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Understanding Formulas

Understanding Formulas. Spring 2014 Dr. Yau (loosely based on Chap. 1.5 & 1.6 in Jespersen, Brady & Hyslop, 6th edition). Proof Of Atoms. STM of palladium. Early 1980’s, use Scanning Tunneling Microscope (STM) Surface can be scanned for topographical information

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Understanding Formulas

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  1. Understanding Formulas Spring 2014 Dr. Yau (loosely based on Chap. 1.5 & 1.6 in Jespersen, Brady & Hyslop, 6th edition)

  2. Proof Of Atoms STM of palladium • Early 1980’s, use Scanning Tunneling Microscope (STM) • Surface can be scanned for topographical information • Image for all matter shows spherical regions of matter • Atoms

  3. How Do We Visualize Atoms? Atoms are too small for our eyes to see, but we can use models to help us understand the concepts. • Atoms represented by spheres • Different atoms have different colors • Standard scheme given in Fig. 1.11 is represented on the right.

  4. Molecules • Atoms combine to form more complex substances • Discrete particles • Each composed of 2 or more atoms Ex. • Molecular oxygen, O2 • Carbon dioxide, CO2 • Ammonia, NH3 • Sucrose, C12H22O11

  5. Chemical Formulas • Specify composition of substance • Chemical symbols • Represent atoms of elements present • Subscripts • Given after chemical symbol • Represents relative numbers of each type of atom Ex. Fe2O3 : iron & oxygen in 2:3 ratio

  6. Chemical Formulas FreeElements • Element not combined with another in compounds • Just use chemical symbol to represent Ex. Iron Fe Neon Ne Sodium Na Aluminum Al Diatomic Molecule • Molecules composed of 2 atoms each • Many elements found in nature Ex. Oxygen O2 Nitrogen N2 Hydrogen H2 Chlorine Cl2

  7. Depicting Molecules • Want to show: • Order in which atoms are attached to each other • 3-dimensional shape of molecule • Three ways of visualizing molecules: • Structural formula • Ball-and-Stick model • Space filling model

  8. 1. Structural Formulas H2O water CH4 methane • Use to show how atoms are attached • Atoms represented by chemical symbols • Chemical bonds attaching atoms indicated by lines

  9. 3-D Representations of Molecules Hydrogen molecule, H2 Oxygen molecule, O2 Nitrogen molecule N2 Chlorine molecule, Cl2 Use touching spheres to indicate molecules Different colors indicate different elements Relative size of spheres reflects differing sizes of atoms

  10. 2. “Ball-and-Stick” Model Chloroform, CHCl3 Methane, CH4 • Spheres = atoms • Sticks = bonds

  11. 3. “Space-Filling” Model Methane CH4 Water H2O Chloroform, CHCl3 Shows relative sizes of atoms Shows how atoms take up space in molecule

  12. More Complicated Molecules Ball-and-stick model Space-filling model • Sometimes formulas contain parentheses • How do we translate into a structure? Ex. Urea, CO(NH2)2 • Expands to CON2H4 • Atoms in parentheses appear twice

  13. Hydrates Blue = CuSO4 •5H2O White = CuSO4 • Crystals that contain water molecules Ex. plaster: CaSO4∙2H2O calcium sulfate dihydrate • Water is not tightly held • Dehydration • Removal of water by heating • Remaining solid is anhydrous (without water)

  14. Counting Atoms • Subscript following chemical symbol indicates how many of that element are part of the formula • No subscript implies a subscript of 1. • Quantity in parentheses is repeated a number of times equal to the subscript that follows. • Raised dot in formula indicates that the substance is a hydrate • Number preceding H2O specifies how many water molecules are present.

  15. Counting Atoms Ex. 1 (CH3)3COH • Subscript 3 means 3 CH3 groups So from(CH3)3, we get 3 × 1C = 3C 3 × 3H = 9H #C = 3C + 1C = 4 C #H = 9H + 1H = 10 H #O = 1 O Total # of atoms = 15 atoms

  16. Counting Atoms Ex. 2 CoCl2 · 6H2O • The dot 6H2O means you multiple both H2 & O by 6 • So there are: #H 6 × 2 = 12 H #O 6 × 1 = 6 O #Co 1 × 1 = 1 Co #Cl 2 × 1 = 2 Cl Total # of atoms = 21 atoms

  17. Your Turn! Count the number of each type of atom in the chemical formula given below • ___Na, ___C, ___ O • ___N, ___H, ___S, ___O • ___Mg, ___P, ___O • ___Cu, ___S, ___O, ___H • ___C, ___H, ___N 2 1 3 2 8 1 4 3 2 8 9 10 1 1 12 4 2 • Na2CO3 • (NH4)2SO4 • Mg3(PO4)2 • CuSO4∙5H2O • (C2H5)2N2H2

  18. Dalton’s Atomic Theory • We now have the tools to explain this theory & its consequences • All molecules of compound are alike & contain atoms in same numerical ratio. Ex. Water, H2O Ratio of oxygen to hydrogen is 1 : 2 1 O atom : 2 H atoms in each molecule O weighs 16 times as much as H 1 H = 1 mass unit 1 O = 16 mass units

  19. Atoms in Fixed Ratios by Mass For water in general: • mass O = 8 mass H • Regardless of amount of water present

  20. Dalton’s Atomic Theory Successes: • Explains Law of Conservation of Mass • Chemical reactions correspond to rearranging atoms. • Explains Law of Definite Proportions • Given compound always has atoms of same elements in same ratios. • Predicted Law of Multiple Proportions • Not yet discovered • Some elements combine to give 2 or more compounds Ex.SO2 & SO3

  21. Law Of Multiple Proportions • When 2 elements form more than one compound, different masses of one element that combine with same mass of other element are alwaysin ratio of small whole numbers. • Atoms react as complete (whole) particles. • Chemical formulas • Indicate whole numbers of atoms • Not fractions

  22. Using Law Of Multiple Proportions sulfur sulfur dioxide trioxide Mass S 32.06 g 32.06 g Mass O 32.00 g 48.00 g • Use this data to prove law of multiple proportions

  23. Law of Multiple Proportions Sulfur dioxide 64.06 g 32.06 g 32.06 g Sulfur trioxide 80.06 g 32.06 g 48.00 g Ratio of

  24. Molecules Small and Large • So far we’ve only discussed small molecules • Some are very large, especially those found in nature • Same principles apply to all Ex. DNA - short segment

  25. How Do We Know Formulas? • Hardly “out of the blue” • Don’t know formula when compound 1st isolated • Formulas & structures backed by extensive experimentation • Use results of experiments to determine • Formula • Chemical reactivity • Molecular Shape • Can speculate once formula is known • Determine from more experiments

  26. Visualizing Mixtures a. b. • Look at mixtures at atomic/molecular level • Different color spheres stand for 2 substances • Homogeneous mixture/solution – uniform mixing • Heterogeneous mixture – 2 phases

  27. Chemical Reactions • When 1 or more substances react to form 1 or more new substances Ex. Reaction of methane, CH4, with oxygen, O2, to form carbon dioxide, CO2, & water, H2O. Reactants = CH4 & O2 Products = CO2 & H2O • How to depict? • Words too long • Pictures too awkward

  28. Chemical Equations • Use chemical symbols & formulas to represent reactants & products. • Reactants on left hand side • Products on right hand side • Arrow () means “reacts to yield” Ex. CH4 + 2O2 CO2 + 2H2O • Coefficients • Numbers in front of formulas • Indicate how many of each type of molecule reacted or formed • Equation reads “methane & oxygen react to yield carbon dioxide & water”

  29. Conservation of Mass in Reactions CH4 + 2O2CO2 + 2H2O 4 H + 4O + C = 4 H + 4O + C • Mass can neither be created nor destroyed • This means that there are the same number of each type of atom in reactants & in products of reaction • If # of atoms same, then mass also same

  30. Balanced Chemical Equation 4 C & 10 H per molecule 2 H & 1 O per molecule 2 O per molecule 1 C & 2 O per molecule Subscripts • Define identity of substances • Must not change when equation is balanced Ex. 2C4H10 + 13O2 8CO2 + 10H2O

  31. Balanced Chemical Equation 10 molecules of C4H10 2 molecules of C4H10 13 molecules of O2 8 molecules of CO2 Coefficients • Number in front of formulas • Indicate number of molecules of each type • Adjusted so # of each type of atom is same on both sides of arrow • Can change Ex. 2C4H10 + 13O2 8CO2 + 10H2O

  32. Balanced Chemical Equations • How do you determine if an equation is balanced? • Count atoms • Same number of each type on both sides of equation? • If yes, then balanced • If no, then unbalanced Ex. 2C4H10 + 13O2 8CO2 + 10H2O Reactants Products 2×4 = 8 C 8×1 = 8 C 2×10 = 20 H 10×2 = 20 H 13×2 = 26 O (8×2)+(10×1)= 26 O

  33. Learning Check Fe 1 1 O 3 + (2×3) = 9 (3×3) + 2 = 11 3 + 2 = 5 (2×2) = 4 H N 2 3 Fe(OH)3 + 2 HNO3 Fe(NO3)3 + 2 H2O • Not Balanced • Only Fe has same number of atoms on either side of arrow.

  34. Learning Check: How many atoms of each element appear on each side of the arrow in the following equation? 4NH3 + 3O2 → 2N2 + 6H2O

  35. Learning Check: Count the number of atoms of each element on both sides of the arrow to determine whether the following equation is balanced. 2(NH4)3PO4 + 3Ba(C2H3O2)2 → Ba3(PO4)2 + 6NH4C2H3O2

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