Understanding Chemical Compounds for a Safer World

1. 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 manahans@missouri.edu

2. 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 • H2O • NH3 • NaCl Bonds holding chemical compounds together • Covalent bonds composed of shared electrons • Ionic bonds consisting of positively charged cations and negatively charged anions

3. 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

4. Molecular Structure Molecular structure refers to the shape of molecules Consider Cl2CF2, which does not have a flat structure Instead 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

5. 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

6. 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

7. 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

8. 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 bonds Refer to the Lewis symbols in the periodic table (next slide)

9. Abbreviated Periodic Table Showing Lewis Symbols

10. 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

11. Examples of the Octet Rule

12. Stable Outer Electron Shells from Covalent Bonding

13. Ball and Stick Model of CH4

14. 3.3. Sodium Chloride and Ionic Bonds Recall that ions are charged atoms or groups of atoms, cations are positively charged ions, and anions are negatively charged ions Ionic compounds bonded together by ionic bonds • Mutual attraction of oppositely charged ions Formation of ions based on the octet rule

15. 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

16. 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

17. 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 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- ion, which releases energy 5. 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

18. 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 Cl2

19. 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 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-

20. Formation of Calcium Chloride Byproduct of making other chemicals Effective road salt, but a less polluting alternative is calcium acetate, Ca(C2H3O2)2, which biodegrades: Ca(C2H3O2)2 + 4O2 (bacteria)  CaCO3 + 3CO2 + 3H2O Yields a harmless calcium carbonate (limestone) product

21. Aluminum Oxide Formation of aluminum oxide, Al2O3: • Bauxite, the ore from which aluminum is produced

22. 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)2

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

24. 3.4. Covalent Bonds in H2 and Other Molecules Although 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 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, H2, below:

25. 3.5. Covalent Bonds in Compounds Criteria 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

26. 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 H2 is 75 pm long

27. Bond Energy A 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)

28. 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.

29. N2 as an Energy-Intensive Raw Material The 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.

30. 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

31. Weak Bonds and Smog Weak 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

32. 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

33. 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

34. 3.7. Predicting Formulas of Covalently Bound Compounds

35. Double Bonds in Carbon Dioxide

36. Hydrogen Peroxide, a Reactive Green Compound Hydrogen peroxide: Hydrogen peroxide is unstable and decomposes to release oxygen: 2H2O2(liquid)  O2(gas) + H2O(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

37. 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 • 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

38. Unequal Sharing of Electrons Unsymmetrical water molecule: 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 and the O-H bonds are polar covalent bonds 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

39. 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:

40. 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

41. Information in a Chemical Formula

42. 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

43. 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

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

45. 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, N2O5 is called dinitrogen pentoxide • SiCl4, silicon tetrachloride • Si2F6, disilicon hexafluoride • PCl5, phosphorus pentachloride • SCl2, sulfur dichloride

46. Ionic Compounds Names Ionic 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 SO42- 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

47. 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. 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

48. 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 PO43- ion is called disodium monohydrogen phosphate • KH2PO4 in which each formula unit is composed of 1 K+ ion, 2 H+ ions, and 1 PO43- ion is called monopotassium dihydrogen phosphate

49. 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+ ion or produces it when it dissolves in water Sulfuric acid, H2SO4, contains 2 H+ ions per molecule

50. Carbon Dioxide As Acid Carbon 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