HEAD A_UND OBJ TEXT_UND 1 STOICHIOMETRIC RELATIONSHIPS Introduction There is a broad community o people working within a wide variety o scientif c disciplines and approaching their inquiry with common methodology, terminology and reasoning processes. Chemistry can be regarded as the central science, and mathematics the language o science. In this chapter we begin to lay down many o the oundations on which an understanding o chemistry is based. From the classif cation o matter to the IUPAC organization o the nomenclature o organic and inorganic compounds and the representations o chemical reactions by equations, this chapter discusses the comprehensive language o chemistry. For chemists, the mole concept is o undamental importance. Its def nitions in relation to the number o particles, mass and the volume o a gas elicit universal understanding and stoichiometry, the quantitative method o examining the relative amounts o reactants and products in a particular chemical reaction is developed. Treatment o the gas laws and the application o volumetric analysis complete this introductory chapter. 1.1 Introduction to the particulate nature of matter and chemical change Applications and skills Deduction o chemical equations when reactants and products are speci ed. Application o the state symbols (s), (l), (g), and (aq) in equations. Explanation o observable changes in physical properties and temperature during changes o state. Nature of science Making quantitative measurements with replicates to ensure reliability de nite and multiple proportions. Understandings Atoms o dif erent elements combine in xed ratios to orm compounds, which have dif erent properties rom their component elements. Mixtures contain more than one element and/ or compound that are not chemically bonded together and so retain their individual properties. Mixtures are either homogeneous or heterogeneous. 1
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HEAD A_UND
OBJ TEXT_UND
1 S TO I CH I O M E TR I C R E LAT I O N SH I PS
IntroductionThere is a broad community o people working
within a wide variety o scientif c disciplines
and approaching their inquiry with common
methodology, terminology and reasoning
processes. Chemistry can be regarded as the
central science, and mathematics the language
o science. In this chapter we begin to lay
down many o the oundations on which an
understanding o chemistry is based. From the
classif cation o matter to the IUPAC organization
o the nomenclature o organic and inorganic
compounds and the representations o chemical
reactions by equations, this chapter discusses the
comprehensive language o chemistry.
For chemists, the mole concept is o
undamental importance. Its def nitions in
relation to the number o particles, mass and the
volume o a gas elicit universal understanding
and stoichiometry, the quantitative method o
examining the relative amounts o reactants
and products in a particular chemical reaction
is developed. Treatment o the gas laws and the
application o volumetric analysis complete this
introductory chapter.
1.1 Introduction to the particulate natureof matter and chemical change
Applications and skills Deduction o chemical equations when
reactants and products are speci ed.
Appl ication o the state symbols (s) , ( l ) , (g) ,
and (aq) in equations.
Explanation o observable changes in
physica l properties and temperature during
changes o state.
Nature of science Making quantitative measurements with repl icates to ensure rel iabil ity de nite and multiple
proportions.
Understandings Atoms o d if erent elements combine in xed
ratios to orm compounds, which have d if erent
properties rom their component elements.
Mixtures contain more than one element and/
or compound that are not chemical ly bonded
together and so retain their individual properties.
Mixtures are either homogeneous or
heterogeneous.
1
The atomic theoryA universally accepted axiom o science
today is that all matter is composed o atoms.
However, this has not always been so. During
the seventeenth century the phlogiston theory
was a widely held belie. To explain the process
o combustion it was proposed that a fre- like
element called phlogiston , said to be ound
within substances, was released during burning.
Quantitative investigations o burning metals
revealed that magnesium in act gains rather than
loses mass when it burns in oxygen, contradicting
the phlogiston theory.
Scientists use a wide range o methodologies,
instruments, and advanced computing power
to obtain evidence through observation and
experimentation. Much o the technology
commonly used today was not available to
scientists in the past, who oten made ground-
breaking discoveries in relatively primitive
conditions to eed their appetite or knowledge.
Over time, theories and hypotheses have been
tested with renewed precision and understanding.
Some theories do not stand the test o time.
The best theories are those that are simple and
account or all the acts.
The atomic theory states that all matter is
composed o atoms. These atoms cannot be
created or destroyed, and are rearranged during
chemical reactions. Physical and chemical
properties o matter depend on the bonding and
arrangement o these atoms.
TOK
Antoine Lavoisier (17431794) is
oten reerred to as the ather o
modern chemistry. H is contribution
to science is wel l documented. I n
1772 Lavoisier d iscovered through
experimentation that when su lur
and phosphorus were combusted
they gained mass. These resu l ts
contrad icted the bel ie that mass
would be lost during combustion
as phlogiston was released.
Cou ld phlogiston have a negative
mass? Empirica l data derived
rom Lavoisiers experiments was
eventual ly accepted by the scientiic
community . H is work conta ined
some o the i rst examples o
quanti tative chemistry and the
l aw o conservation o mass. H is
experiments may appear simple by
present-day standards but they were
ground-breaking in their day .
The discovery o oxygen by Joseph
Priestly and Carl Scheele inval idated
the phlogiston theory. This is an
example o a paradigm shit. The
dominant paradigm or bel ie is
replaced by a new paradigm. Is this
how scientifc knowledge progresses?
States of matterMatter is everywhere. We are made up o matter, we consume it, it
surrounds us, and we can see and touch many orms o matter. Air is a
orm o matter which we know is there, though we cannot see it. Our
planet and the entire universe are made up o matter and chemistry
seeks to expand our understanding o matter and its properties.
Figure 1 The characteristics of matter
made up of
particles
atoms,
molecules,
or ions
particles are
in constant
motion
has a mass
MATTER
occupies a
volume in
space
2
1 STO I CH I O M E TR I C R E L AT I O N SH I PS
The way the particles o matter move depends on the temperature. As
the temperature increases the average kinetic energy o the particles
increases the particles in a solid vibrate more. The particles in liquids
and gases also vibrate, rotate, and translate more.
Temperature There are a number o dierent temperature scales. The most commonly
used are the Fahrenheit, Celsius, and Kelvin scales. All three are named
in honour o the scientist who developed them.
The S I unit or temperature is the kelvin (K) . The Kelvin scale is used in
energetics calculations ( see topic 5 ) .
Absolute zero is zero on the Kelvin scale, 0 K (on the Celsius scale
this is 273 C ) . It is the temperature at which all movement o particles
stops. At temperatures greater than absolute zero, all particles vibrate,
even in solid matter.
You can convert temperatures rom the Celsius scale to the the Kelvin
scale using the algorithm:
temperature (K) = temperature ( C) + 2 73 .1 5
Changes of state I you heat a block o ice in a beaker it will melt to orm liquid water. I
you continue heating the water, it will boil to orm water vapour. Figure 2
shows a heating curve or water it shows how its temperature changes
during these changes of state. We shall look at the relationship between
temperature and the kinetic energy o particles during these changes o state.
Soi liqi gas
f xed volume
f xed shape
cannot be compressed
attractive orces between
particles hold the particles in
a close-packed arrangement
particles vibrate in f xed
positions
f xed volume
no f xed shape takes the shape
o the container it occupies
cannot be compressed
orces between particles are
weaker than in solids
particles vibrate, rotate, and
translate (move around)
no f xed volume
no f xed shape expands to
occupy the space available
can be compressed
orces between particles are
taken as zero
particles vibrate, rotate, and
translate aster than in a liquid
SI (Systme International) units are a
set of standard units that are used in
science throughout the world. This wil l be
discussed in great detail in sub-topic 1.2.
When describing room temperature, we
might say 25 degrees Celsius (25 C) or
298 kelvin (298 K) (to the nearest kelvin) .
Note that we use just the word kelvin, not
degrees kelvin. The boiling point of water
is 100 C or 373 K, and the melting point of
water is 0 C or 273 K.
Figure 2 The heating curve for water
temperatu
re/8C
energy input
water
melting
freezingice
evaporation steam
condensation
100
0
The properties o the three states of matter are summarized below.
3
1 . 1 I n T r O d u c T I O n TO T h e pAr T I c u l AT e n AT u r e O f m AT T e r An d c h e m I c Al c h An g e
Elements and compoundsAn element contains atoms o only one type. Atoms o elements combine
in a fxed ratio to orm compounds composed o molecules or ions. These
rearrangements o the particles o matter are the undamental cornerstone
o chemistry, represented in ormulae and balanced chemical equations.
(Atoms are covered in detail in sub-topic 2 .1 . )
What happens to the particles during
changes of state? As a sample o ice at 1 0 C (263 K) is heated, the water molecules
in the solid lattice begin to vibrate more. The temperature increases
until it reaches the melting point o water at 0 C (273 K) .
The ice begins to melt and a solidliquid equilibrium is set up.
Figure 2 shows that there is no change in temperature while
melting is occurring. All o the energy is being used to disrupt the
lattice, breaking the attractive orces between the molecules and
allowing the molecules to move more reely. The level o disorder
increases. (The nature o the orces between molecules is discussed
in sub-topic 4.4. )
Once all the ice has melted, urther heating makes the water
molecules vibrate more and move aster. The temperature rises
until it reaches the boiling point o water at 1 00 C (373 K) , and
the water starts to boil.
At 1 00 C a liquidgas equilibrium is established as the water boils.
Again the temperature does not change as energy is required to
overcome the attractive orces between the molecules in the liquid
water in order to ree water molecules rom the liquid to orm a
gas. (Equilibrium is covered in sub-topic 7 .1 . )
The curve in fgure 2 shows that while the water is boiling its
temperature remains at 1 00 C . Once all the liquid water has been
converted to steam, the temperature will increase above 1 00 C .
Melting and boiling are endothermic processes. Energy must
be transerred to the water rom the surroundings to bring about
these changes o state. The potential energy ( stored energy) o the
molecules increases they vibrate more and move aster.
Cooling brings about the reverse processes to heating the
condensation o water vapour to orm liquid water, and the
freezing o liquid water to orm a solid.
Condensation and reezing are exothermic processes. Energy
is transerred to the surroundings rom the water during these
changes o state. The potential energy o the molecules decreases
they vibrate less and move slower.
Vaporization is the change o state rom liquid to gas which may
happen during boiling, or by evaporation at temperatures below
the boiling point. In sublimation matter changes state directly rom
the solid to gas phase without becoming a liquid. Deposition is the
reverse process o sublimation changing directly rom a gas to a solid.
Activity
1 Expla in why the
temperature o a boi l ing
l iqu id does not increase
despite energy being
constantly appl ied .
2 Deduce which wou ld be
more pa inu l , sca ld ing your
skin with water vapour or
bo i l ing water.
3 Explain why you might eel
cold and shiver when you get
out o the water at the beach
on a very hot, windy day .
Freeze-drying is a ood
preservation technique
that uses the process o
sublimation . Foods that require
dehydration are rst rozen and
then subjected to a reduced
pressure. The rozen water
then sublimes directly to water
vapour, efectively dehydrating
the ood. The process has
widespread applications in
areas outside the ood industry
such as pharmaceuticals
(vaccines) , document recovery
or water-damaged books, and
scientic research laboratories.
deposit ionsu
blimation
free
zing
melting
vaporization
solid
liquid gas
condensation
Figure 3 Changes of state for water
4
1 STO I CH I O M E TR I C R E L AT I O N SH I PS
Figure 5 The structure of sodium chloride. It consists of a crystall ine
lattice of sodium ions (purple) and chloride ions (green)
The halogen chlorine is a gas at room temperature. Chlorine, C l2, is
highly irritating to the eyes, skin, and the upper respiratory tract.
The highly reactive elements sodium and chlorine combine to orm
the ionic crystalline compound sodium chloride, commonly called
table salt and consumed daily in the ood we eat. The properties
and uses o sodium chloride are very dierent rom those o its
constituent elements.
Mixtures
A pure substance is matter that has a constant composition. Its
chemical and physical properties are distinct and consistent. Examples
include the elements nitrogen, N2 and argon, Ar and compounds such as
water, H2O, table salt, NaC l, and glucose, C
6H
12O
6.
Pure substances can physically combine to orm a mixture . For
example, sea water contains mainly sodium chloride and water. Pure
substances can be separated rom the mixture by physical techniques
such as fltration, ractional distillation, or chromatography. The
Figure 4 Elemental sodium is a reactive a lkal i metal
Chemists study how elements and compounds react with one another,
the many dierent chemical and physical properties o the substances
created in these reactions, and how they can be used in many
important applications.
The compound sodium chloride, NaCl, is made up o the elements
sodium and chlorine.
The group 1 alkali metal sodium is a sot metal that undergoes rapid
oxidation in air and violently reacts with water, creating alkaline solutions.
Sodium is stored under oil to prevent these reactions. It is the sixth most
abundant element on the planet, (2 .26% by mass) .
5
1 . 1 I n T r O d u c T I O n TO T h e pAr T I c u l AT e n AT u r e O f m AT T e r An d c h e m I c Al c h An g e
elements or compounds that make up a mixture are not chemically
bound together.
Homogeneous mixtures have both uniorm composition and uniorm
properties throughout the mixture. Examples include salt water or a
metal alloy such as brass. Heterogeneous mixtures have a non-uniorm
composition and hence their properties vary throughout the mixture.
Examples include oods such as tom yum goong (Thai hot and sour
prawn soup) or Irish stew (a mixture o cubed meat and vegetables) .
Figure 9 summarizes the classifcation o matter into elements,
compounds, and mixtures.
Figure 6 Chlorine reacts vigorously
with sodium metal
Figure 8 Paper chromatography is used to
investigate industrial dyes by separating them
into their pure constituent components
The language of chemistryChemistry has a universal language that transcends borders and
enables scientists , teachers , and lecturers, s tudents , and citizens o
the wider community to communicate with each other. Chemical
symbols and equations are a language that requires no translation.
Knowledge o the symbols or e lements and compounds and their
relationship to one another as displayed in a balanced equation
unlocks a wealth o inormation, allowing understanding o the
chemical process being examined.
Chemical symbols are a way o expressing which elements are present
and in which proportions, in both organic and inorganic compounds.
The International Union o Pure and Applied Chemistry (IUPAC ) is
an organization that develops and monitors a system o standardized
nomenclature or both organic and inorganic compounds. IUPACs role
is to provide consistency in the naming o compounds, resulting in a
language o symbols and words that require no translation rom one
country or cultures language to another.
usefl resoce
The IUPAC Gold Book (http://goldbook.iupac.org/index.html) is IUPACs
compendium of chemical terminology.
Figure 7 Table salt is the compound sodium
chloride, NaCl(s) . It has very diferent properties
rom those o its constituent elements
Figure 9 Elements, compounds, and mixtures
matter any substance that occupies
space and has mass
mixture a combination
of two or more pure
substances that reta in their
individual properties
homogeneous mixture
has both uniform
composition
and properties
throughout,
eg salt water,
metal a l loys
heterogeneous mixture
has non-uniform
composition and
varying properties,
eg sa lad dressing,
paint, garden soil
element made up of
atoms that each have
the same atomic
number, eg lead, Pb,
mercury, Hg,
bromine, Br
compound made up
of a combination of
atoms or ions in a xed
ratio and having d ierent
properties from the
constituent elements, eg
water, H2O, carbon d ioxide,
CO2, sodium chloride, NaCl
pure substance has a
denite and constant
composition
posts law of constant comosition
(1806) stated that compounds have
distinct properties and the same
elemental composition by mass.
6
1 STO I CH I O M E TR I C R E L AT I O N SH I PS
TOK
Language is a crucial component in the communication o
knowledge and meaning. Does the language o chemistry with
its equations, symbols, and units promote or restrict universal
understanding? What role does l inguistic determinism play?
For example, the concept o equilibrium is oten initially
misinterpreted. Preconceived ideas ocus on a 50:50 balance
between reactants and products. It requires an understanding
that equilibrium means that both the orward and reverse
reactions are occurring at the same rate beore we can see that
an equilibrium reaction might avour the ormation o products
or reactions, or that such a reaction could be non-spontaneous.
Common combinations of elements: Background
to writing equationsAn ion is a charged species. Anions are negatively charged and cations are
positively charged.
There are a number of common polyatomic ions that exist in many of
the substances you will s tudy and work with. You need to be familiar
with the names and formulae of these ions, shown in tables 1 to 3 .
na o ai oa
hydrochloric acid HCl
nitric(V) acid HNO3
phosphoric(V) acid H3PO
4
suluric(VI ) acid H2SO
4
ethanoic acid CH3COOH
Table 2 Common acids
na o
oyatoi io
oa na o
oyatoi io
oa
ammonium ion NH4
+ phosphate(V) ion PO4
3
carbonate ion CO3
2 phosphonate ion PO3
3
hydrogencarbonate
ionHCO
3
sulate(VI ) ion SO4
2
hydroxide ion OH sulate(IV) ion SO3
2
nitrate(V) ion NO3
ethanedioate ion C2O
4
2
nitrate(I I I ) ion NO2
peroxide ion O2
2
Table 1 Common polyatomic ions
Writing and balancing equationsAn ability to write equations is essential to chemistry and requires
a full understanding of the language of equations. At the most
fundamental level, formulae for the reactants are put on the left-
hand side along with their state symbols ( s) , ( l) , ( g) , ( aq) , and those
for the products on the right-hand side. The arrow represents a
boundary between reactants and products. S tate symbols can be
deduced by referring to the solubilities of ionic salts and the state of
matter of the element or compound at a given temperature.
A reaction may be described in terms of starting materials and products.
The process of transforming these words into a balanced chemical
equation starts with the construction of chemical formulae. Writing
ionic and covalent formulae will be discussed in depth in topic 4.
na o aio oa nai sfx
sulfde ion S2 -ide
sulate(VI ) ion SO4
2 -ate
sulate(IV) ion SO3
2 -ate
Table 3 Naming anions. The prex identies the
element present and the sufx the type o ion
(eg element or polyatomic ion)
Qik qstios
Write equations or the ollowing chemical reactions, including state symbols. Reer
to the working method on the next page on balancing equations i you need to.
1 Zinc meta l reacts with hydroch loric acid to orm the sa l t zinc ch loride. Hydrogen
gas is evolved .
2 Hydrogen gas and oxygen gas react together to orm water.
3 At a h igh temperature, ca lcium carbonate decomposes in to ca lcium oxide and
carbon d ioxide.
Worked example
Magnesium burns in oxygen to
form a white powder known as
magnesium oxide. Write a chemical
equation to represent this change,
including state symbols.
Solution
The reactants are the metal
magnesium, a solid at room
temperature, and the diatomic
molecule, oxygen, which is a
gas. The product is the oxide of
magnesium, magnesium oxide
which is a solid substance.
2Mg(s) + O2(g) 2MgO(s)
7
1 . 1 I n T r O d u c T I O n TO T h e pAr T I c u l AT e n AT u r e O m AT T e r An d c h e m I c Al c h An g e
Working method: how to balance
chemical equationsThe examples below involve reactions o metals.
Figure 1 0 reminds you that metals are below and
to the let o the metalloids in the periodic table.
Remember that to balance an equation you
change the coefcient o a ormula (add a number
in ront o the ormula) . You do not change the
ormula itsel.
Step 1 : First balance the metallic element on
each side o the equation add a number
in ront o the symbol on one side i
necessary so that there is the same number
o atoms o this element on each side.
Step 2 : B alance any elements that occur in
only one ormula on the reactant and
products side. Sometimes polyatomic
ions remain unchanged in reactions and
they can be balanced easily at this stage.
Step 3 : Balance the remaining elements i necessary.
Figure 10 Metals are below and to the left of the metal loids in
the periodic table
Boron
5
B
Carbon
6
C
Nitrogen
7
N
Oxygen
8
O
Fluorine
9
F
Aluminium
13
Al
Sil icon
14
Si
Phosphorus
15
P
Sulfur
16
S
Chlorine
17
Cl
Gal l ium
31
Ga
Zinc
30
Zn
Germanium
32
Ge
Arsenic
33
As
Selenium
34
Se
Bromine
35
Br
Indium
49
In
Cadmium
48
Cd
Tin
50
Sn
Antimony
51
Sb
Tel lurium
52
Te
Iodine
53
I
Thal l ium
81
Tl
Mercury
80
Hg
per
9
u
er
7
g
ld
9
u
Lead
82
Pb
Bismuth
83
Bi
Polonium
84
Po
Astatine
85
At
Ne
1
N
Ar
1
Kry
3
Xe
5
X
Ra
8
R
1131121 114 115 116 117 1
semi-metals non-metalsmetals
Example 1The alkaline earth metal calcium reacts with
water to produce an alkaline solution. Balance the
ollowing equation.
Step 1 : B alance the metal Ca frst.
It is balanced.
Ca(s) + H2O( l) C a(OH)
2(aq) + H
2(g)
Ca(s) + 2H2O(l) Ca(OH)
2(aq) + H
2(g)
Step 3 :
You can now see that hydrogen
has been balanced by step 2 , which oten
happens. Always check to make sure.
The equation is now balanced overall.
Example 2Potassium hydroxide is a soluble base that can
neutralize the diprotic acid suluric acid. D iprotic
acids produce two hydrogen ions when they
dissociate. Balance the ollowing equation.
Step 1 : B alance K by doubling KOH on
the reactant side.
H2SO
4(aq) + KOH(aq) K
2SO
4(aq) + H
2O( l)
Step 2 : B oth O and H
occur in two compounds on both
sides o the equation. The sulate ion is
unchanged in the reaction and is balanced, so
the coefcient or H2SO
4 will stay the same.
There are 4 H atoms on the reactant
side, so multiply H2O by 2 .
H2SO
4 (aq) + 2KOH (aq) K
2SO
4(aq) + H
2O( l)
H2SO
4(aq) + 2KOH(aq) K
2SO
4(aq) + 2H
2O( l)
The equation is now balanced.
Step 2 : B alance O next,
as it occurs in only one ormula on
each side. (H occurs in both products. )
Multiply H2O by 2 to balance O .
8
1 STO I CH I O M E TR I C R E L AT I O N SH I PS
Some applications and reactions of butane
Fuels and rerigerantsButane, C
4H
10 is mixed with other hydrocarbons such as propane to
create the uel liquefed petroleum gas (LPG) . This is used in a wide
variety o applications.
Methylpropane (also called isobutane) is an isomer o butane. Isomers
have the same chemical ormula but their atoms are arranged structurally
in a dierent way. Methylpropane is used as a rerigerant, replacing the
CFCs that were previously used or this purpose.
Ozone occurs naturally in the stratosphere, in the upper atmopshere.
Ozone flters out most o the harmul ultraviolet rays rom the sun.
Without this protection the ultraviolet radiation would be harmul to
many orms o lie, causing skin cancer in humans and other problems.
O
O
O
C
C
C
H
H
H
H
HH
H
HH
H
C
Figure 11 Ozone, O3 Figure 12 Methylpropane is used as a refrigerant
CFCs undergo reactions with the ozone in the stratosphere, causing it
to break down. The ozone hole is a thinning o the ozone layer that
appears over the polar regions o the Earth each spring. The use o
CFCs has caused this depletion o the ozone layer, so they have now
been replaced by methylpropane.
The names and symbols o
the elements can be ound in
section 5 o the Data booklet.
So tys o atio
cobiatio or sytsis reactions involve the combination o two or more
substances to produce a single product:
C(s) + O2(g) CO
2(g)
doositio reactions involve a single reactant being broken down into two or
more products:
CaCO3(s) CaO(s) + CO
2(g)
Si at reactions occur when one element replaces another in a
compound. An example o this type o reaction is a redox reaction (topic 9) :
Mg(s) + 2HCl(aq) MgCl2(aq) + H
2(g)
dob at reactions occur between ions in solution to orm insoluble
substances and weak or non-electrolytes, a lso termed tatsis reactions:
HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l )
This example is an acid-base reaction d iscussed urther in topic 8.
cfcs a t iat o si a tooy
The process o rerigeration
involves the energy changes o
a condensationevaporation
cycle using volati le l iquids.
Chlorofuorocarbons (CFCs)
were traditional ly used
in rerigerators and air-
conditioning units. They cause
depletion o the ozone layer
in the atmosphere, which
protects us rom the harmul
eects o ultraviolet radiation
in sunl ight.
CFCs are now banned in many
countries, and non-halogenated
hydrocarbons such as propane
are more commonly used
instead. There is more about this
in sub-topic 5.3.
9
1 . 1 I n T r O d u c T I O n TO T h e pAr T I c u l AT e n AT u r e O f m AT T e r An d c h e m I c Al c h An g e
Figure 13 The ozone hole was frst noticed in the 1970s and is monitored by scientists
worldwide
Balancing the equation or the combustion o butaneThe combustion o butane is an exothermic reaction.
C4H
10(g) + O
2(g) CO
2(g) + H
2O( l)
Step 1 : There are no metal atoms to balance, so balance the carbon
atoms frst by multiplying CO2 by 4.
C4H
10(g) + O
2(g) 4CO
2(g) + H
2O( l)
Step 2 : Oxygen is ound in two compounds on the product side so
leave this until last. Hydrogen has 1 0 atoms on the let and 2 atoms on
the right, so multiply H2O by 5 .
C4H
10(g) + O
2(g) 4CO
2(g) + 5H
2O( l)
Step 3 : The products now contain 1 3 oxygen atoms, an odd number.
To balance the equation 6 .5 molecules o oxygen are required.
C4H
10(g) + 6 .5O
2(g) 4CO
2(g) + 5H
2O( l)
Fractions are not used in balanced equations, except when calculating
lattice enthalpy ( see topic 1 5 ) . We thereore multiply the whole
equation by 2 .
2C4H
10(g) + 1 3O
2(g) 8CO
2(g) + 1 0H
2O( l)
The complex coefcients in this example show why the method o
balancing equations on page 8 is more efcient than just trial and error.
The combustion o
hydrocarbons, CxH
y produces
carbon dioxide and water.
Since 1997, taxis in Hong Kong
have been powered by l iquefed
petroleum gas (LPG) . Today
there are over 18 000 LPG
taxis and 500 LPG l ight buses
operating there. LPG, consisting
o butane and/or propane,
undergoes combustion to
release energy to power the
vehicle. The reaction produces
carbon dioxide and water
(sub-topic 10.2) . LPG burns
much more cleanly than petrol
or diesel.
Figure 14 Rush hour in Hong Kong
10
1 STO I CH I O M E TR I C R E L AT I O N SH I PS
The atom economyThe global demand or goods and services along with an increasing world
population, rapidly developing economies, increasing levels o pollution,
and dwindling fnite resources have led to a heightened awareness o the
need to conserve resources. Synthetic reactions and industrial processes
must be increasingly efcient to preserve raw materials and produce
ewer and less toxic emissions. Sustainable development is the way o
the uture.
To this end the atom economy was developed by Proessor Barry
Trost o S tanord University S tanord, CA, USA. This looks at the level
o efciency o chemical reactions by comparing the molecular mass o
atoms in the reactants with the molecular mass o useul compounds.
percentage
atom economy =
Molecular mass o atoms o useul products
____
Molecular mass o atoms in reactants 1 00%
The atom economy is important in the discussion o Green Chemistry,
which we will discuss later in this book. In an ideal chemical process
the amount o reactants = amounts o products produced. So an atom
economy o 1 00% would suggest that no atoms are wasted.
Ativity
a) Suggest why even i a chemical reaction has a y ield close to 100% , the atom
economy may be poor. Carry out some research into this aspect.
b) Discuss some other ways a chemical process may be evaluated other than
the atom economy, eg energy consumption etc.
) Deduce the percentage atom economy or the nucleophi l ic substitution
reaction:
CH3(CH
2)3OH + NaBr + H
2SO
4 CH
3(CH
2)3Br + H
2O + NaHSO
4
Qik qstios
Identiy the type o reaction and then copy and balance
the equation, using the smallest possible whole number
coefcients.
1 SO3(g) + H
2O(l ) H
2SO
4(aq)
2 NCl3(g) N
2(g) + Cl
2(g)
3 CH4(g) + O
2(g) CO
2(g) + H
2O(g)
4 Al(s) + O2(g) Al
2O
3(s)
5 KClO3(s) KCl(s) + O
2(g)
6 C3H
8(g) + O
2(g) CO
2(g) + H
2O(g)
7 Ni(OH)2(s) + HCl(aq) N iCl
2(aq) + H
2O(l)
8 AgNO3(aq) + Cu(s) Cu(NO
3)2(aq) + Ag(s)
9 Ca(OH)2(s) CaO(s) + H
2O(l)
11
1 . 1 I n T r O d u c T I O n TO T h e pAr T I c u l AT e n AT u r e O f m AT T e r An d c h e m I c Al c h An g e
Understandings The mole is a f xed number o particles and
reers to the amount, n , o substance.
Masses o atoms are compared on a scale
relative to 1 2C and are expressed as relative
atomic mass (Ar) and relative ormula/
molecular mass (Mr) .
Molar mass (M) has the units g mol 1 .
The empirical ormula and molecular ormula
o a compound give the simplest ratio and the
actual number o atoms present in a molecule
respectively.
1.2 T o ot
Applications and skills Calculation o the molar masses o atoms, ions,
molecules and ormula units.
Solution o problems involving the
relationships between the number o particles,
the amount o substance in moles and the
mass in grams.
Interconversion o the percentage composition
by mass and the empirical ormula.
Determination o the molecular ormula o
a compound rom i ts empirical ormula and
molar mass.
Obtaining and using experimental data or
deriving empirical ormulas rom reactions
involving mass changes.
Nature of science Concepts the concept o the mole developed rom the related concept o equivalent mass in the early
19th century.
SI: the international system of measurementThroughout history societies have developed dierent orms o
measurement. These may vary rom one country and culture to another,
so an internationally agreed set o units allows us to understand
measurements regardless o the language o our culture.
Units o measurement are essential in all walks o lie . The
f nancial world speaks in US dollars , the resources industries use
million tonnes (MT) , precious metals are measured in ounces,
agricultural manuacturing uses a range o measures including yield
per hectare , and environmental protection agencies , amongst others ,
talk about parts per million ( ppm) o particulate matter. Which units
do chemists use?
The desire or a s tandard international se t o units led to the
development o a system that transcends all languages and
cultures the Systme International d Units ( S I) . Table 1 shows
the seven base units o the S I system. All o ther units are derived
rom these seven base units .
12
1 STO I ch I O m e Tr I c r e l AT I O n Sh I pS
prorty uit Sybo
mass kilogram kg
temperature kelvin K
time second s
amount mole mol
electric current ampre A
luminosity candela cd
length metre m
Table 1 The seven base units o the SI system
Table 2 shows two quantities that are used throughout the study o
chemistry, along with their units. Table 3 is a list o standard prefxes
used to convert S I units to a suitable size or the application you are