pcm0035 chapter01 the components of matter stoichiometry-june2013

8/10/2019 PCM0035 Chapter01 the Components of Matter Stoichiometry-June2013

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PCM 0035

General Chemistry

Foundation in Engineering

ONLINE NOTES

Chapter 1

THE COMPONENTS OF MATTERAND STOICHIOMETRY

Faculty of Engineering and Technology

(FET)


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PCM0035 General Chemistry

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FET 2013/14 The Components of Matter and Stoichiometry 1/ 22

Contents

1.1

Elements, Compounds and Mixtures

1.2 An atomic View of Matter1.3

Daltons Atomic Theory

1.4

The Atomic Theory Today1.5

Compounds; Formulas, Names, and Masses

1.6 Mixtures: Classification and Separation1.7

The Mole

1.8 Determining the Formula of an Unknown Compound1.9

Writing and Balancing Chemical Equations

1.10 Calculating Amounts of Reactant and Product

OBJECTIVES

Upon completion of this chapter, you should be able to:

1. Classify matter and understand the changes it undergoes2. Explain the structure of atom which consists of a very dense central nucleus

containing protons and neutrons, with electrons moving about the nucleus

3. Define atomic number and mass number.

4. Express the composition of molecules and ionic compound in terms of chemical

symbols and understand the rules of naming compounds5. Understand the atomic mass and molecular mass.

6. Understand chemical reactions or called as chemical changes, represented bychemical equations.

7. Understand stoichiometry, the quantitative study of products and reactants in

chemical reactions.

8. Understand the limiting reagent, the reactant that is present in the smalleststoichiometric amount and explain reaction yield.


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Chemistry is the study of matter and the changes it undergoes. Chemistry is often called

the central science, because a basic knowledge of chemistry is essential for students of

biology, physics, geology, ecology, and many other subjects. Indeed, it is central to ourway of life; without it, we would be living shorter lives in what we would consider

primitive conditions, without automobiles, electricity, computers, CDs, and many other

everyday conveniences.

Chemistry also helps us to comprehend the nature of our environment, our universe, and

ourselves. It provides essential information about issues such as atmospheric ozone

depletion, acid rain, and global warming. Chemistry plays a vital role in ourunderstanding and treatment of diseases such as cancer and AIDS, and it helps unravel

the mysteries of the human mind. In fact, the theories of chemistry illuminate our

understanding of the material world from tiny atoms to giant galaxies. It is a journey we

have barely begun, but we can be sure that a knowledge of chemistry will light the pathtoward a better understanding of our natural world.

Moreover, if we cook, then we are a practicing chemist! From experience gained in thekitchen, we know that oil and water do not mix and that boiling water left on the stove

will evaporate. We apply chemical and physical principle when we use baking soda to

leaven bread, choose a pressure cooker to shorten the time it takes to prepare soup,

squeeze lemon juice over sliced pears to prevent them from turning brown or over fish tominimize its odor, and etc. Everyday we observe such changes without thinking about

their chemical nature. The purpose of this subject is to make you think like a chemist, to

look at the macroscopic worldthe things we can see, touch, and measure directly andvisualize the particles and events of the microscopic world that we cannot experience

without modern technology and our imaginations.

1.1 ELEMENTS, COMPOUNDS, AND MIXTURES: AN ATOMIC

OVERVIEW

Matteris anything that occupies space and has mass. Matter includes things we can see

and touch (such as water, earth, and tree), as well as things we cannot (such as air). The

classifications of matter include substances, mixtures, elements, and compounds.

Substances and Mixtures.A substanceis a form of matter that has a definite (constant)

composition and distinct properties. Examples are water, table sugar, and gold.

Substances differ from one another in composition and can be identified by theirappearance, smell, taste, and other properties.

A mixture is a combination of two or more substances in which the substances retain

their distinct identities. Examples are air, soft drink, and cement. Mixtures do not haveconstant composition. Mixtures are either homogeneous or heterogeneous. When a

spoonful of sugar dissolves in water we obtain a homogeneous mixture in which the

composition of the mixture is the same throughout. If sand is mixed with iron filings,however, the sand grains and the iron filings remain separate. This type of mixture is


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called a heterogeneous mixture because the composition is not uniform. Any mixture,

whether homogeneous or heterogeneous, can be created and then separated by physical

means into pure components without changing the identities of the component.

Elements and Compounds. An element is a substance that cannot be separated into

simpler substances by chemical means. Table 1.1 shows the names and symbols of someof the common elements. Compoundis a substance composed of atoms of two or moreelements chemically united in fixed proportions. Compound can be separated only by

chemical means into their pure components.

The relationship among elements, compounds, and other categories of matter are

summarized in figure below.


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1.2 AN ATOMIC VIEW OF MATTER

Mass Conservation

The most fundamental chemical observation of the 18th

century was the law of mass

conservation: the total mass of substances does not change during a chemical reaction.The number of substances may change, and by definition their properties must, be the

total amount of matter remains constant.

Even in a complex biochemical change within an organism, such as the metabolism of

the sugar glucose, which involves many reactions, mass is conserved:

180 g glucose + 192 g oxygen gas 264 g carbon dioxide + 108 g water

372 g material before change 372 g material after changeMass conversion means that, based on all chemical experience, matter cannot be created

or destroyed.

Definite Composition

Another fundamental chemical observation is summarized as the law of definite (orconstant) composition: no matter what its source, a particular compound is composed of

the same elements in the same parts by mass. The mass fraction is that parts of the

compounds mass contributed by the element. It is obtained by dividing the mass of each

element by the total mass of compound. The percent by mass (mass %) is the fraction bymass expresses as a percentage.

1.3 DALTONS ATOMIC THEORY

Daltons atomic theory can be summarized as follows:

1. Elements are composed of extremely small particles called atoms. All atoms of agiven element are identical, having the same size, mass and chemical properties.

The atoms of one element are different from the atoms of all other elements.

2. Compounds are composed of atoms of more than one element. In any compound,the ratio of the numbers of atoms of any two of the elements present is either an

integer or a simple fraction.

3. A chemical reaction involves only the separation, combination, or rearrangementof atoms; it does not result in their creation or destruction.

On the basis of Daltons atomic theory, we can define an atomas the basic unit of anelement that can enter into chemical combination. Dalton imagined an atom that wasboth extremely small and indivisible.


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1.4 THE ATOMIC THEORY TODAY

Structure of the Atom

A series of investigations later clearly demonstrated that atoms actually possess internal

structure; that is, they are made up of even smaller particles electrons, protons, andneutrons.

Electron is a negatively charged particles

Proton is a positively charged particles in the nucleus. (Nucleus a dense central

core within the atom)

Neutron is an electrically neutral particles having a mass slightly greater than that

of protons

Figure below shows that the protons and neutrons of an atom are packed in an extremely

small nucleus. Electrons are shown as clouds around the nucleus.

Atomic Number, Mass number, and Atomic Symbol

The atomic number (Z) is the number of protons in the nucleus of each atom of an

element. In a neutral atom the number of protons is equal to the number of electrons, so

the atomic number also indicates the number of electrons present in the atom.

The mass number (A)is the total number of neutrons and protons present in the nucleus

of an atom of an element.

mass number = number of protons + number of neutrons

= atomic number + number of neutrons


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X

The number of neutrons in ad atom is equal to the difference between the mass number

and the atomic number, or (AZ).

Atoms of a given element do not all have the same mass. Most elements have two or

more isotopes, atoms that have the same atomic number but different mass numbers. For

example, there are three isotopes of hydrogen. One simply known as hydrogen, has oneproton and no neutrons. The deuterium isotope contains one proton and one neutron, andtritium has one proton and two neutrons. The accepted way to denote the atomic number

and mass number of an atom of an element (X) is as follows:

Mass numberA

ZAtomic number


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1.5 COMPOUNDS: FORMULAS, NAMES, AND MASSES

Types of Chemical formula

Chemical formula is used to express the composition of molecules and ionic compound

in terms of chemical symbols. By composition we mean not only the elements present butalso the ratios in which the atoms are combined.

1. Molecular Formulas. A molecular formula shows the exact number of atoms of each

element in the smallest unit of a substance. For example, H2is the molecular formula forhydrogen, O2is oxygen, O3is ozone, and H2O is water. The subscript numeral indicates

the number of atoms of an element present. Note that oxygen (O2) and ozone (O3) are

allotropes of oxygen. An allotrope is one of two or more distinct forms of an element.

Another example, two allotropic forms of the element carbon are diamond and graphite.

2. Empirical Formulas. The molecular formula for hydrogen peroxide is H2O2. This

formula indicates that each hydrogen peroxide molecule consists of two hydrogen atomsand two oxygen atoms. The empirical formula of hydrogen peroxide is HO. Thus the

empirical formula tells us which elements are present and the simplest whole number

ratio of their atoms, but not necessarily the actual number of atoms in a given molecule.

In short, empirical formulas are the simplest chemical formulas; they are written byreducing the subscript in molecular formula to the smallest possible whole numbers.

Molecular formulas are the trueformulas of molecule.


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3. Structural Formulas. The structural formula shows the number of atom and the bond

between them. For instant, the structural formula of hydrogen peroxide is H O O H.

Formula of Ionic Compound

The formula of ionic compounds is usually the same as their empirical formulas because

ionic compounds do not consist of discrete molecular units. For example, a solid sample

of sodium chloride (NaCl) consists of equal number of Na+ and Cl ions. In such a

compound there is a 1 : 1 ratio of cations to anions so that the compound is electrically

neutral. In fact, each Na+ ion is equally held by six surrounding Cl ions and vice versa.

Thus NaCl is the empirical formula for sodium chloride.

In order for ionic compounds to be electrically neutral, the sum of the charges on the

cation and anion in each formula unit must be zero. If the charges on the cation and anion

are numerically different, the following rule to be applied : The subscript of the cation isnumerically equal to the charge on the anion, and the subscript of the anion is

numerically equal to the charge on the cation. This rule follows from the fact that

because the formulas of ionic compounds are empirical formulas, the subscript must

always be reduced to the smallest ratios.

Naming Compounds

To organize and simplify the venture into naming compounds, inorganic compounds can

be divided into four categories : ionic compounds, molecular compounds, acids and

bases, and hydrates.

A. Ionic Compounds

Ionic compounds are made up of cations and anions. Refer to Table 2.2 and Table 2.3 for

the common anions and cations names. Many ionic compounds are binary compounds,

or compounds formed from just two elements.


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Certain metals, especially the trasition metals, can form more than one type of cation. Eg.

Iron can form two cations : Fe2+

and Fe3+

. The name of the compounds that these iron

ions form with chloride would be ferrous chloride (FeCl2) and ferric chloride (FeCl3).However, by using the Stock System, which provide information regarding the actual

charges of the two cations, ferrous chloride becomes iron (II) chloride; and ferric chloride

is called iron (III) chloride.

B. Molecular Compounds

Molecular compounds are usually composed of nonmetallic elements. Many molecularcompounds are binary compounds. The name of the first element is placed in the formula

first, and the second element is named by adding ide to the root of the element name.

Eg. HCl (hydrogen chloride), HBr (hydrogen bromide), and SiC (silicon carbide). For

elements with several compounds, confusion in naming compound is avoided by the useof Greek prefixes to denote the number of atoms of each element present. Eg. CO (carbon

monoxide), CO2 (carbon dioxide), SO2 (sulfur dioxide), SO3 (sulfur trioxide), NO2(nitrogen dioxide), and N2O4(dinitrogen tetroxide).


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C. Acids and Bases

An acidscan be described as a substance that yields hydrogen ions (H+) when dissolved

in water. In some cases, two different names seem to be assigned to the same chemical

formula. Eg. HCl (hydrogen chloride), and HCl (hydrochloric acid).

The name assigned to the compound depends on its physical state. In the gaseous or pureliquid state, HCl is a molecular compound called hydrogen chloride. When it is dissolvedin water, the molecules break up into H

+and Cl

-ions; in this state, the substance is called

hydrochloric acid. Oxoacids are acids that contain hydrogen, oxygen, and another

element (the central element). The formulas of oxoacids are usually written with the Hfirst, followed by the central element and then O, as illustrated by the following

examples:

HNO3 nitric acid

H2CO3 carbonic acidH2SO4 sulfuric acid

A basecan be described as a substance that yields hydroxide ions (OH

-

) when dissolvedin water. Eg. NaOH (sodium hydroxide), and KOH (potassium hydroxide).

D. Hydrates

Hydrates are compounds that have a specific number of water molecules attached tothem. For example, in its normal state, each unit of copper (II) sulfate has five water

molecules associated with it. The systematic name for this compound is copper (II)

sulfate pentahydrate, and its formula is written as CuSO45H2O. The water molecule can

be driven by heating. When this occurs, the resulting compound is CuSO4, which issometimes called anhydrous copper (II) sulfate; anhydrous means that the compound

no longer has water molecules associated with it.

1.6 MIXTURES: CLASSIFICATION AND SEPARATION

There are two broad classes of mixtures. A heterogeneous mixture has one or more

visible boundaries between the components. Thus, its composition is not uniform. A

homogeneous mixture has no visible boundaries because the components are mixed as

individual atoms, ions, and molecules. Thus, its composition is uniform.

A homogeneous is also called a solution. They can exist in three physical states. For

example, air is a gaseous solution of mostly oxygen and nitrogen molecules, and wax is asolid solution of several fatty substances. Solutions in water, called aqueous solutions, are

especially important in chemistry and comprise a major portion of the environment and

of all organism.


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Basic Separation Techniques

All the separation method depend on the physical properties of the substances in themixture; no chemical changes occur.

Filtrationseparates the components of a mixture on the basis of differences in particlesize. It is used most often to separate a liquid from a solid. In vacuum filtration, reducedpressure within the flask speeds the flow of the liquid through the filter.

Crystallizationis based on differences in solubility. The solubility of a substance is theamount that dissolves in a fixed volume of solvent at a given temperature. The purified

compound crystallizes as the solution is cooled.

Distillation separates components through differences in volatility, the tendency of asubstance to become gas. Ether, for example, is more volatile than water, which is much

more volatile than sodium chloride.

Extraction is also based on differences in solubility. In a typical procedure, a natural

material is ground in a blender with a solvent that extracts (dissolves) soluble compounds

embedded in insoluble material. This extraction is separated further by the addition of a

second solvent that does not dissolve in the first. After shaking in a separatory funnel,some component are extracted into the new solvent.

Chromatographyis a third technique based on differences in solubility. The mixture isdissolved in a gas or liquid called the mobile phase, and the components are separated as

this phase moves over a solid (or viscous liquid) surface called the stationary phase. A

component with low solubility in the stationary phase spends less time there, thus

moving faster than a component is highly soluble in that phase.

1.7 THE MOLE

The mole (abbreviated mol) is the SI unit for amount of substance. It is defined as the

amount of a substance that contains the same number of entities as there are atoms inexactly 12 g of carbon-12. This number is called Avogadros number (NA). The currently

accepted value is

NA= 6.0221367 X 1023

Generally, it is rounded to 6.022 X 1023. Thus, 1 mole of hydrogen atoms contains 6.022

X 1023H atoms.

1 mole of carbon-12 atoms has a mass of exactly 12 g and contains 6.022 X 1023

atoms.This mass of carbon-12 is its molar mass, defined as the mass (in grams or kilograms) of

1 mole of units (such as atoms or molecules) of a substance. Note that the molar mass of

carbon-12 (in grams) is numerically equal to its atomic mass in amu. Thus, if we knowthe atomic mass of an element, we also know its molar mass.


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Molecular Mass. The molecular massis the sum of the atomic masses (in amu) in the

molecule. Eg. The molecular mass of H2O is

2(atomic mass of H) + atomic mass of O

or 2(1.008 amu) + 16.00 amu = 18.02 amu

From the molecular mass, the molar mass of a molecule or compound can be determined.

The molar mass of a compound (in grams) is numerically equal to its molecular mass (inamu). For example, the molecular mass of water is 18.02 amu, so its molar mass is 18.02

g. Note that 1 mole of water weighs 18.02 g and contains 6.022 X 1023

H2O molecules.


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1.8 DETERMINING THE FORMULA OF AN UNKNOWN COMPOUND

Empirical Formulas

An analytical chemist investigating a compound decomposes it into simpler substances,

finds the mass of each component element, converts these masses to numbers of moles,

and then arithmetically converts the moles to whole-number (integer) subscripts. Thisprocedure yields the empirical formula, the simplest whole-number ratio of moles of

each element in the compound.

Molecular Formulas

If we know the molar mass of a compound, we can use the empirical formula to obtain

the molecular formula, the actual number of moles of each element in 1 mol ofcompound. In some cases, such as water (H2O), ammonia (NH3), the empirical and

molecular formulas are identical, but in many others the molecular formula is a whole-

number multiple of the empirical formula. Hydrogen peroxide, for example, has theempirical formula HO and the molecular formula H2O2. dividing the molar mass of H2O2

(34.02 g/mol) by the empirical formula (17.01 g/mol) gives the whole-number multiple,

2.


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1.9 WRITING AND BALANCING CHEMICAL EQUATIONS

Chemical reactionis a process in which a substance (or substances) is changed into one

or more new substances. To communicate with one another about chemical reactions, a

chemical equation is used. Chemical equation use chemical symbols to show what

happens during a chemical reaction.

For example, when hydrogen gas (H2) burns in air (which contains oxygen, O2) to form

water (H2O). This reaction can be represented by the chemical equation :

H2 + O2 H2O

Where the plus sign means reacts with and the arrow means to yield. Thus, this

symbolic expression can be read : Molecular hydrogen reacts with molecular oxygen toyield water. The reaction is assumed to proceed from left to right as the arrow indicates.

However, the above equation is not complete. To confirm with the law of conservation of

mass, there must be the same number of each type of atom on both sides of the arrow,that is we can balancethe equation by placing the appropriate coefficient in front of H2

and H2O.

2H2 + O2 2H2O

This balanced chemical equation shows that two hydrogen molecules can combine or

react with one oxygen molecule to form two water molecules.

Base on the equation, H2and O2are referred as reactant, which are the starting materials

in a chemical reaction.H2O is the product, which is the substance formed as a result of a

chemical reaction.

To provide additional information, physical states of the reactants and products can be

indicated by using the lettersg, l, ands to denote gas, liquid, and solid, respectively.


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1.10 CALCULATING AMOUNTS OF REACTANT AND PRODUCT

Stoichiometryis the quantitative study of reactants and products in a chemical reaction .Whether the units given for reactants (or products) are moles, grams, liters, or some other

units, moles can be used to calculate the amount of product formed in a reaction. This

approach is called the mole method, which means simply that the stoichiometriccoefficients in a chemical equation can be interpreted as the number of moles of eachsubstance.

Limiting Reagents. In general, for a reaction, the reactant are usually not present in

exact stoichiometric amounts, that is , in the proportions indicated by the balanced

equation. Because the goal of a reaction is to produce the maximum quantity of a useful

compound from the starting materials, frequently a large excess of one reactant issupplied to ensure that the more expensive reactant is completely converted to the desired

product. Consequently, some reactant will be left over at the end of the reaction. The

reactant used up first in a reactionis called the limiting reagent, because the maximumamount of product formed depends on how much of this reactant was originally present.

Excess reagentsare the reactants present in quantities greater than necessary to react

with the quantity of the limiting reagent.


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Answer:

pcm0035 chapter01 the components of matter stoichiometry-june2013

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