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Chapter 3

Chapter 3. Molecules, Compounds, and Chemical Equations. Compounds. When two or more elements are combined chemically, they are called a compound .

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Chapter 3

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  1. Chapter 3 Molecules, Compounds, and Chemical Equations

  2. Compounds • When two or more elements are combined chemically, they are called a compound. • Just like letters of the alphabet can be combined to form an endless number of words, elements can be combined to form an endless number of compounds. • Life would not possible without the diversity of compounds.

  3. Compounds • Hydrogen, a diatomic and colorless gas • Oxygen, a diatomic and colorless gas • Water, a clear liquid which is essential for most life forms

  4. Compounds

  5. Compounds • Most elements are not found by themselves in nature, rather they occur in compounds like NaCl or MgO • Some occur as diatomic molecules: N2 , O2 , F2 ,Cl2 , Br2 , and I2. • Others like P and S are polyatomic. • Only the metals of Gold, Copper, and Silver can sometimes be found in their pure state.

  6. Molecular Elements

  7. Chemical Bonds • Compounds are held together by one of two types of bonds. • Ionic bonds are the electrostatic attraction between cations and anions. • Covalent bonds are when two atoms share at least a pair of electrons.

  8. Compounds • A chemical formula is used to describe our compounds. • Uses subscripts after each element in the formula. • Subscript of “1” is implied if no subscript. • Empirical formula = simplest whole number ratio of the elements in compound. • Molecular formula = actual number of atoms bonded together. • Structural formula = shows how the atoms are bonded together with lines representing the bonds.

  9. Molecular Models • A model may be used to represent a molecule. • Ball-and-stick • Space-filling • No model is completely accurate!

  10. Molecular View of Elements and Compounds

  11. Classifying Elements & Compounds • Atomic elements = elements whose particles are single atoms • Molecular elements = elements whose particles are multi-atom molecules • Molecular compounds = compounds whose particles are molecules made of only nonmetals • Ionic compounds = compounds whose particles are cations and anions

  12. Ionic vs. Molecular Propane – contains individual C3H8 molecules Table salt – contains an array of Na+ ions and Cl- ions

  13. Learning Check • Classifying each as either an Atomic element, Molecular element, Molecular compound, or Ionic compound. • A) Formaldehyde, CH2O • B) Red phosphorous, P4 • C) Cobalt, Co • D) Magnesium chloride, MgCl2

  14. Ionic Compounds • Ionic compounds – are substances that contain both cations and anions. • Formulas are always empirical – lowest whole number ratio of cations and anions. • Writing ionic formulas – charges must cancel to yield a neutral species. • Most common charges for cations = +1, +2, or +3 • Most common charges for anions are -1, -2, or -3.

  15. Ionic Compounds

  16. Ionic Compounds • Learning Check • Write the correct formula between the ions of… • A) Ca and Br • B) K and S • C) Li and N • D) Al and O • E) Mg and P

  17. Naming Inorganic Compounds • Chemical Nomenclature ‑ the systematic method of naming chemical compounds. • Different system for naming Ionic and Molecular compounds. • Ionic = cation (usually a metal) with an anion (usually a non-metal. • Molecular = two or more non-metals or metalloid with a non-metal. • Identify compound BEFORE going to the rules!!!

  18. Ionic or Molecular • Learning Check… • Identify as Ionic or Molecular • A) Na2S • B) PCl3 • C) SiH4 • D) FeBr3 • E) ZnO • F) CCl4

  19. Polyatomic Ions • An ion composed of two or more elements with a net charge. • Only one is a cation – NH4+ = ammonium ion. • All the rest are anions. • Any time a polyatomic ion is present, then the compound is ionic. • Ex) Na2SO4, MgCO3, NH4Cl • When more than one polyatomic ion is needed to balance charges, then use (poly)x. • Ex) Al(NO3)3, (NH4)2S

  20. Naming Ionic Compounds • Two subcategories: • Metals with only one valence (mostly the main group metals) • Metals with more than one possible valence charge. • Metals with only one valence • Name the metal first • Name the non-metal second and change its suffix to ide. • For polyatomic ions, they ALWAYS keep their same name – DO NOT CHANGE TO ide ending.

  21. Learning Check • What is the name of… • A) NaBr • B) MgSO4 • C) K2S • D) Li3PO4

  22. Naming Ionic Compounds • Metals with more than one valence • Name metal first followed by its valence in Roman Numerals and in ( ). This means that you will have to figure out the charge for that metal based on what it is bonded to. • Name the non-metal second and change the suffix to ide. • As before, polyatomics remain the same.

  23. Learning Check • What is the name of… • A) Cu2O • B) Fe2S3 • C) Mn(NO3)2 • D) Co3(PO4)2

  24. Naming Molecular Compounds • Remember, these contain only non-metals or a metalloid with a non-metal. • Name the first non-metal in the formula. • Name the second non-metal in the formula and change the suffix to ide. • Add prefixes for all subscripts - except if the first one is a “1”. • 1 = mono, 2 = di, 3 = tri, 4 = tetra, 5 = penta, and 6 = hexa.

  25. Learning Check • What is the name of… • A) CS2 • B) PCl5 • C) AsBr3 • D) N2O4 • E) N2O

  26. Nomenclature Overview

  27. Learning Check • What is the name of… • A) NiCl2 • B) K2CrO4 • C) Cl2O • D) SF4 • E) (NH4)2SO4

  28. Naming Acids • Acids are molecular compounds that release an H+ when added to water. • Formula starts with an “H”. • Binary acids = H with one other element. • Hydro + base name of non-metal + ic + acid • HCl = hydrochloric acid • Oxoacids = HXOy • Polyatomic ion ends in –ate, then suffix changes to –ic • Polyatomic ion ends in –ite, then suffix changes to -ous

  29. Learning Check • What is the name of… • A) HBr • B) HNO3 • C) H2SO3 • D) H3PO4

  30. Quantitative Aspect • A chemical formula also has a quantitative aspect. • A Formula Weight for an element or compound is found using the periodic table. • Formula weights can refer to a single element’s weight or an ionic compound. • Molecular weight refers to a molecular compound’s weight. • Weights from periodic table should be rounded to the nearest 0.1 amu at the bare minimum!

  31. Molar Mass • The molar mass of any compound is equal to the sum of the atomic weights expressed in grams. • Ex) The molar mass of CO2 is 44.0 grams. • Thus, one mole of CO2 = 44.0 grams. • 1 mol CO2 = 6.02 x 1023 molecules. • Calculate the molar mass for… • A) PCl3 • B) C6H12O6 • C) Fe2(SO4)3

  32. Using Molar Masses • A molar mass can be used to convert grams to moles or moles to grams. • Calculate: • A) the moles present in 2.85g of CO2 • B) the grams present in 0.552 moles of NH3 • C) the number of molecules present in 0.255g of H2O • D) the number of O atoms in 3.00g of C6H12O6

  33. Percent Composition • A formula weight can be used to calculate the mass percentage of any element in the formula by: • This is one place to also test nomenclature! • Find the mass percent of each element in the compound Calcium nitrate.

  34. Empirical Formulas • We can use moles to find an empirical (simplest) formula from mass percentages by: • Assume a 100 gram sample (%  grams). • Convert grams of each element to moles use the formula weights. • Divide each mole amount by the smallest one. • Using a multiplier to eliminate fractions like: 0.25, 0.33, 0.50, 0.67, and 0.75.

  35. Empirical Formulas • A compound contains 17.6% Na, 39.7% Cr, and 42.7% O by mass. What is its empirical formula?

  36. Molecular Formulas • An empirical formula may not be the actual formula since molecular formulas do not have to be the lowest whole number subscripts. • The multiplier, n, can be found if we know the overall molecular weight of the compound. • A compound is contains 40.9% C, 4.6% H, and 54.5% O by mass with a molecular weight of about 176g/mol. What is the molecular formula for this compound?

  37. Combustion Analysis Tro: Chemistry: A Molecular Approach, 2/e

  38. Combustion Analysis • Combustion of a 0.8233 g sample of a compound containing only carbon, hydrogen, and oxygen produced the following: CO2 = 2.445 g H2O = 0.6003 g • Determine the empirical formula of the compound

  39. Chemical Equations • Chemical reactions are represented in a concise method by a chemical equation. • Ex) 2 H2(g) + O2(g) 2 H2O(l) Products Reactants

  40. Chemical Equations • Ex) 2 H2(g) + O2(g) 2 H2O(l) Phase Symbols Coefficient

  41. Balancing an Equation • A subscript in a chemical formula tells us how many of each type of atom are in the compound. • Ex) C6H12O6 • Subscripts cannot be altered!!! • Atoms can be created nor destroyed in a chemical reaction. • Thus, we balance a reaction by adding coefficients in front of each substance.

  42. Balancing an Equation • Balance by inspection. • Use a tally sheet. • Start with elements that occur once on each side. • Combustion – do C, then H, then O. • Balance each reaction… • __ Al + __ Cl2 __ AlCl • __ H3PO3 __ H3PO4 + __ PH3

  43. Balancing an Equation • __ Na3PO4 + __ CaBr2 __ Ca3(PO4)2 + __ NaBr • __ C3H8 + __ O2 __ CO2 + __ H2O • __ C4H8O + __ O2 __ CO2 + __ H2

  44. Classifying CompoundsOrganic vs. Inorganic • In the 18th century, compounds from living things were called organic; compounds from the nonliving environment were called inorganic • Organic compounds easily decomposed and could not be made in the 18th-century lab • Inorganic compounds very difficult to decompose, but able to be synthesized Tro: Chemistry: A Molecular Approach, 2/e

  45. Modern Classifying CompoundsOrganic vs. Inorganic • Today we commonly make organic compounds in the lab and find them all around us • Organic compounds are mainly made of C and H, sometimes with O, N, P, S, and trace amounts of other elements • The main element that is the focus of organic chemistry is carbon Tro: Chemistry: A Molecular Approach, 2/e

  46. Carbon Bonding • Carbon atoms bond almost exclusively covalently • compounds with ionic bonding C are generally inorganic • When C bonds, it forms four covalent bonds • 4 single bonds, 2 double bonds, 1 triple + 1 single, etc. • Carbon is unique in that it can form limitless chains of C atoms, both straight and branched, and rings of C atoms

  47. Carbon Bonding Tro: Chemistry: A Molecular Approach, 2/e

  48. Classifying Organic Compounds • There are two main categories of organic compounds, hydrocarbons and functionalized hydrocarbons • Hydrocarbons contain only C and H • When all C-C bonds are single bonds, they are called alkanes. • Formula for any alkane is: CnH2n+2.

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