C h e m i s t r y 1 2 C h 5 : M o l e c u l e s a n d C o m p o u n d s P a g e 1 Chapter 5: Molecules and Compounds Read Chapter 5 Check for MasteringChemistry due dates. Pure Substances and Mixtures: Pure substances have one invariable composition (elements and compounds) Mixtures have a variable composition. Homogeneous mixtures (solutions) mix uniformly throughout the mixture (coffee, salt water, air) while Heterogeneous mixtures are not uniform, they vary in texture and have regions of different composition (soil, pencil). Law of Constant Composition: Joseph Proust (1754-1826) All samples of a pure compound have the same proportions of their elements.
Chemicals: C h e m i s t r y 1 2 C h 5 : M o l e c u l e s a n d C o m p o u n d s P a g e 2 The properties of chemicals (pure substances) vary widely and greatly depend on how they are organized. Table salt is sodium chloride, but its properties are very different than elemental sodium or elemental chloride. We have learned about elements such as where to find metals or nonmetals on the periodic table, their symbols and names, as well as compounds which have a fixed ratio of two or more different elements. Now we will study a how to write out chemical formulas and name them (nomenclature). Chemical Formula: Chemicals are represented by element symbols and subscripts following those elements, indicating the number of atoms. By convention, a subscript of 1 is omitted. Parentheses are used when grouping more than one of the same polyatomic ions together. Example 1: How many atoms of each and atoms total are in the following chemicals? a) H 2 O b) NH 4 Cl c) Al 2 (SO 4 ) 3 d) C 4 H 10 S e) Cholesterol, C 27 H 45 OH Generally, the cation or more metallic element comes first followed by the anion or more nonmetallic elements. When dealing with organic formulas the order is usually, C x H y others as listed in the table below. The anion hydroxide is one exception to this general order: OH -1 Example 2: Write the formulas when the following atoms make a compound. Remember to write the more metallic element first, except for organic compounds write C x H y others. a) 4 chlorine and 1 carbon b) 3 oxygen, 2 iron (rust) c) 1 cobalt, 3 bromine d) 8 carbon, 18 hydrogen (octane) e) 2 oxygen, 4 hydrogen, 2 carbon
C h e m i s t r y 1 2 C h 5 : M o l e c u l e s a n d C o m p o u n d s P a g e 3 Chemical Bonds: Compounds and elements may have covalent bonds (molecules) Covalent bonds share electrons and are found in molecules made up when two or more nonmetals combine. Examples: elemental oxygen, O 2 ; compound sucrose table sugar, C 12 H 22 O 11 Compounds may have ionic bonds (formula units) Ionic bonds transfer electrons and are found in compounds made up from combining cations (metals, positively charged) with anions (nonmetals, negatively charged). Examples: table salt, sodium chloride, NaCl; rust, iron(iii) oxide, Fe 2 O 3 Chemical Formulas: Structural Formula will show the specific connections between atoms within a molecule and gives more information than a chemical formula alone. Example: formula = C 2 H 6 O, condensed structural formula = CH 3 CH 2 OH or structural formula = Empirical Formula gives the relative number of atoms of each element in a compound. Molecular Formula gives the actual number of atoms of each element in a molecule of the compound. Molecular N2O4 Empirical NO2 Comparison microscopic views for methane, CH4: Note: the empirical and molecular formulas of CH4 are the same.
C h e m i s t r y 1 2 C h 5 : M o l e c u l e s a n d C o m p o u n d s P a g e 4 Acids and Bases (Arrhenius Definition): Nomenclature: Acids are substances that increase H +1 ions in water. The cation is H +1. Bases are substances that increase OH -1 ions in water. The anion is often OH -1. Elements: Elements may be atomic or molecular. (Know your elements by name and symbol) Most elements are written in an atomic form and given the element name: Cu-copper, He-Helium, Zn-zinc Seven elements are diatomic and are called by their element name: H 2 -hydrogen, N 2, O 2, F 2, Cl 2, Br 2, I 2 A couple elements are polyatomic: P 4 is phosphorus, sulfur can be S, S 2, or S 8 (most common pure form) Some elements form more than one molecule/structure (allotropes) O 2 is oxygen, O 3 is ozone; C (graphite), C (diamond), C 60 (buckminsterfullerene) Common names Some molecules have been around so long they go by a common name: H 2 O-water, NH 3 -ammonia, CH 4 -methane, SiH 4 -silane Binary molecules (Made up from nonmetallic elements) CO 2, P 2 O 4, CCl 4 Mono, di, tri, tetra, penta, hexa, hepta, octa, nona, deca Prefix (not mono) element name prefix root of element plus ide Carbon dioxide Drop the a before an o, pentoxide, not pentaoxide. Organic molecules (carbon based) Alkanes (C n H 2n+2 ) Methane CH 4, ethane C 2 H 6, propane C 3 H 8, butane C 4 H 10, pentane C 5 H 12, hexane C 6 H 14, heptane C 7 H 16, octane C 8 H 18, nonane C 9 H 22, decane C 10 H 22 Organic molecules have functional groups that help identify properties. Combinations of carbon based organic molecules are enormous.
C h e m i s t r y 1 2 C h 5 : M o l e c u l e s a n d C o m p o u n d s P a g e 5 Ionic compounds (write cation name then anion name) In the chemical formula, the sum of positive charges must equal the sum of negative charges. If possible, reduce the subscripts to the smallest whole number ratios. Cations with known oxidation state of metal (Type I) Group 1A (+1), 2A (+2), Al and Ga (+3), Zn and Cd (+2), Ag (+1) Name of ion is identical to the name of the atom for cations Variable oxidation state of metal (Type II) Transition metals and metals below the nonmetal on the right have a variable oxidation state that must be indicated by Roman Numerals in parenthesis (this method is what I expect you to learn. Fe +3, iron (III); Fe +2, iron (II); Cu +1, copper (I); Sn +4, tin (IV) An alternative method differentiates from the higher oxidation number and lower oxidation number using the old form of the name and ic or ous as an ending respectively. (you should be aware of this method, but it will not be on a test) Fe +3, ferric Fe +2, ferrous; Cu +2, cupric; Cu +1, cuprous; Sn +4, stannic; Sn +2, stannous. Elemental Anions Group VA (-3); VIA (-2), VIIA (-1) Name of the element root followed by ide. N -3, nitride; S -2, sulfide, Br -1, bromide Polyatomic cations and anions Memorize the polyatomic ions. Be able to evaluate ion names, formulas, and charges following some basic rules.
C h e m i s t r y 1 2 C h 5 : M o l e c u l e s a n d C o m p o u n d s P a g e 6 Hydrates Ionic compounds can be chemically attached to a small number of water molecules in a solid form. CuSO 4. 5H 2 O, copper (II) sulfate pentahydrate Acids Binary acids-h + cation and an anion that ends with ide. Hydro root of element ic acid. H 2 S, hydrosulfuric acid Ternary oxyacids-h +1 cation with and anion ending in ate or ite ate changes to ic acid, do not use hydro H 2 SO 4 becomes sulfuric acid ite changes to ous acid, do not use hydro H 2 SO 3 becomes sulfurous acid
C h e m i s t r y 1 2 C h 5 : M o l e c u l e s a n d C o m p o u n d s P a g e 7 Formula Mass: Formula mass is the sum of the weighted average atomic masses of all the atoms in the chemical formula. Other names that apply include molar mass, formula weight, atomic mass/weight, molecular mass, molecular weight. This is the mass in grams for one mole (6.022 x 10 23 particles). Nomenclature Practice: Fill in the table with formulas and names of the ionic compounds. Cl * CO 3-2 PO 4-3 OH -1 Na +1 Zn * Sn +4 NH 4 +1 Al* H +1 *predict the known oxidation number
C h e m i s t r y 1 2 C h 5 : M o l e c u l e s a n d C o m p o u n d s P a g e 8 Nomenclature Practice: Names Formulas Chlorine Aluminum sulfide Sodium carbonate Carbon dioxide Nickel (II) chloride methane Ammonium phosphate Hydrosulfuric acid Sulfuric acid Sulfurous acid Cobalt (II) chloride hexahydrate Nomenclature Practice: H 3 PO 4 H 2 SO 4 (aq) H 2 O HBr (g) HBr (aq) MgSO 4 7H 2 O CCl 4 KMnO 4 CuSO 4 Al(HCO 3 ) 3 Formula Mass practice: solve for the formula masses for the species above.