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9 Focus on Health & Medicine: Covalent Drugs and Medical Products

9 Focus on Health & Medicine: Covalent Drugs and Medical Products

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CHAPTER HIGHLIGHTS



SAMPLE PROBLEM 4.10



115



Glycolic acid is a starting material used to manufacture dissolving sutures. (a) Place lone pairs

where needed in the Lewis structure. (b) Give the shape around each atom in red. (c) Label all

polar bonds.



H



O



H



O



C



C



O



H



H

glycolic acid



ANALYSIS AND SOLUTION



a. Each O atom needs two lone pairs to make an octet.

b. Count groups to determine shape.



H



O



H



O



C



C



H

bent shape

• two atoms

• two lone pairs



trigonal planar

• three atoms around C

O



H



tetrahedral

• four atoms around C



c. Since O is much more electronegative (3.5) than hydrogen (2.1) and carbon (2.5), all C O

and O H bonds are polar and are labeled in red.



PROBLEM 4.24



In each compound: [1] determine the geometry around each carbon; [2] label each bond as

polar or nonpolar.



a.



H



H



H



C



C



O



b.



H



H



H



O



C



C



H



H

acetaldehyde



H H

ethanol



Ethanol, the “alcohol” in alcoholic beverages, is the world’s most widely abused drug. It

is metabolized to acetaldehyde, a toxic compound that produces some of the ill effects of

ingesting too much ethanol. Ethanol is discussed in greater detail in Chapter 14.



CHAPTER HIGHLIGHTS

KEY TERMS

Covalent bond (4.1)

Diatomic molecule (4.1)

Dipole (4.7)

Double bond (4.2)

Double-headed arrow (4.4)

Electronegativity (4.7)

Hybrid (4.4)



Lewis structure (4.1)

Lone pair (4.1)

Molecular formula (4.2)

Molecule (4.1)

Multiple bond (4.2)

Nonbonded electron pair (4.1)



Nonpolar bond (4.7)

Polar bond (4.7)

Resonance structure (4.4)

Triple bond (4.2)

Valence shell electron pair repulsion

theory (4.6)



KEY CONCEPTS

❶ What are the characteristic bonding features of covalent

compounds? (4.1)

• Covalent bonds result from the sharing of electrons between

two atoms, forming molecules. Atoms share electrons to

attain the electronic configuration of the noble gas nearest



smi26573_ch04.indd 115



them in the periodic table. For many main group elements,

this results in an octet of electrons.

• Covalent bonds are formed when two nonmetals combine

or when a metalloid bonds to a nonmetal. Covalent bonds

are preferred with elements in the middle of the periodic



12/2/08 10:56:59 AM



116



COVALENT COMPOUNDS



table that would have to gain or lose too many electrons to

form an ion.

• Except for hydrogen, the common elements—C, N, O, and

the halogens—follow one rule: the number of bonds + the

number of lone pairs = four.

❷ What are Lewis structures and how are they drawn? (4.2)

• Lewis structures are electron-dot representations of

molecules. Two-electron bonds are drawn with a solid line

and nonbonded electrons are drawn with dots (:).

• Lewis structures contain only valence electrons. Each H gets

two electrons and main group elements generally get eight.

• After placing all electrons in bonds and lone pairs in a Lewis

structure, it may be necessary to use lone pairs to form

double or triple bonds if an atom does not have an octet.

❸ What are resonance structures? (4.4)

• Resonance structures are two Lewis structures having the

same arrangement of atoms but a different arrangement of

electrons.

• The hybrid is a composite of all resonance structures that

spreads out electron pairs in multiple bonds and lone pairs.

❹ How are covalent compounds with two elements named?

(4.5)

• Name the first nonmetal by its element name and the second

using the suffix -ide

-ide.. Add prefixes to indicate the number of

atoms of each element.



❺ How is the molecular shape around an atom determined?

(4.6)

• To determine the shape around an atom, count groups—

atoms and lone pairs—and keep the groups as far away

from each other as possible.

• Two groups = linear, 180° bond angle; three groups =

trigonal planar, 120° bond angle; four groups = tetrahedral,

109.5° bond angle.

❻ How does electronegativity determine bond polarity? (4.7)

• Electronegativity is a measure of an atom’s attraction for

electrons in a bond.

• When two atoms have the same electronegativity value, or

the difference is less than 0.5 units, the electrons are equally

shared and the bond is nonpolar.

• When two atoms have very different electronegativity

values—a difference of 0.5–1.9 units—the electrons are

unequally shared and the bond is polar.

❼ When is a molecule polar or nonpolar? (4.8)

• A polar molecule has either one polar bond, or two or more

bond dipoles that do not cancel.

• A nonpolar molecule has either all nonpolar bonds, or two

or more bond dipoles that cancel.



PROBLEMS

Selected in-chapter and end-of-chapter problems have brief answers provided in Appendix B.



Covalent Bonding

4.25



4.26



4.27

4.28

4.29



4.30



For each pair of compounds, classify the bonding as ionic

or covalent and explain your choice.

a. LiCl and HCl

b. KBr and HBr

For each pair of compounds, classify the bonding as ionic

or covalent and explain your choice.

a. BeH2 and BeCl2

b. Na3N and NH3

How many bonds and lone pairs are typically observed

with each element: (a) C; (b) Se; (c) I; (d) P?

How many bonds and lone pairs are typically observed

with each element: (a) O; (b) Si; (c) Ge; (d) B?

Fill in the lone pairs needed to give the main group

elements (except hydrogen) an octet. Acrylonitrile is a

starting material used to manufacture synthetic Orlon

and Acrilan fibers. Cysteine is an amino acid used to

synthesize proteins.

a.



H



C



C



C



H H

acrylonitrile



smi26573_ch04.indd 116



N



b.



H



S



H



H



O



C



C



C



H



N



H



O



Fill in the lone pairs needed to give the main group

elements (except hydrogen) an octet. Glycerol is a

product of the metabolism of fats. Acrylamide is used to

make polyacrylamide, which is used in some cosmetics

and food packaging.



a.



H



O



H



H



H



C



C



C



H



O



H



H

glycerol



O

O



H



b.



H



C

H



C



C



N



H



H

H

acrylamide



Lewis Structures

4.31

4.32



Draw a valid Lewis structure for each molecule.

a. HI

b. CH2F2

c. H2Se

d. CO

e. C2Cl6

Draw a valid Lewis structure for each molecule.

a. CH3Br

b. PH3

c. HBr

d. SiF4

e. C2HCl



H



H

cysteine



12/2/08 10:56:59 AM



PROBLEMS



4.33



117



Draw a valid Lewis structure for each compound using

the given arrangement of atoms.



Resonance Structures

4.41



H



a. CH5N

b. HNO2



H



C



N



H



H



O



N



4.42



H



4.43

H



O



What is the difference between a resonance structure and

a resonance hybrid?

Briefly explain why having two resonance structures for a

molecule stabilizes it.

Draw a second resonance structure for the following

anion:



H



c. C3H4



C



H



C



C



H



H



H



4.34



4.44



H

C



H



H



C



4.35



4.36



4.37

4.38

4.39



4.40



smi26573_ch04.indd 117



C



O



H

N



H



H



H



O

N



C



O



O



H



nitromethane



H

H



C



N



O



c. C3H6



C







Draw a second resonance structure for nitromethane,

a compound used in drag racing fuels and in the

manufacture of pharmaceuticals, pesticides, and fibers.



H



b. HNO3



O



H



Draw a valid Lewis structure for each compound using

the given arrangement of atoms.



a. C2H3N



H



4.45



C



C



C



H



H



H



H



Draw a valid Lewis structure for tetrafluoroethylene,

C2F4, the industrial starting material used to prepare

Teflon. Teflon is most widely used as a nonstick surface

on pots and pans, but it has also found application in tape

used by plumbers to seal joints, nail polish, and coatings

on eye glasses. Assume that each carbon is bonded to two

fluorine atoms.

Draw a valid Lewis structure for phosgene, CCl2O, which

contains a central carbon atom. Phosgene is an extremely

toxic gas used as a chemical weapon during World War I.

It is now an important industrial starting material for the

synthesis of Lexan, a lightweight transparent material used

in bike helmets, goggles, and catcher’s masks.

Draw a valid Lewis structure for each ion: (a) NH2–;

(b) H3O+.

Draw a valid Lewis structure for each ion: (a) OCl–;

(b) CH3O–.

Keeping in mind that some elements violate the octet

rule, draw a Lewis structure for each compound:

(a) BCl3; (b) SO3.

Keeping in mind that some elements violate the octet

rule, draw a Lewis structure for each compound:

(a) BeH2; (b) PCl5.



Label each pair of compounds as resonance structures or

not resonance structures.





a.



N



C

H



b.



H



C



N



C



H

O



H



4.46







and



O



C



H



and



H



H



O

H



H



C



C



H



H







b.



N



H



C



H



O



C



C







C



and



O



N



O







N



H



and



H



H



4.48



H



Label each pair of compounds as resonance structures or

not resonance structures.

a.



4.47



O



H



O



C



C







N



H



H



Draw three resonance structures for the carbonate anion

(CO32–) that contain a central carbon atom.

Draw three resonance structures for the nitrate anion

(NO3–) that contain a central N atom.



Naming Covalent Compounds

4.49

4.50



Name each covalent compound.

a. PBr3

b. SO3

c. NCl3

Name each covalent compound.

a. SF6

b. CBr4

c. N2O



d. P2S5

d. P4O10



12/2/08 10:57:00 AM



118



4.51



4.52



4.53

4.54



COVALENT COMPOUNDS



Write a formula that corresponds to each name.

a. selenium dioxide

b. carbon tetrachloride

c. dinitrogen pentoxide

Write a formula that corresponds to each name.

a. silicon tetrafluoride

b. nitrogen oxide

c. phosphorus triiodide

What is the common name for dihydrogen oxide?

What is the systematic name for H2S, the compound we

commonly call hydrogen sulfide?



4.59



4.60

4.61



Give the molecular shape around the boron atom in BCl3

and the nitrogen atom in NCl3 and explain why they are

different.

Give the molecular shape for the oxygen atom in H2O

and H3O+ and explain why they are different.

Predict the bond angles around the indicated atoms in

each compound. Don’t forget to draw in lone pairs where

needed to give octets.

H



a.



H



C



c.



F



H



H



H

H



C



O



c.



H



H



H



b.

4.56



H



C



C



C



C



H



H



H



H



b.



H



4.62

H



c.



H



H



C



C



Cl



4.63



H



H



b.



b.



C



PCl3



4.64



Add lone pairs where needed to give octets and then

determine the shape around each indicated atom.

H



a.



b.



H



H



N



O



H



O



C



C



c.



H



H



H



H



C



N



H



H



+



H



4.65



H



4.66



Add lone pairs where needed to give octets and then

determine the shape around each indicated atom.

H



a.



H



N



N



c.



H



H



H



C

H



smi26573_ch04.indd 118



O



C



C



O



H



4.67

4.68



H



4.69



H



b.



H



H



O



O



H



O



H



H



H



H



C



C



H



H



c.



Cl



H



C

H



C



C



H



H



H



C



C



C



N



H



H



H



Draw Lewis structures for CCl4 and C2Cl4. Give the

molecular shape around each carbon atom. Explain why the

carbon atoms in the two molecules have different shapes.

Draw a Lewis structure for N2H4 and explain why the

shape around each N atom should be described as trigonal

pyramidal.



Electronegativity and Polarity



H



4.58



C



H



H

S



C



Predict the bond angles around the indicated atoms in

each compound. Don’t forget to draw in lone pairs where

needed to give octets.

a.



C



C



H



NF3



H



4.57



H



Add lone pairs where needed to give octets and then

determine the shape around each indicated atom.

a.



O



H



Add lone pairs where needed to give octets and then

determine the shape around each indicated atom.

a.



C



H



H



Molecular Shape

4.55



C



Rank the atoms in each group in order of increasing

electronegativity.

a. Se, O, S

c. Cl, S, F

b. P, Na, Cl

d. O, P, N

Rank the atoms in each group in order of increasing

electronegativity.

a. Si, P, S

c. Se, Cl, Br

b. Be, Mg, Ca

d. Li, Be, Na

What is the difference between a polar bond and a

nonpolar bond? Give an example of each.

What is the difference between a polar covalent bond and

an ionic bond? Give an example of each.

Using electronegativity values, classify the bond formed

between each pair of elements as polar covalent or ionic.

a. hydrogen and bromine

c. sodium and sulfur

b. nitrogen and carbon

d. lithium and oxygen



12/2/08 10:57:01 AM



PROBLEMS



4.70



4.71



4.72



4.73



4.74



4.75

4.76

4.77



4.78



4.79

4.80

4.81

4.82

4.83



119



Using electronegativity values, classify the bond formed

between each pair of elements as polar covalent or ionic.

a. nitrogen and oxygen

c. sulfur and chlorine

b. oxygen and hydrogen

d. sodium and chlorine

Label the bond formed between carbon and each of the

following elements as nonpolar, polar, or ionic.

a. carbon

c. lithium

e. hydrogen

b. oxygen

d. chlorine

Label the bond formed between fluorine and each of the

following elements as nonpolar, polar, or ionic.

a. hydrogen

c. carbon

e. sulfur

b. fluorine

d. lithium

Which bond in each pair is more polar—that is, has the

larger electronegativity difference between atoms?

a. C —O or C —N

b. C —F or C —Cl

c. Si—C or P—H

Which bond in each pair is more polar—that is, has the

larger electronegativity difference between atoms?

a. Si—O or Si—S

b. H—F or H—Br

c. C —B or C —Li

Label each bond in Problem 4.73 with δ+ and δ– to show

the direction of polarity.

Label each bond in Problem 4.74 with δ+ and δ– to show

the direction of polarity.

Explain why the carbon atom in CH3NH2 bears a partial

positive charge (δ+), but the carbon atom in CH3MgBr

bears a partial negative charge (δ–).

Explain why the carbon atom in CH3Cl bears a partial

positive charge (δ+), but the carbon atom in CH3Li bears

a partial negative charge (δ–).

Can a compound be polar if it contains all nonpolar

bonds? Explain.

Can a compound be nonpolar if it contains some polar

bonds? Explain.

Can a compound be nonpolar if it contains one polar

bond? Explain.

Is a compound that contains polar bonds always polar?

Explain.

Label the polar bonds and then decide if each molecule is

polar or nonpolar.

H



a.



C



c.



C



H



H



Cl



Cl



b.



C

Cl



smi26573_ch04.indd 119



Cl



C

Cl



H



H



O



C



C



H



Cl



4.84



Label the polar bonds and then decide if each molecule is

polar or nonpolar.

H



a.



Cl

C



c.



C



H



H



C



4.86



C



C



H



H

C



C



H



4.85



O



H



H



b.



H



Cl



H



C



H



H



Explain why CHCl3 is a polar molecule but CCl4 is not.

Explain why H2O is a polar molecule but H2S is not.



General Questions

4.87



4.88



Answer the following questions about the molecule Cl2O.

a. How many valence electrons does Cl2O contain?

b. Draw a valid Lewis structure.

c. Label all polar bonds.

d. What is the shape around the O atom?

e. Is Cl2O a polar molecule? Explain.

Answer the following questions about the molecule OCS.

a. How many valence electrons does OCS contain?

b. Draw a valid Lewis structure.

c. Label all polar bonds.

d. What is the shape around the C atom?

e. Is OCS a polar molecule? Explain.



Applications

4.89



Glycine is a building block used to make proteins, such

as those in heart muscle (Figure 4.2).

H



H



O



N



C



C



H



H



O



H



glycine



4.90



a. Add lone pairs where needed, and then count the total

number of valence electrons in glycine.

b. Determine the shape around the four indicated atoms.

c. Label all of the polar bonds.

d. Is glycine a polar or nonpolar molecule? Explain.

Lactic acid gives sour milk its distinctive taste. Lactic

acid is also an ingredient in several skin care products that

purportedly smooth fine lines and improve skin texture.

H



H



H



O



C



C



C



H



O



O



H



H

lactic acid



a. Add lone pairs where needed, and then count the total

number of valence electrons in lactic acid.

b. Determine the shape around the four indicated atoms.

c. Label all of the polar bonds.

d. Is lactic acid a polar or nonpolar molecule? Explain.



12/2/08 10:57:02 AM



120



4.91



COVALENT COMPOUNDS



Serotonin (C10H12N2O) is a neurotransmitter that is

important in mood, sleep, perception, and temperature

regulation. Fill in all lone pairs and double bonds to give

every atom its usual bonding pattern.



C



O



H



H



C



C



H



H

H



H



4.92



C



C



C



C

C



C



N



H



Phenylephrine (C9H13NO2) is the decongestant in

Sudafed PE. Phenylephrine replaced the decongestant

pseudoephedrine, which was readily converted to the

illegal stimulant methamphetamine. Fill in all lone pairs

and double bonds to give every atom its usual bonding

pattern.

H



H H

C H



N



H



C



C



C



C



C



H



H

serotonin



H



O



H



C



H

O



H



H



H



C



C



N



C



H



H



H



H



H

phenylephrine



CHALLENGE QUESTIONS

4.93



Cyclopropane is a stable compound that contains three

carbon atoms in a three-membered ring.

H



H

C



H



C



C



H



H

H

cyclopropane



4.94



Although carbon has four bonds in stable molecules,

sometimes reactive carbon intermediates that contain

carbon atoms without four bonds are formed for

very short time periods. Examples of these unstable

intermediates include the methyl carbocation (CH3)+ and

the methyl carbanion (CH3)–. Draw Lewis structures for

both unstable ions and predict the shape around carbon.



a. What is the predicted shape around each carbon

atom in the ring, given the number of groups around

carbon?

b. What is the predicted C — C — C bond angle, given the

shape and size of the ring?

c. Explain why cyclopropane is less stable than similar

three-carbon compounds that do not contain a ring.



smi26573_ch04.indd 120



12/2/08 10:57:02 AM



5

CHAPTER OUTLINE

5.1



Introduction to Chemical Reactions



5.2



Balancing Chemical Equations



5.3



The Mole and Avogadro’s Number



5.4



Mass to Mole Conversions



5.5



Mole Calculations in Chemical

Equations



5.6



Mass Calculations in Chemical

Equations



5.7



Percent Yield



5.8



Oxidation and Reduction



5.9



FOCUS ON HEALTH & MEDICINE:

Pacemakers



CHAPTER GOALS

In this chapter you will learn how to:

➊ Write and balance chemical equations

➋ Define a mole and use Avogadro’s

number in calculations

➌ Calculate formula weight and molar

mass

➍ Relate the mass of a substance to its

number of moles

➎ Carry out mole and mass calculations

in chemical equations

➏ Calculate percent yield

➐ Define oxidation and reduction and

recognize the components of a redox

reaction

➑ Give examples of common or useful

redox reactions



Thread for suturing wounds is made from nylon, one of the countless products synthesized by the

chemical industry using chemical reactions.



CHEMICAL REACTIONS

HAVING learned about atoms, ionic compounds, and covalent molecules in Chapters 2–4, we now turn our attention to chemical reactions. Reactions are at the heart

of chemistry. An understanding of chemical processes has made possible the conversion of natural substances into new compounds with different and sometimes superior

properties. Aspirin, ibuprofen, and nylon are all products of chemical reactions utilizing substances derived from petroleum. Chemical reactions are not limited to industrial

processes. The metabolism of food involves a series of reactions that both forms new

compounds and also provides energy for the body’s maintenance and growth. Burning

gasoline, baking a cake, and photosynthesis involve chemical reactions. In Chapter 5 we

learn the basic principles about chemical reactions.



121



smi26573_ch05.indd 121



12/2/08 3:37:16 PM



122



CHEMICAL REACTIONS



5.1 INTRODUCTION TO CHEMICAL REACTIONS

Now that we have learned about compounds and the atoms that compose them, we can better

understand the difference between the physical and chemical changes that were first discussed

in Section 1.2.

• A physical change alters the physical state of a substance without changing its

composition.



Changes in state—such as melting and boiling—are familiar examples of physical changes.

When ice (solid water) melts to form liquid water, the highly organized water molecules in

the solid phase become more disorganized in the liquid phase, but the chemical bonds do not

change. Each water molecule (H2O) is composed of two O H bonds in both the solid and liquid

phases.

solid H2O



liquid H2O

physical

change

melting

H2O molecules are

unchanged before

and after melting.

2 O–H bonds



2 O–H bonds



• A chemical change—chemical reaction—converts one substance into another.



Chemical reactions involve breaking bonds in the starting materials, called reactants, and

forming new bonds in the products. The combustion of methane (CH4), the main constituent of

natural gas, in the presence of oxygen (O2) to form carbon dioxide (CO2) and water (H2O) is an

example of a chemical reaction. The carbon–hydrogen bonds in methane and the oxygen–oxygen

bond in elemental oxygen are broken, and new carbon–oxygen and hydrogen–oxygen bonds are

formed in the products.

CO2

CH4



chemical

reaction



new bonds formed



O2



H2O



reactants



products



new bonds formed



A chemical reaction may be accompanied by a visible change: two colorless reactants can form

a colored product; a gas may be given off; two liquid reactants may yield a solid product. Sometimes heat is produced so that a reaction flask feels hot. A reaction having a characteristic visible

change occurs when hydrogen peroxide (H2O2) is used to clean a bloody wound. An enzyme

in the blood called catalase converts the H2O2 to water (H2O) and oxygen (O2), and bubbles of

oxygen appear as a foam, as shown in Figure 5.1.

• A chemical equation is an expression that uses chemical formulas and other symbols to

illustrate what reactants constitute the starting materials in a reaction and what products

are formed.



smi26573_ch05.indd 122



12/2/08 3:37:26 PM



INTRODUCTION TO CHEMICAL REACTIONS



123







FIGURE 5.1



Treating Wounds with Hydrogen Peroxide—A Visible

Chemical Reaction



O2



H2O

H2O



H2O2



H2O2(aq)



The enzyme catalase in red blood converts hydrogen peroxide (H2O2) to water and oxygen

gas, which appears as a visible white foam on the bloody surface. Hydrogen peroxide does not

foam when it comes in contact with skin because skin cells do not contain the catalase needed

for the reaction to occur.



Chemical equations are written with the reactants on the left and the products on the right,

separated by a horizontal arrow—a reaction arrow—that points from the reactants to the products. In the combustion of methane, methane (CH4) and oxygen (O2) are the reactants on the left

side of the arrow, and carbon dioxide (CO2) and water (H2O) are the products on the right side.

coefficient

Chemical

equation



CH4



+



2 O2



reactants



coefficient

CO2



+



2 H2O



products



The numbers written in front of any formula are called coefficients. Coefficients show the

number of molecules of a given element or compound that react or are formed. When no

number precedes a formula, the coefficient is assumed to be “1.” In the combustion of methane,

the coefficients tell us that one molecule of CH4 reacts with two molecules of O2 to form one

molecule of CO2 and two molecules of H2O.

When a formula contains a subscript, multiply its coefficient by the subscript to give the total

number of atoms of a given type in that formula.

2 O2



= 4 O atoms



2 H2O = 4 H atoms + 2 O atoms



Coefficients are used because all chemical reactions follow a fundamental principle of nature, the

law of conservation of mass, which states:

• Atoms cannot be created or destroyed in a chemical reaction.



smi26573_ch05.indd 123



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124



CHEMICAL REACTIONS



TABLE 5.1 Symbols Used

in Chemical Equations

Symbol



(s)

(l)

(g)

(aq)



Meaning



Although bonds are broken and formed in reactions, the number of atoms of each element in the

reactants must be the same as the number of atoms of each type in the products. Coefficients are

used to balance an equation, making the number of atoms of each element the same on both

sides of the equation.



Reaction arrow

Heat

Solid

Liquid

Gas

Aqueous solution



+



CH4



2 O2



CO2



Atoms in the reactants:

• 1 C atom

• 4 H atoms

• 4 O atoms



+



2 H2O



Atoms in the products:

• 1 C atom

• 4 H atoms

• 4 O atoms



Two other important features are worthy of note. If heat is needed for a reaction to occur, the Greek

letter delta (∆) may be written over the arrow. The physical states of the reactants and products are

sometimes indicated next to each formula—solid (s), liquid (l), or gas (g). If an aqueous solution

is used—that is, if a reactant is dissolved in water—the symbol (aq) is used next to the reactant.

When these features are added, the equation for the combustion of methane becomes:

Combustion

of methane



CH4(g)



+



2 O2(g)







CO2(g)



+



2 H2O(g)



The symbols used for chemical equations are summarized in Table 5.1.



SAMPLE PROBLEM 5.1



Label the reactants and products, and indicate how many atoms of each type of element are

present on each side of the equation.

C2H6O(l) + 3 O2(g)



2 CO2(g) + 3 H2O(g)



ANALYSIS



Reactants are on the left side of the arrow and products are on the right side in a chemical

equation. When a formula contains a subscript, multiply its coefficient by the subscript to give

the total number of atoms of a given type in the formula.



SOLUTION



In this equation, the reactants are C2H6O and O2, while the products are CO2 and H2O. If no

coefficient is written, it is assumed to be “1.” To determine the number of each type of atom

when a formula has both a coefficient and a subscript, multiply the coefficient by the subscript.

1 C2H6O = 2 C’s + 6 H’s + 1 O

3 O2

= 6 O’s

2 CO2

= 2 C’s + 4 O’s

3 H2O



= 6 H’s + 3 O’s



Multiply the coefficient 3 by the subscript 2.

Multiply the coefficient 2 by each subscript;

2 × 1 C = 2 C’s; 2 × 2 O’s = 4 O’s.

Multiply the coefficient 3 by each subscript;

3 × 2 H’s = 6 H’s; 3 × 1 O = 3 O’s.



Add up the atoms on each side to determine the total number for each type of element.

C2H6O(l)



+



3 O2(g)



2 CO2(g)



+



3 H2O(g)



O



H

C

Atoms in the reactants:

• 2 C’s 6 H’s 7 O’s



smi26573_ch05.indd 124



Atoms in the products:

• 2 C’s 6 H’s 7 O’s



12/2/08 3:37:31 PM



BALANCING CHEMICAL EQUATIONS



PROBLEM 5.1



125



Label the reactants and products, and indicate how many atoms of each type of element are

present on each side of the following equations.

a. 2 H2O2(aq)

2 H2O(l) + O2(g)

b. 2 C8H18 + 25 O2

16 CO2 + 18 H2O

c. 2 Na3PO4(aq) + 3 MgCl2(aq)

Mg3(PO4)2(s) + 6 NaCl(aq)



PROBLEM 5.2



One term in a balanced chemical equation contained the coefficient 3 in front of the formula

Al2(SO4)3. How many atoms of each type of element does this represent?



PROBLEM 5.3



Write a chemical equation from each of the following descriptions of reactions.

a. One molecule of gaseous methane (CH4) is heated with four molecules of gaseous chlorine

(Cl2), forming one molecule of liquid carbon tetrachloride (CCl4) and four molecules of

gaseous hydrogen chloride (HCl).

b. One molecule of liquid methyl acetate (C3H6O2) reacts with two molecules of hydrogen

gas (H2) to form one molecule each of liquid ethanol (C2H6O) and methanol (CH4O).



ENVIRONMENTAL NOTE



5.2



BALANCING CHEMICAL EQUATIONS



In order to carry out a reaction in the laboratory, we must know how much of each reactant

we must combine to give the desired product. For example, if we wanted to synthesize aspirin

(C9H8O4) from a given amount of salicylic acid (C7H6O3), say 100 g, we would have to determine how much acetic acid would be needed to carry out the reaction. A calculation of this sort

begins with a balanced chemical equation.

H

H

C



C



C

H



The reaction of propane with oxygen

forms carbon dioxide, water, and

a great deal of energy that can be

used for cooking, heating homes

and water, drying clothes, and

powering generators and vehicles.

The combustion of propane and

other fossil fuels adds a tremendous

amount of CO2 to the atmosphere

each year, with clear environmental

consequences (Section 12.8).



HOW TO

EXAMPLE

Step [1]



H

C



O



C



H



H

+



O



C



C



H



O



H



H



salicylic acid

C7H6O3



C

H



H



O



C



C

O



H



acetic acid

C2H4O2



C



C

H



O

C



O



C



C

O



C



C



H



O



CH3



+



H2O



H



aspirin

C9H8O4



In this example, the equation is balanced as written and the coefficient in front of each formula is

“1.” Thus, one molecule of salicylic acid reacts with one molecule of acetic acid to form one molecule of aspirin and one molecule of water. More often, however, an equation must be balanced

by adding coefficients in front of some formulas so that the number of atoms of each element

is equal on both sides of the equation.



Balance a Chemical Equation

Write a balanced chemical equation for the reaction of propane (C3H8) with oxygen (O2) to form carbon dioxide

(CO2) and water (H2O).

Write the equation with the correct formulas.

• Write the reactants on the left side and the products on the right side of the reaction arrow, and check if the

equation is balanced without adding any coefficients.

C3H8 + O2



CO2 + H2O



Continued



smi26573_ch05.indd 125



12/2/08 3:37:31 PM



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