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
Group 13: metals and non-metals
exemplifies almost all periodic trends:
- change from covalent (B) to ionic bonding (Al down) = Decreasing Q/r ratio
- nonmetal (B) and metal behaviour (Al down) = Decreasing Q/r, more orbitals
- diagonal relationships (B →Si and Be →Al) = Similar Q/r: increased charge
offsets increasing size
inert pair effect
s-electrons have electron density near the nucleus and the very high Z means electrons experience an increase in mass if close to the nucleus - a relativistic effect. This lowers the s-electron energy enough that it takes too
much energy to remove them, i.e. they become ‘inert’
Ex: Oxidation states: +3 for most but heaviest member Tl dominated by +1.
Where and how aluminum metal
- Most abdundant metal in Earth’s crust
- Huge energy cost to mine
- Prepared by electrolysis (Hall-Héroult Process)
- Al2O3 (Na3AlF6 ) + 2C (s) → 2 Al (l) + CO (g) + CO2 (g)
Aluminum hydroxides and oxides
- Dissolves in acids or bases: amphoteric
- Conversion of hydroxides to oxides is condensation (- H2O)
- Al2O3 and B2O3 is like quartz
- SiO2 (silica) and borosilicates are similar to pyrex
Boranes and boric acid
- Boranes: B/H compunds
- Types: Arachno (Web, B4), Nido (Nest B5) and Closo (cage, B6 or C2B4)
- Boric acid: B(OH)3, weak acid (pKa 9.5), extensive hydrogen bonding in a sheet structure, used as a mild antiseptic: eye/mouth wash
Group 14
- group includes non-metals (C), semi-metals (Si, Ge) and metals (Sn, Pb)
- all elements catenate to some degree
- inert pair effect for Sn and especially Pb (2+ state is important)
Allotropes of carbon
- Graphite (sp2): conductor
- Very soft: layers can slide over one another. Intercalation compounds: e.g. C6K
- Diamond (sp3): insulator
- Extremely hard, must break several bonds to move C atoms
- Graphenes (sp2):
- Fullerenes
- Nanotubes
Silicones/siloxanes uses
- Produced on a massive scale
industrially - Low toxicity and cheap leads
to many uses: - Oil baths, hydraulic fluids (stable to many chemicals and high T)
- Personal care products as lubricants: shampoo, conditioner, antiperspirant, hair gels, toothpaste, shaving cream
- Greases, sealants, gaskets
- Medical implants
Group 15: pnictogens
- non-metals except for Bi
- tend to catenate
- multiple bonding rapidly declines down the group:
- N≡N bond strength (942 kJ/mol) is >50% stronger than 3 times the N-N bond strength (600 kJ/mol)
- P≡P bond strength (480 kJ/mol) is 30% less than 3 times the P-P single bond energy (630 kJ/mol)
Arsenic compounds
- Bind strongly to S and disrupts S-S bonds
- Poisonous: Napoleon was poisoned with As – deliberately or not!
- Strong antibacterial properties
- Ehrlich’s Magic Bullet: Developed the cure for Syphilis. Originator of chemotherapy
Group 15 pesticides and nerve agents
- Pesticides: Parathion, diazinon, malathion (the safest)
- Nerve agents: Sarin and soman
- Mechanism: Acetylcholine esterase (ACE) inhibitors – interfere with the ‘off’ switch leading to uncontrolled nerve impulses and death
Group 16: chalcogenides
- Oxygen: O2 or O3 gases, insulator, pπ-pπ bonding, very little catenation, strong oxidizing ability
- Sulfur: S8 and many other Sn, insulator, bonding mainly with O, extensive catenation, weak oxidizing ability
- Selenium: Sen helical chains, semiconductor, no multiple bonding, significant catenation, no oxidizing ability
Tellurium: Ten helical chains, semiconductor, no multiple bonding, significant catenation, no oxidizing ability
Group 16 catenation
- Sulphur has the 2nd greatest tendency to catenate after carbon
- S8: yellow sulfur, S chain: red liquid, cooled: black plastic sulfur
- But…many other catenation compounds of S, Se, and Te are known
- MoS2 (lubricant) and FeS2 (Fool’s gold) contain S2 2- anions
Group 16: selenium and tellurium
- Se and Te are photosensitive semiconductors: they conduct when exposed to light
- The Xerox process: old printing method using a selenium-coated drum and charge
Sulfur oxides
- Combustion of any S-containing material (especially coal) produces SO2 (g)
- SO2 slowly converts to SO3 which dissolves in water to form H2SO 4 (part of acid rain):
- degrades limestone used in buildings
- limits ability of freshwater to support fish/plants
- But, SO2 can be useful:
- Used in bleaching of wood pulp in pulp+paper industry
- Antibacterial sprayed on fruit or added to wine as a preservative: dissolves in
water and converts to sulphite (SO32-) salts
Group 17: halogens
- Fluorine: only -1 valence in compounds, weak F-F bond (extreme lone pair repulsion), reacts with virtually all elements (except N and O), extremely oxidizing, Eº = +2.87V, very strong and unreactive C-F bonds, e.g. teflon
- Chlorine: yellow gas, Eº = +1.36V, PVC, other polymers, bleach (ClO -), solvents
- Bromine: red liquid, Eº = +1.09V, Photographic film, flame retardants
- Iodine: purple solid, Eº = +0,54V, disinfectant, X-ray contrast agent, catalysts
Stereoisomers and types
- Stereoisomers: same chemical formula, same bonds but different relative orientation of those bonds
- Enantiomers: Mirror image, same physical
- Diatereomers: Non mirror images, different physical properties (ex: cis vs trans or mer vs. fac)
Strucral isomers and types
- Structrual isomers: same formula, different atom connectivity
- Ionization isomers: interchange of an anionic ligand in 1st coordination sphere with anion outside the coordination sphere
- Hydration isomers: interchange another ligand in 1st coordination sphere with outer sphere water
- Coordination isomers: interchange of ligands in first coordination spheres of salt where both cation and anion are coordination complexes
- Linkage isomers: same ligand bonded to the metal in different ways (an ambidentate ligand)
π donors vs. π acceptors
- π donors: ligand with filled π orbitals, greater M character in anti-bonding, greater L character in bonding, weak field, smaller delta o
- π acceptors: ligand with empty π orbitals, greater L character in anti-bonding, greater M character in bonding, strong field, larger delta o
Generalizations about delta o
1) For a given M n+ ion, the position of L in the spectrochemical series predicts the magnitude of delta o
2) For a given ML6 with M in different oxidation states: the higher oxidation state has the higher delta o
3) For a given MLn in a single triad: heavier metal has the higher delta o
4) BUT, there is NO TREND in delta o across the series for different M with the same ox. state and ligand set
Types of radio active decay
- Beta (β) radiation involves emission of high-speed electrons. Neutron converts to proton + ejected e-
- Gamma (γ) radiation involves emission of a very short wavelength (high energy) photon. It usually accompanies other radioactive decay because it represents the energy lost when the nucleus changes into a more stable arrangement
- Positron emission involves an anti-electron being generated. Proton converts to neutron + ejected positron
- When a positron encounters an electron, they annihilate each other and produce two gamma rays
- Electron capture involves a nucleus capturing an orbital electron. Electron is captured and combines with a
proton to give a neutron
N and Z ratios in stability belt
- Z ≥ 84 decay by alpha emission (decreases both N and Z)
- High N/Z ratio nuclides usually decay by beta-emission (Nˇ, Zˆ)
- Low N/Z ratio nuclides usually decay positron emission or electron capture (Nˆ, Zˇ)
