UCSB CHEM 6AL FINAL REVIEW EXAM QUESTIONS ANSWERED CORRECTLY LATEST UPDATE 2026

UCSB CHEM 6AL FINAL REVIEW EXAM QUESTIONS ANSWERED CORRECTLY LATEST UPDATE 2026 Intermolecular Forces and how they affect physical properties/purification techniques - Answers 1. Dipole-Dipole: polar covalent molecules such as aldehydes and ketones. positive and negative endings. polar molecules must be close together for force to be significant. weaker than ion-dipole forces. strength increases with the polarity of the molecule. 2. Electrostatic: occurs between charged species, cations, and anions, and is responsible for very high MP and BP of ionic compounds and metals 3. Hydrogen Bonding: hydrogen atom in a polar bond (NOF). considered a dipole-dipole interaction; quite polar 4. Ion-dipole: interaction between a charged ion and a polar molecule. cations are attached to the negative end of the dipole, and anions are attached to the positive end 5. London Dispersion Forces: only types of forces non-polar covalent molecules can experience. dependent on surface area and polarizability of the surface e of the molecule. results from the movement of electrons in the molecule, which generate temporary positive and negative regions in the molecule. Finding limiting reagents/percent yields/filling out reagent tables - Answers Limiting reagents: The compound that runs out first in a reaction. When the reagent runs out, reactions can no longer proceed and another compound is considered in excess. To determine the limiting reagent, convert the mass to moles using the molar mass of the compound. Then use stoichiometric ratios to see which is the smaller compound in moles. Percent Yield: Actual yield/theoretical yield multiplied by 100. If you expect 10g but only get a yield of 8.4g, the the percent yield is 84%. Get theoretical yield using molar ratio of the limiting reagent. Melting Point Ranges- How to measure them - Answers Using a capillary, take a small sample and heat it up using a melting point range apparatus and observe the solid as it turns into a liquid and record the given range right before it has fully dissolved. Melting Point Ranges- How impurities affect melting point range - Answers Solid's have an ordered crystal lattice structure. Heat makes this structure move enough to the point of liquidation. The MP represents the amount of energy needed to make the lattice structure move. Impurities disrupt the crystal structure, making the compound easier to melt. Therefore, the MP lowers and broadens due to the impurities. Ideal Liquids/Rault's Law - Answers Ideal Liquid: a liquid that is incompressible (density is constant), irrotational (no turbulence, flow is smooth), and non-viscous (fluid has no internal friction). These liquids do not exist in nature and obey Rault's law. Rault's law: Total pressure can be found by adding up partial pressure. Partial pressure can be found using this law. To find the mole fraction, divide the moles of compound A by the total moles of all compounds in the solution. Simple vs. Fractional Distillation - Answers Simple distillation: Used to purify almost already purified compounds, used to separate liquid from solid impurities, and removes volatile solvents from a given solution. There are few cycles. Boiling stones are placed in the liquid. Vapor travels up and condenses back down in a different tube. This distillation works well when there is a large difference in boiling points (60-70 degrees C) Fractional distillation: Fractionating columns increase theoretical plates (where vaporization/condensation can potentially occur) Comparison: Fractional distillation results in better separation but a little bit of liquid doesn't vaporize and total yield decreases due to the need for more cycles during fractionation. Vapor Pressure/Partial Pressure/Mole Ratios - Answers Azeotropes - Answers Azeotropes: a mixture of two liquids that has a constant boiling point and composition throughout distillation. IMF determines differences in boiling points. No longer acts as an ideal solution. Occurs when the liquid composition is equal to the vapor composition. EX: water and ethanol Chromatography - Answers Chromatography: separation of complex mixtures of compounds based on their polarity and distribution between a solid and mobile phase. There are 3 types of chromatography: 1. Liquid Chromatography (LC): column chromatography and thin-layer chromatography 2. High-Pressure LC (HPLC) 3. Gas Chromatography (GC) Solid/Mobile Phase - Answers Solid phase: a stationary composite through which the compound and the mobile phase interact. Mobile phase: Liquid or gas that carries compounds through the solid phase, AKA the solvent. A compound either interacts with the liquid or the mobile phase. A solvent (mobile phase) travels through a solid (solid phase) and carries the solute with it. The solvent with the dissolved solutes can be collected as they leave the column. This process is similar to the distillation process for vapors. Migration rates will differ depending on how the molecules absorb into the solid and liquid phases. Thin Layer Chromatography (TLC) - Answers A type of chromatography that uses silica gel or alumina on a card as the medium for the stationary phase. 1. Injection - load the sample onto the TLC plate - being the TLC spotting 2. Separation - run the solvent through the TLC plate - develop the TLC plate after a short time 3. Detection - Determine the chromatography results and measure the Rf values Migration Rates - Answers Determined by intermolecular interactions. Mobile phase has a slower migration rate because the solid phase is polar and the mobile phase is non-polar. The polar molecules have stronger interactions with the solid phase than the non-polar molecules, and a stronger interaction means slower movement. Monitoring TLC Reactions - Answers Compound spots move differently on TLC plates dues to the polarity of the compound. The solid phase is normally polar and polar compounds bind to the solid phase via H-bonding, van der Waal forces, and dipole-dipole interactions. Compounds move slower because they interact with things they like and are drawn to. Compounds move slower in non-polar solvents. Effect of Solvent Polarity - Answers ??Compound spots move differently on TLC plates dues to the polarity of the compound. The solid phase is normally polar and polar compounds bind to the solid phase via H-vonding, van der Waal forces, and dipole-dipole interactions. Compounds move slower because they interact with things they like and are drawn to. Compounds move slower in non-polar solvents. Common Errors during TLC - Answers 1. Having giant spots/blobs or spots have tails meaning the sample on the plate is too concentrated and the spotting was too heavy. 2. No compound on the TLC after developimng. This could be because the sample is not concentrated enough, the UV lamp is not set to short-wave, compounds are volatile, or you didn't use the right solvent. 3. Overlapping spots. This could be because of not choosing the correct solved, the sample was spotted too heavily, or it is too concentrated Extraction/Drying - Answers Extraction: removing one or more compounds from a solvent using higher solubility in another solvent. Drying: involves the removal of any residual solvent in the solution. Once the drying process is complete, the purified compound is typically stored in a dry, cool place to prevent any moisture or impurities from being absorbed. The purity of the final product can be verified using a variety of analytical techniques, such as thin-layer chromatography Partition Coefficients - Answers The ratio of solubilities in two immiscible solvents at equilibrium? How to determine how much you can extract? - Answers How does acid/base chemistry effect extraction? - Answers Acid/base chemistry is used to separate the organic compounds from one another based on their acid/base properties. For example, acidic compounds can be extracted using a basic solvent and vice versa. This is because acidic and basic solvents can protonate or deprotonate the target compounds, making them more soluble in the solvent. Additionally, acid/base chemistry can be used to modify the properties of the target compounds themselves. Emulsions - Answers Appears as a milky cloudy substance. You can tell when one layer stops and when one begins, unless an emulsion forms. To fix an emulsion, either give the funnel time to sit, adde brine, centrifugate, filter, or gently stir while releasing gas buildup within the funnel. How to dry organic solutions - Answers After using brine to get any remaining water out of the organic layer, you should use a drying agent, such as anhydrous sodium sulfate. This salt hydrate with H2O, absorbs it all up, isolating the liquid from the solid. Recrystallization/Sublimation - Answers Recrystallization: The process of dissolving a solute in a hot solvent which then precipitates as the solution cools. Sublimation: The process of DIRECTLY moving from the solid phase to a vapor phase (does not include melting) How to perform a recrystallization? - Answers 1. Add a minimum hot solvent to a solid compound to dissolve the crude product. Gently heat the solvent just below boiling point to obtain a saturated solution. Continually add small amounts of the solvent to the crude product. 2. Cool the solution slowly. This step is important because impurities want to come in during this stage and destroy the crystal lattice structure. Compounds reject impurities while cooling, thus leaving the impurities in the solution. 3. Collect crystal using vacuum filtration. Use filter paper, pour the solvent in slowly while washing over it with a cold solvent. 4. Let the crystal dry over a vacuum and weigh the pure product at the end. What to do if no crystal form? - Answers If crystal do not form, you can deploy a seed crystal that provides a nucleation site in the beaker from crystals to grow on. Also, you can scratch the edges of the flask, also known as etching, to help crystals begin to form. If no crystal form, you may have used the wrong solvent or too much of it was used, so boiling some off might induce crystallization. How to pick a solvent? - Answers When picking a solvent, you should experiment with some solvent until you fine one where the solute does not dissolve at room temperature but does when heated. This might take time initially, but results in a pure yield of crystals at the end if done correctly. IR spectroscopy - Answers connecting macro to nano. interaction of light with matter. allows us to ID molecules of a substance. Bends vs. Stretches - Answers Bends: change of the bond ANGLE Stretch: change of the bond LENGTH Factors that effect intensity and frequency - Answers Factors that influence intensity: 1. change in dipole moment -stronger dipole= more intense band (C=O C=C) - O-H N-H C-H The frequency (wavenumber) are affected by 1. bond strength: C triple bond C C = C C - C 2. mass: the heavier the atoms involved in the vibration, the lower the frequency of the peak - Solution: get a good, clean background when testing the IR sample and test the right amount of sample How to interpret an IR spectrum - Answers 1. Look at the double bond region ()(cm-1) -Aldehyde: 2 weak bands around -carboxylic acid: broadband around -ester: two strong bands around -if C=O is present and nothing else, likely a ketone 2. Look at the O-H, N-H region -Alcohols are represented as strong bands around -Amines are 1 or 2 medium bands around 3. Check the C-H bond region -sp= C-H bond around 3300 -sp2= C-H bond around -sp3= C-H bond around 4. Check the triple bond region () 5. Check for C-O single bond () - carboxylic acid around 1250 - carboxylic ester around - alcohol around 1050 Absorption bands to know - Answers 1. Double bond region - CO (1720)(cm-1) - Amides (1640) - Esters/Carboxylic Acids (1750) 2. Single-bond region -CO () - Alcohol/Ether (1050) - Ester (1250 or 1050) - Carboxylic Acid (1250) 3. O-H, N-H bonds (strong and broad) - Alcohol () - Carboxylic anhydride () - N-H tends to be less intense than O-H - N-H bend around 1600 (broad) - secondary amine () 4. C-C/ C-H - C triple bond C (2200) - C = C (1650) - C - C (1300-800) -sp- C - H at 3300 -sp2- C - H at -sp3- C - H at NMR spectroscopy - Answers A method of determining molecular structure that uses the relative position of carbons and hydrogens determined by the relative shielding and spins of electrons observed when a molecule is exposed to a magnetic field. More specific than IR; probes nuclei instead of bonds Number of Signals - Answers The number of signals tells us how many peaks are in the NMR. It tells us the number of sets of equivalent protons in the compound. By comparing the number of signals in the spectrum to the molecular formula and known chemical shifts for different functional groups, we can identify the different types of protons present in the molecule and determine their positions in the molecular structure. Chemical Shift/Shielding - Answers Chemical shift: the distance at peak from TMS/operating frequency of spectrometer. The greater the chemical shift, the higher the frequency. The chemical shifts often depends on the extent of hydrogen bonding. Electronegativity affects it. Carbon sub also affects it; the more hydrogens means the more upfield. Carbons closer to the least dense electron group are more downfield. Hydrogen near Cl will be more upfield than hydrogen near F. It also tells us about the nature of the chemical environments: alkyl, alkene, benzene, etc. Shielding Formula: Beffective=Bapplied=Bshielding