Chem 117 Exam 2

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chemical bonding: the forces that hold compounds together. the strength of bonds determines the physical properties of the compound. chemical bonds are a balance in the forces of attraction and repulsion between electrically charged particles. bonding lowers the potential energy of the atoms
bond making: energy is released. exothermic -ΔH. all bonds, when theyre formed, release energy
bond breaking: energy is used. endothermic +ΔH
intramolecular bonds: the forces that hold molecules together. (covalent, ionic). stronger that intermolecular
intermolecular bonds: the forces between adjacent bonds that hold molecules and leads to physical properties
ionic bonding: electrostatic attraction between ions of opposite charge.
covalent bonding: nonmetal-nonmetal. bonds between density atoms that have small differences in electron density, they bond by sharing electrons. atoms lose, gain, or share electrons resulting in the formation of chemical bonds (and the resulting noble gas configuration)
salts: have high solubility in water. if you change something to a salt, you increase it solubility.
sodium and chlorine gas reaction: transfer of one electron. produces a violent reaction. ionic bonding in characterized by a transfer of electrons.
aluminum with bromine gas: transfer of 3 electrons which results in a large combustion. (larger than sodium chloride)
formation of an ionic lattice: sodium (losing an electron) gets smaller and chlorine (gaining an electron) gets bigger. ionic compounds are highly ordered and form an ionic crystal lattice
lattice energy: the amount of energy required to convert a mole of ionic solid to its constituent ions in the gas phase.
breaking a sodium chloride lattice: NaCl(s)---> Na+(g) + Cl-(g) ΔH=+788 kJ/mol
making a sodium chloride lattice: Na+(g) + Cl-(g) ---> NaCl ΔH=-788 kJ/mol. making the lattice RELEASES energy
effect of ionic size on lattice energy: as ionic size increases (down a group) the attraction between the anion and cation decreases. as you go down a group, the lattice energy decreases
effect of ionic charge on lattice energy: as the charge of the ion increases, the electrostatic force increases. increases in lattice energy with increase in magnitude of charges
how to figure out the biggest lattice energy: 1. figure out charges. 2. pick one with largest charges. 3. pick smaller radii
strongest lattice energy: highest magnitude of charge, smallest ionic size
valence electrons in electronic configurations: n of highest value
covalent bond length: as atoms approach each other, they decrease in energy (become more stable) as they are attracted to each other. at some point, the repulsions overcome attractions and the atoms push away from each other
average bond length in covalent bonds: the optimal distance between two atoms
bond length in covalent bonds: is actually fluxional at any temperature above absolute zero (0 Kelvin). in polyatomic molecules, electrons are unequally shared, so bond length is different than radii
bond length and strength: shorter is NOT stronger. when atoms are at zero potential energy, they are the furthest away from each other. when atoms are too close together, they repel each other which has the highest energy. optimal bond energy and bond length is when they are at the right distance for a stable bond. they are constantly oscillating
covalent bond length: larger atoms make longer bond lengths. smaller atoms form smaller bonds. multiple bonds are shorter than single bonds. multiple bonds are stonger than single bonds.
electronegativity: the ability of an atom to attract electron density from other atoms in a molecule. flourine is the most electronegative element. different than electron affinity because the electron has not yet been transferred.
pure covalent bond: neutral atoms held together by equally shared electrons. <0.5
polar covalent bond: partially charged atoms held together by unequally shared electrons. 0.5-2.0
ionic bond: oppositely charged ions held together by electrostatic attraction. >2.0
dipole moment: quantitative measure of polarity. charge x distance between the charges
compound with greatest ionic character: greatest difference between values. also furthest apart.
impact of electronegativity in life (example): changing elements in toxic nerve agents to something less electronegative makes it less toxic to humans (insecticide)
H2: noble gas configuration for helium
group 4A: 4 valence electrons, 4 bonds
group 5A: 5 valence electrons, 3 bonds;1 lone pair
group 6A: 6 valence electrons, 2 bonds; 2 lone pairs
group 7A: 7 valence electrons, 1 bond; 3 lone pairs
formal charge=0: if element has the same number of electrons as it does in its elemental form. formal
formal charge= -1: if it has 1 more electron than its elemental form
formal charge= +1: if it has one less electron than its elemental form
single bond: covalent bond in which 1 pair of electrons is shared
double bonds: 4 shared electrons, represented by 2 lines
triple bonds: 6 shared electrons, represented by 3 lines
multiple bonding: occurs to obey the octet rule or lower formal charge of all atoms. made to minimize formal charge. HYDROGEN NEVER MAKES MULTIPLE BONDS. every time you make a bond, you lower the potential energy (release in energy)
lewis structures: the most electropositive element goes in the center. add up number of valence electrons and distribute. place electrons on most electronegative elements first. make sure every element, except hydrogen has a full octet.
resonance structures: different plausible structures for lewis structures where double bonds/electrons are moved among several positions
best resonance structures: have the greatest number of covalent bonds and place the negative charge on the electronegative element, positive charge on the electropositive element
bond order: number of bonds/number of locations
rules of engagement: 1. all resonance stuctures must have the same number of valence electrons 2. the octet rule must be obeyed 3. nuclei do not change positions in space between resonance structures
nitrogen lewis dot: 3 bonds with 1 lone pair
oxygen lewis dot: 2 bonds 2 lone pairs
condensed structural formula: read like a book from left to right
line drawings: bends and ends are carbons. when you have a hydrogen that is not on a carbon it must be drawn
dashes/wedges: dashes are going away from you wedges are coming toward you
bond enthalpy: the enthalpy change associated with breaking a bond in 1 mole of gaseous molecule
ΔH°=: bonds broken-bonds formed.
-ΔH° means: release of energy. exothermic
+ΔH° means: use of energy. endothermic
determine bond enthalpy: 1. draw lewis dot structures to determine number of bonds broken and formed 2. multiply the number of bonds by their specific bond enthalpy
electron deficient molecules: molecules derived from groups 1, 2, and 3 (B, Al, Ga) elements being the central atom often have the central atom being electron deficient
expanded valence shell: occurs when non metal elements from period 3 or higher (i.e. S, O, P). has accessible d orbital
VSEPR: molecules bond such that all regions of electron density are far from each other
molecular geometry: considers all electron domains (lone pairs and bonding pairs, single, double, and triple bonds)
molecular shape: considers only spatial arrangements of atoms, not lone pairs
lone pairs: take up more room then bonding pairs
linear shape: 2 electron domains. linear geometry. 180°. CO2, HCN
trigonal planar shape: 3 electron domains. trigonal planar geometry. 120°. BF3
bent shape (trigonal planar): 3 electron domains. 1 lone pair. trigonal planar geometry. <120°. SO2
trigonal planar shape (double bond): 3 electron domains. trigonal planar geometry. >120°. COH2
tetrahedral shape: 4 electron domains. tetrahedral geometry. 109.5°. CH4
trigonal pyramidal shape: 4 electron domains. 1 lone pair. tetrahedral geometry. <109.5°. NH3
bent shape (tetrahedral): 4 electron domains. 2 lone pairs. tetrahedral geometry. <109.5°. H2O
triangular bipyramidal shape: 5 electron domains. triangular bipyramidal geometry. 90° and 120°. PF5
seesaw shape: 5 electron domains. 1 lone pair. trigonal bipyramidal geometry. 90° and 120°. SF4, SeCl4
t shaped shape: 5 electron domains. 2 lone pairs. trigonal bipyramidal geometry. 90°. ClF3
linear shape (trigonal bipyramidal): 5 electron domains. 3 lone pairs. trigonal bipyramidal geometry. 180°. XeF2
octahedral shape: 6 electron domains. octahedral geometry. 90°. SF6
square pyramidal shape: 6 electron domains. 1 lone pair. octahedral geometry. 90°. BrF5
square planar shape: 6 electron domains. 2 lone pairs. octahedral geometry. 90°. XeF4
polarity: is determined by knowing a molecules shape and the polarity of the individual bonds in the molecule and adding these up to get a net vector called a dipole moment
Valence bond model: atoms share electrons when atomic orbitals overlap: 1. a bond forms when a singly occupied atomic orbitals on two atoms overlap. 2. the two electrons shared in the region of orbital overlap must be of opposite spin. 3. formation of a bond results in a lower potential energy for the system
orbital overlap: orbitals overlapping of similar energy form better overlaps and stronger bonds (2s-2s stronger than 2s-5s).
node: no electron density. can't have a lobe with an overlapping node.
hybridization: mixing of atomic orbitals can account for observed bond angles in molecules that could not be described by he direct overlap of atomic orbitals.
number of orbitals mixed=: number of orbitals obtained
lone pairs always live: in hybrid orbitals
sp: 2 electron domains
sp2: 3 electron domains
sp3: 4 electron domains
sp3d: 5 electron domains
sp3d2: 6 electron domains
sigma bond: forms when hybrid orbitals overlap on axis. stronger than a pi bond. free rotation
pi bond: forms when unhybridized orbitals overlap on a parallel (off axis) fashion. made above and below the axis.
sigma bond and a pi bond together: make a double bond. sigma bond has limited rotation due to pi bond because pi bond is less energetic than sigma
constitutional isomer: same molecular formula but different bond connectivity
conformers: single bonds are axes of rotation. pentane with two methyls pointing up or down
cis "kink": two electronegative elements are on the same side
trans: electronegative elements on opposite sides.
cis and trans: geometric isomers. completely different molecules. different boiling, freezing points etc
trans fat: look like saturated fat. form solid at room temperature because they lay perfectly in line
cis fat: has a kink. oil at room temperature
atoms hybridize to...: make sigma bonds or hold lone pairs
hybridization with multiple bonds: triple bond is 1 sigma and 2 pi bonds
H-C=C-H: 3 sigma bonds- 1 between C-C (sp and sp). 2 between C-H (sp and 1s). 1 Pi bond between C-C (2p and 2p)
non hybridized p orbitals: used to make pi bonds. must be on the same axes!
conjugation: delocalization of electrons. pi bonds in benzene are delocalized (spread out over the entire structure). delocalized electrons are always moving around p orbitals. single double single double= conjugation! brings out color
color and conjugation: color comes from extended conjugation. fading occurs due to breaking of weak pi bonds and a decrease in overall conjugation. UV light causes fading
single bonds: overlap of orbitals along the internuclear axis. the first bond between any two atoms is always a sigma bond
multiple bonds: combination of sigma and pi bonds. PI BONDS ARE WEAKER THAN SIGMA BONDS
lewis theory strength: qualitative prediction of bond strength and bond length
lewis theory weakness: two dimensional model
VSEPR strength: predict the shape of many molecules and polyatomic ions
VSEPR weakness: fails to explain why bonds form based on lewis theory
valence bond theory strength: covalent bonds form when atomic orbitals overlap
valence bond theory weakness: fails to explain the bonding in many molecules
hybridization of atomic orbitals strength: (extension of valence bond theory) using orbitals it is possible to explain the bonding and geometry of more molecules
hybridization of atomic orbitals weakness: fails to predict some important properties, such as magnetism
molecular orbital theory strength: most accurate model; predicts the magnetic and other properties of molecules
gases are compressible because: they are 99.9% empty space. solids and liquids are 30% empty space.
gas properties: low viscosity and low density and are highly variable depending on temperature and pressure
gases are miscible: gas mixtures are always homogeneous - completely soluble with each other. gases have no intermolecular forces
conversion of pressure: 1 atm=760 torr=760 mmHg
ideal gas law: PV=nRT (R=0.08206)
STP: 1 atm, 0°C (273.15K), 22.4 L/mol
Daltons law of partial pressure: the total pressure exerted by a gas mixture is the sum of the partial pressures exerted by each component of the mixture
mole fractions: number of the moles of a certain component/the total number of moles. always less than 1. sum of mole fractions for all components of a mixture =1.
what % of (specific gas) is responsible for the pressure?: use mole fraction
calculate the partial pressure when given moles and total pressure: mole fraction x total pressure
vapor pressure: the amount of pressure exerted over a liquid or solid. as temperature increases, there are more molecules in the vapor phase.
normal boiling point: temperature at 1 atm. when liquid and gas molecules are in constant equilibrium in a closed system
kinetic molecular theory: explains how the molecular nature of gases gives rise to their macroscopic properties.
assumptions of kinetic molecular theory: gas is composed of molecules separated by large distances. volume occupied by individual molecules is negligible. gas molecules in constant motion with perfectly elastic collisions (don't lose energy). gas molecules do not have attractive or repulsive forces
root-mean-square (rms) speed: the speed of a molecule with the average kinetic energy in a gas sample. directly proportional to temperature. inversely proportional to molecular mass. lightest particles move fastest
diffusion: the mixing of gases as the result of random motion and frequent collisions
effusion: the escape of a gas molecules from a container to a region of vacuum
ring strain: electron domains-think of ideal bond angle and if that is being fulfilled

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