CHAPTER 3COMPOUNDS: SAFER MATERIALS FOR A SAFER

WORLD

From Green Chemistry and the Ten Commandments of Sustainability, Stanley E. Manahan, ChemChar Research,

Inc., 2006manahans@missouri.edu

3.1. Chemical Bonds and Chemical CompoundsChemical compounds consist of molecules or aggregates of ions consisting of two or more elements held together by chemical bonds

• H2O • NH3 • NaClBonds holding chemical compounds together

• Covalent bonds composed of shared electrons• Ionic bonds consisting of positively charged cations and

negatively charged anions

Importance of Bond Strengths

• Chlorofluorocarbons, such as Cl2CF2, have very strong C-Cl bonds and C-F bonds and persist into the stratosphere where they cause ozone destruction

• These are not green chemicals because the practice of green chemistry requires that substances that get released to the environment break down readily

• Bonds that break very readily are characteristic of reactive compounds, such as explosives, that may be hazardous

Molecular StructureMolecular structure refers to the shape of molecules

Consider Cl2CF2, which does not have a flat structure

• Shapes of molecules especially important in living systems, such as in the interaction of biological catalyst enzymes with the substrates upon which they react

F

F

C ClCl C

F

Cl Cl

F

Instead

Shapes of molecules determine the ways in which they interact with other molecules

What Are Green Chemical Compounds?Dichlorodifluoromethane, Cl2CF2, a chlorofluorocarbon (Freon) compound, would not be regarded as a green material because it is so stable and persistent in the atmosphere and causes stratospheric ozone destruction• Green replacement hydrofluorocarbons and hydrochlorofluoro-

carbons are much more green because they do not last long in the atmosphere and the hydrofluorocarbons do not contain ozone-destroying chlorine

Characteristics of Green Compounds• Preparation from renewable or readily available resources by

environmentally friendly processes• Low tendency to undergo sudden, violent, unpredictable reactions,

such as explosions that may cause damage, injure personnel, or cause release of chemicals and byproducts to the environment

• Nonflammable or poorly flammable• Low toxicity• Absence of toxic or environmentally dangerous constituents,

particularly heavy metals• Facile degradability, especially biodegradability, in the

environment• Low tendency to undergo bioaccumulation in environmental food

chains

Sodium Stearate, Hand Soap, is Green• Prepared by reacting byproduct animal fat with sodium hydroxide,

which is prepared by passing an electrical current through saltwater

• Flushed down the drain, sodium stearate reacts with calcium in water to form solid calcium stearate that biodegrades readily

3.2. Electrons Involved in Chemical Bonds and Octets of Electrons

Valence electrons are the ones in the outermost shell of atoms that can become involved in chemical bondsRefer to the Lewis symbols in the periodic table (next slide)

Abbreviated Periodic Table Showing Lewis Symbols

Elements in the Periodic Table and the Octet Rule

The three elements on the right of the table are noble gases that are chemically content with their filled outer shells containing 2 electrons in the case of helium and 8 each for neon and argonOther elements try to attain the filled electron shells of their nearest-neighbor noble gases by sharing, losing, or gaining electronsHydrogen, H, seeks to have 2 electrons (like noble gas helium) shared in covalent bondsThe other elements considered here, carbon and higher, attain 8 electrons in their outer shells by chemical bonding

• Tendency to attain 8 electrons is the basis of the octet rule

Cl Cl Cl ClTwo chlorine atoms, eachlacking only 1 electron fora complete octet in theirouter shells

share 2 electrons so thatthey are held together bya single covalent bond inthe Cl2 molecule.

Na Cl NaA sodium atom donates itsouter-shell electron to achlorine atom, leaving theresulting Na+ cation withits underlying second shellelectrons as its filled outershell octet

+Cl

-

to produce the ionic compoundsodium chloride (NaCl) in whichboth the Na+ cation and the Cl-anion have filled outer electronshells consisting of stable octets.

Examples of the Octet Rule

Stable Outer Electron Shells from Covalent Bonding

CHHHH2 electrons around each HBonding pair of electrons

Stable octet of outershell electronsaround CNNNNTwo nitrogen atoms, eachwith 5 outer-shell electronsDiatomic N2 molecule with the stableoctets of both atoms circled.

Ball and Stick Model of CH4

3.3. Sodium Chloride and Ionic BondsRecall that ions are charged atoms or groups of atoms, cations are positively charged ions, and anions are negatively charged ionsIonic compounds bonded together by ionic bonds• Mutual attraction of oppositely charged ionsFormation of ions based on the octet ruleNa+Cl Na+ClSodium atom’sinner shell of 8 e-Sodium atom’ssingle outer e-Chlorine atom’souter shell of 7 e-+Sodium ion’souter shell of 8 e- -Chloride ion’souter shell of 8 e-

Placement of Ions in NaClIn the sodium chloride crystalline structure, the six nearest neighbors of each negatively charged Cl- anion are positively charged Na+ cations and the six nearest neighbors of each positively charged Na+ cation are negatively charged Cl- anionsNa+ cationCl- anion

The nearest neighbors to this Cl- ion are six Na+ ions.

Crystal Structure of Ionic CompoundsFormula unit of NaCl where a formula unit of this compound consists of 1 Na+ ion and 1 Cl- ion, the smallest quantity of a substance that can exist and still be sodium chloride• There are not molecules of NaCl as such

Reaction to Form NaCl

2Na(solid) + Cl2(gas) 2NaCl(solid)This reaction can be broken down into the following steps:1. The atoms in solid Na are taken apart, which requires energy

2. Each molecule of Cl2 is split into Cl atoms, which requires energy3. An electron is taken from each Na atom to produce Na+ ion, which

requires energy4. An electron is added to each Cl atom to produce a Cl- ion, which

releases energy5. All the Na+ cations and 1 Cl- anion are assembled in a 1/1 ratio in a

crystal lattice to produce NaCl, which releases a very large quantity of lattice energy

Energy in Compound Formation

The energy involved in forming a compound is much like that involved in rolling a cart down a hill. In the case of sodium chloride, a lot of energy is released in forming solid crystalline ionic NaCl from solid Na metal and gaseous Cl2Na(solid) and Cl2(gas)

NaCl(solid)

Relative Ion Sizes

F- (133)Na+ (98)Mg2+ (65)Al3+ (45)S2- (190)Cl- (181)K+ (133)

9111213161719

Negative ions are generally larger than positive ions formed from elements that are nearby in the periodic table.

For ions in the same group of elements that have the same charge, the ion from the element with higher atomic number is larger.

As electrons are removed from elements in the same period to form more highly charged cations, ion size shrinks: Na+ > Mg2+ > Al3+.

As electrons are added to atoms to produce more highly charged anions, the anion size increases because more electrons occupy more space: S2- > Cl-

Formation of Calcium Chloride

Byproduct of making other chemicalsEffective road salt, but a less polluting alternative is calcium acetate, Ca(C2H3O2)2, which biodegrades:

Ca(C2H3O2)2 + 4O2 (bacteria) CaCO3 + 3CO2 + 3H2OYields a harmless calcium carbonate (limestone) product

Ca +ClClCl-Cl-Ca2+

Aluminum OxideFormation of aluminum oxide, Al2O3:

• Bauxite, the ore from which aluminum is produced

OO-O-Al3+O-Al3+OOAlAl

Ions Consisting of Covalently Bonded Clusters of Atoms

Many ions consist of groups of atoms covalently bound together, but having a net electrical charge

Acetate anion in calcium acetate, Ca(C2H3O2)2CCHHHOO-

Ionic LiquidsIonic Liquids and Green ChemistryComposed of large ionsUnlike most ionic compounds, which are solids, ionic liquids are liquids under normal conditionsMay substitute for organic solvents in some applications

3.4. Covalent Bonds in H2 and Other MoleculesAlthough more highly charged ions, such as Al3+ and N3- do exist, elements in the middle of periods of the periodic table do not readily lose or gain enough electrons to form highly charged ionsSo, these elements, as well as nonmetals to the right of periods, tend to form bonds by sharing electrons as shown for the elemental hydrogen molecule, H2, below: ++--

3.5. Covalent Bonds in CompoundsCriteria for covalent bonds:

• Number of electrons involved• Two shared electrons give a single bond, four shared electrons

constitute a double covalent bond, six shared electrons make up a triple covalent bond

Bonds may be shown as straight lines each indicating a pair of e-

• Two electrons in H2:

• Four electrons in carbon-carbon bond of C2H4

• Six electrons in N2

• Electrons not involved in bonds are not shown

H HN N

CH

H

HC

H

Covalent Bond LengthCovalent bonds have a characteristic bond length• About the same lengths as sizes of atoms• Expressed in picometers (pm)

• The H-H bond in H2 is 75 pm long

Bond EnergyA third important characteristic of covalent bonds is bond energy

• Expressed in kilojoules (kJ) required to break a mole (6.02 x 1023) of bonds

• The bond energy of the H-H bond in H2 is equal to 435 kJ/mole• Therefore, 435 kJ of energy is required to break all the H-H bonds

in a mole of H2 (2.0 g, 6.02 x 1023 molecules)

3.6. Covalent Bonds and Green Chemistry• High-energy bonds in raw materials require a lot of energy and

severe conditions, such as those of temperature, pressure, and radiation, to take apart in synthesizing chemicals. The practice of green chemistry tries to avoid such conditions.

• Especially stable bonds may make substances unduly persistent in the environment

• Relatively weak bonds may allow molecules to come apart too readily, and compounds with such bonds are often reactive species in the atmosphere or in biological systems

• Unstable bonds or arrangements of bonds may lead to excessive reactivity in chemicals making them prone to explosions and other hazards

• Some arrangements of bonds contribute to chemical toxicity.

N2 as an Energy-Intensive Raw MaterialThe high bond stability of N2 makes it an energy-intensive source of raw material

• Large amounts of energy and severe conditions are required to take the N2 molecule apart in the synthesis of ammonia, NH3, the compound that is the basis for the synthesis of most synthetic nitrogen compounds

• Rhizobium bacteria on the roots of leguminous plants produce some chemically combined nitrogen from elemental N2.

Strong Bonds in Chlorofluorocarbons and Ozone Depletion

Dichlorodifluoromethane, Cl2CF2, implicated in stratospheric ozone depletion• Especially stable bonds contribute to persistence and ultimate

environmental harm• Chlorofluorocarbons penetrate to the stratosphere before

contacting electromagnetic radiation energetic enough to break the molecules of these compounds apart

• Cl atoms split off from chlorofluorocarbons in the stratosphere attack ozone resulting in destruction of protective stratospheric ozone

Weak Bonds and SmogWeak bonds in NO2 near ground level contribute to the generation of photochemical smog• Relatively low energy ultraviolet radiation (h) causes the following

photochemical dissociation reaction to occur:

NO2 + h NO + O• Very reactive O atoms released interact with pollutant

hydrocarbons to produce photochemical smog

Instability and ToxicitySome bonding arrangements are noted for instability

• Bonds in which two N atoms are adjacent or very close in a molecule and are bonded with double bonds

• Arrangements in which N and O atoms are adjacent and double bonds are present

The presence of some kinds of bonds in molecules can contribute to their biochemical reactivity and, therefore, to their toxicity

• An organic compound with one or more C=C double bonds in the molecule is often more toxic than a similar molecule without such bonds

Bonds and Green ChemistryGreen chemistry seeks to avoid generation, use, or release to the environment of compounds with the kinds of bonds likely to cause problemsGreen chemistry seeks to avoid having to protect bonding groups by attaching protective chemical groups

• Protecting groups consume chemicals• Protecting groups generate byproducts

3.7. Predicting Formulas of Covalently Bound CompoundsHC CHHHH HH H C H (CH4)HHN N H NHHH H (NH3)HUnshared pair ofelectronsHO O H OHH (H2O)HHF F H FH (HF)

HHHHH

Double Bonds in Carbon Dioxide

C O C OO C OOO

Hydrogen Peroxide, a Reactive Green CompoundHydrogen peroxide:

• Much safer to handle than elemental chlorine• Does not generate the potentially toxic byproducts that chlorine

produces

OHH OHydrogen peroxide is unstable and decomposes to release oxygen:

2H2O2(liquid) O2(gas) + H2O(liquid)Dilute hydrogen peroxide makes an effective and safe bleaching agent

Can be pumped underground to serve as an oxidant for acclimated bacteria to use in attacking wastes

Exceptions to the Octet Rule

Exceptions occur when a molecule has an uneven number of electrons so that it is impossible for every atom to have an octet (an even number) of electronsNitric oxide is such a molecule having 11 valence electrons, 5 from N, 6 from O• NO exists as 2 resonance structures

• The uneven number of valence electrons in NO means that the molecule cannot accommodate octets around both the N and O atoms simultaneously, so NO exists as a resonance structure between two forms shown by the double arrows

NONON +O

Unequal Sharing of ElectronsUnsymmetrical water molecule:

• This gives each H atom a partial positive charge and the O atom a partial negative charge

• Unequal distribution of charge makes a body polar and the O-H bonds are polar covalent bonds

Relatively larger O atom nucleus has a stronger attraction for the electrons than do the two H atom nuclei, each with only 1 proton

OHH(+)(-)The polar watermoleculeHHO

The polar nature of the water molecule has a lot to do with water as a solvent and how it behaves in the environment and in living systems

When Only One Atom Contributes to a Covalent Bond

A coordinate covalent bond or a dative bond is one in which only one of the two atoms contributes to the bond as shown by the example of NH3 contributing the two electrons to a bond with H+ ion in forming NH4

+ ion below:

Formation of a coordinate covalent bond between a water molecule and H+ ion in water:

H N + H+HHH N H+HHCoordinate covalent bond,both electrons contributedfrom the N atom in NH3H O + H+HH O H+HCoordinate covalent bond,both electrons contributedfrom the O atom in H2O

Chemical Formulas, the Mole, and Percentage Composition

Chemical formulas are the words of chemical language and include

• The elements that compose the compound• The relative numbers of each kind of atom in the compound

• How the atoms are grouped, such as in ions (for example, SO42-)

present in the compound• With a knowledge of atomic masses, the molar mass of the

compound can be calculated• With a knowledge of atomic masses, the percentage composition

of the compound can be calculated

Information in a Chemical Formula(NH4)2SO4Nitrogen, hydrogen, sulfur, and oxygen compose the compoundEach ammonium ion inthe formula has 1 N atomand 4 H atomsThere are 2 ammonium ionsin the formula unit for eachsulfate ionEach sulfate ion in theformula unit contains1 S atom and 4 O atomsUsing atomic masses of N 14.0, H 1.0, S 32.0 and O 16.0, the formulamass of the compound is 2 × 14.0 + 8 × 1.0 + 1 × 32.0 + 4 × 16.0 = 132. 2 8 Each formula unit of the compound contains N atoms and H atoms 2 in the NH4 gr 1 4 oups and S atom and O atoms in the SO4 gr .oup 2 8 Each formula unit of the compound contains N atoms and H atoms 2 in the NH4 gr 1 4 oups and S atom and O atoms in the SO4 gr .oup

The Mole

A mole of a substance is the amount of a substance the number of the mass in grams of which is equal to the number of the formula mass of the compound

The formula mass of water is 18, so a mole of H2O has a mass of 18 g, that is, the molar mass of H2O is 18 g/mol Examples of a mole of a substance • Atoms of neon, atomic mass 20.1: 20.1 grams/mole

• Molecules of H2, atomic mass 1.0, molecular mass 2.0: 2.0 g/mol

• Molecules of CH4, molecular mass 16.0: 16.00 g/mol• Formula units of ionic CaO, formula mass 56.1: 56.1 g/mol

Avogadro’s Number

The number of specified entities in a mole of a substance is always the same regardless of the substance

• The number of specified entities in a mole of a substance is a very large number called Avogadro’s number = 6.02 x 1023

• A mole of neon contains 6.02 x 1023 neon atoms

• A mole of elemental hydrogen contains 6.02 x 1023 H2 molecules (and 2 x 6.02 x 1023 H atoms)

• A mole of CaO contains formula units (pairs of Ca2+ and O2- ions) of CaO

Percentage Composition of (NH4)2SO4, molar mass 132 g/mol

2 mol N x 14.0 g N/mol N = 28.0 g N, %N = 28.0 gx 100 = 21.2% N132 g8 mol H x 1.0 g H/mol H = 8.0 g H, %H = 8.0 gx 100 = 6.1% H132 g1 mol S x 32.0 g S/mol S = 32.0 g S, %S = 32.0 gx 100 = 24.2% S132 g4 mol O x 16.0 g O/mol O = 64.0 g O, %O = 64.0 gx 100 = 48.5% O132 g

Note the earlier slide showing the chemical formula of (NH4)2SO4 and

the calculation of its molar mass

A mole of (NH4)2SO4 contains the following elements:

3.9. What Are Chemical Compounds called?Prefixes: 1-mono, 2-di, 3-tri, 4-tetra, 5-penta, 6-hexa, 7-hepta, 8-octa, 9-nona, 10-decaBinary molecular compounds• The first part of the name is that of the first element in the

compound formula• The second part of the name is that of the second element in the

compound formula modified to have the ending -ide• Prefixes are added to indicate how many of each kind of atoms are

present in the molecule, for example, N2O5 is called dinitrogen pentoxide

• SiCl4, silicon tetrachloride

• Si2F6, disilicon hexafluoride

• PCl5, phosphorus pentachloride

• SCl2, sulfur dichloride

Ionic Compounds NamesIonic compounds are referred to as formula units (rather than molecules) equal to the smallest aggregate of ions that can compose the compound

• For example, a formula unit of sodium sulfate consists of 2 Na+ ions and 1 SO4

2- ion composing a unit of Na2SO4 Every ionic compound must be electrically neutral with the same number of positive as negative charges

• The requirement for electrical neutrality fixes the formula of an ionic compound, so it is usually not necessary to use prefixes to denote relative numbers of ions in the compound formula

• For example, we do not call Na2SO4 disodium monosulfate, but call it simply sodium sulfate

Naming Ionic Compounds ExerciseExercise: Give the formulas and names of compounds formed from each cation on the left, below, with each anion on the right.

1. NH4+ (A) Cl-

2. Ca2+ (B) SO42-

3. Al3+ (C) PO43-

Answers: 1(A) NH4Cl, ammonium chloride

1(B) (NH4)2SO4 ammonium sulfate

1(C) (NH4)3PO4, ammonium phosphate

2(A) CaCl2, calcium chloride

2(B) CaSO4, calcium sulfate

2(C) Ca3(PO4)2, calcium phosphate

3(A) AlCl3, aluminum chloride

3(B) Al2(SO4)3, aluminum sulfate

3(B) AlPO4 aluminum phosphate

Ionic Compounds With More Than One Cation or Anion

Prefixes are used in naming ionic compounds where more than 1 cation or more than 1 anion are present in the formula unit

• Na2HPO4 in which each formula unit is composed of 2 Na+ ions, 1 H+ ion, and 1 PO4

3- ion is called disodium monohydrogen phosphate

• KH2PO4 in which each formula unit is composed of 1 K+ ion, 2 H+ ions, and 1 PO4

3- ion is called monopotassium dihydrogen phosphate

3.10. Acids, Bases, and Salts

Other than covalently bound binary compounds, most inorganic compounds can be classified as acids, bases, or saltsAcidsAcids are characterized by H+ ion in waterH+ ion dissolved in water makes the water acidicAn acid either contains H+ ion or produces it when it dissolves in water

Sulfuric acid, H2SO4, contains 2 H+ ions per molecule

Carbon Dioxide As AcidCarbon dioxide produces H+ ion by reacting with water

• CO2 + H2O H+ + HCO3-

• Only a small fraction of the CO2 molecules dissolved in water undergo the above reaction to produce H+ ion, so water solutions of CO2 are weakly acidic

• Carbon dioxide is classified as a weak acid

• Rainfall is weakly acidic because of dissolved CO2 from air

Strong Acids

Acids such as hydrochloric acid, HCl, are completely dissociated in water to H+ and an anion.

• Such acids are strong acids

• HCl H+ + Cl- (Reaction to 100% complete dissociation)

• HNO3 H+ + NO3- (Complete dissociation)

Acids With Different Numbers of Oxygen Atoms• An acid that contains only H and one other element is a “hydro-ic”

acid, such as HCl, which is called hydrochloric acid• Different rules apply when an acid contains oxygen• With acids containing oxygen, the one with more oxygen, such as

H2SO4, is sulfuric acid, the one with less oxygen, H2SO3, is sulfurous acid

• For greater or lesser amounts of oxygen consider the following example:

HClO4, perchloric acid HClO3, chloric acid

HClO2, chlorous acid HClO, hypochlorous acid

Uses and Occurrence of AcidsSulfuric acid is the top chemical produced at about 40 million metric tons (40 billion kilograms) annually in the United States

• Greatest use is to treat phosphate minerals to produce phosphate crop fertilizers

• Other uses include removal of corrosion from steel (steel pickling), detergent synthesis, petroleum refining, lead storage battery manufacture, and alcohol synthesis

About 7-9 metric million tons of nitric acid, HNO3, are produced in the U.S. each year ranking it 10th in chemical manufactureHydrochloric acid ranks about 25th, with annual production around 3 million metric tons

Acids and Green ChemistryBecause of their widespread use and corrosive nature, acids are very important in the practice of green chemistry

• Reclamation and recycling of acids• Much of sulfuric acid now manufactured uses hydrogen sulfide,

H2S, removed from sour natural gas as a source of sulfurAcetic acid made by the fermentation of carbohydrates can be an excellent green alternative to harsher acidsThe production of acetic acid is a green process that uses biological reactions acting upon renewable biomass raw materialsAs a weak acid, acetic acid is relatively safe to use, biodegradable, contact of relatively dilute solutions with humans is not usually dangerous

H C C

H

H

OH

O H atom that produces H+

Bases

A base either contains hydroxide ion, OH-, or reacts with water to produce hydroxideMost bases that contain hydroxide are metal hydroxides: Sodium hydroxide, NaOH, and calcium hydroxide, Ca(OH)2, are examples.The most common basic substance that does not contain, but produces, hydroxide ion in water is ammonia, NH3:

NH3 + H2O NH4+ + OH-

Weak and Strong Bases

The reaction of ammonia with H2O to produce OH- ion is a reversiblereaction that lies far to the left:

NH3 + H2O NH4

+ + OH-

Only a small fraction of the ammonia reacts• Therefore, ammonia is a weak baseThe metal hydroxides, such as KOH, that completely dissociate in water are strong basesMetal hydroxides are named by the metal followed by “hydroxide”

Mg(OH)2 is a weak base because of low solubility (ingredient of milk of magnesia)

SaltsAn acid and a base react to form a salt, an ionic compound that has a cation other than H+ and an anion other than OH-

• Always produces water and is known as a neutralization reaction

NaOH + HCl NaCl + H2O base acid salt (sodium chloride)Some example salts:• NaCl, sodium chloride

• CaCl2, calcium chloride, used to melt ice

• NaCO3, sodium carbonate, used to make glass, treat water, otherSalts with only one cation and one anion are named with just the name of the cation followed by the name of the anionSalts with more than one cation or anion are named to denote the number of each:

KH2PO4 is potassium dihydrogen phosphate