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Chemical Names and Formulas

Chemical Names and Formulas

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Chemical Names and Formulas

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  1. Chemical Names and Formulas

  2. Molar Mass • Molar mass relates moles to grams • The molar mass is the mass in grams of 1 mole of an element or compound • Use the mass numbers from the periodic table, instead of amu’s, the units are grams

  3. Molar masses are computed in the typical way

  4. Percentage Composition • Percentage composition is the percentage by mass of each element in a compound

  5. The chemical formula gives the ratio of atoms in a compound

  6. The chemical formula can also give the ratio of ions to one another

  7. Significance of a Chemical Formula • For molecules, the formula gives the number of atoms of each element present in the molecule • For ionic compounds, the formula gives the simplest ratio of cations (+) and anions (-) in the crystalline lattice • Called a formula unit

  8. The subscript refers to what is immediately in front of it

  9. Monatomic Ions • By gaining or losing electrons, single atoms will become ions • Cations (+) will keep their atomic name • Anions (-) will change their ending to -ide • Rather than gain or lose, some atoms will share their electrons • This is called a covalent bond

  10. Binary Ionic Compounds • Made of two different elements • The total number of positive and negative charges must be equal • The cation will be written first, the anion second Aluminum +3 Oxide -2 Al2O3 Aluminum Oxide

  11. Stock System of Nomentclature • Some elements form two or more cations • System uses Roman numerals to indicate an ion’s charge

  12. Polyatomic Ions • Atoms that are covalently bonded to one another, but as a group have gained or lost electrons (forming a charge) • Many are oxyanions (contain oxygen) • The most common one ends in –ate • The one with less oxygen ends in –ite • If there is one with less oxygen than –ite, it is also given the prefix hypo- • If there is more oxygen than –ate, it is given the prefix per-

  13. Naming Binary Molecular Compounds • There are two naming systems • Stock system • Prefixes

  14. Covalent-Network Compounds • Some covalent compounds do not consist of individual molecules • Each atom is joined to all of its neighbors in a covalently bonded, 3-dimensional network • The subscripts in these formulas indicate the smallest whole-number ratio of the atoms in a compound • The naming scheme is the same as other molecular compounds (SiC, SiO2, Si3N4) Silicon Carbide, Silicon Dioxide, Trisilicon tetranitride)

  15. Acids and Salts • Most acids are either Binary or Oxyacids • Binary acids consist of two elements • Usually hydrogen and one of the halogens • Oxyacids contain H, O, and a third element • Usually a non-metal

  16. An ionic compound composed of a cation and the anion from an acid is often referred to as a salt.

  17. Oxidation Numbers • In order to indicate the general distribution of electrons among the bonded atoms in a molecular compound or a polyatomic ion, oxidation numbers, also called oxidation states, are assigned to the atoms composing the compound or ion.

  18. p. 216

  19. Saturated Hydrocarbons • Saturated Hydrocarbons are hydrocarbons in which each carbon atom in the molecule forms four single covalent bonds with other atoms • Hydrocarbons that contain only single bonds are alkanes

  20. A homologous series is one in which adjacent members differ by a constant unit • For alkanes, the series formula is CnH2n+2

  21. The systematic naming system for compounds was developed by IUPAC • The basic part is the name of the longest carbon chain • The number of carbon atoms is reflected in the prefix, while the suffix –ane indicates the molecule has all single bonds

  22. The naming of branched-chain alkanes also follows a systematic method • The branches are called alkyl groups • They are formed when a hydrogen is removed from an alkane molecule • Named by changing the suffix from –ane to –yl.

  23. To name this molecule, locate the longest continuous carbon chain that contains the most straight chain branches • Do not be tricked by the way it is drawn, the longest chain may be bent

  24. Name the parent hydrocarbon and add the suffix –ane • Next identify the alkyl groups • Arrange the names in alphabetical order in front of the name of the parent hydrocarbon • If there are multiple groups, use the prefix di, tri, etc. • Place the location numbers of each alkyl group in front of the name (lowest possible number)

  25. Small alkanes have very weak London dispersion forces only (for IM attractions) and have low BP’s • Longer alkanes have greater London dispersion forces and have higher BP’s

  26. Natural gas is a fossil fuel composed primarily of alkanes containing one to four carbon atoms • Petroleum is a complex mixture of different hydrocarbons that varies greatly in composition • Anywhere from 1 to 50 carbon atoms • Petroleum is separated into its different portions through fractional distillation

  27. When alkanes combust, they produce carbon dioxide, water, and heat • The carbon dioxide is a greenhouse gas • When gasoline ignites before the flame front reaches it, there is a decrease in the amount of power (engine knock) • Octane rating is a measure of the anti-knocking properties of gs

  28. Unsaturated Hydrocarbons • Unsaturated hydrocarbons are hydrocarbons in which not all carbon atoms have four single covalent bonds • Alkenes are hydrocarbons that contain double covalent bonds • An alkene with one double bond has two less hydrogens that the corresponding alkane • CnH2n

  29. The rules for naming alkenes are similar those for naming alkanes • The parent hydrocarbon is the longest one that contains the double bond • The suffix –ene is added to the prefix • The number are established so that the first carbon in the double bond has the lowest number (this number is placed at the beginning of the name) • If there is more than one double bond, the suffix is modified to: 2=diene, 3=triene, etc.

  30. Ethene is the smallest alkene • Ethene is the hydrocarbon produced in the greatest quantity in the US • Used in the synthesis of plastic and alchohol • Can cause the ripening of fruit

  31. Hydrocarbons with triple covalent bonds are alkynes • The general formula is CnH2n-2 • The naming rules are similar to the naming rules for alkenes except the suffix –yne is used

  32. The smallest alkyne is ethyne (acetylene) that is used for welding

  33. Effusion and Diffusion • Diffusion is the constant motion of gas molecules that causes them to spread out in their container • Effusion is the process whereby the molecules of a confined gas pass through a tiny opening of their container

  34. We know that • The rates of effusion and diffusion depend on the relative velocities of gas molecules • The velocity of a gas varies inversely with its mass (lighter molecules move faster than heavier molecules at the same temperature) and since the KE for all gases is the same at a given temperature then for gases A & B: The M represents the molar mass

  35. Can be multiplied by 2 to give And then rearranged to give or

  36. This leads to Graham’s law of effusion which states that the rates of effusion of gases at the same temperature and pressure are inversely proportional to the square roots of their molar masses • The law only provides the relative rates of effusion based on the relative velocities of the molecules Density varies directly with molar mass so density can be substituted for M This is used to separate U-238 and U-235 by allowing gaseous uranium to pass through porous membranes (over time and many membranes, the U-235 becomes concentrated)