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Chapter 5 Chemistry Notes - Memorize polyatomic ions in...

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9/10/2013 1 Chapter 5A - Atoms and Elements Chemical Formulas Molecular formulas Formula units Classifying Elements and Compounds Elements as Atomic or Molecular Compounds-Ionic vs Molecular Thinking about Chemistry How should I think about chemical reactions? Macroscopically Submicroscopically Symbolically Models/pictures Mathematical & chemical equations Words, names, & chemical symbols (Ch. 5) Central Theme in Chemistry: Macroscopic properties and behaviors are the results submicroscopic properties and behaviors. Proust’s Macroscopic Observation Law of constant composition- all samples of a given compound have the same proportion of their constituent elements. Example: 9.0 g of water can be separated into 1.0 g of hydrogen and 8.0 g of oxygen. (Note: mass conserved). Two ways to think about this: As a fraction (or %): Oxygen= 8 / 9 (89%) Hydrogen= 1 / 9 (11%) If we had an 18 g sample Oxygen= 8 / 9 *18g=16 g Hydrogen= 1 / 9 *18g=2 g As a proportion (O:H ratio): If a second sample had 2 g of hydrogen: X= 16 g of oxygen 8 1g 8g H O = = 2g Xg 1g 8g H O = = Macroscopic Connected with Sub-microscopic Since water is a pure compound (1) water is chemically the same even if the sample is very small (2) and law of constant composition should still apply to a small sample. *** *** Dalton’s theory says that this is true until we get to the atomic level. At some point we can’t get a smaller size and still be water. Sub-microscopic masses too Macroscopic mass 1g H 8 g Oxygen 1 8 If we know the atomic mass of H (1 amu) and O (16 amu), we can find out the ratio of the atoms of each type. (recall: 1 amu = 1.6x10 -24 g) H’s=1g/1.6x10 -24 g=6x10 23 O’s=8g/(16*1.6x10 -24 g)=3x10 23 **Atom ratio: 2 H per 1 O! (**This is the relative number of H’s and O’s, more about this in Ch. 6) Left most To Right most Chemical Formula We write a Chemical Formula to summarize the elements present in the compound, e.g. H 2 O. use elemental symbol for each atom in compound use subscript to indicate number of atoms of each type. ( “1” is implied if nothing written, and use parentheses when needed.) Elements are written in this order: (metal left , non-metal right ) http://old.iupac.org/publications/books/principles/principles_of_nomenclature.pdf Examples: NaCl not ClNa CO 2 not O 2 C CHCl 3 not HCl 3 C or Cl 3 CH, etc. (exception, OH - ) F O Application: Law of Constant Composition Two Samples of Iron oxide are found, each decomposed to Fe and O Are they the same Iron Oxide? Look at the mass ratios: Analysis: based on 4 sig figs, ratios are different. Conclusion: Not the same Iron Oxide! (FYI: Hematite, Fe 2 O 3 vs Magnetite, Fe 3 O 4 , notice how chemical formulas are often a more meaningful summary than masses).
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