Chemical Formulas & Chemical Compounds. Chemical formula indicates the relative number of atoms of each kind in a chemical compound.

Transcription

1 Unit 6: Chemical Formulas & Chemical Compounds Chemical Names & Formulas Chemical formula indicates the relative number of atoms of each kind in a chemical compound. Molecular compound - it s formula reveals the number of atoms of each element contained in a single molecule of the compound. Ionic compound - consists of a network of pos. & neg. ions held together by mutual attraction; represents one formula unit or the simplest ratio of cations (+) and anions(-). numbers of atoms are shown in the subscript parentheses are used to identify polyatomic ions as a unit. monatomic ions - are formed from a single atom gaining or losing e`. not all main group elements readily form ions, they share electrons and form covalent bonds. monatomic cations - are identified by the element s name monatomic anions - have the element s ending name dropped and -ide is added to the root name. Stock system - the names of the ions use roman numerals to name chemical ions...see table pg 205. Binary compounds - are compounds composed of 2 different elements [their total positive & negative charges must equal].

2 Method to writting out names... symbol for cation is written first (metal). symbol for anion is written last w/ -ide ending (nonmetal) to determine subscripts for ionic formulas, the pos. and neg. charges can be crossed-over. This method balances the charges write the symbols for ions [cation 1st] Al+3 O-2 2. cross-over charges - use the absolute value of each ion s charge as a subscript. Al+3 O check the subscripts and divide by their LCM. Give smallest whole # ratio of ions Al 2 O 3 Nomenclature or naming system involves combining the names of the compound s pos. and neg. ions... Aluminum oxide Al 2 O 3

3 to distinguish ions formed, the stock system is used. This system uses roman numerals to indicate an ion s charge. Fe+2 Fe+3 Iron(II) Iron(III) CuCl2 Copper (II) chloride CuCl Copper (I) chloride Names of metals that commonly form 1 cation do not include a roman numeral NaCl sodium chloride MgCl 2 magnessium chloride Polyatomic Ions (see pg 210) All are negatively charged (except NH4 + ) Most are Oxyanions - ions that contain oxygen when naming oxyanions, the most common ion is given the -ate ending. the ion with one less oxygen ends in -ite... such as... NO 2 - nitrite NO 3 - nitrate one less oxygen than -ite is given the prefix hypoone more oxygen than -ate is given the prefix per- ClO- ClO2 - ClO3 - ClO4 - hypochlorite chlorite chlorate perchlorate

4 Oxidation Numbers [7-2]...or oxidation states are assigned to atoms in compounds according to a set of specific rules. Generally, shared electrons are assumed to belong to the more electronegative atom. Electronegativity is a measure of the ability of an atom in a chemical compound to attract electrons. for many compounds, the negative charge of the valence electron is concentrated closer to one atom than to another. 1. atoms in a pure element have an oxidation number of zero. 2. In binary molecular compounds, the more electronegative element is assigned the neg. charge (anion); the less electronegative element is assigned the pos. charge (cation). 3. [ F ] has an oxidation number of -1 always, b/c it is the most electronegative element. 4. [ O ] has an oxidation number of -2 almost always, except when in a peroxide (H2O2) and when combined w/ such halogens as OF2 (when its +2). 5. [ H ] has an oxidation number of +1, when combining w/ more electronegative elements; and -1, in compounds w/ metals. 6. Sum of oxidation numbers in a neutral compound are equal to zero. 7. The sum of oxidation numbers in a polyatomic ion is equal to the charge of the ion. 8. Can also be assigned a to atoms in ionic compounds. the numbers can sometimes be used in the same manner as ionic charges to be determined formulas. Compounds w/ more than one oxidation state are named using the stock system; stock system names and prefix-system names are interchangeably used for many molecular compounds. By knowing the oxidation numbers, compounds can be named w/o knowing if they are ionic or molecular.

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6 Naming Binary Molecular Compounds composed of covalently bonded molecules newer naming system is stock system (this requires oxidation numbers) older system uses prefixes... Number prefix used 1 mono- 2 di- 3 tri- 4 tetra- 5 penta- 6 hexa- 7 hepta- 8 octa- 9 nona- 10 deca-

7 Rules for the prefix system 1. less-electronegative element is given 1st. 2. second element named by combining: prefix, root name, & -ide ending. 3. -o or -a ending of prefix is usually dropped when root word begins w/ a vowel. oxygen and halogens are usually given second b/c less electronegative element is written first. see examples PCl3 Prefix system Stock system phosphorous trichloride phosphorous (III) chloride PCl5 phosphorous pentachloride phosphorous (V) chloride N2O dinitrogen monoxode nitrogen (I) oxide NO nitrogen monoxide nitrogen (II) oxide Covalent Network Compounds - covalent compounds that do not consist of molecules. Their subscripts indicate the smallest whole-number ratio of the atoms in the compound. Naming is similar to molecular compounds, for example... silicon carbide SiC - SiO2 - Si3N4 - silicon dioxide trisilicon tetranitride

8 Acids and Salts - most acids are classified as binary acids or oxy acids: binary acids - consist of usually hydrogen and a halogen. oxyacids - contain hydrogen, oxygen, and a nonmetal 3rd element. the term acid usually refers to a solution in water, made of one of these compounds; HCl - Hydrochloric acid...refers to hydrogen chloride HBr - Hydrobromic acid...refers to hydrogen bromide many polyatomic ions are produced by the loss of hydrogen ions from oxyacids... sulfuric acid [ H2SO4 ] ---> sulfate SO4-2 nitric acid [ HNO3 ] ---> nitrate NO3 - phosphoric acid [ H3PO4 ] ---> phosphate PO4-3 Anion Ending Example Acid Name Example -ide Cl - hydro (stem) ic acid hydrochloric Chloride acid -ate NO 3 (stem) ic acid nitric acid Nitrate -ite SO 3 (stem) ous acid sulfurous acid sulfite Use Hypo and Per, just like with naming polyatomic ions

9 Common Acids Formula CH 3 COOH H2CO3 H 3 BO 3 HClO4 HClO 3 HClO 2 HClO HBr HCl HF HI H 5 IO 6 HNO 2 HNO 3 H 2 C 2 O 4 H 3 PO 4 H 3 PO 3 H 3 PO 2 H 2 SO 4 H 2 SO 3 Name acetic carbonic boric perchloric chloric chlorous hypochlorous hydrobromic hydrochloric hydrofluoric hydroiodic periodic nitrous nitric oxalic phosphoric phosphorous hypophosphorous sulfuric sulfurous Salts form from a cation and the anion from an acid. Some salts contain anions where one or more hydrogen atoms are retained from the acid. named by adding the word hydrogen or prefix bi- to the anion name... H2CO3 carbonic acid HCO3 - hydrogen carbonate or bicarbonate ion H 2SO4 HSO4 - Sulfuric acid hydrogen Sulfate or bisulfate ion pg 215 #1-4