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James Madison University
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do the values of a,b, and c have any relation to the balanced chemical equation?
what is graphed in kinetics lab?
ln absorbance over time
how is absorbance measured in the kinetics lab?
absorbance is measured experimentally using a spectronic 20 UV-visible spectrometer tuned to the exact wavelength absorbed by the Cr2O72-
what is the wavelength absorbed by Cr2O7?
what is the simplified rxn. rate? Why
if you follow the reaction for a short period of time the amount of IPA and H that react is negligible compared to the initial concentrations. k(1)[cr2O7]
where k(1) = k[IPA]
what changes in the second reaction of kinetics?
the concentration of IPA is doubled
how is a found?
what changes in the third reaction of kinetics?
the concentration of H+ is increased.
what is used to measure % transmittance and observed absorbance in the kinetics lab?
when are time intervals recorded in the kinetics lab?
every 20 secs between 10% and 70%
why is H+ only increased not doubled in the third reaction?
the reaction may proceed too quickly to accurately measure
how is the spectronic 20 turned on?
using the left hand knob
when and how to set spectronic 20 to 0% transmittance?
done before adding cuvettes by using the left-hand know to set the needle to read infinite absorbance
when and how to set spectronic 20 to 100% transmittance?
after blank sample in cuvette has been put in spec-20. using the right hand knob to set the needle to 0 absorbance.
what is kinetics?
speed of a reaction
what are the reagents in the kinetics lab?
Isopropyl alcohol (IPA), Potassium dichromate, and H+
what are the products in the kinetics lab?
why are we able to monitor the reaction in the kinetics lab?
due to the color change, due to changing pH
What is beer's law and what does it stand for?
A=abc....absorbance=(molar absorptivity)(path length)(concentration). It is a correlation between absorbance and concentration.
how is Absorpance calculated?
how is c calculated?
amt of IPA reacted?
3(amt of Cr2O7 reacted)
amt of H+ reacted?
8(amt of Cr2O7 reacted)
amt of Cr2O7 reacted?
initial concentration - concentration after 2 half lives
Rate after 2 half lifes?
k[IPA initial - amt reacted]
[Cr2O7 after 2 half lives]
[H+ initial - amt reacted]
what happens to a molecule under mass spec?
M + e- --> M+ + 2e- . Molecule loses an electron.
what is the x-axis in mass spec?
mass/charge ratio, basically the mass of the molecule being measured
what is the y-axis in mass spec?
is it's abundance
what is the Nitrogen rule?
if odd number assume one nitrogen, at end if mass doesn't add up assume 3. if even number assume zero nitrogens, at end if mass doesn't add up assume 2.
how to detemine if you have Cl or Br in your molecule?
look 2 mass units higher than M+, if there is a peak that is 1/3 the height of M+ peak you have Cl if of equal height you have Br.
what does IR stand for?
what is the x-axis in an IR?
cm-1 or wavenumbers, concentrate on the 1500-3400 range
what is the range below 1500 cm-1 called in an IR
what does an IR look for?
the presence or absence of functional group information.
what does NMR stand for?
nuclear magnetic resonance
what is the x-axis in the 13C NMR?
what do the peaks represent in a 13CNMR
each chemically non-equivalent carbon in the molecule
what is the x-axis in 1H NMR?
what does the integral tell you in a 1H NMR?
how many protons are associated with that signal
what is metalation?
metalation is defined as replacing a group on a molecule (usually H) with a metal.
what is the intermediate in a metalation reaction? what type of reaction is this.
litiocarbanion. R-Li. a simple acid/base reaction.
what type of compounds do carbanions from metalation reacted with?
electrophiles in condensation reactions
R: Li + E ---> R--E + Li
what is only requirement in theory of a metalating agent?
that it is a sufficiently strong base to abstract a proton from the compound one wishes to metalate.
what is the base in the metalation lab? what molecule is it abstracting a proton from?
n-butyllithium is abstracting a proton from diphenylmethane.
what is the specific metalation or acid/base reaction of interest in the metalation lab?
(C6H5)2CH2 + n-C4H9Li <----> (C6H5)2CHLi + C4H10
in which way does equilibrium lie in the metalation of diphenylmethane? Why?
far to the right, due to the fact that the carboanion intermediate is a much weaker base than the n-butylcarboanion. In other words C4H10 is much weaker acid than diphenylmethane
what two contaminants are metalatiopn reactions particulary sensitive too?
O2 and H2O
what electrophile is used in the metalation of diphenylmethane?
CO2 (dry ice)
what is the solvent used in the metalation of diphenylmethane?
what does THF stand for?
what causes the color change in the metalation lab?
the reaction which begins immediately to form a lithiodiphenylmethide (carbanion), which experiences considerable delocalization, is colored.
what solvent is used to cover the dry ice in the metalation lab?
what is the product in the metalation lab?
what is the equation for Ka?
Ka = [products]/[reactants]
how to find pKa
pKa = --logKa
what does Ka stand for?
it is the acid dissociation constant, it indicates the degree of dissociation in water
what does Kb stand for?
it is the basicity constant, it indicates the ability to accept a proton form water
relationship between Ka and Kb
pKa + pKb = 14
base vs acid?
base abstracts a proton, acid donates a proton
what are organometallic reagants sensitive too? what one did we use in the metalation lab?
oxygen and water. n-butyllithium (also a base)
other than diphenylacetic acid what else is formed in the metalation of diphenylmethane?
what does it mean to quench a reaction?
to go from active to neutral
what is Kf?
it is the carl fisher analysis, it measures water in salt
why is crystallization not a feasible method of separating cholesterol from human gallstones?
because the molecules in gallstones are so similar in solubilities.
which type of cholesterol is converted to cholesterol dibromide?
why can't 7-cholesten-3B-ol be converted to cholesterol dibromide? 3B-cholestanol?
to steric. no double bond (saturated).
how is the cholesterol dibromide debrominated?
with zinc dust.
what causes the dirty yellow appearance in gallstones?
it is a brown residue of the bile pigment, bilirubin, a metabolite of hemoglobin
the halogen addition to the alkene in cholesterol occurs in what way?
essentials of recrystallization?
dissolve compound in minimum amount of boiling solvent.
break up lumbs
add slight excess of boiling solvent
boil off excess solvent, making it saturated
remove saturated solution form heat
allow to cool at room temperature
whence crystals have formed place in ice bath
collect crystals via vacuum filtration
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