Chemistry 127

August 23, 2004

Some review notes

What we expect you to know, be comfortable with, and be able to do...

• First you know the symbols and names for common elements and multi-element ions.
  • H, He, Na, K, Ca, Fe, Al, Mg, Ag, Cu, N, O, F, Cl, Br, I, S, Si
    • you also need to know the most common oxidation states (or number of bonds)
    • H (H+ or 1 bond), O(2 bonds), C and Si (4 bonds), Ca, Cu and Mg (+2 ions)
    • Na, K, Ag (+1 ions), Halogens (F, Cl, Br, I, -1 anions), Al(+3 ion or 3 bonds), Fe(+3 or +3 states)
  • Pay attention to elements with symbols that don't match the English name/spelling
    • K, Na, Pb, Cu, Fe, Au, Ag, Hg
    • potassium, sodium, lead, copper, iron, gold, silver, mercury
• You should know the common multi-element ions
  • NH₃ and NH₄+, NO₃-, SO₄2-, PO₄3-, OH-, CO₃2-, HCO₃-
  • ammonia/ammonium, nitrate, sulfate, phosphate, hydroxide, carbonate, bicarbonate
• You should also be able to write formula or name simple compounds
  • water, ammonia, carbon dioxide, carbon monoxide, sodium sulfate, potassium chloride
  • you should know the elements that usually exist as diatomic molecules
    • O₂, N₂, H₂, halogens (F₂, Cl₂, Br₂, I₂)
• You should be able to balance a chemical equation and to work with that equation
  • 2 H₂(g) + O₂ (g) --> 2 H₂O (l)
  • therefore 1 mole of O₂ (and an excess of H₂ can form) 2 moles of water
    • mixing 3.5 moles of H₂ and 2.1 moles of O₂ can yield __ moles of H₂O
    • 1 gram of H₂O can be formed from __ g of H₂ and __ g of O₂
  • obviously you need to be able to evaluate formula weight, convert mass to moles, compute using solution concentration (moles/liter)
• You also need to know simple energy relationships
  • heat is a form of energy
  • calorimetry relates heat to temperature rise of a sample
    • Q (heat) = C (heat capacity) x DT (temperature change)
      • D should be "delta" symbol
      • usually C = C_p(per gram) x mass of sample
  • heat and enthalpy (H) are the same when pressure remains constant
  • tables of enthalpy exist and DH can be computed for any reaction
    • this probably needs review, but we will be working with this soon enough again
• You should have a good understanding of the different types of chemical bonds
  • a. Ionic compounds-- exist as atomic ions (cations, anions); may also include multielement ions such as NO₃-, nitrate.
  • Ionic Solids exist as crystalline lattices or ordered arrays of cations alternating with anions.
    • No "molecules" exist; CaCl₂ means Ca²⁺ ions and twice as many Cl- ions in the lattice.
  • When an ionic compound dissolves, it forms
    • a solution of anions and cations.
  • b. Molecular species held together by covalent bonds, such as water H₂O or methane CH₄.
    • The molecule is the stable or defining entity.
    • Crystals form, but they are regular arrays of molecules.
    • In liquids the molecules are attracted to each other but they move freely and do not form ordered structures.
    • In gases, the molecules remain intact and are well separated.
• c. Covalently bonded solids such as diamond or SiO₂ (silica, sand)
  • A collection of atoms form a linked solid by forming a network of covalent bonds.
  • Again, no "molecules" exist, since most atoms forms a links to other parts of the structure.
• d. Metals are a special class.
  • They really exist as cations surrounded by a "sea" of valence electrons.
  • Again, no "molecules."
  • Metals often form solid solutions or alloys containing two or more metals.

You certainly should be comfortable with the physical properties of the three states of matter.

• Solids--
  • they resist deformation and hold their shape; they can be cut, twisted, extruded,
  • they can break, they are useful structural materials.
  • Solids have densities that vary from about 0.75g/ml for gasoline , 1.0 g/ml for water to about 14 g/l for dense metals like mercury or gold.
• Liquids--
  • they flow and fill the bottoms of containers
  • they do not retain their shape.
  • Some may be thick and viscous and may flow slowly.
  • Liquids maintain their volume and have densities comparable to solids.
  • When cooled, most liquids will freeze to form solids and the solids melt when heated.
• gases--
• these are very low density forms of matter.
  • A gas expands to fill its container, occupying the shape and size.
  • As a result, gases escape readily unless they are confined by sealed containers. (Or, in the case of the atmosphere, held by gravity.)
  • the volume (and density) of a gas is very dependent on the external pressure
  • gases form when liquids evaporate (or when solids sublime.)

Chemical Bonding

• Ultimately, chemical species are held together by electrical forces--
  • the attraction between positive and negative species.
  • The spacing between atoms is ultimately limited by the repulsion between objects of the same charge ( the nucleus of the atoms)
• Ion ions one atom completely loses electrons and the other accepts electrons
  • charges are + or - 1-2-3- complete electron charges
  • at normal atomic spacing, these are very strong attractions
  • the overall strength is reduced because we also need to bring together more ions and some will repel
• Covalent bonds involve the sharing of 2 electrons between two different atoms
  • usually each atom contributes one of the electrons of the pair
    • the electrons might not be evenly shared so the bond might have a weak + and - end
    • these charges can attract neighboring molecules
  • covalent bonds can be very strong bonds
    • Many electrons will not be partners in the chemical bonds
    • Often, molecules contain "lone pairs" or electrons
• Coordinate Covalent (Dative) bonds
  • this is a bond with a pair of electrons shared by two atoms
    • one atom provides both electrons
    • it's a pretty electronegative atom so the electrons are very unevenly shared
  • as covalent bonds go, this is a pretty weak bond
    • it's easily undone