CHAPTER 3 COMPOUNDS: SAFER MATERIALS FOR A SAFER WORLD From Green Chemistry and the Ten Commandments of Sustainability, Stanley E. Manahan, ChemChar Research, Inc., 2006 [email protected]

3.1. Chemical Bonds and Chemical Compounds

Chemical compounds consist of molecules or aggregates of ions consisting of two or more elements held together by chemical bonds • H 2 O • NH 3 • NaCl Bonds holding chemical compounds together • • Covalent bonds composed of shared electrons Ionic bonds consisting of positively charged negatively charged anions cations and

Importance of Bond Strengths

• Chlorofluorocarbons, such as Cl ozone destruction 2 CF 2 , have very strong C-Cl bonds and C-F bonds and persist into the stratosphere where they cause • 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 Structure

Molecular structure refers to the shape of molecules Consider Cl 2 CF 2 , which does not have a flat structure F F Cl C Cl Instead Cl C Cl F F Shapes of molecules determine the ways in which they interact with other molecules • Shapes of molecules especially important in living systems, such as in the interaction of biological catalyst enzymes with the substrates upon which they react

What Are Green Chemical Compounds?

Dichlorodifluoromethane, Cl ozone destruction 2 CF 2 , 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 • 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 can become involved in chemical bonds of atoms that Refer 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 argon Other elements try to attain the filled electron shells of their nearest neighbor noble gases by sharing, losing, or gaining electrons Hydrogen, H, seeks to have 2 electrons (like noble gas helium) shared in covalent bonds The 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

Examples of the Octet Rule

Cl Cl Cl Cl

Two chlorine atoms, each lacking only 1 electron for a complete octet in their outer shells

Na Cl

share 2 electrons so that they are held together by a single covalent bond in the Cl 2 molecule.

Na

+

Cl

A sodium atom donates its outer-shell electron to a chlorine atom, leaving the resulting Na shell octet + cation with its underlying second shell electrons as its filled outer to produce the ionic compound sodium chloride (NaCl) in which both the Na + cation and the Cl anion have filled outer electron shells consisting of stable octets.

Stable Outer Electron Shells from Covalent Bonding

N N N N

Two nitr ogen atoms, each with 5 outer -shell electrons Diatomic N 2 molecule with the s table octets of both atoms cir cled.

Stable octet of outer shell electrons around C

H H C H H

2 electrons around each H Bonding pair of electrons

Ball and Stick Model of CH

4

3.3. Sodium Chloride and Ionic Bonds

Recall that ions are charged atoms or groups of atoms, cations positively charged ions, and anions are negatively charged ions are Ionic compounds bonded together by ionic bonds • Mutual attraction of oppositely charged ions Formation of ions based on the octet rule

Na + Cl

Sodium atom ’s single outer e Sodium atom ’s inner s hell of 8 e Chlorine atom ’s outer shell of 7 e -

Na

+

+ Cl

Sodium ion ’s outer shell of 8 e Chloride ion ’s outer shell of 8 e -

Placement of Ions in NaCl

In 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 anions Na + cation Cl- anion The near est neighbors to this Cl ion ar e six Na + ions.

-

Crystal Structure of Ionic Compounds

Formula 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) + Cl 2 (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 Cl 2 is split into Cl atoms, which requires energy 3. An electron is taken from each Na atom to produce Na + ion, which requires energy 4. An electron is added to each Cl atom to produce a Cl releases energy ion, which 5. All the Na + quantity of cations and 1 Cl lattice energy anion are assembled in a 1/1 ratio in a crystal lattice to produce NaCl, which releases a very large

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 Cl 2

Relative Ion Sizes

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 F- (133) atomic number is

larger

.

9 11 12 13

Mg 2+ (65) Al 3+ (45)

16 17

Na + (98)

19

K + (133) S 2 - (190) Cl- (181) As electrons are removed from elements in the same period to form more highly charged

cations

, ion size

shrinks

: Na + > Mg 2+ > Al 3+ .

As electrons are added to atoms to produce more highly charged anions, the anion size S 2 > Cl -

increases

because more electrons occupy more space:

Formation of Calcium Chloride

Ca + Cl Cl Ca 2+ Cl Cl

Byproduct of making other chemicals Effective road salt, but a less polluting alternative is calcium acetate, Ca(C 2 H 3 O 2 ) 2 , which biodegrades: Ca(C 2 H 3 O 2 ) 2 + 4O 2 (bacteria)



CaCO 3 + 3CO 2 + 3H 2 O Yields a harmless calcium carbonate (limestone) product

Aluminum Oxide

Formation of aluminum oxide , Al 2 O 3 :

O Al O O Al 3+ O Al 3+ O Al O

• Bauxite , the ore from which aluminum is produced

Ions Consisting of Covalently Bonded Clusters of Atoms

Many ions consist of groups of atoms having a net electrical charge covalently bound together, but Acetate anion in calcium acetate, Ca(C 2 H 3 O 2 ) 2

H H O C C O H

Ionic Liquids

Ionic Liquids and Green Chemistry Composed of large ions Unlike most ionic compounds, which are solids, ionic liquids liquids under normal conditions are May substitute for organic solvents in some applications

3.4. Covalent Bonds in H

2

and Other Molecules

Although more highly charged ions, such as Al 3+ and N 3 do exist, elements in the middle of periods of the periodic table do not readily lose or gain enough electrons to form highly charged ions So, 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, H 2 , below:

+ +

3.5. Covalent Bonds in Compounds

Criteria for covalent bonds: • • Number of electrons involved Two shared electrons give a single bond constitute a double covalent bond , six , four shared electrons shared electrons make up a triple covalent bond Bonds may be shown as straight lines each indicating a pair of e • Two electrons in H 2 :

H H

• Four electrons in carbon-carbon bond of C 2 H 4 H H C C H H • Six electrons in N 2 N N • Electrons not involved in bonds are not shown

Covalent Bond Length

Covalent bonds have a characteristic bond length • About the same lengths as sizes of atoms • Expressed in picometers (pm) • The H-H bond in H 2 is 75 pm long

Bond Energy

A third important characteristic of covalent bonds is bond energy • Expressed in kilojoules (kJ) required to break a mole (6.02 x 10 23 ) of bonds • The bond energy of the H-H bond in H 2 is equal to 435 kJ/mole • Therefore, 435 kJ of energy is required to break all the H-H bonds in a mole of H 2 (2.0 g, 6.02 x 10 23 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 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 reactivity hazards may make substances unduly in chemicals making them prone to persistent excessive explosions in and other • Some arrangements of bonds contribute to chemical toxicity .

N

2

as an Energy-Intensive Raw Material

The high bond stability of N 2 raw material makes it an energy-intensive source of • Large amounts of energy and severe conditions are required to take the N 2 molecule apart in the synthesis of ammonia, NH nitrogen compounds 3 , the compound that is the basis for the synthesis of most synthetic • Rhizobium bacteria on the roots of leguminous plants produce some chemically combined nitrogen from elemental N 2 .

Strong Bonds in Chlorofluorocarbons and Ozone Depletion

Dichlorodifluoromethane, Cl 2 CF 2 , 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 Smog

Weak bonds in NO 2 near ground level contribute to the generation of photochemical smog • Relatively low energy ultraviolet radiation (h



) causes the following photochemical dissociation reaction to occur: NO 2 + h

 

NO + O • Very reactive O atoms released interact with pollutant hydrocarbons to produce photochemical smog

Instability and Toxicity

Some 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 Chemistry

Green chemistry seeks to avoid generation, use, or release to the environment of compounds with the kinds of bonds likely to cause problems Green 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 Compounds

H H C H H H H C H H H H C H (CH

4

) H H H H H H N O H H H H H H

Uns hared pair of electrons

3

H O H O H H (H

2

O) ) F H F H F (HF)

Double Bonds in Carbon Dioxide

O C O O C O O C O

Hydrogen Peroxide, a Reactive Green Compound

Hydrogen peroxide:

H

Hydrogen peroxide is unstable and decomposes to release oxygen: 2H 2 O 2 (liquid)



O 2 (gas) + H 2 O(liquid) Dilute hydrogen peroxide makes an effective and safe bleaching agent • Much safer to handle than elemental chlorine • Does not generate the potentially toxic byproducts that chlorine produces 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 electrons Nitric oxide is such a molecule having 11 valence electrons, 5 from N, 6 from O • NO exists as 2 resonance structures

N + O N O N O

• 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

Unequal Sharing of Electrons

Unsymmetrical water molecule:

H O H

Relatively larger O atom nucleus has a stronger attraction for the electrons than do the two H atom nuclei, each with only 1 proton • 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 bonds are polar covalent bonds (+) and the O-H H H O The polar water molecule (-) 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 NH 3 contributing the two electrons to a bond with H + ion in forming NH 4 + ion below: H H H N + H + H N H + H H Coordinate covale nt bond, both e le ctrons contribute d from the N atom in NH 3 Formation of a coordinate covalent bond between a water molecule and H + ion in water: H H O + H + H H O H + Coordinate covale nt bond, both e le ctrons contribute d from the O atom in H 2 O

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, SO 4 2 ) 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

Nitrogen, hydr ogen, s ulfur , and oxygen compos e the compound (NH 4 ) 2 SO 4 Each ammonium ion in the formula has 1 N atom and 4 H atoms There ar e 2 ammonium ions in the formula unit for each s ulfate ion Each sulfate ion in the formula unit contains 1 S atom and 4 O atoms Each formula unit of the compound contains 2 N atoms and 8 H atoms in the 2 NH 4 g roups and 1 S atom and 4 O atoms in the SO 4 g roup.

Us ing atomic mass es of N 14.0 , H 1.0, S 32.0 and O 16 .0, the formula mas s of the compound is 2



14 .0 + 8



1.0 + 1



32 .0 + 4



16 .0 = 132 .

The Mole

A mole of a substance the mass in grams of which is equal to the number of the formula mass of the compound is the amount of a substance the number of The formula mass of water is 18, so a mole of H 2 O has a mass of 18 g, that is, the molar mass of H 2 O is 18 g/mol Examples of a mole of a substance • Atoms of neon, atomic mass 20.1: 20.1 grams/mole • Molecules of H 2 , atomic mass 1.0, molecular mass 2.0: 2.0 g/mol • Molecules of CH 4 , 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 10 23 • A mole of neon contains 6.02 x 10 23 neon atoms • A mole of elemental hydrogen contains 6.02 x 10 23 (and 2 x 6.02 x 10 23 H atoms) • A mole of CaO contains formula units (pairs of Ca 2+ CaO H 2 molecules and O 2 ions) of

Percentage Composition of (NH

4

)

2

SO

4

, molar mass 132 g/mol

Note the earlier slide showing the chemical formula of (NH 4 ) 2 SO 4 and the calculation of its molar mass A mole of (NH 4 ) 2 SO 4 contains the following elements: 2 mol N x 14.0 g N/mol N = 28.0 g N, %N = 28.0 g x 100 = 21.2% N 132 g 8 mol H x 1.0 g H/mol H = 8.0 g H, %H = 8.0 g x 100 = 6.1% H 132 g 1 mol S x 32.0 g S/mol S = 32.0 g S, %S = 32.0 g x 100 = 24.2% S 132 g 4 mol O x 16.0 g O/mol O = 64.0 g O, %O = 64.0 g x 100 = 48.5% O 132 g

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-deca Binary 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, N 2 O 5 is called di nitrogen pent oxide • SiCl 4 , silicon tetrachloride • Si 2 F 6 , disilicon hexafluoride • PCl 5 , phosphorus pentachloride • SCl 2 , sulfur dichloride

Ionic Compounds Names

Ionic compounds the compound are referred to as formula units (rather than molecules) equal to the smallest aggregate of ions that can compose • For example, a formula unit of sodium sulfate consists of 2 Na + ions and 1 SO 4 2 ion composing a unit of Na 2 SO 4 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 it simply sodium sulfate call Na 2 SO 4 disodium monosulfate, but call

Naming Ionic Compounds Exercise

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

1. NH 4 + 2. Ca 3. Al 2+ 3+ (A) Cl (B) SO 4 2 (C) PO 4 3 Answers: 1(A) NH 4 Cl, ammonium chloride 1(B) (NH 4 ) 2 SO 4 ammonium sulfate 1(C) (NH 4 ) 3 PO 4 , ammonium phosphate 2(A) CaCl 2 , calcium chloride 2(B) CaSO 4 , calcium sulfate 2(C) Ca 3 (PO 4 ) 2 , calcium phosphate 3(A) AlCl 3 , aluminum chloride 3(B) Al 2 (SO 4 ) 3 , aluminum sulfate 3(B) AlPO 4 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 • Na 2 HPO 4 H + in which each formula unit is composed of 2 Na ion, and 1 PO 4 3 ion is called disodium monohydrogen + phosphate ions, 1 • KH 2 PO 4 in which each formula unit is composed of 1 K ions, and 1 PO 4 3 + ion, 2 H 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 salts Acids Acids are characterized by H + ion in water H + ion dissolved in water makes the water acidic An acid either contains H + water ion or produces it when it dissolves in Sulfuric acid, H 2 SO 4 , contains 2 H + ions per molecule

Carbon Dioxide As Acid

Carbon dioxide produces H + ion by reacting with water • CO 2 + H 2 O



H + + HCO 3 • Only a small fraction of the CO 2 molecules dissolved in water undergo the above reaction to produce H + ion, so water solutions of CO 2 are weakly acidic • Carbon dioxide is classified as a weak acid • Rainfall is weakly acidic because of dissolved CO 2 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) • HNO 3  H + + NO 3 (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 hydro chlor ic acid

•

• Different rules apply when an acid contains oxygen With acids containing oxygen, the one with more oxygen, such as H 2 SO 4 , is sulfur sulfur ous acid ic acid, the one with less oxygen, H 2 SO 3 , is • For greater or lesser amounts of oxygen consider the following example: HClO 4 , per chlor ic acid HClO 3 , chlor ic acid HClO 2 , chlor ous acid HClO, hypo chlor ous acid

Uses and Occurrence of Acids

Sulfuric 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 , HNO 3 , are produced in the U.S. each year ranking it 10th in chemical manufacture Hydrochloric acid million metric tons ranks about 25th, with annual production around 3

Acids and Green Chemistry

Because 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, H 2 S, removed from sour natural gas as a source of sulfur Acetic acid made by the fermentation of carbohydrates can be an excellent green alternative to harsher acids The production of acetic acid is a green process that uses biological reactions acting upon renewable biomass raw materials As a weak acid, acetic acid is relatively safe to use, biodegradable , contact of relatively dilute solutions with humans is not usually dangerous

H O

H atom that produces H +

H C C H OH

Bases

A base either contains hydroxide ion, OH , or reacts with water to produce hydroxide Most 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, NH 3 : NH 3 + H 2 O



NH 4 + + OH -

Weak and Strong Bases

The reaction of ammonia with H 2 O to produce OH reaction that lies far to the left: NH 3 + H 2 O

 

NH 4 + + OH Only a small fraction of the ammonia reacts • Therefore, ammonia is a weak base ion is a reversible The metal hydroxides, such as KOH, that completely dissociate in water are strong bases Metal 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)

Salts

An 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 + H 2 O base acid salt (sodium chloride) Some example salts: • NaCl, sodium chloride • CaCl 2 , calcium chloride, used to melt ice • NaCO 3 , sodium carbonate, used to make glass, treat water, other Salts with only one cation and one anion are named with just the name of the cation followed by the name of the anion Salts with more than one cation or anion are named to denote the number of each: KH 2 PO 4 is potassium dihydrogen phosphate