CHEM 301: Ch4 The Study of Chemical Reactions
37 Cards in this Set
Front | Back |
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thermodynamics
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energies of the reaction at equilibrium
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mechanism
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step by step pathway from reactants to products
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kinetics
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variation of reaction rates with different conditions and concentrations of reagents
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chlorination of methane
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methane + chlorine + heat/light
--->
chloromethane +hydrogen chloride
blue color absorbed by chlorine, activating it so it initiates reaction with methane
high quantum yield
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Free radical chain reaction
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1. iniation step
2. propogation step
3. termination steps
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mechanism
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generates reactive intermediate
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propagation
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reactive intermediate reacts with a stable molecule to produce another reactive intermediate and product molecule
allows chain to continue until supply of reactants is exhausted or reactive intermediate destroyed
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termination
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side reactions that destroy reactive intermediates and tend to slow or stop the reaction
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initiation
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generates reactive intermediate
blue light splits chlorine into 2
unpaired electron is radical electron
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PROPAGATION I
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when chlorine collides with a methane molecule it removes hydrogen from methane, forming HCl bond and CH3 methyl radical
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Propagation II
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Methyl radical reacts with molecule of chlorine to form chloromethane
chlorine atom is left with odd electron
results in chloromethane and chlorine radical
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Termination reactions
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step that produces fewer reactive intermediates than it consumes
-combination of any two free radicals
-collision with wall
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Equilibrium constant
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a+b > c+d
products /reactants
keq > 1 reaction is favored, left to right
keq<1 reaction moves right to left
from value of keq we can calculate gibbs free energy
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gibbs free energy
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change in free energy (delta G)
free energy of products - free energy of reactants
if products < reactants reaction is favored (-delta G)
-Delta G, energy released
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Equilibrium constants and free energy
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Keq= e^-DeltaG/(RT)
DeltaG= -2.303RT(logv10Keq
R-8.314
T-temp in kelvin
e-2.718
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delta H
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enthalpy, heat change
products - reactants
measure of strength of bonding in products and reactants, reactions favor products with lowest enthalpy (strongest bonds)
-delta H, heat released
+delta H, heat absorbed
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delta S
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entropy products - reactants
disorder
reactions favor greatest entropy
+ denotes freedom of motion
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change in free energy
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delta G= delta H - Tdelta S
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Bond dissociation energies
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Bond breaking requires energy (+)
bond forming releases energy (-)
BDE always positive/endothermic
delta H= (BDE of broken bonds) BDE bonds formed
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homolytic
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each bonded atom retains one of the bonds two electrons
forms free radicals
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heterolytic
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one of the atoms retains both e-s
forms ions
enthalpies rely heavily on solvent to solvate ions that result
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kinetics
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study of reaction rates
how fast
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rate of rxn
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how fast products appear and reactants disappear
depend on concentrations of reactants, greater concentration-more likely to react
rate = kr[a]^a[b]^b
MUST BE DETERMINED EXPERIMENTALLY
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activation energy
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Ea
minimum kinetic energy molecules must have to overcome repulsions between electron clouds when they collide `
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Rate limiting step
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transition states are at energy maximums
intermediates are at energy minimums
the reaction step with the highest activation energy will be the slowest, and the rate determining for the entire reaction
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Rate, Ea, and temperature
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Increasing Ea, rate decreases
Increasing temp. rate increases
fluorine reacts explosively
chlorine reacts moderately
bromine must be heated to react
iodine does not react
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carbanion
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strongly nucleophilic species with negatively charged carbon atom having only three bonds
C atom has nonbonding pair of electrons
8e- on C +lone pair, negative charge
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carbene
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highly reactive species with only two bonds to an uncharged carbon atom with a nonbonding pair of electrons
carbon is neutral
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carbocation
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6 electrons, positive charge carbon sp2 hybridized with vacant p
strongly electrophilic species with positively charged carbon atom only having three bonds
opposite of carbanion
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Hammond postulate
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species closer in energy are also closer in structure
in exo - transition state closer to reactants in energy in energy and structure
in endo- transition state is closet to products in energy snd structure
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inductive effect
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donation or withdrawal of electron density through sigma bonds
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intermediate
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molecule or molecule fragment formed in a reaction and exists for finite length of time before it reacts in the next step
corresponds to relative minimum in the diagram
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radical inhibitor
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compound added to prevent progogation of free radical chain rxns
mostly stabilizes radicals so they dont react
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resonance stabilization
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takes place by delocalization of electrons in pi bonded system
cations, radicals, anions often stabilized through this
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reactive intermediates
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carbocations
free radicals
carbanions
carbene
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inductive effect
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donation of anelectron density along sigma bonds
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hyperconjugatio
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overlap of sigma bonding orbitals with empty orbital
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