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9/27/2010 Principles of Chemistry: A Molecular Approach, 1st Ed. Nivaldo Tro Chapter 3 Molecules, Compounds, and Chemical Equations Roy Kennedy Massachusetts Bay Community College Wellesley Hills, MA Edited by K.M. Hattenhauer Tro, Principles of Chemistry: A Molecular Approach 1 Elements and Compounds • Elements combine together to make an almost limitless number of compounds compounds. • The properties of the compound are totally different from the constituent elements. Tro, Principles of Chemistry: A Molecular Approach 2 Chemical Bonds • Compounds are made of atoms held together by chemical bonds. • Bonds are forces of attraction between atoms. • The bonding attraction comes from attractions between protons and electrons. Tro, Principles of Chemistry: A Molecular Approach 3 1 9/27/2010 Bond Types • Two general types of bonding between atoms are found in compounds, ionic and covalent. • Ionic bonds result when electrons have been transferred between atoms, resulting in oppositely charged ions that attract each other. generally found when metal atoms bonded to nonmetal atoms t • Covalent bonds result when two atoms share some of their electrons. generally found when nonmetal atoms bonded together Tro, Principles of Chemistry: A Molecular Approach 4 Tro, Principles of Chemistry: A Molecular Approach 5 Representing Compounds (Chemical Formula) • Compounds are generally represented with a chemical formula. • The amount of information about the structure of the compound varies with the type of formula. All formulas and models convey a limited amount of i f information—none ti are perfect f t representations. t ti • All chemical formulas tell what elements are in the compound. use the letter symbol of the element Tro, Principles of Chemistry: A Molecular Approach 6 2 9/27/2010 Types of Formula • Empirical Formulas describe the kinds of elements found in the compound and the ratio of their atoms. They do not describe how many atoms, the order of attachment, or the shape. The formulas for ionic compounds are empirical. • Molecular Formulas describe the kinds of elements found in the compound and the numbers of their atoms. does not describe the order of attachment, or the shape Tro, Principles of Chemistry: A Molecular Approach 7 Types of Formula • Structural Formulas describe the kinds of elements found in the compound, the numbers of their atoms, the order of atom attachment, and the kind of attachment. They do not directly describe the three-dimensional shape but an experienced chemist can make a good shape, guess at it. use lines to represent covalent bonds Each line describes the number of electrons shared by the bonded atoms. single line = 2 shared electrons, a single covalent bond double line = 4 shared electrons, a double covalent bond triple line = 6 shared electrons, a triple covalent bond Tro, Principles of Chemistry: A Molecular Approach 8 Representing Compounds (Molecular Models) • Models show the three-dimensional structure along with all the other information given in the structural formula. • Ball-and-Stick Models use b ll tto representt th balls the atoms t and sticks to represent the attachments between them. • Space-Filling Models use interconnected spheres to show the electron clouds of atoms connecting together. Tro, Principles of Chemistry: A Molecular Approach 9 3 9/27/2010 Types of Formula Tro, Principles of Chemistry: A Molecular Approach 10 Elements and Compounds • atomic elements elements whose particles are single atoms • molecular elements elements whose particles are multi-atom molecules • molecular compounds • Tro, Principles of Chemistry: A Molecular Approach compounds whose particles are molecules made of only nonmetals ionic compounds compounds whose particles are 11 cations and anions Molecular Elements • Certain elements occur as two-atom molecules. H2, N2, O2, F2, Cl2, Br2, I2 • Other elements occur as polyatomic molecules. P4, S8, Se8 Tro, Principles of Chemistry: A Molecular Approach 12 4 9/27/2010 Formulas Describe Compounds • A compound is a distinct substance that is composed of atoms of two or more elements. • Describe the compound by describing the number and type of each atom in the simplest unit of the compound. molecules or ions • Each element is represented by its letter symbol. • The number of atoms of each element is written to the right of the element as a subscript. If there is only one atom, the 1 subscript is not written. • Polyatomic groups are placed in parentheses. if more than one Tro, Principles of Chemistry: A Molecular Approach 13 Ionic vs. Molecular Compounds Propane—contains individual C3H8 molecules Tro, Principles of Chemistry: A Molecular Approach Table salt—contains an array of Na+ ions and Cl– ions 14 Ionic Compounds • Compounds of metals with nonmetals are made of ions. Metal atoms: cations; nonmetal atoms: anions. • no individual molecule units, instead they have a three-dimensional array of cations and anions made of formula units • many contain polyatomic ions several atoms attached together in one ion Note: Compound must have no total charge; therefore, we must balance the numbers of cations and anions in a compound to get 0 charge. Tro, Principles of Chemistry: A Molecular Approach 15 5 9/27/2010 Writing Formulas: Ionic Compounds 1. Write the symbol for the metal cation and its charge. 2. Write the symbol for the nonmetal anion and its charge. 3 Charge (without sign) becomes subscript for other 3. ion. 4. Reduce subscripts to smallest whole number ratio. 5. Check that the sum of the charges of the cation cancels the sum of the anions. Tro, Principles of Chemistry: A Molecular Approach 16 Formula-to-Name Rules for Ionic Compounds • made of cation and anion • Some have one or more nicknames that are only learned by experience. NaCl = table salt, NaHCO3 = baking soda • Write systematic name by simply naming the ions ions. if cation is: metal with invariant charge = metal name metal with variable charge = metal name(charge) polyatomic ion = name of polyatomic ion if anion is: nonmetal = stem of nonmetal name + -ide polyatomic ion = name of polyatomic ion Tro, Principles of Chemistry: A Molecular Approach 17 Metal Cations • metals with invariant charge metals whose ions can have only one possible charge Groups 1A1+ & 2A2+, Al3+, Ag1+, Zn2+, Sc3+ cation name = metal name Tro, Principles of Chemistry: A Molecular Approach 18 6 9/27/2010 Metal Cations • metals with variable charges metals whose ions can have more than one possible charge g determine charge of cation by charge on anion name = metal name with Roman numeral charge in parentheses Tro, Principles of Chemistry: A Molecular Approach 19 Naming Monatomic Nonmetal Anion • Determine the charge from position on the Periodic Table. • To name anion, change ending on the element name to -ide. 4A = 4− 5A = 3− 6A = 2− 7A = 1− C = carbide N = nitride O = oxide F = fluoride Si = silicide P = phosphide S = sulfide Cl = chloride Tro, Principles of Chemistry: A Molecular Approach 20 Naming Binary Ionic Compounds for Metals with Invariant Charge • contain metal cation + nonmetal anion • metal listed first in formula and name 1. name metal cation first, name nonmetal anion second 2. cation name is the metal name 2 3. nonmetal anion named by changing the ending on the nonmetal name to -ide Tro, Principles of Chemistry: A Molecular Approach 21 7 9/27/2010 Naming Binary Ionic Compounds for Metals with Variable Charge • contain metal cation + nonmetal anion • metal listed first in formula and name 1. Name metal cation first, name nonmetal anion second. 2. Metal cation name is metal name followed by a Roman numeral in parentheses to indicate its charge. determine charge from anion charge common ions Table 3.4 3. nonmetal anion named by changing the ending on the nonmetal name to -ide Tro, Principles of Chemistry: A Molecular Approach 22 Compounds Containing Polyatomic Ions • Polyatomic ions are single ions that contain more than one atom. • often identified by (ion) in formula • Name and charge of polyatomic ion do not change. • Name any ionic compound by naming cation first d th i and then anion. Tro, Principles of Chemistry: A Molecular Approach 23 Some Common Polyatomic Ions Name Formula Name Formula acetate C2H3O2– hypochlorite ClO– chlorite chlorate ClO2– ClO3– perchlorate p ClO4– sulfate sulfite SO42– SO32– 2– carbonate CO3 hydrogen carbonate HCO3– (aka bicarbonate) hydroxide OH– nitrate nitrite NO3– NO2– chromate CrO42– dichromate ammonium Cr2O72– NH4+ Tro, Principles of Chemistry: A Molecular Approach hydrogen sulfate HSO4– (aka bisulfate) hydrogen sulfite HSO3– (aka bisulfite) 24 8 9/27/2010 Patterns for Polyatomic Ions 1. Elements in the same column form similar polyatomic ions. same number of O’s and same charge ClO3– = chlorate BrO3– = bromate 2. If the polyatomic ion starts with H, add hydrogen- prefix before name and add 1 to the charge. CO32– = carbonate HCO3– = hydrogen carbonate Tro, Principles of Chemistry: A Molecular Approach 25 Periodic Pattern of Polyatomic Ions -ate groups 3A 3− BO3 borate 4A 5A 2− 6A 7A − CO3 carbonate NO3 nitrate SiO32− silicate PO43− phosphate SO42− sulfate ClO3− chlorate AsO43− arsenate SeO42− selenate BrO3− bromate TeO42− tellurate IO3− iodate Tro, Principles of Chemistry: A Molecular Approach 26 Patterns for Polyatomic Ions • -ate ion chlorate = ClO3– • -ate ion + 1 O same charge, per- prefix perchlorate = ClO4– • -ate ate ion – 1 O same charge, -ite ite suffix chlorite = ClO2– • -ate ion – 2 O same charge, hypo- prefix, -ite suffix hypochlorite = ClO– Tro, Principles of Chemistry: A Molecular Approach 27 9 9/27/2010 Hydrates • Hydrates are ionic compounds containing a specific number of waters for each formula unit. water of hydration often “driven off” by heating • • in formula, attached waters follow · Prefix No. of Waters hemi mono ½ 1 di 2 tri tetra 3 4 penta hexa 5 6 CoCl2·6H2O • in name, attached waters indicated by prefix+hydrate after name of ionic compound CoCl2·6H2O = cobalt(II) chloride hexahydrate CaSO4·½H2O = calcium sulfate hemihydrate Anhydrous CoCl2 Hydrate CoCl2·6H2O Tro, Principles of Chemistry: A Molecular Approach hepta 7 octa 8 28 Writing Names of Binary Molecular Compounds of Two Nonmetals 1. Write the name of the first element in the formula. 2. element furthest left and down on the Periodic Table Use the full name of the element, Write the name of the second element in the formula with an -ide suffix. as if it were an anion—However,, remember these compounds p do not contain ions! 3. Use a prefix in front of each name to indicate the number of atoms. a) Never use the prefix mono- on the first element. Tro, Principles of Chemistry: A Molecular Approach 29 Subscript—Prefixes • 1 = mono- • 6 = hexa- not used on first nonmetal • • • • • • • • • 2 = di3 = ttrii 4 = tetra5 = penta- 7 = hepta8 = octa9 = nona10 = deca- Drop last “a” if name begins with a vowel. Tro, Principles of Chemistry: A Molecular Approach 30 10 9/27/2010 Acids • Acids are molecular compounds that form H+ when dissolved in water. contain H+ cation and anion (in aqueous sol’n) To indicate the compound is dissolved in water, (aq) is written after the formula. not named as acid if not dissolved in water • sour ttaste t • dissolve many metals like Zn, Fe, Mg; but not Au, Ag, Pt • formula generally starts with H (eg. HCl, H2SO4) • Two Types: Binary acids have H+ cation and nonmetal anion. Oxyacids have H+ cation and polyatomic anion. Tro, Principles of Chemistry: A Molecular Approach 31 Acids Naming Binary Acids • • • • Write a hydro prefix. Follow with the nonmetal name. Change ending on nonmetal name to -ic. Write the word acid at the end of the name. Naming Oxyacids • If the polyatomic ion name ends in -ate, then change ending to -ic suffix. • If the polyatomic ion name ends in -ite, then change ending to -ous suffix. • Write word acid at the end of all names. Tro, Principles of Chemistry: A Molecular Approach 32 Writing Formulas for Acids • when name ends in acid, formula starts with H • Write formulas as if ionic, even though it is molecular. • Hydro prefix means it is a binary acid, no prefix means it is an oxyacid. f oxyacid, id if ending di iis -ic, i polyatomic l t i iion ends d iin • for ate; if ending is -ous, polyatomic ion ends in -ous Tro, Principles of Chemistry: A Molecular Approach 33 11 9/27/2010 Mass Formula Mass • the mass of an individual molecule or formula unit also known as molecular mass or molecular weight • sum of the masses of the atoms in a single molecule or formula unit whole = sum of the parts! Molar Mass of Compounds • The relative masses of molecules can be calculated from atomic masses. Tro, Principles of Chemistry: A Molecular Approach 34 Percent Composition • percentage of each element in a compound by mass • can be determined from 1. 2. • the formula of the compound the experimental mass analysis of the compound The percentages may not always total to 100% due to rounding. Tro, Principles of Chemistry: A Molecular Approach 35 Mass Percent as a Conversion Factor • The mass percent tells you the mass of a constituent element in 100 g of the compound. The fact that CCl2F2 is 58.64% Cl by mass means that 100 g of CCl2F2 contains 58.64 g Cl. • This can be used as a conversion factor. 100 g CCl2F2 : 58.64 g Cl Tro, Principles of Chemistry: A Molecular Approach 36 12 9/27/2010 Conversion Factors in Chemical Formulas • Chemical formulas have inherent in them relationships between numbers of atoms and molecules. or moles of atoms and molecules • These relationships can be used to convert between amounts of constituent elements and molecules. such as percent composition Tro, Principles of Chemistry: A Molecular Approach 37 Empirical Formula • simplest, whole-number ratio of the atoms of elements in a compound • can be determined from elemental analysis masses of elements formed when decompose or react combustion analysis percent composition Tro, Principles of Chemistry: A Molecular Approach 38 Finding an Empirical Formula 1) Convert the percentages to grams. a) b) assume you start with 100 g of the compound skip if already grams 2) Convert grams to moles. a) use molar mass of each element 3) Write a pseudoformula using moles as subscripts. 4) Divide all by smallest number of moles. a) If result is within 0.1 of whole number, round to whole number. 5) Multiply all mole ratios by number to make all whole numbers. a) b) if ratio ?.5, multiply all by 2; if ratio ?.33 or ?.67, multiply all by 3; if ratio 0.25 or 0.75, multiply all by 4; etc. skip if already whole numbers Tro, Principles of Chemistry: A Molecular Approach 39 13 9/27/2010 Molecular Formulas • • The molecular formula is a multiple of the empirical formula. To determine the molecular formula, you need to know the empirical formula and the molar mass of the compound compound. Tro, Principles of Chemistry: A Molecular Approach 40 Combustion Analysis • A common technique for analyzing compounds is to burn a known mass of compound and weigh the amounts of product made. generally used for organic compounds containing C, H, O • By knowing the mass of the product and composition of constituent elements in the product product, the original amount of constituent elements can be determined. All the original C forms CO2, the original H forms H2O, the original mass of O is found by subtraction. • Once the masses of all the constituent elements in the original compound have been determined, the empirical formula can be found. Tro, Principles of Chemistry: A Molecular Approach 41 Chemical Reactions • Reactions involve chemical changes in matter resulting in new substances. • Reactions involve rearrangement and exchange of atoms to produce new molecules. Elements are not transmuted during a reaction. Reactants Tro, Principles of Chemistry: A Molecular Approach Products 42 14 9/27/2010 Chemical Equations • shorthand way of describing a reaction • provides information about the reaction formulas of reactants and products states of reactants and products relative numbers of reactant and product molecules that are required can be used to determine weights of reactants used and products that can be made Tro, Principles of Chemistry: A Molecular Approach 43 Combustion of Methane • Methane gas burns to produce carbon dioxide gas and gaseous water. Whenever something burns, it combines with O2(g). CH4(g) + O2(g) CO2(g) + H2O(g) This equation reads “1 molecule of CH4 gas combines with 1 molecule l l off O2 gas tto make k 1 molecule l l off CO2 gas and d1 molecule of H2O gas”. O H H C H + O H + C O H O H O Tro, Principles of Chemistry: A Molecular Approach 44 Combustion of Methane, Balanced • To show the reaction obeys the Law of Conservation of Mass, the equation must be balanced. We adjust the numbers of molecules so there are equal numbers of atoms of each element on both sides of the arrow. CH4(g) + 2 O2(g) CO2(g) + 2 H2O(g) (g) H H C H H O + O O + O C O Tro, Principles of Chemistry: A Molecular Approach O + H H O + O H H 45 15 9/27/2010 Chemical Equations CH4(g) + 2 O2(g) CO2(g) + 2 H2O(g) • CH4 and O2 are the reactants, and CO2 and H2O are the products. • The (g) after the formulas tells us the state of the chemical. • The number in front of each substance tells us the number of those molecules in the reaction. called the coefficients Tro, Principles of Chemistry: A Molecular Approach 46 Chemical Equations CH4(g) + 2 O2(g) CO2(g) + 2 H2O(g) • This equation is balanced, meaning that there are equal numbers of atoms of each element on the reactant and product sides. To obtain the number of atoms of an element element, multiply the subscript by the coefficient. 1C1 4H4 4O2+2 Tro, Principles of Chemistry: A Molecular Approach 47 Symbols Used in Equations • symbols used to indicate state after chemical (g) = gas; (l) = liquid; (s) = solid (aq) = aqueous = dissolved in water • energy symbols used above the arrow for p reactions decomposition = heat h = light shock = mechanical elec = electrical Tro, Principles of Chemistry: A Molecular Approach 48 16 9/27/2010 Classifying Compounds Organic vs. Inorganic • In the18th century, compounds from living things were called organic; compounds from the nonliving environment were called inorganic. • Organic compounds easily decomposed and could not be made in an 18th-century lab. • Inorganic compounds are very difficult to decompose, but can be synthesized. Tro, Principles of Chemistry: A Molecular Approach 49 Modern Classifying Compounds Organic vs. Inorganic • Today, we commonly make organic compounds in the lab and find them all around us. • Organic compounds are made mainly of C and H, sometimes with O O, N N, P P, S S, and trace amounts of other elements. • The main element that is the focus of organic chemistry is carbon. Tro, Principles of Chemistry: A Molecular Approach 50 Carbon Bonding • Carbon atoms bond almost exclusively covalently. Compounds with ionic bonding C are generally inorganic. • When C bonds, it forms four covalent bonds. 4 single bonds, 2 double bonds, 1 triple + 1 single, etc. • Carbon is unique in that it can form limitless chains of C atoms, both straight and branched, and rings of C atoms. Tro, Principles of Chemistry: A Molecular Approach 51 17 9/27/2010 Classifying Organic Compounds • There are two main categories of organic compounds, hydrocarbons and functionalized hydrocarbons. • Hydrocarbons contain only C and H. • Most fuels are mixtures of hydrocarbons. Tro, Principles of Chemistry: A Molecular Approach 52 Tro, Principles of Chemistry: A Molecular Approach 53 18