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Elements and Compounds: Molecular Approach

Explore how elements combine to form compounds with distinct properties, bond types like ionic and covalent, molecular formulas, moles, and percent compositions in this comprehensive chemistry guide. Learn about chemical bonds, molecular elements, and allotropes for a deeper understanding of chemistry principles.

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Elements and Compounds: Molecular Approach

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  1. Elements and Compounds • elements combine together to make an almost limitless number of compounds • the properties of the compound are totally different from the constituent elements • H2(g) + O2(g)  H2O(l) Tro, Chemistry: A Molecular Approach

  2. Tro, Chemistry: A Molecular Approach

  3. Tro, Chemistry: A Molecular Approach

  4. Formation of Water from Its Elements Tro, Chemistry: A Molecular Approach

  5. Allotropes:one of two or more forms of an element that differ in their basic structure diamond graphite Tro, Chemistry: A Molecular Approach

  6. Allotropes of carbon diamond graphite Tro, Chemistry: A Molecular Approach

  7. Allotrope of carbon

  8. Chemical Bonds • compounds : made of atoms held together by chemical bonds • Bonds:forces of attraction between atoms • attractions between protons and electrons Tro, Chemistry: A Molecular Approach

  9. Bond Types • two general types of bonding ionic and covalent • ionic bonds electrons transferred between atoms, results in oppositely charged ions that attract each other • Metals + nonmetal • covalent bonds two atoms share some of their electrons • Nonmetal + nonmetal Tro, Chemistry: A Molecular Approach

  10. Molecular View of Elements and Compounds Tro, Chemistry: A Molecular Approach

  11. Homonuclear vs heteronuclear Tro, Chemistry: A Molecular Approach

  12. 7A 7 H2 N2 O2 F2 Cl2 Br2 I2 Molecular Elements • Certain elements occur as 2 atom molecules • Rule of 7’s • Other elements occur as polyatomic molecules • P4, S8, Se8 Tro, Chemistry: A Molecular Approach

  13. Classifying Materials • atomic elements = Fe • molecular elements = N2 • molecular compounds = H2O • ionic compounds = SrCl2 MgCO3 Tro, Chemistry: A Molecular Approach

  14. Representing Compoundswith Chemical Formula • compounds represented with: chemical formula • Type of formula determines amount of information given

  15. Types of FormulaEmpirical Formula • Empirical Formula • Tells atoms and their ratio • do not describe how many atoms, the order of attachment, or the shape • the formulas for ionic compounds are empirical

  16. Empirical formula Empirical formula Molecular formula H2O2 C8H8 C6H12O6 _________ Hg2Cl2 • HO • __________ • __________ • H2O • __________

  17. Types of Formula

  18. Expanded structural formula vs. condensed structural formula

  19. Formula Mass • the mass of an individual molecule or formula unit (FW) amu (D for Dalton) • molecular mass or molecular weight (MW or μ) g/mol • sum of the masses of the atoms in a single molecule or formula unit mass of 1 molecule of H2O = 2(1.01 amu H) + 16.00 amu O = 18.02 amu Tro, Chemistry: A Molecular Approach

  20. What Is a Mole? Tro, Chemistry: A Molecular Approach

  21. How Big is a Mole? • N=6.022x1023 • One mole of Marshmallows would cover the entire earth 12 miles high • One mole of marshmallows would fill the Grand Canyon and still be enough left over to displace all the water from Lake Michigan and more

  22. Mole = 6.022x1023itemsAvogadro’s number N

  23. Molar Mass of Compounds • the relative masses of molecules can be calculated from atomic masses Formula Mass = 1 molecule of H2O = 2(1.01 amu H) + 16.00 amu O = 18.02 amu • since 1 mole of H2O contains 2 moles of H and 1 mole of O Molar Mass = 1 mole H2O = 2(1.01 g H) + 16.00 g O = 18.02 g so the Molar Mass of H2O is 18.02 g/mole Tro, Chemistry: A Molecular Approach

  24. Moles as Conversion factors • Moles to molecules or atoms • Use Avogadro’s number • Ex: How many molecules of water are in 9.01 g of water? • Grams to moles • Use Molar Mass • Ex: How many moles are in 9.01 g of water?

  25. Moles • How many molecules of Chlorine are in 19.5 grams of chlorine? • How many moles of sodium atoms is 29.0 grams of sodium? Tro, Chemistry: A Molecular Approach

  26. Tro, Chemistry: A Molecular Approach

  27. Molecular formula • In 5.0 moles of the compound Mg(H2PO4)2 • How many moles of Mg are there?____ • How many moles of phosphorus are there?___ • How many moles of oxygen are there?______ Tro, Chemistry: A Molecular Approach

  28. Molar mass • What is the formula weight (FW) of Mg(H2PO4)2 including units? ________ • What is the molar mass (MW or µ) of Mg(H2PO4)2 including units?_________ Tro, Chemistry: A Molecular Approach

  29. Practice - Converting Grams to Molecules How many molecules are in 50.0 g of PbO2? (PbO2 = 239.2 g/mol) Tro, Chemistry: A Molecular Approach

  30. Percent Composition • Percentage of each element in a compound • Mass of element/mass of compound • The percentages may not always total to 100% due to rounding Tro, Chemistry: A Molecular Approach

  31. C=12.01, Cl=35.45, F=19.00 Find the mass percent of Cl in CCl2F2

  32. Mass Percent as a Conversion Factor • the mass percent tells you the mass of a constituent element in 100 g of the compound • the fact that CCl2F2 is 58.64% Cl by mass means that 100 g of CCl2F2 contains 58.64 g Cl • this can be used as a conversion factor • 100 g CCl2F2: 58.64 g Cl

  33. g Na g NaCl Example 3.14 – Find the mass of table salt containing 2.4 g of Na 2.4 g Na, 39% Na g NaCl Given: Find: 100. g NaCl : 39 g Na Concept Plan: Relationships: Solution: Check: since the mass of NaCl is more than 2x the mass of Na, the number makes sense

  34. Benzaldehyde is 79.2% carbon. What mass of benzaldehyde contains 19.8 g of C?

  35. Empirical Formula • simplest, whole-number ratio of the atoms of elements in a compound • can be determined from elemental analysis • masses of elements formed when decompose or react compound • combustion analysis • percent composition Tro, Chemistry: A Molecular Approach

  36. Finding an Empirical Formula • convert the percentages to grams • assume you start with 100 g of the compound • skip if already in grams • convert grams to moles • use molar mass of each element • write a pseudoformula using moles as subscripts • divide all by smallest number of moles • if result is within 0.1 of whole number, round to whole number • multiply all mole ratios by number to make all whole numbers • if ratio ?.5, multiply all by 2; if ratio ?.33 or ?.67, multiply all by 3; if ratio 0.25 or 0.75, multiply all by 4; etc. • skip if already whole numbers Tro, Chemistry: A Molecular Approach

  37. Calculating Empirical formula • 1. A sample of an unknown compound with a mass of 2.571 g was found to contain 1.102 g of carbon and 1.469 g of oxygen. What is its empirical formula?

  38. Calculating empirical formula • An oxide of iron called black iron oxide, is analyzed. In a 2.448 g sample the ore was found to contain 1.771 g of Fe and 0.6766 g of O. Calculate the empirical formula of this compound

  39. Example 3.17 • Laboratory analysis of aspirin determined the following mass percent composition. Find the empirical formula. C = 60.00% H = 4.48% O = 35.53%

  40. Finding empirical formula mol C, H, O mol ratio empirical formula g C, H, O Tro, Chemistry: A Molecular Approach

  41. Determine the empirical formula of hematite, which contains 72.4% Fe (55.85) and the rest oxygen (16.00) Tro, Chemistry: A Molecular Approach

  42. Molecular Formulas • The molecular formula is a multiple of the empirical formula • To determine the molecular formula you need to know the empirical formula and the molar mass of the compound

  43. Example 3.18 – Find the molecular formula of butanedione empirical formula for butanedione = C2H3O; MW of molecular formula = 86.03 g/mol

  44. Benzopyrene has a molar mass of 252 g/mol and an empirical formula of C5H3. What is its molecular formula? (C = 12.01, H=1.01)

  45. hydrates

  46. Hydrates • When the hydrate of RuCl3 is heated water is driven off to form the anhydrous salt as shown below: • RuCl3.xH2O  RuCl3 + xH2O • If 1.056 g of hydrated compound is heated and 0.838 g of anhydrous salt remains what is the value of x?

  47. Naming hydrates: name the ionic compound then the water part, use prefixes for number of water molecules Ionic formula • x H2O x = prefix (di, tri etc from covalent naming) H2O = hydrate Ex:   CuSO4• 5 H2O   copper(II) sulfate pentahydrate Ex.   CaCl2• 2 H2O   calcium chloride dihydrate

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