Subscript 4 refers to 4 oxygen atoms in sulfate ion.

Transcription

1 Chapter 7: Chemical Formulas and Chemical Compounds Section 7-1: Chemical Names and Formulas Explain the significance of a chemical formula. The subscript indicates that there are 8 carbon atoms in the molecule of octane. The subscript indicates that there are 18 hydrogen atoms in the molecule of octane. Subscript 2 refers to 2 aluminum atoms. Subscript 4 refers to 4 oxygen atoms in sulfate ion. Subscript 3 refers to everything inside parentheses giving 3 sulfate ions, with a total of 3 sulfur atoms and 12 oxygen atoms. Ionic Binary Compounds Determine the formula of an ionic compound formed between two given ions. 1. Write the symbols for the ions side by side. Write the cation first: Al 3+ O 2 2. Cross over the charges by using the absolute value of each ion s charge as the subscript for the other ion: Al 2 3+ O Multiplying the charge by the subscript shows that the charge on two Al 3+ cations (2 3+ = 6+) equals the charge on three O 2 anions (3 2 = 6 ). The largest common factor of the subscripts is 1. The formula is: Al 2 O 3

2 4. Check the subscripts and divide them by their largest common factor to give the smallest possible whole-number ratio of ions. Then write the corrected formula. Al 2 O 3 1. Write the symbols for the ions side by side. Write the cation first. 2. Place the Cation s charge as a subscript for the Anion. 3. Place the absolute value of Anion s charge as a subscript for the Cation. 4. Place the Cation next to the Anion. 5. Check the subscripts and divide them by their largest common factor to give the smallest possible whole-number ratio of ions.

3 1. Write the symbols for the ions side by side. Write the cation first. 2. Place the Cation s charge as a subscript for the Anion. 3. Place the absolute value of Anion s charge as a subscript for the Cation. 4. Place the Cation next to the Anion. 5. Check the subscripts and divide them by their largest common factor to give the smallest possible whole-number ratio of ions Name an ionic compound given its formula. Nomenclature - naming system, of binary ionic compounds Name of cation aluminum Page 207 Practice: Name of anion oxide

4 The Stock System of Nomenclature: Some elements (mostly transition metals) form two or more cations with different charges. To distinguish these ions, the Stock system of nomenclature is used. The Stock system - uses a Roman numeral to indicate an ion s charge. The numeral is enclosed in parentheses and placed immediately after the metal name. Stock system Examples Fe 2+ Fe 3+ iron(ii) iron(iii) Names of metals that commonly form only one cation do not include a Roman numeral. Na + Ba 2+ Al 3+ Sodium barium aluminum Using the stock system: Name of cation + Roman numeral indicating charge copper (II) Name of anion (Drop the ending add ide to name) chloride Put it together and you get copper (II) chloride. The way we know that the copper is a +2 is by analyzing the formula and see that there are 2 Cl 1 s and to be neutral copper has to have a +2: (+2) + (2( 1) = 0 thus a neutral compound Do Practice Exercises: page 209 # s 1 & 2

5 Oxyanions - polyatomic ions of oxygen Two or more oxyanions are formed by the same two elements. When naming compounds containing such oxyanions, the most common ion is given the ending -ate. The ion with one less oxygen atom is given the ending -ite. Examples: Nitrate, Nitrite; Sulfate, Sulfite NO 3 NO 2 SO 4 2 SO 3 2 When more than two different oxyanions form, the anion with one less oxygen than the - ite anion is given the prefix hypo-. An anion with one more oxygen than the -ate anion is given the prefix per-.this nomenclature is illustrated by the four oxyanions formed between chlorine and oxygen. Hypochlorite ClO Chlorite ClO 2 Chlorate ClO 3 Perchlorate ClO 4 SAMPLE PROBLEM 7-3 Write the formula for tin(iv) sulfate. SOLUTION: Write the symbols for the ions side by side. Write the cation first. Sn 4+ SO 4 2 Cross over the charges to give subscripts. Add parentheses around the polyatomic ion if necessary. Sn 4+ 2 (SO 4 ) 4 2 Check the subscripts and write the formula. The total positive charge is 2 4+ = 8+.The total negative charge is 4 2 = 8. The overall charge is neutral. Check. The largest common factor of the subscripts is 2, so the smallest whole-number ratio of ions in the compound is 1:2. The correct formula is therefore Sn(SO 4 ) 2. Do Practice Exercise page 211 # s 1 & 2 Homework: Page 235 #s 1-6 Binary Ionic compounds: A summary Why do metals form cations? They have low ionization energy & low electron affinity, so they don't particularly care to keep their outer shell electrons; they fall back to Nobel Gas Configuration of the next lower energy level.

6 Why do nonmetals form anions? They have high ionization energy & high electron affinity; they want to keep not only their electrons, but also what ever electrons are around to fill their outer shell. So for main group elements (the s and p blocks), cations & anions have an octet of electrons. Covalent Bonds (review): Covalent bonds consist of shared electron pair(s) and are formed between nonmetals. Octet rule as applied to Covalent bond formation, atoms try to complete octet by sharing electron pairs. The Question arises How do we name the Covalent Compound? The old system of naming molecular compounds is based on the use of prefixes. Naming Binary Molecular Compounds: Using prefixes, name a binary molecular compound from its formula. First question is what are the prefixes? A prefix indicates how many atoms of an element are in a molecule. The newer system is the Stock system for naming molecular compounds, which requires an understanding of oxidation numbers. This system will be discussed in Section 2. The old system of naming molecular compounds is based on the use of prefixes; so we will start with prefixes!

7 First things first, we must know the prefixes. TABLE 7-3 page 212 The rules for the prefix system of nomenclature of binary molecular compounds are as follows. 1. The less electronegative element is given first. It is given a prefix only if it contributes more than one atom to a molecule of the compound. Number Prefix 1 mono- 2 di- 3 tri- 4 tetra- 5 penta- 6 hexa- 7 hepta- 8 octa- 9 nona- 10 deca- 2. The second element is named by combining (a) a prefix indicating the number of atoms contributed by the element, (b) the root of the name of the second element, and (c) the ending -ide. With few exceptions, the ending - ide indicates that a compound contains only two elements. 3. The o or a at the end of a prefix is usually dropped when the word following the prefix begins with another vowel, e.g., monoxide or pentoxide. Because the less electronegative element is written first, oxygen and the halogens are usually given second. In general, the order of nonmetals in binary compound names and formulas is C, P, N, H, S, I, Br, Cl, O, F.

8 Prefix needed if less electronegative element contributes more than one atom Name of Less electronegative element tetraphosphorus Prefix indicating number of atoms contributed by more electronegative element decoxide Root name of more electronegative element + ide Put it all together you get tetraphosphorus decoxide More Examples: C 3 P 4 tricarbon tetraphosphide H 2 S dihydrogen monosulfide CCl 4 carbon tetrachloride H 2 O dihydrogen monoxide Turn to page 213: example 7-4 Homework: Practice Problems page 213

9 Examples: Binary Compounds of Nitrogen and Oxygen Formula N 2 O NO NO 2 N 2 O 3 N 2 O 4 N 2 O 5 Prefix-system name dinitrogen monoxide nitrogen monoxide nitrogen dioxide dinitrogen trioxide dinitrogen tetroxide dinitrogen pentoxide Write the formulas using the prefixes to describe the number of the elements in the compound for: Dihydrogen monoxide, Water (H 2 O) Carbon tetrachloride, CCl 4 Carbon dioxide, CO 2 Carbon monoxide CO Acids and Salts Acids Most acids can be classified as binary acids or oxyacids. Binary acids are acids that consist of two elements, usually hydrogen and one of the halogens: fluorine, chlorine, bromine, iodine. Oxyacids are acids that contain hydrogen, oxygen, and a third element (usually a nonmetal). Many polyatomic ions are produced by the loss of hydrogen ions from oxyacids. A few examples of the relationship between oxyacids and oxyanions are shown next page

10 (in your text on page 214). sulfuric acid H 2 SO 4 sulfate 2- SO 4 nitric acid HNO 3 nitrate NO - 3 phosphoric acid H 3 PO 4 phosphate 3- PO 4 Oxyacids of the -ate ending polyatomic ion are the -ic acids. Sulfate Sulfuric Acid Oxyacids of the -ite ending polyatomic ion are the -ous acids. Sulfite Sulfurous Acid Also from page 214 learn the Common Binary Acids and Oxyacids Salts Salt is an ionic compound composed of a cation and the anion from an acid. Table salt, NaCl, contains the anion from hydrochloric acid. Calcium sulfate, CaSO 4, is a salt containing an anion from sulfuric acid. Finally, some salts contain anions in which one or more hydrogen atoms from the acid are retained. Such anions are named by adding the word hydrogen or the prefix bi- to the anion name. The best known such anion comes from carbonic acid, H 2 CO 3 HCO - 3 hydrogen carbonate ion (bicarbonate ion) Ex of the salt: (sodium dissolved in weak carbonic acid) Sodium Bicarbonate NaHCO 3 Or Sodium Hydrogen Carbonate Best known as Baking soda