Upload
lyquynh
View
215
Download
0
Embed Size (px)
Citation preview
Chemical Reactions 1:Energy and Chemical Dynamics
CHE-5042-2
Learning Guide
CHEMICAL REACTIONS 1: ENERGY AND CHEMICAL DYNAMICS
CHE-5042-2LEARNING GUIDE
0.1
Chemical Reactions 1: Energy and Chemical Dynamics is the second of three Learning
Guides prepared for the three courses making up the Secondary V Chemistry
program, which comprises the following three courses:
Gases
Chemical Reactions 1: Energy and Chemical Dynamics
Chemical Reactions 2: Equilibrium and Oxidation–Reduction
The three Learning Guides are accompanied by the workbook, Experimental Activities
of Chemistry, which covers the “experimental method” component of the program.
CHEMICAL REACTIONS 1: ENERGY AND CHEMICAL DYNAMICS
This Guide was produced by the Société de formation à distance des commissions
scolaires du Québec.
Project Coordinator: Jean-Simon Labrecque (SOFAD)
Project Coordinator: Mireille Moisan (first edition)
Coordinator: Céline Tremblay (FormaScience)
Author: André Blondin
Illustrators: Gail Weil Brenner (GWB)
Jean-Philippe Morin (JPM)
Content Revisors: Céline Tremblay (FormaScience) (French Version)
Stéphanie Belhumeur (English Version)
Layout: I. D. Graphique inc. (Daniel Rémy)
Translator: Claudia de Fulviis
Linguistic Revisor: Patricia Fillmore
Proofreader: Gabriel Kabis
First Edition: November 2000
September 2008
© Société de formation à distance des commissions scolaires du Québec
All rights for translation and adaptation, in whole or in part, are reserved for all countries.
Any reproduction by mechanical or electronic means, including microreproduction, is
forbidden without the written permission of a duly authorized representative of the Société
de formation à distance des commissions scolaires du Québec.
Legal Deposit – 2000
Bibliothèque et Archives nationales du Québec
Bibliothèque et Archives Canada
ISBN 978-2-89493-192-9
TABLE OF CONTENTS
GENERAL INTRODUCTION
OVERVIEW ................................................................................................................... 0.10
HOW TO USE THIS LEARNING GUIDE ............................................................................. 0.10
Learning Activities ................................................................................................. 0.11
Exercises .............................................................................................................. 0.11
Self-evaluation Test ............................................................................................... 0.12
Appendices ........................................................................................................... 0.12
Materials .............................................................................................................. 0.12
CERTIFICATION ............................................................................................................. 0.13
INFORMATION FOR DISTANCE EDUCATION STUDENTS .................................................... 0.13
Work Pace ............................................................................................................ 0.13
Your Tutor ............................................................................................................ 0.13
Homework Assignments ....................................................................................... 0.14
CHEMICAL REACTIONS 1: ENERGY AND CHEMICAL DYNAMICS ...................................... 0.15
CHAPTER 1 – HEAT: ENERGY IN MOTION .................................................................... 1.1
1.1 ENERGY ............................................................................................................... 1.3
Forms of Energy .................................................................................................... 1.4
Chemical Energy in Action ............................................................................... 1.6
Potential Energy .............................................................................................. 1.7
Energy Conversions .............................................................................................. 1.8
Conservation of Energy ......................................................................................... 1.13
1.2 HEAT .................................................................................................................... 1.15
Kinetic Molecular Model of Matter ......................................................................... 1.16
Thermometers and Heat Transfers ......................................................................... 1.21
Experimental Activity 1: Heat Transfers ..................................................... 1.22
Mercury Thermometers ................................................................................... 1.23
Thermal Equilibrium ........................................................................................ 1.26
Temperature Scales ........................................................................................ 1.26
The Mechanical Equivalent of Heat ........................................................................ 1.29
Heat and Thermal Energy ...................................................................................... 1.30
1.3 HEAT EXCHANGES ................................................................................................. 1.33
Experimental Activity 2: Final Temperature of a Mixture .............................. 1.33
Specific Heat Capacity ........................................................................................... 1.34
Sea Breezes and Land Breezes ....................................................................... 1.36
Heat Exchange Equation ........................................................................................ 1.37
Calorimeters ................................................................................................... 1.38
Applications ................................................................................................... 1.39
Chemical Reactions 1 - Table of Contents
0.5
Energy in Phase Changes ...................................................................................... 1.44
Melting ........................................................................................................... 1.46
Boiling ............................................................................................................ 1.48
1.4 TECHNICAL APPLICATIONS ..................................................................................... 1.51
The Bread Oven .................................................................................................... 1.51
Re-entering the Atmopshere ................................................................................... 1.52
Geysers ................................................................................................................ 1.52
Key Words in This Chapter ............................................................................................ 1.54
Summary ..................................................................................................................... 1.54
Review Exercises .......................................................................................................... 1.56
CHAPTER 2 – DISSOLUTION: AN ENERGY PHENOMENON ............................................. 2.1
2.1 MIXTURES AND AQUEOUS SOLUTIONS ................................................................... 2.3
Mixtures ............................................................................................................... 2.3
The Water Molecule ............................................................................................. 2.5
Capillary Rise ................................................................................................. 2.9
2.2 DISSOLUTION AND SOLUBILITY .............................................................................. 2.10
Molecular Dissolution ............................................................................................ 2.10
Ionic Dissolution ................................................................................................... 2.12
Hydration ........................................................................................................ 2.14
Salt Waters .................................................................................................... 2.16
Electrolytes ..................................................................................................... 2.16
Solubility and Precipitation ..................................................................................... 2.18
Precipitation ................................................................................................... 2.20
2.3 DISSOLUTION: AN ENERGY PHENOMENON ............................................................. 2.23
The Process of Dissolution .................................................................................... 2.25
Molar Heat of Solution .......................................................................................... 2.28
Experimental Activity 3: Molar Heat of Solution .......................................... 2.30
2.4 SOLUTIONS IN EVERYDAY LIFE ............................................................................... 2.31
Gases in Aqueous Solutions .................................................................................. 2.31
The Senses of Taste and Smell ............................................................................. 2.31
Pharmaceuticals, Beauty Products and Perfumes ................................................... 2.33
Other Solvents ...................................................................................................... 2.34
Key Words in This Chapter ............................................................................................ 2.35
Summary ..................................................................................................................... 2.35
Review Exercises .......................................................................................................... 2.37
CHAPTER 3 – CHEMICAL REACTIONS AND ENERGY .................................................... 3.1
3.1 HEAT OF REACTION ............................................................................................... 3.3
Definition and Convention ...................................................................................... 3.4
Measuring the Heat of Reaction ............................................................................. 3.7
Change in Enthalpy ............................................................................................... 3.9
Chemical Reactions 1 - Table of Contents
0.6
Enthalpy Diagrams ............................................................................................... 3.12
Activation Energy ................................................................................................... 3.16
3.2 COMBUSTION REACTIONS ..................................................................................... 3.18
Rapid Combustion and Slow Combustion ............................................................... 3.18
A Little Bit of History ....................................................................................... 3.21
Fossil Fuels: A Useful Commodity .......................................................................... 3.24
Industrialization and Social Changes ................................................................ 3.27
A Technical Application .................................................................................... 3.29
Fossil Fuels: The Drawbacks .................................................................................. 3.31
Carbon Dioxide (CO2) ....................................................................................... 3.31
Carbon Monoxide (CO) .................................................................................... 3.34
Nitrogen Oxides and Sulphur Dioxide ............................................................... 3.37
A Few Possible Solutions ................................................................................ 3.37
3.3 HESS’S LAW ......................................................................................................... 3.39
Experimental Activity 4: Hess’s Law .......................................................... 3.41
Summation of Heats of Reaction ........................................................................... 3.41
Application of Hess’s Law ..................................................................................... 3.44
Hess’s Law and Chemical Bonds ........................................................................... 3.48
Key Words in This Chapter ............................................................................................ 3.53
Summary ..................................................................................................................... 3.53
Review Exercises ......................................................................................................... 3.55
CHAPTER 4 – THE RATE OF CHEMICAL REACTIONS ..................................................... 4.1
4.1 PROGRESS OF A CHEMICAL REACTION ................................................................. 4.3
Rate of a Reaction ................................................................................................ 4.4
Using Graphs to Represent Reaction Rates ............................................................ 4.8
Reaction Rate As a Function of Time ...................................................................... 4.10
4.2 DETERMINING FACTORS ........................................................................................ 4.20
Experimental Activity 5: Rate of a Chemical Reaction ................................. 4.21
Nature of Reactants .............................................................................................. 4.21
Concentration of Reactants ................................................................................... 4.23
Pressure and Gaseous Reactants .......................................................................... 4.24
Temperature ......................................................................................................... 4.25
Surface Area ........................................................................................................ 4.27
Catalysts .............................................................................................................. 4.30
An Overview .......................................................................................................... 4.34
Preparation of a Reference Solution ................................................................. 4.34
Effect of Concentration on the Rate of Reaction ............................................... 4.34
Effect of Temperature on the Rate of Reaction ................................................. 4.35
Key Words in This Chapter ............................................................................................ 4.37
Summary ..................................................................................................................... 4.37
Review Exercises ......................................................................................................... 4.39
Chemical Reactions 1 - Table of Contents
0.7
CHAPTER 5 – ENERGY AND THE RATE OF REACTION ................................................... 5.1
5.1 MOLECULAR ENERGY ............................................................................................ 5.3
Kinetic Energy of Molecules ................................................................................... 5.4
A Nornal Distribution for Large Populations ............................................................. 5.6
Maxwell-Boltzmann Distribution ............................................................................. 5.10
5.2 CONSEQUENCES OF THE MAXWELL-BOLTZMANN DISTRIBUTION .............................. 5.16
Temperature ......................................................................................................... 5.16
Threshold Energy and Activation Energy .................................................................. 5.18
Other Applications ................................................................................................. 5.24
Kinetic Energy in Our Lives .................................................................................... 5.24
5.3 ENERGY AND THE MECHANISM OF REACTION ........................................................ 5.27
Reaction Mechanism ............................................................................................. 5.27
Catalysts .............................................................................................................. 5.32
Spontaneity of Reactions ....................................................................................... 5.36
Key Words in This Chapter ............................................................................................ 5.40
Summary ..................................................................................................................... 5.40
Review Exercises ......................................................................................................... 5.42
CONCLUSION
SELF-EVALUATION TEST ............................................................................................... C.5
ANSWER KEY ............................................................................................................... C.13
CHAPTER 1 – Heat: Energy in Motion ..................................................................... C.13
CHAPTER 2 – Dissolution: an Energy Phenomenon .................................................. C.27
CHAPTER 3 – Chemical Reactions and Energy ........................................................ C.36
CHAPTER 4 – The Rate of Chemical Reactions ....................................................... C.49
CHAPTER 5 – Energy and the Rate of Reaction ....................................................... C.61
ANSWER KEY TO THE SELF-EVALUATION TEST ........................................................ C.68
APPENDIX A – The International System of Units (SI) ...................................................... C.73
APPENDIX B – Mathematical Prerequisites ..................................................................... C.75
Ratios and Proportions .......................................................................................... C.75
Formulas .............................................................................................................. C.76
APPENDIX C – Chemical Prerequisites ........................................................................... C.78
Balancing Equations .............................................................................................. C.78
Calculating Molar Mass ......................................................................................... C.81
APPENDIX D – Table of Figures ..................................................................................... C.83
BIBLIOGRAPHY ............................................................................................................. C.87
GLOSSARY ................................................................................................................... C.89
INDEX .......................................................................................................................... C.97
Chemical Reactions 1 - Table of Contents
0.8
GENERAL INTRODUCTION
OVERVIEW
Welcome to the course entitled Chemical Reactions 1: Energy and Chemical Dynamics,
which is part of the Secondary V Chemistry program. This program comprises the
following three courses:
CHE-5041-2 Gases
CHE-5042-2 Chemical Reactions 1: Energy and Chemical Dynamics
CHE-5043-2 Chemical Reactions 2: Equilibrium and Oxidation—Reduction
The three main components of the Chemistry program are related content, the
experimental method and the history-technology-society perspective. Whereas the
experimental method is developed in the workbook Experimental Activities of
Chemistry, the related content and the history-technology-society perspective are
covered in the three Learning Guides accompanying the three courses which must
be taken in sequential order.
Chemical Reactions 1: Energy and Chemical Dynamics is the second in the set of three
Learning Guides. It is divided into five chapters corresponding to the five terminal
objectives of the program.1 This Guide is to be used in conjunction with the workbook
Experimental Activities of Chemistry. You will find references to the appropriate sections
of the Workbook throughout the Guide.
The course Chemical Reactions 1: Energy and Chemical Dynamics will help you gain
a better understanding of chemical dynamics and the energy transfers involved in
chemical reactions, together with the related technical applications, social changes
and environmental consequences.
HOW TO USE THIS LEARNING GUIDE
This Guide is the main work tool for this course and has been designed to meet the
needs of adult students enrolled in individualized learning programs, or distance
education courses.
Each chapter covers a certain number of themes, using explanations, tables,
illustrations and exercises designed to help you to master the different program
objectives. A list of key words, a summary and review exercises are included at the
end of each chapter.
Chemical Reactions 1 - General Introduction
0.10
1. The terminal objectives and associated objectives are listed at the beginning of each chapter.
The conclusion contains a summary covering all the courses in the program along
with a self-evaluation test. It also includes an Answer Key for the self-evaluation test,
for the exercises in each chapter and for the review exercises. A glossary with definitions
of the key words, a bibliography, appendices and an index are also provided in the
conclusion. You may wish to consult the books and publications in the bibliography
for further information on the topics covered in this course.
Learning Activities
The Guide contains theoretical sections as well as practical activities in the form of
exercises. The exercises come with an Answer Key.
Start by skimming through each part of the Guide to familiarize yourself with the
content and the main headings. Then read the theory carefully:
– Highlight the important points.
– Make notes in the margins.
– Look up new words in the dictionary.
– Summarize important passages in your own words, in your notebook.
– Study the diagrams carefully.
– Write down questions relating to ideas you don’t understand.
Exercises
The exercises come with an Answer Key, which is located in the coloured section at
the end of the Guide.
• Do all the exercises.
• Read the instructions and questions carefully before writing your answers.
• Do all the exercises to the best of your ability without looking at the Answer Key.
Reread the questions and your answers, and revise your answers, if necessary. Then
check your answers against the Answer Key and try to understand any mistakes
you made.
• Complete each chapter before doing the corresponding review exercises. Doing these
exercises without referring to the Guide is a good way to prepare for the final
examination.
Chemical Reactions 1 - General Introduction
0.11
Self-evaluation Test
The self-evaluation test is a step that prepares you for the final evaluation. You must
complete your study of the course before attempting to do it. Reread your notebook
and the definitions of the key words in the chapters. Make sure you understand how
they relate to the course objectives listed at the beginning of each chapter. Then do
the self-evaluation test without referring to the main body of the Guide or the Answer
Key. Compare your answers with those in the Answer Key and review any areas you
had difficulty with.
Appendices
The appendices contain a review of some concepts you should be familiar with before
beginning the course. The complete list of appendices appears in the table of contents.
Materials
Have all the materials you will need close at hand:
• Learning material: this Guide and a notebook in which you will summarize important
concepts relating to the objectives (listed in the introduction of each chapter). You
will also need to use your periodic table and the workbook Experimental Activities
of Chemistry.
• Reference material: a dictionary.
• Miscellaneous material: a calculator, a pencil for writing your answers and notes
in your Guide, a coloured pen for correcting your answers, a highlighter (or a pale-
coloured felt pen) to highlight important ideas, a ruler, an eraser, etc.
Chemical Reactions 1 - General Introduction
0.12
CERTIFICATION
To earn credits for this course, you must obtain at least 60% on the final examination
which will be held in an adult education centre.
The evaluation for the course Chemical Reactions 1: Energy and Chemical Dynamics
is divided into two separate parts.
Part I consists of a two-hour written examination made up of multiple-choice, short-
answer and essay-type questions. This part is worth 75% of your final mark and deals
with the objectives covered in this Guide. You may use a calculator.
Part II is designed exclusively to evaluate the experimental method. It will be held
in the laboratory during one 90-minute session. It is worth 25% of your final mark
and deals with the course objectives covered in Section B of Experimental Activities
of Chemistry.
INFORMATION FOR DISTANCE EDUCATION STUDENTS
Work Pace
Here are some tips for organizing your work:
• Draw up a study timetable that takes into account your personality and needs, as
well as your family, work and other obligations.
• Try to study a few hours each week. You should break up your study time into several
one- or two-hour sessions.
• Do your best to stick to your study timetable.
Your Tutor
Your tutor is the person who will give you any help you need throughout this course.
He or she will answer your questions and correct and comment on your homework
assignments.
Don’t hesitate to contact your tutor if you are having difficulty with the theory or the
exercises, or if you need some words of encouragement to help you get through this
course. Write down your questions and get in touch with your tutor during his or
Chemical Reactions 1 - General Introduction
0.13
her available hours. The letter included with this Guide or that you will receive shortly
tells you when and how to contact your tutor.
Your tutor will assist you in your work and provide you with the advice, constructive
criticism, and support that will help you succeed in this course.
Homework Assignments
In this course, you will have to do three homework assignments: the first after
completing Chapter 2, the second after completing Chapter 4, and the third after
completing Chapter 5. Each homework assignment also contains questions on the
experimental method you studied in Experimental Activities of Chemistry.
These assignments will show your tutor whether you understand the subject matter
and are ready to go on to the next part of the course. If your tutor feels you are not
ready to move on, he or she will indicate this on your homework assignment, providing
comments and suggestions to help you get back on track. It is important that you
read these corrections and comments carefully.
The homework assignments are similar to the examination. Since the exam will be
supervised and you will not be able to use your course notes, the best way to prepare
for it is to do your homework assignments without referring to the Learning Guide
and to take note of your tutor’s corrections so that you can make any necessary
adjustments.
Remember not to send in the next assignment until you have received the corrections
for the previous one.
Chemical Reactions 1 - General Introduction
0.14
CHEMICAL REACTIONS 1: ENERGY AND CHEMICAL DYNAMICS
Forest fires, sudden frost, melting snow, respiration, the shuttle launch and archery
all involve changes that have one thing in common—energy. While we cannot observe
energy directly, we perceive it as light, heat, motion and noise, among others things.
Energy takes different forms and is named according to the way it manifests itself.
For instance, we speak of mechanical energy to describe the thrust, firing or rotation
of a mechanical part, of kinetic energy for the movement of molecules or other bodies,
of radiant energy for light, and of potential energy for all forms of stored energy.
Associated with hot and cold sensations, heat is transferred between systems when
they are at different temperatures. We might say that heat is a mode of energy transfer.
Heat and changes in temperature are therefore closely linked. Heat can either be
absorbed or released during a chemical reaction. For instance, energy is released when
we burn natural gas to heat our homes, whereas enormous amounts of energy are
consumed in aluminum production.
The last topic covered in the course entitled Gases was the energy balance of chemical
reactions. The second course, Chemical Reactions 1: Energy and Chemical Dynamics,
examines energy transfers from a broader perspective as well as the rate of reactions
and the factors that affect it. The third course, Chemical Reactions 2: Equilibrium and
Oxidation—Reduction, provides an in-depth study of two categories of chemical
reactions.
Chapter 1 of this Guide analyzes energy transfers between two liquids that are mixed
together. Energy transfers depend on the amounts and types of liquids involved, and
can be observed through changes in temperature. The first chapter also reviews the
heating curve of a substance and gives a more detailed explanation for rises in
temperature and phase changes.
Chapter 2 deals with dissolution reactions, and describes them macroscopically, that
is, according to what can be observed with the naked eye. They are then examined
at the molecular level, using the kinetic molecular model of matter, which is an
expanded version of the model based on the kinetic theory of gases. You will learn
why water is the most common solvent in nature and the solvent of choice in the
laboratory. This chapter focuses on the energy involved in dissolutions.
Chapter 3 deals with the energy associated with chemical reactions. It also reviews
the energy balance involved in the dissociation and formation of bonds. In this course,
Chemical Reactions 1 - General Introduction
0.15
however, the subject of energy balance is examined in greater detail. The energy
diagrams provide more information about the progress of reactions, and the sum of
the energies involved in the steps of a reaction yields the overall energy of the reaction.
This analysis will be based on combustion reactions.
Chapters 4 and 5 examine kinetic chemistry or the rate of chemical reactions. This
rate depends on the nature of the reactants, their concentration, their surface area
and on environmental conditions such as temperature, pressure and acidity. Catalysts
speed up reactions, but do not alter the nature of the products. Speed is part and
parcel of any process involving energy. The fifth and last chapter in this course examines
the relationship between energy, reaction rate and the various factors that affect this
rate. It also introduces the content of the next course.
As in the first two Guides, a table of contents diagram at the beginning of each chapter
shows you how the chapter fits into the course as a whole. The content of the chapter
you are about to begin is in bold type and in larger characters, whereas the content
of completed chapters is in italics. For example, the table of contents diagram for
Chapter 2 is reproduced below. The section for Chapter 2 is in bold type and the content
of Chapter 1 is in italics and smaller type. You will find this diagram a very useful
tool as you go through the course.
Good luck!
1. Heat: Energy in MotionEnergy: forms, conservationHeat: model, thermometerHeat exchangesEnergy in phase changesApplications
2. Dissolution: An Energy PhenomenonMixtures, solutions
Molecular and ionic dissolutionSolubility, precipitation
Heat of solutionSolutions in everyday life
5. Energy and the Rate of ReactionMaxwell-Boltzmann distributionEnergy threshold and activation energyReaction mechanismsSpontaneity of reactions
CHEMICAL REACTIONS 1:
Energy and Chemical Dynamics
4. Rate of Chemical ReactionsRate of reactionGraphsDetermining factors
3. Chemical Reactions and EnergyHeat of reaction
Rapid and slow combustionFossil fuelsHess’s law
Chemical Reactions 1 - General Introduction
0.16
CHAPTER 1
HEAT: ENERGY IN MOTION
GWB
Terminal Objective 1
To analyze the energy transfers that occur in phase changes and mixtures of substances at different temperatures.
Intermediate Objectives
1.1 To recognize the different forms of energy acting in the phenomena observed in their environment.
1.2 To associate a macroscopic phenomenon with corresponding changes occurring at the atomic or molecularlevel.
1.3 To describe a heat transfer in terms of kinetic energy and the variation in temperature.
1.4 To classify physical and chemical phenomena according to whether they represent endothermic orexothermic reactions, on the basis of observations.
1.5 To determine, through experimentation, the factors that influence the final temperature of a mixture.
1.6 To establish relationships between the definition of specific heat capacity and its units of measurement.
1.7 To describe the energy transfers produced during phase changes of a pure substance.
1.8 To describe briefly how Joule established a relationship between heat and mechanical energy.
1.9 To give examples of energy conversions involving heat.
1.10 To solve problems related to energy transfers that occur during phase changes and mixtures of substancesat different temperatures.
1. Heat: Energy in MotionEnergy: forms, conservationHeat: model, thermometerHeat exchangesEnergy in phase changesApplications
2. Dissolution: An Energy PhenomenonMixtures, solutions
Molecular and ionic dissolutionSolubility, precipitation
Heat of solutionSolutions in everyday life
5. Energy and the Rate of ReactionMaxwell-Boltzmann distributionEnergy threshold and activation energyReaction mechanismsSpontaneity of reactions
CHEMICAL REACTIONS 1:
Energy and Chemical Dynamics
4. Rate of Chemical ReactionsRate of reactionGraphsDetermining factors
3. Chemical Reactions and EnergyHeat of reaction
Rapid and slow combustionFossil fuelsHess’s law
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.2
Have you ever wondered about all the different ways in which we use the word
“energy”? You are no doubt aware that this term has several meanings. For instance,
in the expression “My child is full of energy,” it refers to the ability to move and do
things; however, when we say that “Solar energy powers satellites that orbit the earth,”
it refers to light; and in “Water heaters consume a lot of energy,” it refers to the source—
more often than not electric—which heats the water.
In this chapter, we will review the various forms of energy and pay particular attention
to heat. We will explore the concept of temperature in greater detail, explain heat
transfers at the molecular level and represent them by means of a mathematical
relation. We will then examine the energy associated with phase changes more closely
and end with an overview of a few applications of heat energy.
1.1 ENERGY
Human beings have used what we now call energy since the dawn of time and have
tried to harness it in different ways. At first, humans used energy simply to eat, keep
warm and bask in the Sun’s light. They then went on to “tame” fire and invent the
wheel, levers and other, more complex devices, such as windmills and watermills.
More recently, humans have mastered the use of certain mixtures of explosive
substances: this has contributed to the development of firearms, combustion engines
and dynamite, used to dig mines and construct roads through mountains.
Humans are still actively involved in the quest for technical advances to meet their
ever-changing needs—the construction of completely automated plants and more
effective power-generating stations are a few examples. Others include launching
satellites into orbit around the Earth, developing the Moon’s mineral resources and
exploring the planet Mars, projects which have been undertaken by NASA1 and other
space agencies worldwide.
Although the word is used frequently, “energy” remains a difficult concept to grasp.
Strictly speaking, energy is defined as the capacity for doing work, or the capacity
for producing an effect such as movement or light, among other things.
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.3
1. National Aeronautics and Space Administration: the organization that oversees the entire American space program.In Canada, the Canadian Space Agency fulfills the same function.
Energy cannot be observed directly. Rather, we observe its manifestations, the main
ones being light, heat and motion. Our eyes take in light in a very precise manner
and very small corpuscles2 in the skin detect sensations of warmth or cold. And of
course, our eyes, ears and muscles can detect motion.
Exercise 1.1
Complete the following table by naming sources of energy that are detected as light,
heat or motion. Write two sources for each form of energy indicated.
Form Sources
Light Le soleil, une ampoule électrique allumée, une flamme, etc.nnn
Heat L’élément électrique d’un rond de poêle allumé, d’un grille-pain, d’unebouilloire, d’un radiateur-plinthe ou d’une ampoule électrique ; le composten décomposition, une flamme, etc.
Motion L’énergie d’un trampoline compressé, d’un arc bandé, d’un élan de bâtonde golf ou de baseball ; une voiture, un vélo ou un train en mouvement, unenfant qui court, la rotation des aiguilles d’une horloge, etc.
FORMS OF ENERGY
Forms of energy are usually named after their source or the process of transformation
that produces it. For instance, mechanical energy is produced by the movement of
mechanical parts in machinery and engines which cut, strike or launch objects; solar
energy includes visible light, ultraviolet rays and the heat released by the Sun;
gravitational energy is associated with the force of gravity which causes objects to
fall to the ground; nuclear energy is produced by the fusion or fission of atomic nuclei
and fuels nuclear power stations and aircraft carriers; magnetic energy causes two
magnets to repel each other and aligns the electron stream that produces the image
on a television screen; electrical energy lights our homes and powers engines of all
kinds; wind energy refers to the wind’s capacity to turn the blades of a mill;
hydraulic energy is provided by rivers, waterfalls and tides; muscular energy refers
to a muscle’s capacity to move an arm or a leg; electromagnetic energy includes visible
light, X-rays, microwaves and other types of electromagnetic radiation.
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.4
?
2. Ruffini’s corpuscles detect sensations of warmth and Krause’s corpuscles detect sensations of cold.
More specifically, kinetic energy is the name given to the energy associated with the
straight-line or spinning motion of an object. By contrast, heat, which is also named
calorific energy, involves a transfer of energy between two systems.
In subsequent chapters, we will focus on a form of energy called chemical energy,
so named because it is associated with chemical reactions that, as you may recall,
change the nature of substances. For instance, the combustion of propane gas releases
heat that comes from the chemical energy contained in the propane and oxygen. Thus,
the bond energy3 discussed at the end of the last course is chemical energy.
Exercise 1.2
State the form of energy (wind energy, nuclear energy or hydroelectric energy) that
best corresponds to the descriptions given below.
The electrical energy flowing out of a generator that is driven by arotating turbine powered by a waterfall.
The light, heat and kinetic energy released by the fission of heavy andunstable atomic nuclei such as those of uranium 235.
The mechanical energy generated by the movement of air across theEarth’s surface.
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.5
3. Lalancette, Pauline and M. Lamoureux, Gases (Chemistry, Secondary V), Chapter 1, Learning Guide producedby SOFAD.
?
Benjamin Franklin (1706-1790)4
Born in the Unites States and the fifteenth child in a family of English immigrants,
Benjamin Franklin was a self-educated and very talented man. In 1752, Franklin
performed his famous experiment that involved going out during a thunderstorm and
flying a kite with a metal key attached to it. He wanted to prove that lightning was
similar to electricity. Without knowing it, he was taking an enormous risk. Franklin
is an excellent example of the long line of curious-minded, ingenious people who
were determined to explore nature.
Franklin invented the stove that bears his name, the Franklin stove, the lightning
conductor (a metal rod placed on buildings to protect them against lightning) and bifocal lenses. He also
developed a theory to explain the absorption of heat. A master of many trades, Franklin was elected to
Pennsylvania’s legislative assembly and helped draft the American Declaration of Independence, which he
signed at the age of 70!
Chemical Energy in Action
Chemical energy is behind the powerful thrust needed to propel space rockets into
orbit. In American space shuttles, the central external tank fuels three engines placed
in a triangular arrangement at the back of the shuttle. Chemical energy is released
by the intense reaction between hydrogen and oxygen.5 In order to save space, the
two gases are first liquefied and stored separately in refrigerated tanks. The formation
of water vapor occurs, and the acceleration produced by the chemical energy that is
released causes it to be forcefully expelled from the rocket. Guided by the nozzles,
the expelled vapour propels the rocket. The shuttle’s three main engines provide a
thrust of more than two million newtons,6 or, in other words, the thrust needed to
propel the equivalent of 165 cars into orbit! At the outset, the shuttle weighs about
2 500 tonnes. The white trail behind the rocket as it rises into the sky results from
the condensation of water vapour, which cools upon contact with air.
The general combustion reaction for hydrogen is:
2 H2 + O2 → 2 H2O + energy
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.6
4. A light bulb indicates additional information: this information is not part of the course as such and will not becovered on the final examination.
5. “Propellant” is the term used to designate one or more substances that react chemically to produce the energyrequired to propel rockets into space. Hydrogen and oxygen constitute the main propellants for space shuttles.
6. The newton is the unit of force in the International System of Units (SI).
GWB
a) Hydrogen and oxygen form the space shuttle’s liquid propellant which is contained incompartments within the central external tank. The side rockets contain the solid propellant.
b) The central rocket engine is composed of tanks and fuel pumps. H2 and O2 combustion takes place in the three engines attached to the back of the shuttle.
Potential Energy
Regardless of its form, energy may be stored in such a way that it can be recovered
at a later time. Any form of stored energy that is ready for use is called potentialenergy. We will now look at three examples that will help us better understand this
concept: the propellant in a rocket, a match and an electric battery.
Let’s consider the two liquids (oxygen and hydrogen) stored in the refrigerated tanks
of a rocket that is about to be launched. Until the rocket has been launched, the liquids
remain in their respective chambers, the combustion reaction does not occur and
no energy is released. Upon ignition, however, the two liquids combine in the
combustion chamber, the reaction takes place and the energy released propels the
rocket into space. Prior to the reaction between the two liquids, the potential chemical
energy was stored in the substances, ready to be released.
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.7
External tank
Oxygen tankO2(l)
H2(l)
H2 + O2
H2 O+
Energy
Hydrogen tank
Combustionchamber
Nozzles
Solidpropellantrocket
Figure 1.1 - Liquid propellant rocket
Fuel pump
GWB
a) b)
}
A match in a matchbox will not light up by itself. By striking the match, we release
the energy contained in the substances that cover the match head. More precisely,
the potential chemical energy which is stored in the substances that make up the match
(sulphur, potassium chlorate, etc.) and in the oxygen in the air, is activated as soon
as the match is rubbed against a rough, specially prepared surface. We then perceive
heat and light.
When an electric battery is connected to a circuit, the substances in the battery will
react chemically to produce the energy needed to light a lamp or drive an engine.
The substances cannot react if the battery is not connected. The battery therefore
contains potential chemical energy.
All forms of energy can be described as potential, provided the energy is not activated,
that is, as long as it cannot be perceived.
Exercise 1.3
Briefly explain in your own words why we can say:
a) that a stone on the edge of a ravine has potential energy.
b) that a drawn arrow has potential energy.
ENERGY CONVERSIONS
The potential energy stored in a battery can be released in different forms depending
on whether the battery is used to power a flashlight, the motor of a toy car or any
other device. We speak of a conversion when energy changes form, that is, when it
is converted or changed from one form to another.
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.8
?
Let’s consider an archer who shoots an arrow into the sky at a 45° angle. Between
the moment the archer takes aim and releases the arrow and the moment the arrow
hits the target, energy is converted from one form to another several times.
Figure 1.2 - An arrow in flight
The archer uses his muscles to release the arrow at a 45° angle above the horizontal plane.Muscular energy is transferred from the archer to the arrow. The arrow curves downwards
and strikes a target about 30 m away. Energy is then transferred from the arrow to the target; a part of this energy serves to drive the arrow into the wood and the rest is converted into heat.
Let’s look at this example more closely. Read on and try to imagine what happens
when the arrow is released. You will see that throughout the arrow’s flight, energy,
whether stored or active, is converted from one form to another.
By inhaling air and digesting his food, the archer provides his cells with the sugar
and oxygen they need to release the chemical energy that will allow him to contract
his muscles (muscular energy) and draw the bow (the bow stores potential mechanical
energy). The slow combustion of sugar in the cells converts the potential energy stored
in the sugar and oxygen into muscular energy that can be used to draw the bow.
When the arrow is released, the bow straightens out (mechanical energy of rotation)
and pulls on the bowstring that then propels the arrow upwards at a 45° angle (kinetic
energy). As the arrow rises into the air, the energy in the arrow serves to counteract
the gravitational force that is pulling it towards the ground. As it gains height, however,
the arrow stores potential gravitational energy. At the highest point along its
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.9
GWB
trajectory,7 the arrow changes direction and gains speed as it starts to fall (it acquires
kinetic energy). In short, the energy stored in the bow has been converted into the
kinetic energy of the arrow, which in turn is converted into potential gravitational
energy as the arrow rises and then converted again into kinetic energy as the arrow
falls.
The arrow eventually hits and becomes embedded in its target (mechanical energy).
Immediately after impact, the arrow’s metallic tip will feel slightly warm (heat). In
this case, the kinetic energy of the arrow has been converted once again, but this time
into heat and work, since the arrow has become lodged in the target.
You no doubt noticed in the description you have just read that energy takes a different
form with each conversion. For discussion purposes, imagine that in each case a
quantity of energy is transferred from a source to a receptor, which in turn becomes
the source for the next energy conversion. In this way, we have a continuous chain
of energy conversions.
Let’s now go back to our example of an arrow in flight. This time, however, we will
use the concepts of source and receptor. This exercise will allow you to become familiar
with these two terms used often in this chapter. Figure 1.3 illustrates the description
of the sequence of energy conversions.
Food is the primary source of energy. The potential energy that it contains is transferred
to the first energy receptor, namely, the archer’s muscles that contract. These then
become the source of power needed to draw the bow, which is the new receptor. The
drawn bow then becomes the source of the motion of the bowstring, which in turn
becomes the new receptor. The bowstring then propels the arrow, which receives the
bowstring’s energy. The arrow uses up this energy by gaining altitude. At the same
time, however, it stores potential gravitational energy. This energy is recovered when
the arrow falls towards the ground. Upon impact, the target and the materials that
make up the arrow absorb the arrow’s energy. The partial perforation of the target
and the heating up of the arrow’s tip are proof that they have become the new receptors.
In short, at all moments during the arrow’s path, we have been able to identify a source
of energy and at least one receptor which absorbs the source’s energy.
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.10
7. An arrow’s path always follows a more or less elongated parabola, depending on the arrow’s initial velocity andangle. The vertex is the highest point on the parabola.
Figure 1.3 - An unbroken chain of energy conversions
In the second step in the energy chain, the archer’s (source) muscular energy is transferred to thebow (receptor) in the form of potential mechanical energy. In turn, the bow becomes a source,
and its energy is transferred to the arrow (receptor). Thus, each person or object participating inthe action is both a receptor and a source of energy. The target is the last receptor in the sequence.
Let’s consider a second example that is similar to the arrow in flight. The figure below
illustrates a shotgun being fired.
Figure 1.4 - Trigger mechanism of a shotgun
Detail of a shotgun’s trigger mechanism
When the gunman presses the trigger, the shotgun’s firing pin strikes the detonator (cap) of thecartridge. The heat causes the gunpowder to explode. The shots are forced into the barrel and then
through the air, before reaching the target.
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.11
Chemicalenergy(foods)
Muscularenergy
Potentialmechanical
energy(bow)
Mechanicalenergy ofrotation
(bowstring)
Kineticenergy(arrowrises)
Potentialgravitational
energy (arrowon top)
Kineticenergy(arrowfalls)
Mechanicalenergy +
heat (arrow,target)
→ → → → → → →
GWB
Firing pin Shots
Detonator
Powder
Gun barrel
Firing chamberSpring releasemechanism
Firing pin spring
IDG GWB
Exercise 1.4
The following six steps describe the firing of shots with a shotgun. The steps are not
in chronological order.
• Released gases force the shots to travel down the barrel of the shotgun.
• A finger presses the trigger.
• The firing pin hits the detonator of the cartridge.
• The shots travel through the air and hit the target (a bottle).
• The bottle shatters and the pieces are projected into different directions.
• The heat released by the impact initiates combustion of the gunpowder in the
cartridge case.
Complete the following table by answering questions a) and b). The first line in the
table has been completed for you.
a) Place the steps in the order in which they occur.
b) For each step, describe the energy conversion that takes place and identify the energy
source and receptor.
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.12
?
CONSERVATION OF ENERGY
Just as it is difficult to believe that a rabbit actually disappears inside a magician’s
hat (just because we cannot see it does not mean that it no longer exists!), it is equally
difficult to believe that energy can be destroyed. Rather, we observe, as have the leading
scientists of our time, that energy is converted into work or other forms without losing
its force. This principle, which was implicit in the examples we have looked at so far,
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.13
No. Step
1. A finger presses thetrigger.
2. Le percuteur dufusil frappe ledétonateur de laballe.
3. La chaleur dégagéepar l’impact amorcela combustion de lapoudre à canondans la douille.
4. Les gaz libéréspoussent lesplombs dans lecanon.
5. Les plombsvoyagent dans l’airet frappent la cible.
6. La bouteille éclateet les morceauxsont projetés dansdifférentesdirections.
Conversion
Muscular energy in thefinger → mechanical energyin the release mechanism
Énergie potentielle duressort → énergiemécanique + chaleur
Énergie potentielle chimiquedes substances → chaleur+ énergie cinétique desmolécules de gaz
Énergie cinétique desmolécules → énergiecinétique des plombs dechasse
Énergie cinétique desplombs → travail d’impactsur la bouteille + chaleur
Énergie cinétique desmorceaux de bouteille →impact sur les surfacesenvironnantes + chaleur
Source
Muscles in thefinger
Mécanisme dedétente (ressort)
Détonateur
Gaz libérés
Plombs
Morceaux debouteille
Receptor
Releasemechanism(spring)
Détonateur de laballe
Poudre
Plombs
Bouteille
Surfacesenvironnantes
is called the law of conservation of energy. It states that in a closed system, the
total amount of energy remains constant, regardless of the conversions it undergoes.
In other words, the total amount of energy is the same (it is conserved) before and
after the conversion.
Law of conservation of energy:
Total energy before conversion = Total energy after conversion
Remember that a system is a collection of objects that form a whole. In the example
of the bow and arrow, the archer, his bow, the arrow, the ground, the ambient air
and the target are all part of a system. The description implied that when the archer
contracted his muscles, all the chemical energy released by the cells was transferred
to his muscles, which in turn transferred all of this energy to the bow, arrow and
target. The same assumption applies to each energy conversion in the chain. We
therefore intuitively applied the law of conservation of energy. As we mentioned above,
this law states that in a closed system the total amount of energy remains constant.
However, because we wanted to keep the description simple, we omitted certain details.
For example, when the arrow falls, friction with the air produces a small amount of
heat that is transferred to the air. The rest of the arrow’s potential energy is converted
into kinetic energy. The law of conservation of energy still applies, but if we wanted
to be more precise, we would write:
Potential gravitational energy = Kinetic energy (arrow) + Heat (air)
Energy is conserved, since the two receptors, namely, the arrow and the air, absorb
all the potential energy. In fact, in real life situations, several receptors often share
the energy, even though only one of the receptors is relevant to the action being studied.
Exercise 1.5
Jehane Benoît describes the preparation of hard-boiled eggs as follows: “Place the
eggs in a saucepan and cover them with cold water. Place a lid on the pan and bring
the water to a boil over medium heat. As soon as the water starts to boil, remove the
saucepan from the source of heat and wait from 3 to 10 minutes before removing
the eggs from the water, depending on whether you like your eggs soft-boiled or hard-
boiled.”8
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.14
?
8. Benoît, Jehane, La nouvelle encyclopédie de la cuisine (Montréal: Les messageries du Saint-Laurent Ltée, 1971),p. 283. (translation)
Using this recipe, illustrate the principle of conservation of energy by either writing
a description or drawing a diagram. Keep in mind all the receptors, including those
that are not directly involved in cooking the eggs.
1.2 HEAT
Heat, which is also called calorific energy, is one of the most common manifestations
of energy. Examples of heat are abundant: the Sun heats beaches, the electromagnetic
energy of microwaves heats and cooks our food, and so on. You may have already
noticed that the head of a nail is warm after it has been struck vigorously with a
hammer. If not, try it—but watch your fingers!
Now that you have felt this heat, how would you explain it? In other words, how does
the hammer’s kinetic energy heat the nail? Do you have any idea? Write it down in
the next exercise. We will come back to this exercise later and you will have a chance
to complete your answer then, if necessary.
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.15
Exercise 1.6
Try touching the head of a nail that you have just hit forcefully with a hammer. How
can you explain the heat emanating from the nail? In other words, what happened
inside the nail? Give an explanation based on what you currently know about energy.
In order to provide an adequate answer to the preceding exercise, we have to try to
imagine what happens inside the nail. But first we will review the model of matter
that we developed in the previous course.9
KINETIC MOLECULAR MODEL OF MATTER
First, let’s look again at the kinetic theory of gases which, as you may recall, describes
the model of an ideal gas. We will then apply this theory to all matter, whether solid,
liquid or gaseous. This model will help us form a mental picture of the fundamental
structure of matter, given that it cannot be observed directly.
At the molecular level, the structure of matter is invisible and cannot be observed
with an ordinary microscope. The purpose of the model is therefore to provide a
microscopic representation of matter. The model suggests images and types of
behaviours that provide an explanation for those properties of matter we can
perceive with our senses and measure with instruments. These observable properties
are said to be macroscopic. The term “microscopic” refers to invisible phenomena
and the behaviour that the model attempts to describe and explain. By contrast, the
term “macroscopic”refers to properties and phenomena that are normally visible to
the naked eye. This will be better understood if we consider the macroscopic and
microscopic views of a gas confined in a cylinder.
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.16
?
9. Lalancette, Pauline and M. Lamoureux. Gases (Chemistry, Secondary V), Chapter 1. Learning Guide producedby SOFAD.
Figure 1.5 - Gas confined in a cylinder
a) The gas appears uniform and translucent.
b) The gas is composed of very small particles (atoms or molecules) that are separated from each other by great distances, that move freely in all directions and that do not attract
or repel one another. Note that this drawing is not to scale. A single millilitre of gas contains billions upon billions of particles.
Figure 1.5 shows a gas confined in a cylinder with an immobile piston in the top. At
the molecular level, the model provides the following explanation for what we see:
the gas particles are in motion and collide with the walls of the container and the
piston. The numerous collisions between the particles and the underside of the piston
prevent it from falling, even though its weight pulls it down.10
The model of an ideal gas, as it was described in the previous course in connection
with the kinetic theory of gases, can be summed up by the following hypotheses.
• All gases are composed of very small particles, either atoms or molecules, separated
by a vacuum. The distance between the particles is large compared to their size.
• The particles of a gas are independent, that is, they do not attract or repel one another.
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.17
a) Macroscopic view b) Microscopic view
10.Bernoulli (1728) held that the pressure exerted by a gas on the walls of its container is due to the billions of collisionsbetween the molecules of the confined gas and the walls (kinetic theory of gases). This view, which confirms Boyle’slaw on the compressibility of gases, also accounts for the fact that the temperature of a gas is related to the motionof its particles and therefore to its kinetic energy. From this point onwards, the foundation for the microscopicinterpretation of heat had been laid.
• The particles of a gas are in constant motion (translation, rotation and vibration).
They collide regularly with one another or with the walls of the container in which
they are confined.
• The average kinetic energy of the particles is a function of the temperature of a
gas. An increase in temperature will cause the molecules of the gas to become more
agitated. Conversely, a decrease in temperature will cause them to become less
agitated.
Matter exists not only in the gaseous state, but also in the liquid and solid states. As
a result, the hypotheses outlined above do not provide a satisfactory explanation for
the three states of matter at the microscopic level. If the ideal gas model states that
gas particles neither attract nor repel one another, how can we explain the solidity
of a candy or a stone?
It may be useful to look at an example of a solid to help us answer this question. Try
to imagine the structure of a penny at the microscopic level.
Figure 1.6 - A penny
a) Macroscopic view b) Microscopic view (to be completed)
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.18
Exercise 1.7
Take a penny and observe it carefully. It is made of copper. Complete Figure 1.6b by
drawing a microscopic representation of the penny as you imagine it.
The coin is a solid. It keeps its shape and it is difficult to cut. In your opinion, can
the model of the kinetic theory of gases explain these properties? Can your drawing
explain them? Let’s take a closer look.
Since the coin keeps its shape, we can conclude that the particles—the atoms of
copper—stick together and occupy fixed positions in relation to one another.
According to this description, although the atoms are agitated, their motion is limited
to vibration movements. Furthermore, it is difficult to cut the coin, which, on a
molecular level, means that it is difficult to separate the atoms. We can conclude from
this that an attractive force (cohesive force) is keeping the atoms together, acting
somewhat like a glue. This force explains why the atoms remain together and why
it is difficult to separate them. We can use the same explanation for other solids since
it accounts for the solidity of a stone just as well as that of a grain of sugar.
The model of a solid that we developed in the preceding paragraph is therefore not
consistent with all the hypotheses outlined earlier, which let us recall, are valid for
gases. The second hypothesis is particularly inexact. However, by reformulating these
hypotheses, we can obtain a more general model that describes the behaviour of gases,
liquids and solids. Because this model is based on the kinetic theory of gases, we will
refer to it as the “kinetic molecular model of matter.” It can be summed up by the
following hypotheses.
Kinetic Molecular Model of Matter
• All matter is composed of very small particles (atoms or molecules) separated by
a vacuum. In a gas, the particles are far apart from one another, whereas in a liquid
and in a solid, they are close to one another.
• There are forces of attraction between the particles of a substance. These forces
are negligible in gases but strong in liquids and solids.
• All the particles are in constant motion, be it translation, rotation, vibration, or a
combination of the three.
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.19
?
• The average kinetic energy of the particles of a substance is a function of its
temperature. An increase in temperature will cause the particles to become more
agitated. Inversely, a decrease in temperature will cause them to become less agitated.
Before you continue, you may find it useful to compare these hypotheses with those
given for gases earlier. Most scientists today use this model of matter to explain many
common phenomena. You may want to refer to it often to help you understand what
cannot be observed by the naked eye. The table below compares the macroscopic
properties of gases, liquids and solids and the corresponding microscopic view provided
by the model. Take the time to study it carefully.
Figure 1.7 - The three states of matter: properties and model
MACROSCOPICGAS LIQUID SOLIDPROPERTIES
Shape Indefinite Indefinite Definite
Volume Indefinite Definite Definite
Compressibility High Negligible Negligible
MODELGAS LIQUID SOLID(microscopic view)
Diagram
Distance between molecules Large Very small Very small
Principal types of movements Vibration, rotation Vibration Vibrationand translation and rotation
Attractive forcesbetween the molecules
No Yes Yes
Order No No Yes
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.20
Exercise 1.8
Consider again the nail that heats up when it is struck with a hammer (Exercise 1.6).
A nail is made up of atoms of iron.
a) Use the kinetic molecular model of matter to explain the presence of heat in the
head of the nail.
b) Compare your answer with the answer you gave in Exercise 1.6.
THERMOMETERS AND HEAT TRANSFERS
The phenomena that involve a change in temperature are numerous. The nail that
is hit with a hammer is just one example. The nail heats up (its temperature rises)
because the hammer transferred a part of its energy to the nail. Consider a second
example: a drop of alcohol on your skin produces a sensation of coolness as it
evaporates. How can we explain this? The temperature of the skin’s surface decreases
because the skin provides the energy necessary to evaporate the alcohol. A transfer
of energy has occurred between the skin and the alcohol. In the following experimental
activity, you will prepare a series of mixtures and, for each one, you will observe whether
a change in temperature occurs. You will also determine the direction of the energy
transfer.
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.21
?
Experimental Activity 1: Heat Transfers
In this first activity, you will observe a series of physical and
chemical changes. For each one, you will determine whether
or not a heat transfer has occurred. In most cases, you will
use a thermometer to detect the changes in temperature.
This is your first “hands-on” experience with the scientific
method in this second chemistry course. You will explore
different types of mixtures, compare the results obtained and interpret them while
remembering to take into account the thermometer’s degrees of accuracy. Allow
approximately 30 minutes to carry out all the steps in the experiment. Although the
steps are relatively simple, you must be meticulous in order to obtain meaningful results.
All of the information you need to carry out this activity is given in Section B of the
workbook Experimental Activities of Chemistry. Enjoy your work!
When a physical or chemical change causes the temperature of its surroundings to
increase, it is said to be exothermic. For instance, the dissolution of NaOH, which
you observed in the experimental activity, is exothermic. By releasing heat, it acted
as a source of energy and transferred heat to the solution (receptor).
Inversely, when a physical or chemical change produces a decrease in energy, energy
is absorbed from the surroundings or from an external source and the change is said
to be endothermic. For instance, ice melting is an endothermic change because energy
is absorbed in the process. In the experiment you conducted, the surrounding water
(source) provided the energy and the ice was the receptor. The temperature of the
water decreased because it gave up some of its energy to melt the ice.
In most of the changes studied in the activity, the thermometer was used to determine
whether a heat transfer had occurred and, if so, the direction of the energy flow. Have
you ever wondered how exactly a thermometer works? How does it measure temperature?
A thermometer is a common instrument used to determine whether a system is hotter
or colder than another by means of the temperature displayed on a scale. Remember
that temperature is associated with the kinetic energy of the molecules of a substance.
If the temperature rises, the kinetic energy of the molecules rises and, inversely, if the
temperature decreases, the kinetic energy also decreases. To better understand how a
thermometer works, let’s take a look at what happens at the microscopic level.
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.22
Mercury Thermometers
A mercury thermometer consists of a glass bulb filled with mercury attached to a
graduated tube whose inside diameter is as fine as a hair (capillary tube). Widely
used in the chemistry laboratory, thermometers are usually graduated in degrees
Celsius. According to this scale, water boils at 100°C, ice forms at 0°C and the
temperature of the human body is 37°C.
Figure 1.8b shows a microscopic view of a thermometer. To keep the diagram simple,
the solid walls of the bulb and of the capillary tube are represented as a single layer
of tightly packed glass particles firmly held together. The bulb contains liquid mercury
(Hg) whose atoms are less tightly packed than the glass molecules. The crowded
mercury atoms are in continuous motion, colliding with each other and with the
molecules that make up the glass wall. However, because the glass molecules are held
in place more firmly, they do not separate but vibrate in fixed positions, thus retaining
the atoms of Hg.
Figure 1.8 - Mercury thermometer
a) b)
a) Macroscopic view: the base of the thermometer consists of a bulb attached to a long graduated capillary tube.
b) Schematic diagram of a thermometer at the molecular level. To keep the diagram simple, only a few particles have been represented. In fact, a thermometer is made
up of billions upon billions of particles.
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.23
Capillary tube
Wall ofbulb
Mercuryatoms
Bulb
Immerse the bulb of a thermometer in a hot gas, such as the vapour escaping from
a saucepan of steaming vegetables, for instance. Be careful! The temperature of the
vapour can go up to 130°C. You may want to use a candy thermometer to conduct
this experiment.
Figure 1.9 - Thermometer immersed in a hot gas
a) b)
a) The bulb of the thermometer is immersed in the vapour escaping from the saucepan.
b) Bombarded by vapour molecules, the bulb of the thermometer heats up. The atoms of mercury (Hg) that it contains become more agitated and tend to occupy more space.
The liquid mercury expands and rises in the capillary tube attached to the bulb.
What happens at the microscopic level? The vapour molecules, which are very agitated,
bombard the walls of the bulb, transmitting a part of their kinetic energy to the glass.
The collisions cause the glass molecules to become agitated. In turn, the glass molecules
transmit energy to the Hg atoms. As they become more agitated, the distance between
the Hg atoms increases and, as a result, the mercury will try to occupy a greater volume.
We say that the mercury expands. The Hg atoms then move into the only available
space, the opening of the capillary tube, and we see the mercury rise. The height reached
by the mercury depends on the energy that has been transmitted to it.
Exercise 1.9
Now place the thermometer in the freezer. The thermometer’s bulb is now immersed
in cold air, whose molecules have a lower average kinetic energy than that of the
mercury atoms in the bulb.
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.24
?
Describe what happens:
a) at the macroscopic level.
b) at the microscopic level.
What happens when the molecules in the thermometer reach the same level of agitation
as those in the medium in which the thermometer is immersed? This is a specific
case with very important implications. If we immerse a thermometer in a gas whose
molecules are moving at the same speed as the molecules in the thermometer’s bulb,
then the molecules in the thermometer and those in the ambient gas cannot
accelerate each other’s movements. In this case, there is no transfer of energy between
the gas and the bulb. The mercury level does not change and the thermometer displays
a constant temperature.
The manner in which the mercury moves inside the thermometer is of special
significance. When it rises, the bulb absorbs energy; when it drops, the bulb loses
energy. When it is stable, there is no transfer of energy from the thermometer’s bulb
to the outside. The temperature of the thermometer then equals the temperature of
its surroundings.
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.25
Exercise 1.10
A thermometer is placed in a bowl of water and, after a few minutes, the level of the
mercury stabilizes. Then it suddenly starts to climb. How would you interpret this?
Thermal Equilibrium
A thermometer that is immersed in a liquid for any length of time will display a constant
temperature. The temperature will remain constant provided the surrounding
conditions remain the same, that is, the water does not cool down or heat up. We
say that the thermometer is in thermal equilibrium with the liquid. In other words,
the thermometer and the liquid are at the same temperature. At the microscopic level,
the degree of molecular motion is, on average, the same in both the thermometer
and the liquid.
We can therefore use a thermometer to detect heat transfers, or transfers of kinetic
energy between molecules. If a system comes in contact with an energy source, the
mercury in the thermometer rises. Inversely, if the system gives up energy to a receptor,
then the temperature drops and the mercury contracts.
Temperature can be expressed according to various scales. The most commonly used
ones are the Celsius, Kelvin and Fahrenheit scales. Let’s review here some of the
highlights in the history of thermometers.
Temperature Scales
As we have seen, our skin detects sensations of hot and cold. These sensations are
therefore very familiar to us. The need to define these sensations and quantify them
more objectively led scientists such as Galileo (circa 1592), Torricelli (circa 1672) and
especially Fahrenheit (1714) and Celsius (1742) to determine fixed reference points
between which a numerical scale could be established.
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.26
?
Delancé, in 1688, and Newton, in 1701, came up with the first scales for measuring
degrees of heat. For instance, Newton had arbitrarily set 0° as the point at which snow
melts, 12° as the temperature of the human body and 34° as the point at which water
boils vigorously.
Fahrenheit chose the coldest temperature obtained with a mixture of snow and
ammonia salt for the zero on his thermometer, and the boiling point of mercury, or
600°, for the highest point. He then divided the interval between these two points
into 600 equal divisions. On this scale, water freezes at 32°, water boils at 212° and
the temperature of the human body is 98.6°.
Today, Celsius’ reference points have been widely adopted and integrated into the
International System of Units (SI). The zero on the Celsius scale was obtained by
immersing the thermometer in ice water and the 100° mark was obtained by immersing
the thermometer in boiling water at standard atmospheric pressure (101.3 kPa).
Figure 1.10 - The most commonly used temperature scales
The reference points on the Celsius scale are the freezing and boiling points of water, set at 0°Cand 100°C respectively. On the Fahrenheit and Kelvin scales, the freezing point of water is 32°F
and 273 K respectively. Note that the Kelvin scale has only positive values.
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.27
373
273
0
212
32
−459
100
˚C ˚F K
0
−273Absolute 0All molecular
movement ceases.
Exercise 1.11
On my oven, I set the baking temperature for a raisin pie at 450°F. The red pilot light
came on right away and went off about five minutes later. I then opened the oven
door just long enough to put the pie in the oven. The red pilot light came on again
briefly and then went off. Ten minutes later, following Julia Child’s instructions, I
lowered the temperature to 350°F and continued to bake the pie for another
30 minutes. The pilot light did not come on again. The pie was delicious. As it was
piping hot, I had to wait 20 minutes before I could top it with a scoop of vanilla ice
cream and start eating it.
a) What is the purpose of the red pilot light?
b) Had we been able to read the oven thermometer, what would the mercury have
done during the procedure outlined in the problem above? Answer by completing
the following table.
PhasesPilot
EnergyMovement of mercury
light thermometer
Oven turned on, On Elements heating up Mercury rises.T = 450°F
After 5 minutes Éteint Les éléments cessent Le mercure est stablede chauffer.
Door opened Rouge Il y a perte de chaleur ; Le mercure monte.les éléments chauffent.
After 5 minutes Éteint Les éléments cessent Le mercure est stable.de chauffer.
T reduced to 350°F Éteint Les éléments Le mercure descend.ne chauffent pas
After 10 minutes Éteint Les éléments Le mercure descend ne chauffent pas. ou reste stable.
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.28
?
Caloric Fluid
Early scientists had formulated a theory to explain the propagation of heat. These men, including Antoine-
Laurent de Lavoisier (1789) and Sadi Carnot (1824), had imagined that heat was an invisible fluid that could
flow from one body to another. They named this fluid “caloric.” This is how they explained the heat and light
released during combustion.11 To measure heat, this elusive caloric was “broken down”into small units defined
as the quantity of caloric needed to raise the temperature of a given quantity of water by one degree. This
is how the unit of heat called the “calorie” originated.
THE MECHANICAL EQUIVALENT OF HEAT
While it seems natural to us today to consider heat as a form of energy, this relationship
was not so obvious in the early 19th century, when heat and energy were considered
to be two apparently unrelated phenomena. If you stop to think about it, how much
heat is needed to provide enough energy to raise a block of concrete onto a wall ten
metres high? Not so simple, is it?
James Prescott Joule (1818-1889) was the first to determine the number of units of
mechanical energy equivalent to one unit of heat (1 calorie). This relationship is called
the mechanical equivalent of heat. Joule established this correlation by comparing
the mechanical energy required to rotate paddles in a container of water with the
quantity of heat resulting from this action.
He defined his experiment after Count Rumford (Benjamin Thomson) had shown,
in 1798, that the drills used to bore cannons produced frictional heat that raised the
temperature of both the tube and the metal shavings. This is how Joule got the idea
of producing frictional heat by rotating paddles in water and measuring the resulting
rise in temperature. By definition, the calorie is the quantity of heat needed to raise
the temperature of one gram (1 g) of pure water by one degree (1°C) at standard
atmospheric pressure (101.3 kPa). Today, his name denotes one of the units used to
measure energy. For instance, we say that 4.18 joules of work are equivalent to one
calorie of heat.
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.29
11.Brock, William H. The Fontana History of Chemistry, New York: W. W. Norton (1993), p. 119.
Joule published the results of his work for the first time in 1843, but he had to wait
four years before his ideas were noticed. This occurred in Oxford, England, in August
1847. He was 28 years old. He had now obtained more precise measurements with
his paddle-wheel apparatus and wanted to make his results known to the British
Association. All the leading British scientists of the time belonged to this organization
and they periodically held meetings to discuss and exchange their ideas. The
president of the organization had given Joule a very short amount of time in which
to present his findings.
His work would have passed unnoticed if a young scientist named William Thomson,
who later came to be known as Lord Kelvin, had not agreed with Joule and started
a lively discussion. The ideas presented by Joule contradicted the caloric theory of
heat which was upheld by scientists at that time12 (see section entitled “Caloric Fluid”
above). From this point on, the scientific community started to take an interest in
Joule’s ideas, which are at the origin of the law of conservation of energy, considered
to be one of the cornerstones of contemporary science. The unit of work and energy
is named in his honour.
HEAT AND THERMAL ENERGY
Thermal energy and calorific energy (better known as heat) are examples of the two
views of matter we have discussed extensively so far. Heat is a macroscopic
phenomenon because it can be perceived by touch, while thermal energy is the sum
of the kinetic energies of all the particles in a system. It is therefore a microscopic
phenomenon. But what exactly is the link between heat and thermal energy? Can we
define heat in terms of thermal energy? Let’s find out.
When two systems at different temperatures come into contact, the warmer system
cools down and the cooler one heats up, until they both reach the same temperature.
In the process, a part of the thermal energy of the warmer system is transferred to
the cooler system. Today, science tells us that heat is the portion of thermal energy
that is exchanged between the two systems. The sensation of heat that we feel when
we touch a hot object is therefore produced by the thermal energy transferred to us
by the object. Inversely, when we touch an object that is colder than our body
temperature, the sensation of cold that we feel is due to the thermal energy we lose
to that object.
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.30
12.Boorse H., Motz L. Weaver. The Atomic Scientists, a Biographical History, New York: John Wiley & Sons, 1989,p. 61.
An analogy with the operation of a lock may help us distinguish between thermal
energy and heat. The transfer of heat between two systems at different temperatures
can be compared to the changes in water level in a lock (Figure 1.11). In this analogy,
the systems come into contact with each other when the sluice is opened, the water
that is exchanged represents the heat, and the total amount of water represents the
thermal energy. The different levels represent different temperatures.
Figure 1.11 - A boat making its way through a lock
A lock is a structure designed to move vessels from one elevation to another, either upstream ordownstream. A lock consists mainly of gates equipped with sluices that retain or let water outdepending on the direction in which the vessel is being moved. The lock chamber is the central
portion of the lock, and is located between the two gates.
a) A vessel is about to move downstream, that is, from the higher level to the lower level. The twogates and the sluices are closed. The upstream sluice is opened and gravity causes the water to
flow from the higher to the lower level. The lock chamber fills with water.
b) When the water in the lock chamber reaches the level of the water upstream, the gate is openedand the vessel enters the lock chamber. The gate and the upstream sluice are then closed and the
downstream sluice is opened. Gravity causes the water to flow from the lock chamber into the tail bay. The level of water in the lock chamber descends.
c) When the water in the lock chamber reaches the level downstream, the downstream gate isopened and the vessel moves into the tail bay. The vessel has then overcome the changes in
elevation and can continue on its course.
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.31
Lock chamber
a)
b)
c)
Tail bay
Head bay
Closed gate
Closed gate
Closed gate
Closed gate and sluice
Closed gateand sluice
Open gate and sluice
Open sluice
Closed sluice
Open sluice
As in the analogy, when two systems at different temperatures come into contact (open
sluice), a part of the thermal energy of the warmer system is transmitted to the molecules
of the cooler system until the levels of molecular motion are the same in both systems.
The kinetic energy that is transferred takes the form of heat. After the transfer, the two
systems are at the same temperature and have attained thermal equilibrium.
Kinetic energy, which results from the collisions between the molecules, is always
transferred from a warmer system to a cooler system. This idea can be expressed in
three different ways. For instance, we can say that heat flows from the warmer system
to the cooler system; from the system with the higher temperature to that with the
lower temperature; or from the system with the higher average kinetic energy to that
with the lower average kinetic energy.
The sensation of heat that we perceive results from the kinetic energy associated with
molecular motion and which is transferred from molecule to molecule and from atom
to atom as these particles collide. In short, a change in temperature indicates a transfer
of heat at the macroscopic level. At the microscopic level, however, this change indicates
a transfer of kinetic energy between the molecules or the atoms of the two systems
in question.
Exercise 1.12
How can you explain the fact that heat flows from a warm system to a cold system
and not the reverse?
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.32
?
1.3 HEAT EXCHANGES
When two systems at different temperatures come into contact, heat flows from the warmer
system to the cooler one until both reach thermal equilibrium, that is, the same temperature.
This is what happens when we mix a hot and a cold liquid together. After a while, the
temperature of the resulting mixture is uniform and falls somewhere between the initial
temperatures of the two liquids. Because heat is a form of energy, we can measure it and
assign numerical values to it. In the following experimental activity, you will study the
final temperatures of mixtures in order to derive a mathematical relationships for
determining the quantity of heat that is transferred from one liquid to another.
Experimental Activity 2: Final Temperature of a Mixture
In this activity, you will determine whether your predictions
about the final temperature of a mixture of hot and cold water
are true. You will then analyze the factors that affect the final
temperature of a mixture of liquids.
Allow approximately 50 minutes to conduct the experiment.
In order to obtain meaningful results, be meticulous in
carrying out the procedure. In this activity, you will also learn
more about experimental errors and uncertainty in measurements and become familiar
with writing instructions for the experimental procedure.
All of the information you require in order to carry out this activity is given in Section B
of the workbook Experimental Activities of Chemistry. Enjoy your work!
In the experimental activity, you derived an equation that explains heat transfers on
a macroscopic level. For the mixture of hot and cold water, you found that m1 × ΔT1
= –m2 × ΔT2. This equation confirms the law of conservation of energy. In fact, all
things being equal, the energy lost by the mass of hot water (m1) has been gained by
the mass of cold water (m1). In other words, the quantity of heat released by the source
(hot water) is equal to the quantity of heat gained by the receptor (cold water). If we
generalize the results of the experiment, we obtain the well-known relation that governs
heat exchanges and that we write as:
Qlost = Qgained or Qs = Qr,
where Q stands for heat, and the subscripts “s”and “r” for the source and receptor
respectively.
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.33
SPECIFIC HEAT CAPACITY
In the second part of Experimental Activity 2, you observed that windshield washer
fluid cools more rapidly than water when placed in the refrigerator. The nature of
the liquids involved is therefore a determining factor in heat transfers. More
generally, every substance may be characterized by its capacity to absorb or give up
heat. This property is called specific heat capacity13 and is designated by a
lowercase c. Specific heat capacity expresses the ratio between the heat Q supplied
to a system and the product m × ΔT. Mathematically, this is expressed as:
Qc = ––––––
m × ΔT
where Q is the quantity of heat gained or lost,
m, the mass of the substance,
ΔT, the change in temperature,
c, the heat capacity.
The table in Figure 1.12 gives the specific heat capacity of selected substances. Specific
heat capacity is expressed in terms of cal/g•°C or J/g•°C. The values are based on the
calculations done by engineering firms and are published in reference books which
can be consulted in science libraries.
You will note from the table that water has by far one of the highest specific heat
capacities on the planet. Only two gases found in relatively small amounts in the
atmosphere, hydrogen and helium, have a greater specific heat capacity than water.
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.34
13. Also called “heat capacity”
Figure 1.12 - Specific heat capacity of selected substances14
Substances Average specific heat capacity (c) Temperaturein the temperature interval interval
(ΔT) (ΔT)
(J/g•°C) (cal/g•°C) (°C)or
(kcal/kg•°C*)
Solids
Paraffin 2.97 0.710 0-20
Lumber, hard 2.93 0.700 10-60
Ice (water) 2.03 0.485 (−20)-(0)
Leather 1.50 0.360 0-100
Paper 1.30 0.310 20-60
Lumber, soft 1.25 0.300 10-60
Chalk 0.92 0.220 0-200
Clay, dry 0.92 0.220 20-100
Sandstone 0.90 0.215 0-100
Aluminum 0.89 0.214 0-700
Sand, dry 0.82 0.195 0-100
Glass 0.75 0.180 0-100
Nickel ($0.05) 0.46 0.111 0-100
Iron 0.45 0.107 20-100
Copper 0.39 0.094 20-1100
Tin 0.25 0.060 0-100
Tungsten 0.13 0.031 25
Liquids
Water (reference substance for the calorie) 4.19 1.000 0-100
Seawater 3.89 0.930 0-80
Methanol (also called “wood spirit”; highly poisonous) 2.51 0.600 20-25
Acetic acid 2.27 0.542 20-90
Mercury 0.14 0.033 0-100
Gases (constant pressure)
Hydrogen 14.18 3.392 20
Helium 5.23 1.250 20
Vapour (water) 1.90 0.455 20
Nitrogen 1.04 0.248 20
Air 0.99 0.238 20
* The units cal/g•°C and kcal/kg•°C are equivalent.
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.35
14. Based on Tuma, Jan J. Handbook of Physical Calculations. McGraw-Hill Book Company, 1976, pp. 301-303.
Exercise 1.13
a) Compare the specific heat capacity of ice, water and water vapour.
b) What requires more energy: heating a block of ice from –20°C to –10°C or heating
the same mass of water from 10°C to 20°C? Explain your answer.
Sea Breezes and Land Breezes
Along a shore, a wind can usually be felt blowing in from the sea in the daytime.
This is called a “sea breeze.” When the sun sets, the wind dies down and gradually
changes direction, blowing out to sea. This is called a “land breeze.” This phenomenon
is due to the considerable difference between the specific heat capacity of seawater
and that of the substances that make up the land mass (see the specific heat capacity
for sand and sandstone in the table).
While 0.82 J of solar heat are sufficient to raise the temperature of one gram of sand
by one degree, five times as much heat is needed to raise the temperature of one gram
of seawater by one degree. During the day, with the same amount of solar heat, seawater
heats up more slowly than rocks and sand. Land is therefore generally warmer than
the surface of the water. For this reason, the air over the shore is warmer than the
air over the water. Consequently, a current of warm air rises over the shore and a
current of cool air descends over the water. The combined effect of these two currents
creates a cool wind along the shore blowing in from the water. This is called a sea
breeze.
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.36
?
Figure 1.13 - Land breezes and sea breezes
a) Sea breeze b) Land breeze
In daytime, under the influence of the Sun’s rays, the air over the shore warms up more quicklythan the air over the water, producing a sea breeze. At night, the opposite phenomenon occurs. As
the seawater has stored more heat than the ground, a land breeze develops.
At night, the Sun disappears below the horizon. At this point, both the water and the
land begin to cool, but the water loses heat more slowly than the shore. The air over
the water therefore becomes warmer than the air over the land. Consequently, a
descending air current forms over the shore and a rising air current develops over
the sea. The combined effect of these currents produces a wind moving from the shore
out to the sea, or a land breeze.
HEAT EXCHANGE EQUATION
Recall the following equation that defines specific heat capacity:
Qc = ––––––
m × ΔT
If we rearrange this equation, we get the more familiar heat exchange equation shown
below. This equation means that the quantity of heat (Q) that flows from one system
to another is a function of the mass of the substance (m), its nature (c) and the change
in temperature observed (ΔT). We write:
Q = mcΔT = mc(Tf – Ti)
where Tf and Ti stand for the final and initial temperatures.
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.37
GWB
We can interpret this equation in three different ways.
• The quantity of heat (number of calories or joules) needed to raise the temperature
of a substance is proportional to the mass of the substance (Q ∝ m).
• The quantity of heat needed to heat a given mass of a substance is proportional
to the desired increase in temperature (Q ∝ ΔT).
• The ratio of the heat Q to the product (m × ΔT) is characteristic of the substance
and corresponds to c, or the specific heat capacity.
Combined with the law of conservation of energy, the heat exchange equation allows
us to calculate the amount of heat exchanged between two quantities of matter that
come into contact with one another. The operation of the calorimeter is based on
this equation. After taking a look at this instrument, we will consider other
applications of this equation.
Calorimeters
The transfer of heat between two substances is generally measured using a calorimeter.
This instrument is used mainly to determine the amount of energy released or absorbed
by a chemical reaction. The Styrofoam cup used in Experimental Activity 2 is a
simplified version of this instrument and is generally adequate for simple experiments
that do not require a great deal of precision.
Figure 1.14 - Calorimeter
A calorimeter consists of an insulated container at the centre of which is a reaction chamber. Thestirrer keeps the temperature of the water in the container uniform. The quantity of heat
transferred to the water (or absorbed from the water) during the reaction can bedetermined by measuring the temperature of the water before and after the reaction.
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.38
Stirrer
Thermometer
Water
Reactionchamber
A calorimeter consists essentially of an insulated container with a lid through which
a thermometer and a stirrer can be inserted. The container holds a known quantity
of water. The heat absorbed or lost by a reaction is measured by having the reaction
occur in a chamber immersed in the water in the calorimeter. The temperature of
the water is measured before the reaction and after, when the water and the reaction
chamber are in thermal equilibrium. In general, we assume that the thermal energy
lost or gained by the reaction remains in the calorimeter and we do not take into
account any heat lost through the openings or the spaces around the lid. Under these
conditions, all the heat released by the source is transferred to the water that serves
as the receptor. Of course, in special cases, another liquid can be substituted for water.
From now on and in all the following examples, we will assume that heat exchanges
occur within a perfect calorimeter. The systems will therefore be perfectly insulated
and losses will be considered negligible. Mathematically, this situation corresponds
to the equation we saw earlier on in this chapter: the quantity of heat released by
the source (Qs) is equal to that absorbed by the receptor (Qr).
Qs = Qr
If we replace each term in this equation with the terms in the heat exchange equation
and identify each symbol with the subscripts “s”and “r,” we get:
mscs ΔTs = mrcr ΔTr
If the two substances that come into contact with each other are of the same nature,
as in the case of hot and cold water, the specific heat capacity is the same on both
sides (cs = cr) and the equation is simplified. The equation is then the same as that
obtained in Experimental Activity 2:
ms ΔTs = mrΔTr which is equivalent to m1ΔT1 = –m2ΔT2.
Applications
This section includes several examples and a number of exercises. Take the time to
analyze the examples carefully, and try to understand and visualize the concepts which
underlie the equations. Equations represent a phenomenon or situation that you can
observe or that is described in a problem. Identify the source and the receptor, and
assign the appropriate subscripts to the quantities given. Check the values assigned
to each term in the chosen equation. Lastly, solve it by carrying out the necessary
algebraic operations and calculations.
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.39
Be careful to make the distinction between two concepts that are sometimes
confused with each other: heat and temperature. While heat refers to the amount of
energy that flows from one substance to another, a change in temperature indicates
the direction of the heat transfer. In short, remember this basic principle: heat is always
transferred from a warmer system (higher temperature) to a cooler system (lower
temperature). As the change in heat occurs, the temperature of the source drops while
that of the receptor rises.
Example
a) How much heat is needed to raise the temperature of a 100-g strip of copperfrom 20°C to 100°C? Give your answer in calories.
The heat source here is a given, and we assume that it provides all the heat required.The receptor is the strip of copper, whose specific heat capacity is given in thetable in Figure 1.12. The heat absorbed by the copper is proportional to its mass,and the rise in temperature depends on the specific heat capacity of copper. Theheat exchange equation can be used to describe what happens.
The mass of the strip of copper is 100 g, its initial temperature is 20°C and thefinal temperature is 100°C. The table gives c = 0.094 cal/g•°C for copper. Weare looking for Q. Let’s substitute the variables in the equation with the valuesin our problem. We get:
Q = mcΔTcal
Q = 100 g × 0.094 –––– × (100°C − 20°C)g•°C
Q = 752 cal
The heat source will therefore have to supply 752 calories to raise the temperatureof the strip of copper to 100°C.
b) The copper in a) is now replaced by a strip of aluminum. The aluminum has thesame mass, its initial temperature is the same and it absorbs the same amountof heat. Which of the two strips of metal is hotter? Answer first without doing anycalculations.
The strip of copper will be hotter. In fact, the specific heat capacity of aluminumis greater than the specific heat capacity of copper (see the table in Figure 1.12).
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.40
If the two strips of metal have the same mass, more heat is needed to raise thetemperature of the aluminum by one degree. The aluminum will therefore have alower final temperature.
c) Calculate the final temperature of the strip of aluminum.
The quantity of heat absorbed by the strip of aluminum (receptor) is the same asthat absorbed by the strip of copper, that is, 752 cal. Again, we can use the heatexchange equation. We are looking for the final temperature of the aluminum (Tf).We have m = 100 g, Ti = 20°C, c = 0.214 cal/g•°C and Q = 752 cal. By applyingthe heat exchange equation, we get:
Q = mcΔTQ
ΔT = ––––mc 752 cal × g•°C
ΔT = ––––––––––––––––100 g × 0.214 cal
ΔT = 35.1°C
ΔT = Tf − Ti
Tf = ΔT + Ti = 35.1°C + 20°C = 55.1°C
The temperature of the strip of aluminum will therefore be 55.1°C.
Exercise 1.14
How much heat is needed to raise the temperature of a 100-g strip of iron from 20°C
to 90°C? Give your answer in joules.
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.41
?
Example
Peter is about to wash the dishes and is preparing a basin of water. He pours 10 litresof hot water into the basin (T = 40°C) and then adds 1 litre of cold water (T = 12°C).What is the final temperature of the mixture? Remember that the mass of 1 litre ofwater is 1 000 g, since the density of water is 1 g/mL, or 1 kg/L.
The hot water is the source of heat and the cold water is the receptor. This situationis similar to the one you encountered in Experimental Activity 2, where you mixedknown quantities of hot and cold water. According to the law of conservation of energy:
Qs = Qr
mscsΔTs = mrcrΔTr
Since in this case water is both the source and the receptor, the value of c is thesame on both sides of the equation. In other words, cs = cr. By simplifying, we get:
msΔTs = mrΔTr (equivalent to m1ΔT1 = −m2ΔT2 )
The temperature of the hot water (source) will go from 40°C (Ti) to a certain finaltemperature (Tf). The mass is 10 kg since we have 10 L of hot water. The initialtemperature of the cold water (receptor) is 12°C and it will rise to the final temperature(Tf), which will be the same as that of the hot water since we are mixing them together.The mass of 1 L of cold water is 1 kg. We get:
msΔTs = mrΔTr (ΔT = Thigher – Tlower)10 kg × (40°C − Tf) = 1 kg × (Tf − 12°C)
By simplifying the kg, we get:10(40°C − Tf) = (Tf − 12°C)400°C − 10 Tf = Tf − 12°C
−11 Tf = −412°C Tf = 37°C
The temperature of the water prepared by Peter is therefore 37°C.
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.42
Example
How long will it take a 1 500-watt electric kettle to bring one litre of water to a boil?The water’s initial temperature is room temperature (20°C). Assume that 30% of theheat produced by the heating element is lost to the air.
First, determine the energy needed to bring the water to a boil. Remember that 1 Lof water has a mass of 1 kg (1 000 g). The heat exchange equation gives us:
Q = mcΔTJ
Q = 1 000 g × 4.18 –––––– × (100°C – 20°C)g•°C
Q = 3.344 × 105 J = 334.4 kJ
Therefore, 334.4 kJ of energy are needed to bring the water to a boil. However, only70% of the energy provided to the kettle goes towards heating the water. More energymust therefore be provided to the element. If we call the energy that goes towardsheating the water efficient energy, Eeff, and the energy that is dissipated by the element,Eel, we can write Eef f = 0.70 Eel since only 70% of the element’s energy serves toheat the water. We have:
Eef f = 0.70 Eel
Eef f 334.4 kJEel = –––––– = –––––––– = 477.7 kJ
0.70 0.70
A total of 477.7 kJ must be supplied to the kettle in order to bring the water to aboil.
Now, we will determine how long it will take to bring the water to a boil. Electric powerrepresents the amount of energy consumed by the kettle each second. One watt isequal to one joule per second (1 W = 1 J/s). The 1 500-W kettle therefore consumes1 500 J per second. We are now trying to calculate how much time it will take toprovide the kettle with the 477.7 kJ needed to bring the water to a boil.
Mathematically, we have:
EP = –––, where P stands for power, E for energy and t for time.
t
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.43
In our case, P = 1 500 W or 1 500 J/s, E = Eel = 477.7 kJ and t, the time we wantto calculate. We have:
EP = ––––
tEelP = ––––tEel 477.7 kJ 477.7 × 103 J × s
t = –––– = –––––––– = –––––––––––––––––– = 318 sP 1 500 W 1 500 J
mint = 318 s × ––––
60 st = 5.3 min
A little more than 5 minutes are required to boil one litre of water.
Exercise 1.15
An 800-g piece of glass has a temperature of 25°C. What will its temperature be after
it has absorbed 1 500 joules?
ENERGY IN PHASE CHANGES
So far, we have studied transfers of thermal energy from one system to another. Forexample, when a strip of hot copper immersed in a bowl of cold water causes thetemperature of the water to rise, it is an indication that a certain amount of energyhas flowed from the copper (source) to the water (receptor). We can calculate thisamount of energy by applying the law of conservation of energy and the heat exchangeequation.
But do these principles apply in all situations? What happens with regard to energywhen the water being heated starts to boil? Or when ice melts?
Let’s now consider what happens when a block of ice is heated continuously. Thissituation is illustrated by the graph of the temperature as a function of time, knownas the heating curve. Most of the curve can be obtained with a setup such as the oneshown in Figure 1.15.
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.44
?
Figure 1.15 - Heating a block of ice
After immersing a thermometer in a container of water, the entire apparatus is placed in thefreezer for some time. When it is removed, the water is frozen and the thermometer reads –20°C.
The container is then placed on a heating plate, and the temperature on the thermometer is recorded every minute.
Figure 1.16 - Heating curve for ice
(a) In the interval between –20°C and 0°C, the temperature rises and the ice absorbs heat from the source.
(b) Starting at 0°C, liquid water appears and the ice starts to melt. The temperature remains stable at 0°C until the ice is completely melted.
(c) If we continue to supply heat, the temperature will rise and the water will boil at 100°C.
(d) The temperature remains stable as long as some liquid water remains.
(e) If we recover the vapour and continue to supply heat, the temperature will start to rise again, as indicated by the last interval on the graph.
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.45
−20˚C
Block of ice
120
100
80
60
40
20
0
−20
Tem
pera
ture
(˚C
)
Time
Water vapour
Water and water vapour(vapourization)
Water
Ice andwater(melting)
Ice
(e)
(a)
(b)
(c)
(d)
The heating curve for water was covered in the first guide.15 This curve includes five
intervals and features two plateaus corresponding to the two phase changes. Let’s
see what happens in these plateaus at the microscopic level. First, we will analyze
the three increases in temperature, that is, intervals (a), (c) and (e) on the curve.
In interval (a), the temperature of the ice rises. The kinetic energy of the molecules
increases, causing their vibrations to become more intense.
In interval (c), the liquid water heats up. The kinetic energy of the molecules increases
and they become more agitated, causing the mercury in the thermometer to rise.
In interval (e), the temperature of the vapour increases and, consequently, so does
the kinetic energy of the molecules. Their movements become increasingly agitated.
Note that intervals (a), (c) and (e) have different slopes. In fact, the slope of the intervals
is a reflection of the capacity of the ice, water or vapour to absorb heat. Since liquid
water has the highest specific heat capacity, more energy is required to heat it and
its temperature rises more slowly. On the graph, the slope of the interval representing
water as a liquid is clearly less steep than the slopes obtained for ice and vapour.
Exercise 1.16
Why are burns caused by water vapour often more serious than those caused by boiling
water?
Melting
Now let’s examine the first plateau on the curve, or interval (b), in more detail. The
temperature remains stable as the ice gradually melts. The average kinetic energy of
the molecules remains the same (constant temperature) even though the heat
supplied is being absorbed. What happens exactly? If the block of ice is to remain a
solid, in other words, if it is to keep its shape, the forces of attraction between the
molecules must be sufficiently strong enough to keep them together, in fixed
positions in relation to each other. The case of liquid water is different because it
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.46
15. Lalancette, Pauline and M. Lamoureux. Gases (Chemistry, Secondary V), Chapter 1. Learning Guide producedby SOFAD.
?
takes the shape of its container. We can conclude from this that the heat provided
during the time the temperature remains constant serves to weaken the attractive
forces until the molecules no longer adhere firmly together in fixed positions. Since
the molecules no longer adhere together as strongly, they start to slide over one another
on the sides of the block of ice and only the walls of the container prevent them from
flowing further. The molecules now form a liquid.
It’s as though the energy from the outside source were “eaten up” by the ice. In more
scientific language, we say that the energy absorbed by the solid is transformed into
potential energy as the attractive forces between the molecules of the solid weaken.
If the water were cooled and once again solidified, the ice would release an amount
of energy equal to the energy that went into melting it, meaning that potential energy
would once again be converted to heat.
The heat of fusion of a substance is the energy required to turn it from a solid into
a liquid at its melting point. It is characteristic of the substance and is measured in
calories per gram (cal/g) or in joules per gram (J/g) of melted solid. This value is
determined experimentally and the heat of fusion of a large number of solids can be
found in specialized reference books and in some commercial periodic tables. The
heat of fusion of ice is 334 J/g or 80 cal/g under standard conditions (0°C and 101.3 kPa).
Exercise 1.17
What is the role of the energy absorbed by a melting solid?
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.47
?
Boiling
Have you ever noticed that boiling water doesn’t get hotter even if it is heated longer?
The second plateau on the heating curve confirms this. The temperature is constant
in interval (d).
This phenomenon is similar to what happens when ice melts. When the water’s
temperature reaches 100°C, bubbles form in the water and the temperature remains
stable as long as there is liquid water remaining. The energy absorbed by the molecules
of water serves to separate them, making them independent of each other. They then
form a gas.
In the example of the heating plate, the heat source is in direct contact with the bottom
of the container, causing the water at the bottom of the container to heat up faster.
It is not surprising then that the first molecules to succeed in breaking away from
the liquid are those at the bottom of the container. These molecules form bubbles
which quickly rise to the surface, in the same way that a submerged cork or a ping-
pong ball floats to the surface when released. The bubbles burst when they reach the
surface of the water and release the “independent” molecules into the air. These then
form water vapour.
As soon as they are in air, the molecules collide with the molecules of nitrogen and
oxygen, which together make up 98% of air. They lose a part of their kinetic energy
to the air, and this slows them down. Some of the molecules adhere together, creating
minute drops of liquid and producing a white cloud. This is why the vapour that forms
above boiling water is white.
The heat of vapourization of a substance is the heat needed to vapourize a given
mass of the substance at its boiling point. It is specific to each substance and is
expressed in cal/g or in J/g. The heat of vapourization for water is 540 cal/g or 2 255 J/g.
Remember that we use the term “boiling” to refer to the rapid vapourization of a liquid
into gas (e.g. boiling of water, of liquid hydrogen) and “evaporation” to refer to the
slow vapourization of liquid into a gas (e.g. the evaporation of water, of alcohol).
The following table shows the typical heats of fusion and of vapourization of selected
substances. Note that in the case of water, the heat of vapourization is much higher
than the heat of fusion. This is why the vapourization plateau is longer than the melting
plateau on the heating curve for water.
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.48
Figure 1.17 - Heats of fusion and of vapourization of selected substances
NameTfusion ΔΔHfusion* Tvapourization ΔΔHvapourization*
(°C) (J/g) (°C) (J/g)
Alcohol (methanol) 64.5 502
Aluminum 658 396 2 970 10 759
Copper 1 083 206 2 567 4 728
Water 0 333 100 2 255
Tungsten 3 410 192 5 660 2 296
*ΔHfusion stands for heat of fusion and ΔHvapourization stands for heat of vapourization.
Example
A 400-g strip of aluminum whose temperature is 25°C is heated to its melting point.Heat is then applied to it until it is completely melted. How much energy did thealuminum absorb?
Using the specific heat capacity for aluminum, we first calculate the energy neededto heat the aluminum and then the energy it absorbs as it melts. The total energyis obtained by adding these two values together.
Heating the solid
We have m = 400 g; c = 0.89 J/g•°C (table in Figure 1.12); Ti = 25°C and Tf = 658°C (table in Figure 1.17). We are looking for Q. Using the heat exchangeequation, we get:
Q = mcΔTJ
Q = 400 g × 0.89 ––––– × (658°C – 25°C)g•°C
Q = 225 348 J ou 225.3 kJ
225.3 kJ of energy are required to heat the aluminum to the melting point.
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.49
Melting
We know, from the table, that the heat of fusion is 396 J/g. Using the property ofproportions, we have:
400 g × 396 J1 g → 396 J –––––––––––––– = 158 400 J400 g → ? J 1 g
158.4 kJ are required to melt the aluminum.
Total energy
Qtotal = Qheating + Qfusion
Qtotal = 225.3 kJ + 158.4 kJQtotal = 383.7 kJ
The aluminum absorbed a total of 383.7 kJ.
Exercise 1.18
A 200-g strip of copper whose temperature is 22°C is heated to its melting point. Then
it is heated again until it is completely melted. How much energy was supplied?
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.50
?
1.4 TECHNICAL APPLICATIONS
THE BREAD OVEN
Readily perceived by the senses, heat is closely associated with human activity. Bread
ovens, whose origins go back to biblical times, are proof of this. Our ancestors built
their ovens with stones capable of storing large amounts of heat. A fire was allowed
to burn in the oven for a certain amount of time and, once the stones were hot, the
fire was put out and the wood was replaced with bread dough. The heat that had
accumulated in the stones then baked the bread. The same principle is used today
in restaurants that serve pizza cooked in a wood oven.
Figure 1.18 - Bread oven
Refractory stones have great thermal capacity. Once heated, they slowly release the stored heat
that serves to bake the bread.
The example of the bread oven is a good illustration of the usefulness of heat. Other
useful applications of heat include the teakettle and heating systems. However, in many
human activities, heat can be a useless, and even undesirable, by-product.
Exercise 1.19
a) List two or three examples where heat is useful for human activity.
b) List two or three examples where heat is undesirable and even useless.
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.51
?
GWB
To conclude, we will focus on two spectacular examples that involve kinetic energy
and heat simultaneously.
RE-ENTERING THE ATMOPSHERE
In orbit, a spacecraft has an enormous amount of kinetic energy. Just think that it
circles the Earth in just 90 minutes and that it travels at a speed of over 25 000 km/h!
To land, the spacecraft must reduce its speed to about 250 km/hr in approximately
10 minutes. As its speed decreases, the spacecraft loses kinetic energy. However, this
energy is transformed into heat, which is the result of friction between the spacecraft
and the atmosphere. This heat must be re-channelled to prevent it from burning up
the passengers and disintegrating the vehicle. For this purpose, special insulating tiles
are used to line the bottom of the spacecraft. These tiles serve to re-channel part of
the heat but they also absorb most of it. In fact, the tiles contain a layer that melts,
thereby absorbing a great deal of heat without an increase in temperature. They have
a very low density and keep their properties up to temperatures of around 3 000°C.
They therefore form a very effective thermal shield.
Figure 1.19 - A space shuttle losing kinetic energy
When the space shuttle slows down, the lost kinetic energy is transformed into heat. The special tiles that line its surface form a heat shield that protects the crew
as well as the spacecraft and the equipment.
GEYSERS
Geysers are natural “machines” that periodically spout columns of water and vapour
up to 100 metres high. They draw energy from the Earth’s core where melting rocks
reach temperatures of several hundreds of degrees Celsius. The water that drains into
the ground settles in cavities hundreds of metres below the surface of the Earth and
in the channels that lead from these cavities to the surface (Figure 1.20). The water
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.52
that accumulates in the channel compresses the water below it. Because of the pressure,
the water boils at much higher temperatures than normal. After a certain time, the
pressure of the vapours formed at the bottom of the cavities becomes so high that it
succeeds in explosively ejecting a large part of the accumulated water and vapour. This
is how a geyser is formed. Geothermic energy, which comes from the heat released
by magma,16 is transformed into kinetic energy that projects the water outside its natural
well. Runoff water drains into the ground and the cycle begins again.
Figure 1.20 - A geyser
Hot water periodically erupts from the geyser. The water, heated by the surrounding rocks, isforced out with great intensity at more or less regular intervals. The water that shoots up often
contains sulphurous matter and mineral deposits.
In this chapter, we examined the topic of energy in greater detail. The kinetic
molecular model of matter enhanced our understanding of heat transfers and
the examples studied, all involved the concept of energy, along with the kinetic
molecular model of matter helped enhance our understanding of heat transfers.
This concept seems to be at the root of all explanations and of all physical and
chemical changes. Did we not define it as “the capacity of an object to produce
an effect”? Its importance will become even more apparent in the rest of this
course. In the next chapter, we will explore the phenomenon of dissolution by
concentrating on the types of energy involved.
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.53
Superheated
water T > 200°C
16. A very hot viscous liquid formed by the melting of the Earth’s crust or mantle and which, after being ejectedthrough fissures (or volcanoes), forms volcanic rock.
GWB
Calorie CalorimeterCohesive force Conservation of energy
Endothermic EnergyExothermic
Heat (calorific energy) Heat of fusionHeat of vapourization
Joule
Kinetic energy
Macroscopic Melting pointMicroscopic
Potential energy
Specific heat capacity (c)
Thermal energy Thermal equilibrium
Work
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.54
KEY WORDS IN THIS CHAPTER
Energy exists in many different forms, for example, electrical, nuclear, mechanical,
and so on. It can be stored just as it can be converted from one form to another. All
types of energy have one thing in common: the ability to produce an effect that can
be perceived, such as light, movement or heat. Heat is a form of energy associated
with numerous phenomena. Some, like combustion, produce heat and are said to be
exothermic. Others, such as ice melting and water evapourating, absorb heat and
are said to be endothermic.
All substances can store an amount of heat that is determined by their specific heatcapacity (c). The sum of the kinetic energies of the molecules in a system makes
up the thermal energy of the system. Temperature is an indication of the level of
this energy. Two systems that come into contact with each other tend to reach thermal
SUMMARY
equilibrium. Heat transfer always occurs in the same direction, that is, from the system
with the higher temperature (source) to the system with the lower temperature
(receptor). Heat is the sensation we perceive during the transfer of thermal energy.
Changes in heat are governed by the following equations:
Qs = Qr law of conservation of energy, and
Q = mcΔT heat exchange equation
The first equation means that the heat released by the source is absorbed by the
receptor. The second equation determines the amount of heat absorbed (Q) by a
substance with mass m and heat capacity c when its temperature is raised by a number
of degrees ΔT.
A calorimeter serves to measure the amount of energy released or absorbed during
a heat transfer or a chemical reaction. The Styrofoam cup used in the experimental
activities is a simplified version of this instrument.
The heating curve for a substance generally features two constant temperature plateaus
that reflect phase changes. The energy absorbed during melting and vapourization
serves to separate the molecules (or atoms) and it is stored in the form of potentialenergy. The same amount of energy is then released in the opposite phase changes
(melting and solidification). Between the plateaus on the curve, the slope of the intervals
indicates the heat capacity of the different phases.
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.55
Exercise 1.20
Each of the following descriptions corresponds to an instrument that detects,
converts or measures energy. From the list, choose the instrument that best
corresponds to the descriptions given in the table.
Television antenna, radio set, sonar, telescope, television set,
thermometer, voltmeter, wattmeter
Description Instrument Form of energy
A bulb containing a liquid and attached to a capillary tube which detects heat loss or heat gain in the surroundings
An instrument used to measure the magnitude of electric potential difference between two terminals electrique
A device similar to a radar, which uses sound waves to detect and locate underwater objects
énergie acoustique
A device used to capture electromagnetic waves emitted by a remote source telévision electromagnétique
A device used to decode and translate the waves from a television antenna into images electromagnétique
An optical instrument used to observe distant objectsTelescope Énergie lumineuse
A device used to decode and translate the waves from an antenna into sound
Poste de radio Énergie electrique
An instrument for measuring the magnitude of the electrical power in a circuit
Wattmètre Énergie electrique
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.56
REVIEW EXERCISES
?
Exercise 1.21
a) Describe the energy conversions that take place when a spring is compressed and
then immediately released. Use the example of the bow and arrow in the “Energy
Conversions” section of this chapter to describe the sequence of conversions that
take place.
b) Discuss how the law of conservation of energy applies in this situation.
Exercise 1.22
Give a microscopic explanation for each of the following phenomena:
a) A wooden stair is gradually worn down.
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.57
?
?
b) A small puddle of water evaporates in the Sun.
c) Two small drops of water brought close together form a single drop.
d) Water boiling in a Pyrex container forms small bubbles at the bottom of the
container.
e) Surface runoff water that seeps into a geyser’s underground channel boils at very
high temperatures, sometimes reaching 250°C.
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.58
Exercise 1.23
Indicate whether the following phenomena are endothermic or exothermic. For each
equation, include the heat term on the appropriate side.
a) The combustion of wax: wax(s) + O2(g) → CO2(g) + H2O(g)
b) The formation of ice: water(l) → water(s)
c) Identification test for hydrogen: 2 H2(g) + O2(g) → 2 H2O(g)
d) Lead melting: Pb(s) → Pb(l)
e) Heating a quantity of water: water at 37°C → water at 88°C
Exercise 1.24
Calculate the quantity of heat absorbed by 180 g of water if its temperature is raised
by 20°C. Give your answer in kJ.
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.59
?
?
Exercise 1.25
A 500-g block of ice (T = 0°C) melts in a dish overnight. In the morning, the temperature
of the water is 14°C. Calculate the energy absorbed by the water during the night.
Exercise 1.26
Thirty litres of hot water at 40°C are mixed with 20 litres of cold water at 3°C. What
is the final temperature of the mixture?
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.60
?
?
Exercise 1.27
A drop of methanol (wood spirit) is placed on your hand. It evaporates immediately,
producing a sensation of coolness.
a) How much energy did your skin supply to the methanol? The mass of the methanol
is 0.1 g and its initial temperature is 22°C.
b) Does most of the energy go into heating the methanol or in evaporating it?
Exercise 1.28
Aluminum’s molar heat of fusion is the amount of energy needed to melt one mole
of aluminum atoms once the aluminum has reached the melting point. It is expressed
in kJ/mol. Calculate the molar heat of fusion of aluminum.
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.61
?
?
Exercise 1.29
In your own words, explain why the comparison of the molar heat of fusion of various
solids may be used to compare the cohesive forces between the molecules (or atoms)
of these solids.
Exercise 1.30
A nail is struck with a hammer. The nail becomes embedded in the plank and
immediately after being struck the head of the nail feels warm. How was the kinetic
energy transferred from the hammer to the nail?
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.62
?
?
Exercise 1.31
Explain how a thermostat functions. Refer to a user’s manual or the dictionary.
Chemical Reactions 1 - Chapter 1: Heat: Energy in Motion
1.63
?