Chapter 5: Molecules and Compounds

Transcription

1 Chapter 5 Page 1 Chapter 5: Molecules and Compounds Compounds-a pure substance composed of more than one type of where the combine chemically infixed, definite proportions. Contrast a compoundto a mixturewhere elements can have ANY proportion. Law of Constant Composition (Proust)-ratio of elements in a compound is always the sameif the compound is pure. Ex. Two samples of pure carbon dioxide gas were broken down into their constituent elements -carbon and oxygen -and the masses of each element were measured. Sample 1 = 4.8 g O, 1.8 g C Sample 2 = 17.1 g O, 6.4 g Are these results consistent with the Law of Constant Composition?

2 Chapter 5 Page 2 Chemical Formulas Chemical Formulas: describe compounds by showing the number and type of each element in the simplest unit of the compound. Formulas use element symbolsand subscripts to the rightof the element symbolto describe the number of atoms of each element in the compound. symbol for carbon and implied subscript of "1" Ex. CO 2 symbol for oxygen and subscript showing 2 oxygen atoms C 3 H 8 -propane Carbon dioxide Order of writing elements in a formula: 1. Metals before nonmetals (You need to know this.) 2. Nonmetals written in order shown below C P N H S I Br Cl O F (You DON"T need to memorize this, but be able to use it to write formula) Give the formula of acetone, a molecule composed of six hydrogen atoms, three carbon atoms, and one oxygen atom.

3 Chapter 5 Page 3 Formulas with Polyatomic Ions Polyatomic ions: groups of atoms that act as a unit. Need to write these in chemical formulas in a way to indicate that the unit is kept together Ex. Symbol of acetate polyatomic ion (in parentheses) Ca(C 2 H 3 O 2 ) 2 Calcium acetate Implied "1" subscript on calcium Subscript indicates two C 2 H 3 O 2 units K 2 SO 4 Determine the number of each type of atom in Mg 3 (PO 4 ) 2. SO 4 is symbol of sulfate polyatomic ion Noparentheses for one polyatomic ion unit Classifying Elements and Compounds Atomic Elements: Elements whose particles are SINGLE ATOMS. Most elements are atomic elements -Fe, C, Au, B, etc. Molecular Elements: Elements whose particles are DIATOMIC MOLECULES (molecule has 2 atomsof the same element) Only a handful of these elements (7). They are: H 2 N 2 O 2 Cl 2 Br 2 I 2 F 2 You need to know these

4 Chapter 5 Page 4 Classifying Compounds Molecular Compounds: Compounds whose particles are molecules made of two or more NONMETALS. Examples: H 2 O, CS 2, C 3 H 6 O 2, NH 3, PCl 3 Ionic Compounds: Compounds whose particles are CATIONS(made from metals) and ANIONS (made from 1 or more nonmetals). No molecules, instead have formula units Have 3-D lattice of cations and anions held together by + &-attractive forces Ex. NaCl, CaO, NiBr 2, K 3 PO 4 Classifying Elements and Compounds NaCl Classify the following as ether an atomic element, molecular element, molecular compound or ionic compound. O 2 CO Na 2 O Co N 2 H 4 FeCl 3

5 Chapter 5 Page 5 Review: Main Group Cation and Anion Charges Recall from Chapter 4: Examples Group IA metals form +1 cations Na +, Li + Group IIA metals form +2 cations Ca 2+, Group IIIA metals form cations Group VIIA nonmetals form -1 anions Group VIA nonmetals form anions Group VA nonmetals form anions F -,,, To form K +, neutral K needs to lose/gain1 electron To form P 3-, neutral P needs to gain electrons

6 Chapter 5 Page 6 Writing Ionic Compound Formulas Formulas for ionic compounds should be NEUTRAL. Ratio in which cations (+) and anions (-) combine is such that the charges cancel. cation anion charge cancellation formula Ex. K + S 2-2K + and 1 S 2- K 2 S Ba 2+ Cl - The symbol for the positively charge cation is written first, followed by the anion. CHARGES ARE NOT SHOWN in final formula Subscripts give the combining ratio Problems: (Hint: need to first determine cation and anion charges) Write the formula of an ionic compound made from: a) Calcium (Ca) and bromine (Br) b) Lithium (Li) and nitrogen (N) c) Aluminum (Al) and oxygen (O) d) Magnesium (Mg) and sulfur (S)

7 Chapter 5 Page 7 Type I & II Ionic Compounds Two types of Ionic Compounds: Type I and Type II Type I -Metal forms only ONE TYPEof positive ion Group IA -IIIA metalsare type I cations Name of cation is the same as the element Ex. Mg 2+ magnesium cation Na + sodium cation TYPE II -Metal forms MORE THAN ONE TYPEof positive ion Most transition metalsare type II cations (exceptions: Zn 2+, Cd 2+, Ag + -always these charges) Name of cation is the element name followed by charge of cation in parentheses Ex. Fe 3+ iron (III) cation Cu 2+ copper (II) cation

8 Chapter 5 Page 8 Anion Nomenclature Monoatomic ANION Names Use element's root (base) name plus "ide" VA VIA VIIA N 3- nitride O 2- oxide F- fluoride P 3- S 2- Cl - Se 2- Br - I - Some Common Polyatomic Ions Name Formula Acetate C 2 H 3 O 2 Carbonate CO 2 3 Hydrogencarbonate HCO 3 (aka bicarbonate) Hydroxide OH Nitrate NO 3 Nitrite NO 2 Chromate CrO 2 4 Dichromate Cr 2 O 2 7 Ammonium NH + 4 Cyanide CN - Name Formula Hypochlorite HClO - Chlorite ClO 2 Chlorate ClO 3 Perchlorate ClO 4 Sulfate SO 2 4 Sulfite SO 2 3 Hydrogen sulfate HSO 4 (aka bisulfate) Hydrogen sulfite HSO 3 (aka bisulfite) Phosphate PO 3-4 You need to know the 10 polyatomic ions in BLACK by quiz #2 (Know Names, formulas and charges -See your syllabus for list there as well)

9 Chapter 5 Page 9 Naming Ionic Compounds Name cation first followed by anion (need to determine whether cation is type I or II to name correctly) Determine the nameof the following ionic compounds from their formulas: Na 2 O FeCl 3 CoCl 2 Li 2 SO 4 CuCl PbBr 2 (NH 4 ) 3 PO 4 SnF 4 CaCO 3 BaS Determine the formulafor the following ionic compounds from their names: copper (II) iodide sodium cyanide calcium nitride zinc oxide lead (II) sulfide iron (III) nitrate ammonium chloride magnesium chlorate Sodium hydroxide Aluminum Phosphide

10 Chapter 5 Page 10 Naming Molecular Compounds Molecular Compounds: contain only nonmetals and have no ions Binary (2 element) molecular compounds are named from the formula using Greek prefixes to show quantity N 2 O: N 2 O 4 : CO: NO 3 : CO 2 : S 2 Cl 2 : Phosphorus trichloride Greek prefixes (for quantity) 1: mono 2: di 3: 4: 5: 6: Carbon tetrachloride Disulfur monoxide

11 Chapter 5 Page 11 Naming Acids Acid -a molecular compound that ionizes to form H + when dissolved in water (H + is the cation). HCl(g) H 2 O H + (aq) + Cl - (aq) To indicate an acid is dissolved in water (aq)is written after the formula i.e. HCl (aq) Acid formulas are written so that the acidic hydrogens(s)is the first atom(s) listedin the formula. Examples of compounds which are acids: HNO 3 HNO 2 HClO 4 H 2 SO 4 H 3 PO 4 HCl H 2 CO 3 HC 2 H 3 O 2 HCN Name (compound not in water) Hydrogen chloride HCl Hydrogen nitrate HNO 3 Hydrogen nitrite H 2 O H 2 O H 2 O Name (dissolved in water) hydrochloric acid nitricacid nitrous acid Not all compounds with H atoms are acids: ex. CH 4, NH 3

12 Chapter 5 Page 12 Naming Acids anion -ate -ite -ide acid -ic acid -ous acid hydro--ic acid Give the acid name for the following compounds: HBr hydrogen bromide H 2 CO 3 hydrogen carbonate H 2 SO 3 hydrogen sulfite H 2 S hydrogen sulfide HNO 3 hydrogen nitrate HClO 3 hydrogen chlorate HF hydrogen fluoride H 3 PO 3 hydrogen phosphite

13 Chapter 5 Page 13 Formula Mass The mass of an individual molecule or formula unit (also known as molecular mass or molecular weight) To calculate formula mass add the masses of the atoms in a single molecule or formula unit Ex. What is the formula mass of H 2 O? 2( amu) amu = = 18.02amu Calculate the formula mass of Al 2 (SO 4 ) 3