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Chemical Formulas And Chemical Compounds

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1 Chemical Formulas And Chemical Compounds
Chapter 7

2 Many compounds go by common names such as salt (NaCl) or limestone (calcium carbonate.) These names do not give information about the compounds. To describe the atomic makeup of compounds, chemists use systematic methods for naming compounds and for writing chemical formulas

3 Chemical Formula A chemical formula tells us the relative number of each type of atom in a chemical compound. Formulas have the element followed by a subscript. Ex: C8H18 Tells us there are 8 carbons and 18 hydrogens. Ionic and covalent compounds have separate own rules that must be obeyed in writing the formula and naming the compound

4 Ionic Compounds We are familiar with determining the charge of monatomic ions in the s and p blocks. Monotomic ions: ions formed from a single atom. Monatomic cations are identified by the element’s name. Ex: Li+ is simply called lithium. Naming monatomic anions is slightly more complicated: Drop the ending of the element’s name add “–ide” to the root name. Examples: F fluoride (fluorine – replace “ine” with “ide”) N-3 nitride O oxide

5 Binary Ionic Compounds
Binary Compounds: Compounds composed of two elements. In a binary ionic compound, the total number of positive and negative charges must equal zero. In other words, the net charge must be zero, and the overall compound is neutral. (criss-cross rule) Example: Mg2+ + Br- = MgBr2

6 Naming Ionic Compounds
Nomenclature: the naming system of binary ionic compounds involves combining the names of the compound’s positive and negative ions. How to name binary ionic compounds: The name of the cation is first, followed by the name of the anion. The ratio of ions is not indicated in the compound’s name because it is understood that the overall charge of the compound is neutral. Examples: BaF2 CaO AlBr3

7 You Practice: Write the formulas for the binary ionic compounds formed between the following elements Potassium and Iodine Magnesium and Chlorine Sodium and Sulfur Aluminum and Sulfur Aluminum and nitrogen

8 Additional practice: #2 on page 223.

9 Transition Metals Some elements, such as iron, form two or more cations with different charges. To distinguish the ions formed by these elements, the Stock system of nomenclature is used. Roman numerals are used to indicate the charge of the ion. The roman numeral is enclosed in parentheses and placed immediately after the metal name. Examples: Silver(I) nitrate Iron (II) sulfate

10 Transition Metals with Charges

11 Examples Name the following compounds:
CuCl2 VBr3 Write the formulas of the following compounds: Iron(III) sulfide Vanadium (II) fluoride

12 Practice: Name the compound or write the formula for each compound (from page 225 practice problems): CuO CoF3 SnI4 K2O Copper(II) bromide Mercury (II) sulfide Lead (IV) sulfide More problems are in your book.

13 Polyatomic Ions Polyatomic ions are groups of atoms that are covalently bonded. They are usually anions. They have a charge that is spread over the ion. Ex: nitrite NO2- Ex: nitrate NO3-

14 Polyatomic Ions Compounds containing polyatomic ions are named the same way as binary ionic compounds. The name of the cation is named first, followed by the name of the anion, but the name of the polyatomic ion is used for the anion. Ex: Silver Nitrate and Silver Nitrite Some other examples: NaOH Sn4+ and SO42-

15 Practice (p. 227 practice) Give the formula or name of the compound, whichever is appropriate (use your polyatomic ion paper!!): Copper(II) sulfate Ca(OH)2 KClO3 Lithium nitrate Ag2O Sodium carbonate Fe2(CrO4)3 Potassium perchlorate

16 Naming Covalent Compounds
Naming covalent compounds made up of two elements are named by similar method as ionic compounds The first element named is usually the first one written in the formula. The second element has the ending “–ide.”

17 Naming Covalent Compounds
Because covalent compounds are often made of the same elements, we must distinguish between two molecules made of the same elements. For example, NO and NO2 cannot have the same name! We use a system of prefixes to show the number of atoms of each element in the molecule (see table 3, page 228). The o or a at the end of a prefix is usually dropped when the word following the prefix begins with another vowel For instance, we would write monoxide and pentoxide instead of mono-oxide and penta-oxide.

18 Prefixes for Covalent compounds
Number Prefix 1 Mono- 2 Di- 3 Tri- 4 Tetra- 5 Penta- 6 Hexa- 7 Hepta- 8 Octa- 9 Nona- 10 Deca-

19 Examples: Name the following or write the formula PF5 XeF4 CCl4
Carbon dioxide Dinitrogen pentoxide Sulfur hexafluoride

20 Practice SO3 ICl3 PBr5 As2O5 P4O10 Carbon tetraiodide
Phosphorus trichloride Dinitrogen trioxide Silicon dioxide Tetranitrogen pentoxide

21 Naming Acids Acid: a distinct type of molecular compound.
We will focus on acids later in the year. Binary acids: Acids that consist of two elements, usually hydrogen and one of the halogen (F, Cl, Br, or I) Oxyacids: Acids that contain hydrogen, oxygen, and a third element (usually a nonmetal)

22 Binary Acids To name binary acids:
Use the prefix “hydro” followed by the second element with the ending “ic acid” HCl Hydrochloric acid HF Hydrofluoric acid HBr Hydrobromic acid HI Hydriodic acid

23 Oxyacids Most oxyacids involve a polyatomic ion.
For acids that contain a polyatomic ion that ends in “ate,” change the “ate” to “-ic acid.” For acids that contain a polyatomic ion that ends in “ite,” change the “ite” to “–ous acid.” H2SO4 SO4 is the sulfate ion, so this is called Sulfuric acid H2SO3 SO3 is the sulfite ion, so this is called Sulfurous acid

24 Practice: Name the following acids or provide the correct formula HF
Sulfuric acid CH3COOH H2CO3 Phophoric acid HI HNO2 Perchloric acid H2SO3

25 Oxidation Numbers Oxidation numbers: indicate the general distribution of electrons among the bonded atoms in a molecular compound or polyatomic ion. Unlike ionic charges, oxidation numbers do not have an exact physical meaning.

26 Assigning Oxidation Numbers
Atoms in a pure element have an oxidation number of zero. The more-electronegative element in a binary compound is assigned the number equal to the negative charge it would have as the anion. The less electronegative atom is assigned the number equal to the positive charge it would have as the cation. Fluorine always has oxidation number of -1. Oxygen has oxidation number of -2 except when in peroxides (-1) and in compounds with fluorine (+2) Hydrogen has oxidation number of +1 when bonded to more electronegative elements. -1 when bonded with metals. The sum of the oxidation numbers in a neutral compound is zero, and sum is equal to charge of ion for a polyatomic ion.

27 Example: UF6 Rule 3 tells us the oxidation number of F is always -1.
Since UF6 is a neutral compound, positive charge must be +6. +6 / 1 = +6 (1 from one U atom)

28 H2SO4 O and S are more electronegative than H, so H has oxidation number of +1 (rule 5.) O is not a peroxide or halogen, so O is -2 (rule 4) 1(2) + -2(4) = 2 – 8 = -6 So sulfurs oxidation state is -6

29 ClO3- Charge of anion is -1, so sum of oxidation numbers will be -1. The oxidation number of Oxygen is -2 (rule 4) -2(3) + Cl = -1 -6 + Cl = -1 Cl = +5

30 Practice See page Work through number 1 and complete for homework.

31 Percent Composition Percentage Composition: the percentage by mass of each element in a compound. Find the % composition of Copper(I)sulfide, Cu2S: Find the molar mass of the compound. Determine the mass of each element present and multiply by its subscript. Divide mass of element by mass of compound.

32 Practice Find the percent compositions of: PbCl2 Ba(NO3)2
Magnesium hydroxide is 54.87% oxygen by mass. How many grams of oxygen are in 175 grams of the compound? How many moles of oxygen is this?

33 Empirical Formulas An empirical formula shows the simplest ratio of the atoms in a compound A molecular formula is a whole number multiple of the empirical formula. Compound Empirical Formula Molecular Formula Molar Mass (g) Formaldehyde CH20 -Same as empirical formula 30.03 Acetic Acid C2H402 -2x empirical formula 60.06 Glucose C6H1206 -6x empirical formula 180.18

34 Determining Empirical Formula
If the identities of the elements in a compound are known, then the empirical formula can be determined from: From % Composition or Mass: If given % composition, assume 100g sample and change % to grams. Convert grams to moles. Divide each number of moles by the smallest value. Use the numbers from step 2 as your subscripts.

35 Determining Empirical Formula
Masses Change grams to moles Determine the mole ratio Percentage Composition (assume 100 g sample)

36 Example problem Analysis of a compound shows that it contains % Na, % S, and % O. Find the empirical formula.

37 Practice: Analysis of a g sample of a compound known to contain only phosphorous and oxygen indicates a phosphorous content of g. What is the empirical formula of this compound. P. 247 practice problems.

38 To determine molecular formula
X(empirical formula) = molecular formula To determine the molecular formula of a compound, you must know the compound’s formula mass. For example: The empirical formula of a compound is BH3. The mass of this molecule is g. The mass of the unknown compound is g. X=27.67/13.84 = 2.000 So the molecular formula is 2(BH3) = B2H6 Try the practice problems on page 249.


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