Chem 225 Flashcards
symmetry elements/operations
Describe specific symmetry relationships between areas within an object.
Examples: rotation axis, plane of symmetry
point group
A collection of symmetry elements that describes the overall symmetry of an object. All point groups include the identity element E.
Examples: C2v, C2h , Td, Oh
irreducible representations
A set of characters within a Character Table describing a mathematically allowed sub-symmetry of the point group.
Examples: within C2v, 1, -1, 1, -1
characters
Positive or negative values within an irreducible representation (within a Character Table), referring to symmetric, antisymmetric, or asymmetric with respect to a symmetry element.
Examples: 0, 1, -1, 2, -2, 3, -3
Identity element, E
Present in all point groups with a positive character of 1, 2 or 3. The identity operation does not move the object (molecule), and is present for mathematical reasons
Rotation axis, Cn
Symmetry with respect to a partial rotation around a central axis that passes through the object. n is an integer that defines the ‘fold’ of the
rotation, which is the number of turns to complete 360°, i.e. 2 (180° each), 3 (120° each), 4 (90° each), 5 (72° each), 6 (60° each), etc.
- For objects with more than one axis of symmetry that are perpendicular, the axis with the highest fold is referred to as the principal axis.
Plane of symmetry, σ
symmetry with respect to a plane through the center of the object. A plane that includes the principal axis is labelled σv and a plane that is perpendicular to the principal axis is
σh.
Center of inversion, i
symmetry with respect to a combination of a C2 and σh. Inversion of all components of the molecule through the center of the molecule
Improper axis, Sn
Symmetry with respect to a combination of a fraction of a rotation Cn (n is greater than 2) and reflection through a perpendicular plane.
Example in red is S 4, 90° rotation about
the C 2 and reflection through the
perpendicular plane.
cubic close packing (ccp)
- ABCABC
- packing of hexagonal 2D layers
- also known as face centered cubic
hexagonal close packing (hcp)
- ABAB
- packing of hexagonal 2D layers
non close-packed arrays
- Simple cubic lattice and body-centered cubic
- These are the simplest lattices
- Both are built from square 2D packed
layers - They differ only by whether alternate layers are placed in “holes” or directly over the previous layer’s
Effect of temperature of metals and semiconductors
- Semiconductors: the temperature dependence of the number of mobile electrons is more important
than the temperature dependence on lattice vibrations. As a result the resistivity of decreases with increasing temperature - Metals: the temperature dependence of resistivity is determined by the lattice vibrations. As a result the resistivity increases with increasing temperature
1:1 structures
- Rock salt (NaCl) structure: fcc (ccp) lattice for both Na+ and Cl-. Many other salts have the same structure, need a 1 to 1 stoichiometry
- Cesium chloride: simple cubic lattice, salts containing large cations. Octahedral holes in close-packed anion structure only have room for cations 41% of anion size. So this adopts a non-close packed structure
1:2 structures
- Fluorite (CaF2): a face-centered cubic lattice of Ca2+ ions with F- in all tetrahedral holes
- Zinc blended: Only half the tetrahedral holes filled. Preferred coordination number of 4
Oxidation vs. reduction
Oxidation: Loss of electrons of the reducing agent
Reduction: Gain of electrons of the oxidizing agent
Galvanic cells vs. electrolytic cells
- In electrolytic cells, the passage of electrical current causes a chemical reaction to occur whereas in a Galvanic (or Voltaic) cell, a spontaneous chemical reaction causes electrons to flow
Hydrogen compounds
- ionic hydrides (E+H-): True ‘saline’ (ionic) hydrides only form with groups 1 and 2
- covalent hydrides
-hydridic: H slightly - , polymeric solids, electron bridging, BeH2, B2H6, MgH2, SiH4, SnH4, etc- neutral: CH4, PH3, AsH3
- protic: H slightly + , high Bp gases or liquids, NH3, H2O,H2S, HX
Uses of hydrogen
- Fuel cells, energy storage
- Chemical synthesis (e.g. addition to double bonds w/ catalyst)
- Reduction of salts to metals instead of C ( e.g. MoO3 + 2H2 → Mo + 3H2O )
- Used in synthesis of ammonia by the Haber-Bosh process
Group 1: Alkali metals
- Silvery, low melting metals by electrolysis of molten salts
- Form mostly soluble salts that are powerful reductants and very reactive as a result
- Dissolved in liquid NH3 to form ‘solvated electron’ solutions
Flame tests for group 1 metals
- Atomic emission spectra
- Li: crimson, Na: yellow, K: purple, Rb: red-violet, cesium: blue
Group 1 uses
- Lithium ion batteries: Rechargeable, High energy density, Used in portable electronics, flammable (e.g. LiC6, lithium graphite)
- LiC6: INTERCALATION COMPOUND
- Organolithium reagents: C-C bond formation (similar to, but more reactive than, Grignard reagents, RMgX)
- M-C bond formation
- Anionic polymerization initiator (e.g. styrene to polystyrene)
Group 2: Alkaline earth metals
- Silvery metals, higher mp than group 1, less reactive bc harder to oxidize due to increasing Zeff.
- reactivity with water: M= Mg «_space;Ca, Sr< Ba
Group 2 uses
- MgO: A REFRACTORY MATERIAL= Very high mp, insulator, not ‘workable’
- Ceramics, or oven/furnace liners
- Calcium: limestone (CaCO3) is ‘calcined’ with heat to make lime/quick lime CaO, with water it becomes ‘slaked’ lime Ca(OH)2
- Calcium carbide (CaC2): formed from carbon and CaO at high temperatures, hydrolysis releases acetylene gas. Used in miners lamps