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Chemical Formulas And Chemical Compounds Chapter 7.

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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: C 8 H 18 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 elements name. Ex: Li + is simply called lithium. Naming monatomic anions is slightly more complicated: Drop the ending of the elements name add –ide to the root name. Examples: F - fluoride (fluorine – replace ine with ide) N -3 nitride O -2 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: Mg 2+ + Br - = MgBr 2

6 Naming Ionic Compounds Nomenclature: the naming system of binary ionic compounds involves combining the names of the compounds 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 compounds name because it is understood that the overall charge of the compound is neutral. Examples: BaF 2 CaO AlBr 3

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: CuCl 2 VBr 3 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 CoF 3 SnI 4 K 2 O 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: nitriteNO 2 - Ex: nitrate NO 3 -

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 Sn 4+ and SO 4 2-

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 KClO 3 Lithium nitrate Ag 2 O Sodium carbonate Fe 2 (CrO 4 ) 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 NO 2 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 NumberPrefix 1Mono- 2Di- 3Tri- 4Tetra- 5Penta- 6Hexa- 7Hepta- 8Octa- 9Nona- 10Deca-

19 Examples: Name the following or write the formula PF 5 XeF 4 CCl 4 Carbon dioxide Dinitrogen pentoxide Sulfur hexafluoride

20 Practice SO 3 ICl 3 PBr 5 As 2 O 5 P 4 O 10 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 HClHydrochloric acid HFHydrofluoric acid HBrHydrobromic acid HIHydriodic 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. H 2 SO 4 SO 4 is the sulfate ion, so this is called Sulfuric acid H 2 SO 3 SO 3 is the sulfite ion, so this is called Sulfurous acid

24 Practice: Name the following acids or provide the correct formula HF Sulfuric acid CH 3 COOH H 2 CO 3 Phophoric acid HI HNO 2 Perchloric acid H 2 SO 3

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 1.Atoms in a pure element have an oxidation number of zero. 2.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. 3.Fluorine always has oxidation number of Oxygen has oxidation number of -2 except when in peroxides (-1) and in compounds with fluorine (+2) 5.Hydrogen has oxidation number of +1 when bonded to more electronegative elements. -1 when bonded with metals. 6.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: UF 6 Rule 3 tells us the oxidation number of F is always x 6 = -6 Since UF 6 is a neutral compound, positive charge must be / 1 = +6 (1 from one U atom)

28 H 2 SO 4 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 ClO 3 - 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 = Cl = -1 Cl = +5

30 Practice See page 234. 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, Cu 2 S: 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: PbCl 2 Ba(NO 3 ) 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. CompoundEmpirical Formula Molecular FormulaMolar Mass (g) FormaldehydeCH 2 0 -Same as empirical formula Acetic AcidCH 2 0C 2 H x empirical formula GlucoseCH 2 0C 6 H x empirical formula

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: 1.If given % composition, assume 100g sample and change % to grams. 2.Convert grams to moles. 3.Divide each number of moles by the smallest value. 4.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) Change grams to moles Determine the mole ratio

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 compounds formula mass. For example: The empirical formula of a compound is BH 3. The mass of this molecule is g. The mass of the unknown compound is g. X=27.67/13.84 = So the molecular formula is 2(BH 3 ) = B 2 H 6 Try the practice problems on page 249.


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