oxygen (O2) are produced when 7.3 g of lithium

peroxide (Li2O

2) are decomposed.

Li2O

2 ® Li

2O + O

2

1. Balance the equation.

2 Li2O

2 ® 2 Li

2O + O

2

2. Make a data table and record the known values.

3. Determine the molar mass (M) of each reactant and product.

4. Indicate in the table the data value(s) to be determined (in boldface in the table) and

solve the problem.

a) Determine the number of moles of Li2O

2:

2 2Li On = 7.3 g ´ 1 mol

45.88 g = 0.16 mol

b) Determine the number of moles of O2 by cross multiplication:

2On = 0.16 mol x 1

2 = 0.080 mol

c) Determine the mass of O2:

2Om = 0.080 mol ´ 32.00 g1 mol

= 2.6 g

baLanced chemicaL equaTion 2 Li2O

2 ® 2 Li2O + o

2

Number of molesStep 4a

0.16

Step 4b

0.080

Molar mass

(g/mol)

Step 3

45.88

Step 3

29.88

Step 3

32.00

Mass

(g)7.3

Step 4c

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Consider the combustion reaction of butane (C4H

10):

C4H

10(g) + 13

2 O

2(g) ®4 CO

2(g) + 5 H

2O

(g)

a) Assuming that the rate of formation of CO2 is 1.25 mol/s, complete the table below. Pay particular

attention to significant digits.

rC4H10rO2

rCO2rH2O r

g

b) Assuming that the general reaction rate is 0.67 g/h, complete the table below. Pay particular

attention to significant digits.

rC4H10rO2

rCO2rH2O r

g

Using graphs to represent reaction rates

Graphs are very useful for visualizing the progress of a chemical reaction. By way of example, let’s look

at the reaction in which cyclopentadiene (C5H

6), an insecticide, is transformed into an organic solvent

called “Tetralin” (C10

H12

).

2 C5H

6(g) ® C

10H

12(g)

In the reaction, 2 moles of reactant are transformed into 1 mole of product. In this case, it is advisable

to monitor the reaction by measuring the pressure. In the course entitled Chemistry: Gases and

Energy (CHE-5061-2), you saw that pressure is directly proportional to the number of moles of gas

(PV = nRT); this is the ideal gas law. In the reaction under study, as the number of moles of product

is smaller than the number of moles of reactant, the pressure of the gas produced (C10

H12

) increases,

whereas the pressure of the reactant and the total pressure in the system decreases as the reaction

proceeds.

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table 1.1 – partial pressure values anD total pressure of the reaction

graph 1.2 – total anD partial pressure curves versus time

Time (s)

PT

(mmHg)P

5 6C H (mmHg)

(mmHg)

0 500 500 0

10 434 368 66

20 396 292 104

30 370 240 130

40 353 206 147

0 5 10 15 20 25 30 35 40

500.0

400.0

300.0

200.0

100.0

Time (s)

Pre

ssu

re (

mm

Hg)

PC5H6

PC10H12

PT

In the graph, the uppermost curve represents the total pressure as a function of time. As expected,

this pressure decreases. Before the reaction begins (t = 0), only the reactants are present and the total

pressure equals the pressure of the C5H

6; the pressure of the C

10H

12 is zero. After a few seconds, the

C5H

6 molecules begin to react with one another to form C

10H

12. The pressure of the product increases

as the pressure of the reactant decreases. The total pressure decreases since for every 2 moles of

reactant consumed 1 mole of product forms. Thus, there are fewer molecules for the same volume

since volume and temperature are held constant during the reaction.

The shape of the curves shows that the pressures do not vary at a constant rate; consequently, the

reaction rate is not the same throughout the chemical reaction. The pressure of the C5H

6 decreases

rapidly at first and then more slowly. Conversely, the pressure of the C10

H12

increases rapidly at the

beginning and slows down as the reaction proceeds (the curve flattens out). From the viewpoint

of kinetics, this means that the rates at which the reactant disappears and the product appears are

fast at first, and then decrease gradually until they reach a stable value (but not necessarily zero).

Furthermore, if we were to extrapolate the data over a sufficiently long period of time, the curves

would become horizontal plateaus, indicating that the pressures eventually stabilize.

Note, however, that the sum of the partial pressures is always equal to the total pressure in the system

at any given time. The data table shows this clearly: at any given time, the sum of the last two columns

always equals the total pressure given in the second column.

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Chemical kinetics: a little history

The term “kinetics” comes from the Greek word kinêtikos, which means “moving,

putting in motion.” Kinêtikos is itself derived from kinêtos, which means “moveable.”

Of course, we do not observe the actual motion of chemical molecules. Rather, we

observe the signs that chemical compounds are being transformed. Chemical kinetics

can therefore be described as the study of the progress of a chemical reaction.

A number of important discoveries have shaped our current understanding of

chemical kinetics. Among other things, these discoveries have led to the elaboration

of mathematical expressions for reaction rate, the characterization of the rate of a

chemical reaction in terms of an order (first, second and third orders), and the study

of factors that influence the rate of a chemical process, including enzyme catalysis,

which is at the heart of human life. Great scientists such as Arrhenius, Van ’t Hoff and

Hinshelwood have contributed to this field, as illustrated below.

Did you know?

1818 1858 1885 1900 1909 1923

-Kirchhoff-

Decomposition of oxygenated

water

-Thénard-

-Wilhelmy-

-Harcourt- -Esson-

-Guldberg--Waage-

-Van ’t Hoff-

Thermochemistry of reaction rates

-Berthelot-

Theory of ions

-Arrhenius-

Notion of catalyst

-Ostwald- -Brønsted-

Acid catalysis

1812 1850 1867 1889 1907 1913 1930

-Arrhenius-

Presence of radicals in

the synthesis of hydrogen

bromide

-Bodenstein-

Enzymecatalysis

-Michaelis-

Homogeneous and

heterogeneous catalytic

reactions in liquid phase

-Hinshelwood-

Rate law as a function

of temperature

Equality of rates of

forward and reverse

reactions

Glucose inversion as a

function of concentration

Rate of starch

hydrolysis

Action of hydrogen iodide on

oxygenated water

Change in the equilibrium

constant as a function of

temperature

Time scale of some of the discoveries that have contributed to chemical kinetics as we know it today.

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Chemical kinetics: a little history (cont.)

One of the scientists who contributed to the development of chemical kinetics was

Dr. Edgar William Richard Steacie (1900–1962), a Canadian. Born in Montreal,

Steacie studied at McGill University. In 1926, he obtained a doctorate degree after

specializing in physical chemistry under the guidance of Professor Otto Maass, and

then taught in the chemistry department of that university until 1939. He became the

director and then the president of the National Research Council (NRC), where he

set up the prestigious E.W.R Steacie Fellowships, a postdoctoral scholarship program.

From the very beginning of his career, Steacie was interested in chemical kinetics

and coninued his research in this field. He is

responsible, among other things, for major

advances in the nature of chemical reactions

(presence of intermediates, atoms and

free radicals), as well as the development

of modern photochemistry. He received

numerous prizes and and honorary awards for

his contribution to science. In recognition of

his commitment to encouraging young people

to pursue a career in science, each year since 1964

the E.W.R. Steacie Foundation awards the Steacie

Prize to a young scientist under 40 years of age

who has made a significant contribution to research in Canada.

Did you know?

© Museum of Science and Technology of Canada

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Exercises for Activity 1.1

The concept of reaction rate is implied in a number of everyday words. Complete the following table by

explaining how each term is related to the concept of reaction rate. You may use a dictionary or search

the Internet to help you complete this exercise. The first term has been defined for you.

Term definiTion

ExplosionA phenomenon in which gases under pressure are produced in a very short amount

of time. An explosion is a very rapid reaction.

Detonation

Deflagration

Rapid

combustion

Different methods may be used to measure the rate of a reaction, depending on whether the reaction is

slow or rapid. For each reaction described in the left-hand column of the table below, suggest a method

of measuring the reaction rate. Two rows have been completed for you.

sLow reacTions descripTion

Combustion of butane in

a lighter

The reaction is continuous and visible (fire). Its progress could be

measured by recording the amount of butane left in the lighter after each

passing minute.

Combustion of fuel oil

(heating oil)

Formation of rust

on the surface of a piece

of iron

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rapid reacTions descripTion

Mixing equal parts of

the two component

solutions of an epoxy

glue*

The hardening of the mixture and the fact that the glued pieces are

difficult to separate indicate that a reaction has occurred. The reaction

rate could be measured by using a series of identical assemblies consisting

of two boards stuck together with the same amount of epoxy glue. The

boards in each assembly are then pulled apart every 10 seconds and the

force needed to do so is recorded. The change in the amount of force

required as a function of time corresponds to the reaction rate.

A burning match

* Epoxy glue is activated when its two components are mixed. The two components are inactive when separate.

The basic hydrolysis of an ester always produces a carboxylate and an alcohol in the same way as a

reaction between an acid and a base always produces a salt and water. When the following reaction

occurs at room temperature, the alcohol (CH3OH) concentration reaches 1.21 mol/L in 42 minutes.

Calculate the reaction rate for the alcohol.

CH3CH

2COOCH

3(aq) + NaOH

(aq) ® CH

3CH

2COONa

(aq) + CH

3OH

(aq)

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At a given temperature, the rate of decomposition of nitrosyl chloride (NOCl) is 9.67´10-9 mol/L•s.

How long does it take for the chlorine concentration to reach 2.14´10-7 mol/L?

2 NOCl(g)

® 2 NO(g)

+ Cl2(g)

Consider the decomposition of dinitrogen pentoxide at 50ºC:

2 N2O

5(g) ® 4 NO

2(g) + O

2(g)

After 2 hours, the initial N2O

5 concentration of 7.33 ppm is 1.75 ppm.

a) Calculate the rate of disappearance of N2O

5 over this time interval in ppm/h and in ppm/min.

b) Calculate the rate of formation of NO2 in ppm/s over the same time interval.

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Alicia is a scientific consultant for a natural gas company. Her research team has just developed a new

device. To check its calibration, Alicia must determine the rate of formation of carbon dioxide (CO2)

and water (H2O) during propane (C

3H

8) combustion.

a) Write the balanced chemical equation for propane combustion. Check your answer before you go on.

b) Given that the rate of disappearance of propane is 1.35 ´ 10-3 mol/L•s, what is the rate of formation

of carbon dioxide and water?

c) Given that the rate of oxygen (O2) consumption is 5.02 kPa/min, what is the rate of formation of

carbon dioxide and water?

d) Assuming that 0.56 mL of water is produced every hour, what is the value of the general

reaction rate?

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Matthew carried out a kinetics study to determine the rate of a chemical reaction, and obtained the

following results.

concentration of reactant or proDuct versus time

Time (min)

C

on

cen

trat

ion

(m

ol/

L)

1.4

1.2

1.0

0.8

0.6

0.4

0.2

1.6

1 2 3 4 5 6 7 80

a) In your opinion, does this graph represent the disappearance of a reactant or the appearance of a

product? Explain.

b) Is the reaction rate the same throughout the reaction? Explain.

c) What is the rate in the interval between 2 and 6 minutes?

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The rate of the cyclopentadiene reaction was calculated at 10-second intervals. The results are shown

below in table and graph form. The times indicated in the first column of the table correspond to the

midpoints of the time intervals. For instance, the average reaction rate in the interval between 10 and

20 seconds after the start of the reaction is observed at t = 15 s.

table of reaction rates for the Disappearance of c5h6 anD appearance of c10h12

rate of Disappearance of c5h6 anD appearance of c10h12 versus time

Time (s)

raTe of disappearance

of c5h

6

(mmHg/s)

raTe of appearance of c

10h

12

(mmHg/s)

5 13.2 6.6

15 7.6 3.8

25 5.2 2.6

35 3.4 1.7

0 10 20 30 40 Time (s)

R

eact

ion

rat

e (m

m H

g/s)

C5H6

C10H12

12.0

10.0

8.0

6.0

4.0

2.0

14.0

50 60 70

a) Explain why the results obtained in the graph are entirely consistent with what happens at the

molecular level as the reaction proceeds.

b) Extrapolate the curves in order to show that the reaction rate approaches zero after a sufficiently

long period of time. Indicate your answer directly on the graph.

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c) Give the mathematical expression for the rate of disappearance of C5H

6 and for the rate of

appearance of C10

H12

.

d) What is the general reaction rate for each time value shown in the following table?

Time (s) 5 15 25 35

rg (mmHg/s)

Recap of Activity 1.1

We monitor a chemical reaction by measuring a particular characteristic of the reactants or products at

regular time intervals. Most often, we measure either concentration or pressure. On a graph, curves for

concentration as a function of time slope downwards for the reactants and upwards for the products.

The rate of a reaction refers to the rate of disappearance of the reactants or the rate of appearance of

the products. Depending on the substance involved, the values change, but in fact, they represent the

same reaction. The reaction rate versus time curves show that a reaction is faster at the beginning,

and that its rate decreases as the reaction proceeds. When all of the reactants have disappeared, the

reaction stops and the reaction rate is zero.

A chemical reaction is therefore a dynamic process that changes over time, and reaction rate

measurements are indicators of this change. A number of factors affect the rate of reaction, and

these can be modified in order to control the reaction. We will study these factors in the next section.

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Goals

• To determine that the rate of a chemical reaction may be influenced by different factors and

to name these factors

• To understand that, depending on the situation and type of reaction, other factors may

affect the reaction rate

More than 100 years ago, Louis-Camille Maillard carried out

research to gain insight into the reactions that occur when a

mixture of sugars and proteins is heated. Although the main

thrust of his work was medical, focusing in particular on

diabetes, he also contributed to culinary science since the

mechanism he discovered explains why food generally tastes

better when cooked. This mechanism, which occurs in three

stages, is responsible for the characteristic flavour of roasted

meat and other foods.

The Maillard reaction has many applications. Besides the

culinary field, which provides many examples of the mechanism

involved, Maillard’s findings were also beneficial to research on

diabetes and aging, as well as to the petroleum and agri-food

industries.

The kinetics of the Maillard reaction may be controlled by

changing several factors, such as temperature or the types

of sugars involved. Controlling the reaction kinetics is very

important given the associated effects on the preservation,

taste and appearance of foods.

As a stylist for a food magazine, you are asked to prepare some pieces of beef that will be featured

in a photo shoot. Preparing the meat involves browning it by searing it briefly in the oven at a high

temperature. However, you realize that not all the pieces of beef are nicely browned and that the

cooking time varies from one piece to another.

Activity 1.2 Speed: A Question of Taste

© Wikimedia Commons

French chemist Louis-Camille Maillard

(1878–1936) was also recognized for his work

on urea metabolism and kidney disorders.

The basic principles of his work are still used

today to diagnose kidney diseases.

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Your task

In a situation involving the Maillard reaction, you will:

• define the factors that influence the rate of the browning reaction when meat is cooked;

• understand how these factors affect the reaction kinetics;

• make recommendations.

An overview of the Maillard reaction

Before we continue, let’s take a brief look at the Maillard reaction mechanism which comes into play

when meat is cooked. This mechanism occurs in three stages.

The first stage involves the condensation of an amino acid (amino acids are the molecules that make up

food proteins) with a simple sugar. The product of this condensation is glycosylamine.

Rsugar CHO + RA.A. NH2

(sugar) (amino acid)

H

C

N

+ H2O

Rsugar

RA.A.

Not all the molecules react with one another. A part of the sugar, the CHO group, reacts only with the

amine group (NH2) of the amino acid. In the diagram, R represents the part of the molecule that does

not react in the chemical reaction. It may be composed of carbon chains or various other functions; the

exact nature of this group is not important for the purposes of this activity.

In the second stage, glycosylamine undergoes a series of molecular rearrangements which will not be

considered here. Lastly, the third stage in the Maillard reaction is complex. It is important to keep in

mind that caramelization occurs at this stage; in other words, the meat browns.

This a simplified description of the Maillard reaction mechanism which, in reality, is much more

complex. For the purposes of this activity, however, we will concentrate only on the first stage, which

involves condensation. The rate of condensation may be altered by modifying certain variables external

to the reaction. As a result, the kinetics of the mechanism will also change.

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Nature of the reactants

Name two categories of reactants involved in the Maillard condensation reaction.

In your opinion, if a sugar was replaced by another sugar of the same type or an amino acid was

replaced by another amino acid, how would this affect the rate of reaction?

Even though the same sugar group reacts with the same amino acid group, the type of sugar or amino

acid involved nonetheless has an influence on the reaction rate. If one of these reactants is replaced

with a similar compound, the reaction rate will change.

For the vast majority of chemical reactions, replacing one reactant with another of the same type

(e.g. an alcohol with an alcohol that has a bulkier molecular structure) greatly affects the equilibrium of

the reaction and its kinetics. The nature of the reactants is therefore crucial. We will look at this now.

Sugars

A sugar is a chemical compound that is soluble in water and that has a sweetening effect. Saccharose

is the most abundant type of sugar; however, the term “sugar” denotes any type of compound whose

name ends in the suffix “-ose.” These compounds are carbohydrates containing a minimum of three

carbons; the prefix in the name of the sugar indicates the number of carbon atoms in the sugar.

table 1.2 – name anD chemical formula of selecteD “-ose” compounDs

name Triose TeTrose penTose hexose hepTose nonose

General

formulaC

3H

6O

3C

4H

8O

4C

5H

10O

5C

6H

12O

6C

7H

14O

7C

9H

17N

1O

8

In the specific case of the Maillard condensation reaction, it has been shown that the larger the sugar

molecule, the slower the reaction. We will look at why later.

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For each pair of sugars below, choose the molecule that will produce the fastest reaction.

a) Tetrose and triose:

b) Hexose and heptose:

c) Nonose and pentose:

For reasons we will not go into here, you should know that the Maillard condensation reaction is

possible only with pentose (5 carbons) and hexose (6 carbons) sugars.

Which of the following two sugars that may react in a Maillard reaction would produce a faster

reaction? Explain briefly.

HC CH

OH2C

CHHC

OH

HC CH

O

CH2OH

CHHC

HC

(1) (2)

HO HO

OH

OH

OH

OH

OH

Amino acids

Amino acids are essential to life as we know it. There are 20 amino acids, and they are the main

components of proteins and enzymes. All amino acids have the same general H2N–CHR–COOH

structure: an amine group (NH2), a carboxyl group (COOH) and a lateral chain (R) between the two

which is specific to the amino acid in question. This lateral chain is composed mainly of carbon, oxygen,

hydrogen and nitrogen.

Unlike sugars, the size of the molecule does not matter in the case of amino acids. Rather, what is

important is the distance between the carboxyl group and the amine group. The length of the lateral

chain therefore plays a key role with respect to the reaction rate since the longer the chain, the greater

the distance between the carboxyl group and the amine group.

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Given that the carboxyl (COOH) group acts as a natural inhibitor of the reaction, answer the following

questions.

a) Define, in your own words, the term “reaction inhibitor” by taking into account the kinetics of a

chemical reaction.

b) Complete the following sentence by indicating whether the reaction will be faster or slower:

The further the amine group is from the carboxyl group, the the reaction will be.

Since it is the amine group that reacts with the sugar molecule, it must be as far as possible from

the carboxyl group in order to limit its inhibiting effect. In this specific case, the reaction inhibitor

hinders the meat’s browning reaction. However, the inhibitor should not necessarily be seen as a

negative factor. In fact, it could be advantageous to slow down a chemical reaction or even to stop

it. For example, the de-icing salt we use in winter is the main cause of the premature corrosion of

reinforced concrete infrastructures (e.g. bridges, retaining walls, roads, dams). In order to slow down

or control corrosion of these infrastructures, different methods are employed, including the use of

“corrosion inhibitors.” When added to concrete as it is being prepared, these reaction inhibitors help to

prevent water infiltration and slow down the reaction between water and the steel mesh in reinforced

concrete. Ash may also be added to concrete in order to slow down the exothermic reaction between

water and concrete, thus preventing cracks from forming when the concrete is poured.

The principles we have just seen regarding the nature of sugars and amino acids may apply to all

substances and to chemical reactions in general. The rate of a chemical reaction depends on the

nature of the reactants. This is not surprising if we consider that a reaction involves a rearrangement

of atoms in which old bonds are broken and new ones, different from the original bonds, are formed.

Each substance is characterized by unique molecules composed of atoms bonded together with varying

degrees of strength. Consequently, because of the specific nature of the reactants, each reaction has its

own unique characteristics.

The nature of the reactants affects the reaction rate. The rate of a reaction

generally decreases with the number, or the strength, of the bonds to be broken.

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Some fries with that?

There is nothing better to eat with steak than

some crisp golden fries! Did you know that

when potatoes are fried or cooked at high

temperatures, acrylamide, one of the compounds

generated during the Maillard reaction, is

produced? Recent studies have shown that

exposure to acrylamide increases the risk of

certain types of cancers in animals.

Researchers have therefore been working to genetically modify the Simplot potato

(derived from the Idaho potato) so that it produces less asparagine, a precursor to

acrylamide, during its growth. They have succeeded in doing this without sacrificing

either colour or taste. Cultivation of this genetically modified potato was approved

in November 2014 by the U.S. department of agriculture and its production is very

limited for the time being.

So what do we do while we wait for these potatoes to be available here? Should

we stop eating potatoes? Not at all! According to Stanley Omaye, a nutritionist and

toxicologist at the University of Nevada in the United States, the risks to human

health of exposure to acrylamide are not significant. In fact, as long as you maintain

healthy eating habits, eat a variety of different foods and follow the recommendations

in the Canada Food Guide, indulging in some delicious fries from time to time is not

harmful to your health.

Did you know?

© Nitr/Shutterstock.com

Reactant concentration

The concentration of the reactants always influences the rate of a chemical reaction. In general, the

rate of reaction is directly proportional to concentration. The following explanation will help us to

understand why. Molar concentration (c), or molarity, is defined as the number of moles of a solute (n)

dissolved per litre of solution (V), or c = n/V. Thus, the more concentrated a solution, the closer

together the molecules in the reactants are and the greater the chances that they will collide with one

another. The reactant molecules must collide in order for a reaction to occur, and those collisions that

trigger the reaction are called effective collisions. We will take a brief look at the collision theory of

reaction rates in the next learning sequence.

In general, the higher the concentration of the reactants, the faster the rate of

reaction, since the frequency of effective collisions is increased over a given period

of time.

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Reactant concentration may be altered in two ways: either by increasing or decreasing the mass of

reactants or by increasing or decreasing the volume of solvent. For example, in the condensation stage

of the Maillard reaction, you no doubt noticed that water (H2O) is one of the products of the reaction.

However, water is also the solvent in which the reactants are dissolved and the reaction occurs. You will

have guessed that the water content of the food being cooked or the presence of moisture during the

cooking process greatly influences the rate of the condensation reaction, thereby affecting the overall

rate of the Maillard reaction. Research has shown that a water content of between 30% and 60% is

ideal; above or below these values, the condensation reaction is inhibited and the reaction rate greatly

diminished; the reaction may even be halted.

Which factor affects the rate of a reaction involving gaseous reactants in the same way that

concentration affects the rate of a reaction involving liquid reactants? Why?

To observe how water content comes into play when you cook meat, try this: cook two meat patties in

a hot pan at medium heat. Allow one patty to cook by itself without touching it, then flip it over after

about two minutes. Press down on the other patty with a spatula every 30 seconds so that the juices

(essentially water) come gushing out; flip it over after two minutes. You will notice that the patty that

you pressed down is more carbonized (blackened) than the other patty, which is not what you want in

a burger!

In the next learning sequence, we will look more closely at how reactant concentration affects the

rate of reaction by examining the mathematical expression of the rate law.

Temperature

You know intuitively that a rise in temperature generally results in a faster reaction rate. Conversely, a

drop in temperature generally results in a slower reaction rate. Thus, when preparing the pieces of beef

for the photo shoot, it will be impossible to brown them if the oven temperature is too low. However,

if the oven is too hot, this will speed up the reaction rate to such an extent that the pieces of beef will

burn rather than brown. You have to strike the right balance.

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Everyday life is full of examples of how temperature affects the rate of reactions. Give an example

where a change in temperature slows down the reaction rate and another in which it speeds up the

reaction rate.

Example 1:

Example 2:

Tip

As a rule of thumb, reaction rates for many reactions double for every 10°C increase in

temperature.

What happens at the molecular level? According to the kinetic molecular model of matter,

temperature is a measure of how agitated the molecules are. A rise in temperature increases

molecular motion and, conversely, a drop in temperature decreases molecular motion. Consequently,

changes in temperature will influence the rate of chemical reactions. The following graph clearly

shows the effect of temperature on the reaction rate.

graph 1.3 – curves of the Distribution of the kinetic energy of particles at two Different temperatures

Kinetic energy

Nu

mb

er o

f p

arti

cles T1

T2

Ea

The above graph shows that for a given temperature, some particles are more energetic than others.

We can take the average of these energy values to obtain the average kinetic energy. By increasing

the temperature from T1 to T

2, the average kinetic energy of the particles increases; a larger number

of particles now have the energy needed to react, that is, their energy is equal to or greater than the

activation energy of the reaction.

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a) In the grid below, draw a graph of the curves of the distribution of kinetic energies of the particles

which shows a drop in temperature (i.e. T1 > T

2) with respect to the activation energy (E

a).

b) In your own words, explain what is happening in the distribution curves you have drawn.

The higher the temperature, the greater the number of molecules with the

minimum required energy to react. Consequently, an increase in temperature results

in a faster reaction rate.

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Catalysts

Some metals, such as copper, iron and manganese, are known to alter the Maillard condensation

reaction. For safety reasons, nothing is added to meat in order to speed up or slow down the browning

reaction. However, because iron is naturally found in meat, we can conclude that the quality of the

meat will directly affect the rate of the browning reaction; good quality meat contains more iron than

meat of lower quality.

a) What do we call a substance that is added to reactants in order to:

speed up the reaction rate:

slow down the reaction rate:

b) Copper, iron and manganese remain intact once the condensation reaction has occurred. True or

false?

c) What reaction variable is changed by the presence of these metals?

a) Given that the metals copper and iron accelerate the rate of the Maillard condensation reaction,

what happens to the reaction progress curve if one of these catalysts is added to the reaction? Draw

this new curve in the graph below.

Reaction progress

En

ergy

(k

J/m

ol)

Ea

DH

Reactants

Products

Reaction without a catalyst

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b) If the presence of manganese slows down the reaction, how can this be represented in a diagram?

Reaction progress

En

ergy

(k

J/m

ol)

DH

Reaction without an inhibitor

Reactants

Products

Ea

How can the effect of a catalyst on a reaction’s activation energy be represented on the curve of the

distribution of kinetic energies of particles? First, indicate the activation energy Ea on the curve and

then indicate the activation energy in the presence of a catalyst Ea (cat.)

.

Kinetic energy

Nu

mb

er o

f p

arti

cles

A catalyst is a substance that increases the rate of a chemical reaction without

itself taking part in the reaction. It facilitates the interaction between the reactants

by lowering the activation energy of the system. Conversely, a reaction inhibitor

slows down the rate of reaction, without itself taking part in the reaction.

Enzymes are large protein molecules whose shape contributes to the catalysis of biochemical processes

in living organisms. Most of the reactions occurring in our bodies would be impossible without

enzymes. Ptyalin, which is present in saliva, is a good example. The ptyalin molecule speeds up the

digestion of the starch contained in bread and other starchy foods. It facilitates the decomposition of

starch into sugar, a process which could take weeks to complete without a catalyst. In the presence of

ptyalin, starch is decomposed in a matter of seconds. So far, approximately 15 000 enzymes have been

identified in the human body.

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Enzyme and reactantsProduct of the

reaction

Enzyme

Reactants

Enzyme

An enzyme at work. The enzyme makes it easier for the reactants to join together; its presence speeds up the reaction.

Besides their role in the catalysis of biological processes in the human body, enzymes are very useful

proteins in a number of other areas. Examples of this include the bleaching process in the pulp and

paper industry, detergents, the yeasts used to make bread and pastries, and even the strips that come

with glucometers used by diabetics to measure blood sugar levels.

Besides enzymes, effective catalysts include some organic and inorganic compounds. For example, in

the upper atmosphere, chlorine catalyzes the conversion of ozone (O3) to oxygen (O

2). This chlorine

comes mainly from freon (CF2Cl

2), a gas widely used as a refrigerant and which is released into the

atmosphere when refrigeration systems break down. Freon molecules absorb the Sun’s ultraviolet

rays, decompose and release chlorine atoms (Cl). The chlorine atoms are then ready to catalyze the

decomposition of ozone molecules. The following equations describe this reaction:

Stage 1: O3 g( ) + Cl g( )catalyst( )

® ClO g( ) + O2 g( )

Stage 2: ClO g( ) + O g( ) ® Cl g( )catalyst( )

+ O2 g( )

Overall reaction: O3 g( ) + O g( ) ®→ 2 O2 g( )

Thus, chlorine atoms speed up the decomposition of the ozone layer that protects us from the Sun’s

ultraviolet rays. In addition, since the chlorine remains intact, a single chlorine atom can catalyze

the decomposition of a large number of ozone molecules. This explains why the use of freon is

now regulated.

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The catalytic converter is another example of catalysis. The inside surface of the catalytic converter

in the exhaust system of an automobile is treated with a mixture of platinum, rhodium and palladium.

The nitrogen oxide molecules (NO), which are produced during the combustion of gasoline in the

engine, pass through the converter and adhere to its surface. The fact that they are slowed down and

brought closer together facilitates the transformation of NO into the harmless gases O2 and N

2, which

are then released into the air.

NO

Reactants Products

Catalytic surface of platinum, rhodium and palladium

NOIntermediate state

N2

O2

Figure 1.1 A diagram showing how a catalytic converter works.

Surface area

As you prepare to cook the beef, you notice that you have three different cuts of meat, each of a

different size and thickness. However, you have the same quantity of meat for all three cuts. The beef

cubes weigh the same as the steaks, which weigh the same as the roast.

© isantilli/Shutterstock.com © Rudchenko Liliia/Shutterstock.com © Kelvin Wong/Shutterstock.com

Beef cubes Beef steaks Roast beef

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What can you say about cooking (browning) the three cuts of meat in the oven, assuming that the oven

temperature and the cooking time is the same for all three?

Assuming that all the other factors that affect the reaction rate are held constant, what conclusion can

you draw about the effect of surface area on the cooking time for the three cuts of meat?

Increasing the surface area of the reactants increases the reaction rate. In other

words, the greater the surface area, the faster the rate of reaction.

Your recommendations

Now that you are familiar with the factors that influence the rate of the Maillard condensation

reaction and, therefore, the way in which the different cuts of meat cook, you will make and justify

your recommendations for nicely browning the meat. Your recommendations must take into account

these factors with respect to the cut of meat and the quality of meat to be used. Draw up your

recommendations by completing the following table.

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facTors affecTing The reacTion raTe

recommendaTions

Nature of the reactants

Reactant

concentration

Temperature

Catalyst

Surface area

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Exercises for Activity 1.2

For each of the following pairs of chemical reactions, determine which one occurs more rapidly, and

explain your choice.

a) A banana ripening on the counter.

A banana ripening in the fridge.

b) A brick house on fire.

A straw hut on fire.

c) An acetaminophen tablet assimilated by the digestive system.

Acetaminophen granules assimilated by the digestive system.

d) 2 H2O

2 ® 2 H

2O + O

2

2 H2O

2

2Fe +

¾¾¾® 2 H2O + O

2

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e) CH3COOH aq( )Diluted solution

+ NaHCO3 s( )Cubes of 1 cm × 1 cm

→ CH3COONa

(aq) + CO

2(g) + H

2O

(l)

CH3COOH aq( )Concentrated solution

+NaHCO3 s( )Fine powder

® CH3COONa

(aq) + CO

2(g) + H

2O

(l)

State how an increase in temperature affects the following:

a) The activation energy of a reaction

b) The kinetic energy of the reactants

c) The reaction rate

Consider the following reaction which occurs at room temperature:

PbI2(s)

+ 2 KNO3(aq)

® Pb(NO3)

2(aq) + 2 KI

(aq)

For each of the following changes, state in what way the reaction rate will be affected: it will increase, it

will remain the same or it will decrease. Explain briefly.

a) Water is added to the reaction mixture.

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b) PbI2 granules are crushed into a fine powder.

c) The reaction mixture is heated to 35°C.

d) A few milligrams of solid KNO3 (fine powder) are added to the reaction.

e) The KNO3 solution is replaced with a NaNO

3 solution.

Complete the following table by indicating, for each situation described, the factor(s) that affect(s) the

reaction rate.

siTuaTionfacTor(s) affecTing The

reacTion raTe

Gold jewellery oxidizes more slowly than silver jewellery.

Diced potatoes cook more quickly than whole potatoes.

Food in the stomach is digested more readily if specific enzymes

are present.

Ammonia synthesis occurs only at high temperatures (400-500°C)

and in the presence of activated iron.

It would be impossible for the Dead Sea, which is highly saline, to

freeze, even at very low temperatures.

A forest fire spreads more quickly in summer than in winter.

Foods cook faster in a pressure cooker.

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Recap of Activity 1.2

The rate of a chemical reaction depends on a set of factors which can be modified in order to control

this rate. These factors are the nature of the reactants, concentration (or pressure in the case of a gas),

temperature, surface area and presence of a catalyst (or reaction inhibitor).

An increase in concentration, a rise in temperature or a larger surface area will speed up a chemical

reaction. Conversely, a decrease in concentration, a drop in temperature or a smaller surface area will

slow down a chemical reaction.

Catalysts influence the rate of a reaction. In general, catalysts are used to increase the reaction rate.

However, they can also be used to slow down reactions and, in this case, they are called reaction

inhibitors. Catalysts are extremely useful in industrial chemistry because they reduce production costs.

Catalysts involved in reactions occurring in nature are called enzymes. These natural catalysts speed

up biochemical reactions.

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Goal

• To study the different factors that affect the rate of a chemical reaction

In the previous activity, you studied the different factors that affect the rate of reaction. You have

entered a baking contest competition, and your task is to make a winning apple pie. Your challenge is to

make sure that your apple wedges do not brown.

Like several other fruits and vegetables, apples turn brown (or oxidize)

when exposed to air once they have been peeled and cut. Apples oxidize

upon contact with the oxygen (O2) in air, but also under the action of an

enzyme. When we peel, or cut an apple into pieces, the cellular membrane

tears, releasing and dispersing the enzyme that reacts with the proteins in

the flesh of the apple, which turns brown.

You do some research on the Internet and find out that there are several

ways to prevent the apple wedges from turning brown. These methods are all different and involve the

various factors that affect reaction rates, which were covered in the previous activity. You want to test

some of these methods.

Your task

• As a participant in a baking contest, you will prepare a comparative table to help you choose

the best conditions for keeping the apple wedges for a pie from turning brown.

To do this activity, refer to the experimental activity booklet that came with this guide. When you have

completed the activity, answer the following questions.

Activity 1.3 How Do You Like Them Apples?

© Valery121283/Shutterstock.com

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Oxidation in food is not always an undesirable effect. In some cases, it is actually welcomed. Give an

example.

Other factors, which are specific to certain types of reactions, may also affect the reaction rate. Can you

name two?

A chemical reaction is a complex process which can be characterized by the rate at which it proceeds.

All chemical reactions proceed quickly at first and then slow down. Chemical reactions are therefore

dynamic processes. It is possible to alter the rate of a chemical reaction by changing one of the

following factors: the nature of the reactants, concentration (or pressure in the cases of gases),

temperature, surface area and presence of a catalyst.

Knowing these factors, there are several signs that can help us determine the rate of a chemical

reaction. However, actual situations are complex and rarely is only one factor involved at a time; each

reaction has unique characteristics. The factors that affect the rate of a reaction are indicative of

trends, and do not lead to definite conclusions. Chemical kinetics, the branch of chemistry that studies

reaction rates, remains above all an experimental science.

Apples that don’t brown: fiction or soon-to-be fact?

Soon we will see at the grocer’s the first varieties of genetically modified apples that

don’t brown when we cut or bite into them. They are the Golden Delicious, a cousin

of the Red Delicious used in this experimental activity, and the green Granny Smith

apple. It is only a question of time before other varieties of apples, such as the Red

Delicious, are also modified in this way.

© emprize/Shutterstock.com © stevemart/Shutterstock.com

Did you know?

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Integration Exercises

Consider the reaction between calcium carbonate and acetic acid at standard temperature and

pressure. In 2 minutes and 17 seconds, 34.5 mL of CO2 are collected by water displacement.

CaCO3(aq)

+ 2 CH3COOH

(aq) ® Ca(CH

3COO)

2(aq) + CO

2(g) + H

2O

(l)

a) What are the values of standard temperature and pressure?

b) What is the reaction rate in mL/s for the CO2?

c) Given that the partial pressure of water is 2.64 kPa, determine the partial pressure of the CO2.

d) Determine the reaction rate in mol/s.

e) Determine the reaction rate in mol/L•s.

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Consider the decomposition of N2O

5 as well as the following data obtained in a kinetics study.

concentrations of reactant anD proDucts versus time

Time (min) [n2o

5] (mol/L) [no

2] (mol/L) [o

2] (mol/L)

0 1.24 0.00 0.00

10 0.92 0.64 0.16

20 0.68 1.12 0.28

30 0.50 1.48 0.37

40 0.37 1.74 0.44

50 0.27 1.92 0.48

a) Give the balanced chemical equation for the decomposition reaction of N2O

5.

b) For each of the following substances, indicate whether the concentration decreases or increases

during the reaction.

n2o

5no

2o

2

c) Are these variations in concentration consistent with the equation for the reaction?

d) The following graph shows the curve of the NO2 concentration versus time. Use the values in the

results table to complete the curves for the concentrations of the other two gases.

concentrations of reactant anD proDucts versus time

Time (min)

NO2

0 10 20 30 40 50 60 70

2.00

1.50

1.00

0.50

2.50

Co

nce

ntr

atio

n (

mo

l/L

)

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e) On the graph, extrapolate the curves for times t = 60 min and t = 70 min, then give the

concentrations of the substances at these times, below.

f) Are the concentrations different from what they were at t = 50 minutes?

g) Complete the following table of values. Use the lines beneath the table for your calculations. Then,

draw the reaction rate curves on the graph on the next page. The curve for [NO2] has already been

drawn.

reaction rates of reactant anD proDucts versus time

Time (min)

rN2O5 (mol/L•min)

rNO2 (mol/L•min)

rO2 (mol/L•min)

5 0.064

15 0.048

25 0.036

35 0.026

45 0.018

55 0.016

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reaction rates of reactant anD proDucts versus time

Time (min)

NO2

0.060

0.050

0.040

0.030

0.020

0.010

0.070

0 10 20 30 40 50 60

Rat

e (m

ol/

L•m

in)

h) What characteristic is common to all three curves drawn? Explain.

The following data was collected during an experimental study on the kinetics of the combustion

reaction of gasoline, whose chemical equation is given below:

2 C8H

18(aq) + 25 O

2(g) ® 16 CO

2(g) + 18 H

2O

(l)

concentration of c8h18 versus time

Time (s) [c8h

18] (ppm)

0 1.000

20 0.427

40 0.211

60 0.130

80 0.102

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d.

On the next page, make the data table and draw the graph representing the rate of disappearance of

C8H

18 and the rate of appearance of CO

2 versus time. Use the space below for your calculations.

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title of table:

title of graph:

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d.

Use a sketch to represent the effect of each of the following factors on the reaction rate. An example

has been provided for the nature of the reactants.

surface area of reactants concentration of reactants

temperature nature of reactants

Methane Methanol

H

CHHH

O

H

CHHH

Methane burns more quickly than methanol.

catalyst (enzymatic) catalyst (non-enzymatic)

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Give an everyday example that shows the effect of each factor on the reaction rate. Give examples that

are different from those you have seen so far.

a) Nature of the reactants:

b) Surface area:

c) Temperature:

d) Reactant concentration:

e) Presence of a catalyst (or reaction inhibitor):

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d.

A young student is preparing for a science competition where she will present a study on the effects of

certain factors on the rate of decomposition of a blue food dye. The chemical structure of the food dye

is given below.

CCH

CH

HC

C

HC

O3S

H2CN

HC

C

C

CH

CH

H2C

CH3

HC

C

C

CH

HCSO3

CC

HC

CH

CN

HC

HC C

CHC

SO3

CH2

CH2H3C

HC

HC

HC

CH

HC

+

--

-

Blue dye

She has blue food dye, Javel water and 7 glass jars with a capacity of approximately 50 mL each. Each

jar has an airtight lid.

a) In jars 1 and 2, she prepares 25 mL of an aqueous solution of the dye of known concentration. She

adds 2.5 mL of Javel water to each jar of blue solution, and covers them. She puts jar 1 aside; this

will be the reference solution. She then vigorously shakes jar 2. She puts jar 2 beside the reference

solution and measures the time it takes for the blue colour to disappear in each jar.

What can she expect to see?

What happens?

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b) She prepares three other solutions identical to the solution in jar 2; she numbers these jars 3, 4

and 5.

She adds more food dye to jar 3. She shakes the jar vigorously and measures how long it takes for

the blue colour to disappear. She increases the amount of Javel water in jar 4. She shakes the jar

vigorously and then measures how long it takes for the blue colour to disappear. She increases the

amount of water in jar 5. She shakes the jar vigorously and again measures how long it takes for the

blue colour to disappear.

Compare the reaction rates of these three solutions. How do they compare with the rate of reaction

of the reference solution (jar 1)?

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c) She prepares two new solutions identical to the solution in jar 2; she numbers these jars 6 and 7.

She heats the food dye and the Javel water* and mixes them together (jar 6). She then cools the two

reactants and mixes them together (jar 7). In both cases, she vigorously shakes the solutions and

measures the time it takes for the blue colour to disappear.

How do the reaction rates of the warm reaction mixture (jar 6) and the cold reaction mixture (jar 7)

compare with the reference solution in jar 2?

* It is not advisable to heat Javel water, since the fumes may be toxic. This is a hypothetical situation for the purposes

of this exercise.


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Concept Summary

Activity 1.1 – Molar Flow Rate

A chemical reaction is a process that involves the rearrangement of the atoms in a substance. The

original molecules, called the reactants, break apart to form new chemical species, called the products,

with properties that are specific to them.

Reaction rate (r) is a measure of the change in the quantity of reactants that disappear (in terms of

mass, number of particles, volume, pressure or concentration) or the quantity of products that appear

per unit of time.

Reaction rate may be expressed mathematically as follows:

• In terms of the disappearance of the reactants:

r = -DreactantDtime

• In terms of the appearance of the products:

r = DproductDtime

Note the negative sign when the reaction rate is defined in terms of the reactants disappearing, since

rate, or speed, can never be negative.

The stoichiometric coefficients are very useful for determining the general reaction rate (rg); consider the

following general balanced equation:

aA + bB ® cC + dD

where rg is the general reaction rate in mol/L•s or any other

unit of concentration per unit of time,

rA, r

B, r

C and r

D are the reaction rates of reactants A and

B and products C and D respectively,

and a, b, c and d are the stoichiometric coefficients of the

reactants and products.

rg =

rAa

=

rBb

=

rCc

=

rDd

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Activity 1.2 – Speed: A Question of Taste

A chemical reaction is therefore a dynamic process that changes over time. This change

is characterized by the reaction rates for the reactants and products. The reaction rate is

governed by a set of factors which can be modified in order to control this rate.

The nature of the reactants affects the reaction rate. The rate of a reaction generally decreases with

the number, or the strength, of the bonds to be broken.

In general, the higher the reactant concentration, the faster the rate of reaction, since the frequency of

effective collisions is increased over a given period of time.

The higher the temperature, the greater the number of molecules with the minimum required energy to

react. Consequently, an increase in temperature results in a faster reaction rate.

A catalyst is a substance that increases the rate of a chemical reaction without itself taking part in the

reaction. It facilitates the interaction between the reactants by lowering the activation energy of the

reaction.

Increasing the surface area of the reactants increases the rate of a reaction. In other words, the greater

the surface area, the faster the reaction rate.

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CHE-5062-2 CHEMISTRY: KINETICS and EQUILIBRIUM · PDF fileChemistry: Kinetics and Equilibrium This learning guide has been produced by the Société de formation à distance des commissions