chem 241 lab exam
What is a coordination compound?
A compound with a metal center bonded to ligands.
What is a ligand?
A molecule or ion that donates a lone pair to a metal center.
What is the coordination number?
The number of donor atoms directly attached to the metal.
Do charge-balancing ions usually count toward coordination number?
No, only directly bonded donor atoms count.
What is a monodentate ligand?
A ligand that binds through one donor atom.
What is a bidentate ligand?
A ligand that binds through two donor atoms.
Why are bidentate ligands often more stabilizing than monodentate ligands?
Because of the chelate effect.
What is the chelate effect?
The extra stability gained when one ligand binds a metal in multiple places.
What does a bridging ligand do?
It connects two metal centers.
Why is acetate a useful ligand example?
Because it can bind in more than one way: monodentate, bidentate, or bridging.
Why was water needed in the copper acetate synthesis?
Enough water had to be present for the reaction to occur.
Why was too much water bad in the copper acetate synthesis?
Because the product is more soluble in water, so too much water lowers crystallization yield.
Why was concentrated acetic acid useful in the copper acetate synthesis?
Because it helps keep the product less soluble so it can crystallize.
Why was recrystallization unnecessary in the copper acetate synthesis?
Because the product crystallized directly from the reaction mixture in good purity.
Why are some reactions heated?
To increase rate, dissolve reagents, or allow a transformation such as reflux or decomposition.
Why are some reactions cooled?
To control exothermic reactions, reduce side reactions, or promote crystallization.
Why is slow addition sometimes important?
To control heat release, concentration, gas evolution, or reactivity.
Why would a reaction mixture be stirred continuously?
To keep it homogeneous, improve heat transfer, and ensure reagents contact each other.
Why is a reaction sometimes allowed to crystallize after cooling?
Because many solids are less soluble at lower temperature.
What is the general purpose of a wash solvent in product isolation?
To remove impurities while dissolving as little product as possible.
When is gravity filtration preferred?
When you want to remove insoluble impurities from a solution and keep the filtrate.
When is suction filtration preferred?
When you want to collect a solid product quickly.
Why can adding a miscible nonsolvent cause crystallization?
Because the added miscible nonsolvent makes the product less soluble in the solvent mixture, so the dissolved product comes out of solution as crystals instead of staying dissolved.
Why can adding ethanol to an aqueous mixture sometimes cause contamination rather than purification?
Because a salt impurity may also become insoluble and co-precipitate.
What is co-precipitation?
When an impurity precipitates along with the desired product.
If a percent yield is above 100%, what should you suspect first?
Impurities, retained solvent, incomplete drying, or weighing error.
What does an absorption maximum, λmax, represent?
The wavelength where light absorption is strongest.
What is the relationship between wavelength and energy?
They are inversely related; longer wavelength means lower energy.
Why does the observed colour not always perfectly match the colour wheel prediction?
Because absorption bands are broad, concentrations vary, and more than one wavelength region may be absorbed.
Why can a compound still appear coloured if λmax is near or just outside the visible region?
Because the absorption tail may extend into the visible.
Why are broad bands important when predicting observed colour?
Because they can absorb a range of colours, giving mixed or darker shades.
What should you do if a UV-Vis spectrum shows a shoulder rather than a sharp peak?
Recognize it as a partially resolved band and still use it as evidence if appropriate.
What is a shoulder in a spectrum?
A smaller bump attached to the side of a larger peak.
For the Co(III) experiment, which band was most useful for comparison?
The longer wavelength, lower energy d-d band.
Why was the longer wavelength Co(III) band emphasized?
Because the shorter-wavelength band was often hard to see clearly. It could be partly hidden by other absorptions and show up only as a shoulder, so it was less reliable for comparison. The longer-wavelength band was usually clearer, so it was the best one to use when comparing complexes.
Why does a shorter wavelength mean the energy gap is larger?
Because wavelength and energy are inversely related:
ΔE = hc/λ
So if λ gets smaller, ΔE gets bigger.
That means absorbing shorter-wavelength light requires more energy, so the gap between the ground state and excited state is larger.
How does ligand identity affect Co(III) UV-Vis spectra?
Different ligands change how strongly they interact with the metal, which changes the splitting of the metal d orbitals. If the splitting gets bigger, higher-energy light is needed, so λmax moves to a shorter wavelength. If the splitting gets smaller, lower-energy light is enough, so λmax moves to a longer wavelength.
What does a peak or shoulder near 511 nm indicate in the cobalt experiment?
Possible unreacted Co2+ starting material.
What two major types of vibrations appear in IR?
Stretching and bending.
What condition must be met for a vibration to be IR active?
It must produce a change in dipole moment. IR light is absorbed only if the vibration lets the molecule interact with the oscillating electric field of the radiation. That interaction happens when the vibration changes the dipole moment. No dipole change means no IR absorption.
Why are homonuclear diatomics IR inactive?
Because the two atoms are identical, so the electrons are shared equally. There is no bond dipole to begin with, and when the bond stretches, it still stays symmetrical. So the dipole moment remains zero the whole time.
Why is CO2 a good example of selective IR activity?
In the symmetric stretch, both C=O bonds lengthen or shorten equally, so the molecule stays symmetric and the dipole moment does not change. Because those motions make the electron distribution uneven, so the dipole moment changes during the vibration. That makes them IR active.
What does Hooke’s law help explain in IR?
Hooke’s law says vibration frequency depends mainly on:
the bond strength, and
the reduced mass of the two atoms
A stronger bond vibrates faster, so it absorbs at higher frequency.
A smaller reduced mass also gives a higher frequency.
Why do stronger bonds absorb at higher frequency?
Because a stronger bond acts like a stiffer spring. A stiffer spring vibrates faster, so it has a higher frequency.
Why does smaller reduced mass give higher frequency?
Because lighter atoms are easier to move, so the bond vibrates faster.
Why do C–H stretches occur at relatively high frequency?
Because hydrogen is very light, so the reduced mass of a C–H bond is small. Small reduced mass gives high frequency.
Why is assigning every peak in an IR spectrum usually unrealistic?
Because large molecules have many bonds and many possible vibrations. Those bands can overlap, and you can also get combination bands and overtones, so it becomes hard to assign every peak exactly.
What is the fingerprint region?
The region below about 1500 cm−1.
What does an off-scale IR band usually suggest?
The sample is too thick or too concentrated.
How would you fix an off-scale IR spectrum of a film sample?
Use a thinner film or reduce the amount of sample on the plate.
What is a terminal B–H bond?
A B–H bond attached to one boron only.
What is a bridging B–H bond?
A hydrogen shared between two boron centers.
Where do terminal B–H stretches usually appear?
Roughly 2650–2250 cm−1.
Where do bridging B–H stretches usually appear?
Roughly 2200–1500 cm−1.
Why are bridging B–H frequencies lower than terminal B–H frequencies?
Because bridging bonds are weaker and lower in bond order. A terminal B–H bond is a more normal covalent bond between two atoms. A bridging B–H bond shares electrons across three atoms, so the bonding is spread out more and each individual B–H interaction is weaker. Weaker bonds vibrate more slowly. Slower vibration means lower frequency, so bridging B–H stretches appear at lower wavenumber than terminal B–H stretches.
Why is diborane called electron deficient?
Because ordinary 2-center, 2-electron bonds are not enough to describe all its bonding.
What bonding model explains the bridges in diborane?
3-center, 2-electron bonding.
What is luminescence?
Emission of light from a substance after it has been electronically excited, without requiring the substance to be hot.
What is photoluminescence?
Luminescence caused by absorption of light; the substance absorbs photons, becomes excited, then emits light as it relaxes.
What is electroluminescence?
Luminescence caused by an electric current or applied voltage; electrical energy excites the substance, which then emits light.
What is chemiluminescence?
Luminescence caused by a chemical reaction; reaction energy creates an excited product or intermediate that emits light as it relaxes.
Why is [Ru(bipy)3]2+ luminescent?
It absorbs light. A photon promotes an electron from a lower-energy state to a higher-energy excited state and the excited state can relax by emitting a photon.
What kind of transition is most important for [Ru(bipy)3]2+ luminescence?
A charge-transfer transition. An electron is promoted from a metal-based orbital to a ligand-based orbital.
Why are [Ru(bipy)3]2+ compounds useful as photosensitizers?
A photosensitizer is a compound that absorbs light and uses that energy to enter an excited state that can transfer energy or electrons to drive a chemical reaction. [Ru(bipy)3]2+ absorbs light efficiently, forms a relatively long-lived excited state, and that excited state is much more reactive than the ground state, so it can transfer electrons or energy to other molecules and promote photochemical reactions.
What is the difference between the absorbed and emitted wavelengths in a luminescence experiment?
Emission occurs at a longer wavelength than excitation.
Why is emitted light lower in energy than absorbed light?
Some energy is lost nonradiatively before emission.
If a compound emits around 633 nm, what general colour would you expect?
Orange-red.
If a compound absorbs mainly in the UV, how may it appear in natural light?
Nearly colourless or only weakly coloured. Because natural light looks colored only when a compound absorbs light in the visible region. If it absorbs mainly in the UV, that absorption is invisible to our eyes, so little or no visible light is removed and the compound appears colorless.
How do you predict a compound’s glow under UV light?
Use the emission maximum, not the absorption maximum. Because the glow you see is the light the compound emits after excitation, not the light it absorbs. The emission maximum tells you the wavelength and color of that emitted light, so it predicts the glow under UV.
How do you predict its appearance in ordinary light?
Use the absorption spectrum in the visible region.
Why might a bright luminescent compound still look pale in room light?
Because appearance in room light depends on absorption, not emission. Because in room light you usually see the wavelengths of visible light that the compound reflects or transmits after absorbing some of the incoming light. That observed color is controlled mainly by which visible wavelengths are absorbed. Emission only matters if the compound is actively glowing, which most compounds are not under ordinary room light.
What is an inert atmosphere used for?
An inert atmosphere is a nonreactive gas environment used to keep air- and moisture-sensitive chemicals from reacting with oxygen or water and decomposing
Why is oxygen often harmful in organometallic or air-sensitive synthesis?
It can oxidize sensitive compounds or reagents.
Why must glassware be dry in moisture-sensitive synthesis?
Residual water can react with reagents or shift equilibria.
Why is nitrogen used in these experiments?
Nitrogen is used because it is mostly nonreactive and displaces air, protecting air-sensitive reagents or products from reacting with oxygen and moisture.
What is the purpose of a mineral oil bubbler?
To show gas flow and allow pressure relief while maintaining an inert atmosphere.
Why is a Schlenk line more powerful than simple nitrogen purging?
It allows both inert gas handling and vacuum for more air-sensitive work.
Why is syringe transfer useful in air-sensitive chemistry?
It moves solutions without exposing them to air.
Why is a condenser used during reflux?
To return evaporated solvent to the reaction flask.
What is the purpose of an addition funnel?
To add a reagent slowly and in a controlled way.
What is the function of the distillate collection flask on a rotovap?
It collects condensed solvent.
What is the function of the bump trap on a rotovap?
It prevents splashed or bumped liquid from contaminating the condenser and collection flask.
Why does rotating the flask on a rotovap speed evaporation?
It creates a thin film with high surface area.
That helps evaporation because:
more liquid is exposed to air/vacuum at once
the solvent does not stay pooled in one thick blob
molecules can escape the surface more easily
Why does rotating the flask reduce bumping?
It spreads the liquid and prevents localized superheating.
What is the first thing to do if a rotovap sample foams violently?
Release the vacuum.
Why does violent foaming happen on a rotovap?
The pressure is too low too quickly or the bath is too hot.
How do you prevent rotovap foaming?
Lower the bath temperature and apply vacuum gradually.
What does reflux mean?
Heating a reaction at the solvent’s boiling point while condensing vapour back into the flask.
Why is reflux useful?
It heats strongly without losing solvent volume.
Why should disposable gloves not be worn with liquid nitrogen or dry ice?
They can trap cryogenic liquid against the skin and worsen injury.
What is the general formula of a Grignard reagent?
RMgX.
What is the most important practical feature of a Grignard reagent?
A Grignard reagent is extremely reactive toward water and other protic substances, so even small amounts of moisture can destroy it before it does the desired reaction.
What happens when a Grignard reagent reacts with water?
It is quenched and converted into RH, so the Grignard is destroyed instead of reacting with the intended substrate.
Why must THF used with Grignard reagents be very dry?
Because even trace water in THF can react with and destroy the Grignard reagent, lowering yield or stopping the reaction.
Why is THF a common solvent for Grignard chemistry?
Because THF is polar enough to keep the Grignard reagent solvated and its oxygen can coordinate to Mg2+, helping stabilize the reagent in solution.
Why is adding a huge excess of Grignard reagent not a good general way to remove water contamination?
Because the excess Grignard reacts with water to form unwanted byproducts, which can complicate purification and reduce the amount of product that can be isolated. especially for heavier aryl systems.
Why was excess Grignard reagent destroyed with ammonium chloride solution in Experiment 6?
Because ammonium chloride quenches the remaining Grignard in a controlled, mildly acidic way, stopping the reaction safely without using a harsh acid.
Why was ammonium chloride preferred over strong acid for the workup?
It is milder and helps avoid formation of contaminating insoluble magnesium hydroxides.
Why should the workup solution remain slightly acidic when quenching a Grignard reagent?
To prevent Mg(OH)2 formation.
Why can chloride in the wash solution be helpful in the Grignard workup?
Because chloride helps keep the nickel complex as the chloride product and reduces unwanted exchange with other anions present during workup