Presentation on theme: "Chemical Formulas And Chemical Compounds"— Presentation transcript:
1Chemical Formulas And Chemical Compounds Chapter 7
2Many 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
3Chemical FormulaA 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: C8H18Tells 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
4Ionic CompoundsWe 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 nameadd “–ide” to the root name.Examples:F fluoride (fluorine – replace “ine” with “ide”)N-3 nitrideO oxide
5Binary 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
6Naming 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:BaF2CaOAlBr3
7You Practice:Write the formulas for the binary ionic compounds formed between the following elementsPotassium and IodineMagnesium and ChlorineSodium and SulfurAluminum and SulfurAluminum and nitrogen
9Transition MetalsSome 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) nitrateIron (II) sulfate
11Examples Name the following compounds: CuCl2VBr3Write the formulas of the following compounds:Iron(III) sulfideVanadium (II) fluoride
12Practice:Name the compound or write the formula for each compound (from page 225 practice problems):CuOCoF3SnI4K2OCopper(II) bromideMercury (II) sulfideLead (IV) sulfideMore problems are in your book.
13Polyatomic IonsPolyatomic 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-
14Polyatomic IonsCompounds 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 NitriteSome other examples:NaOHSn4+ and SO42-
15Practice (p. 227 practice)Give the formula or name of the compound, whichever is appropriate (use your polyatomic ion paper!!):Copper(II) sulfateCa(OH)2KClO3Lithium nitrateAg2OSodium carbonateFe2(CrO4)3Potassium perchlorate
16Naming Covalent Compounds Naming covalent compounds made up of two elements are named by similar method as ionic compoundsThe first element named is usually the first one written in the formula.The second element has the ending “–ide.”
17Naming 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 vowelFor instance, we would write monoxide and pentoxide instead of mono-oxide and penta-oxide.
18Prefixes for Covalent compounds NumberPrefix1Mono-2Di-3Tri-4Tetra-5Penta-6Hexa-7Hepta-8Octa-9Nona-10Deca-
19Examples: Name the following or write the formula PF5 XeF4 CCl4 Carbon dioxideDinitrogen pentoxideSulfur hexafluoride
21Naming 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)
22Binary Acids To name binary acids: Use the prefix “hydro” followed by the second element with the ending “ic acid”HCl Hydrochloric acidHF Hydrofluoric acidHBr Hydrobromic acidHI Hydriodic acid
23Oxyacids 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 acidH2SO3 SO3 is the sulfite ion, so this is called Sulfurous acid
24Practice: Name the following acids or provide the correct formula HF Sulfuric acidCH3COOHH2CO3Phophoric acidHIHNO2Perchloric acidH2SO3
25Oxidation NumbersOxidation 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.
26Assigning 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.
27Example: 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)
28H2SO4O 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 = -6So sulfurs oxidation state is -6
29ClO3-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 = -1Cl = +5
30PracticeSee page Work through number 1 and complete for homework.
31Percent CompositionPercentage 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.
32Practice 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?
33Empirical FormulasAn empirical formula shows the simplest ratio of the atoms in a compoundA molecular formula is a whole number multiple of the empirical formula.CompoundEmpirical FormulaMolecular FormulaMolar Mass (g)FormaldehydeCH20-Same as empirical formula30.03Acetic AcidC2H402-2x empirical formula60.06GlucoseC6H1206-6x empirical formula180.18
34Determining 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.
35Determining Empirical Formula MassesChange grams to molesDetermine the mole ratioPercentage Composition (assume 100 g sample)
36Example problemAnalysis of a compound shows that it contains % Na, % S, and % O. Find the empirical formula.
37Practice: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.
38To determine molecular formula X(empirical formula) = molecular formulaTo 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 = 2.000So the molecular formula is 2(BH3) = B2H6Try the practice problems on page 249.