Lab exam Flashcards
What is extraction?
refers to the transference of compound(s) from a solid or liquid into a different solvent or phase
Liquid-liquid extraction
process that occurs in a separatory funnel. A solution containing dissolved components is placed in the funnel and an immiscible solvent is added, resulting in two layers that are shaken together
Compounds move from one liquid to another depending on their…?
relative solubility in each liquid
When allowed to equilibrate between two liquids in a separatory funnel,…?
the majority of a compound often ends up in the layer that it is more soluble
Why do we use extraction in a chemistry lab?
principal method for isolating compounds from plant materials
The solution with the lower density will rest ____ and the denser solution will rest on ___?
top, bottom
Most non-halogentaed organic solvents have densities less than __ so they will ____ an aqueous solution (if they are immiscible)?
1 g/mL, float atop
Halogenated solvents are ____ than water so will ___ an aqueous solution?
denser, sink below
What does an organic layer have to be treated with after exposure to water?
drying agent
What are drying agents and examples?
anhydrous inorganic materials that favorably form “hydrates”, which incorporate water molecules into their solid lattice structure.
Magnesium sulfate: fine, loose powder buy hydrate is clumpy and often clings to glass
Common use for gravity filtration
separating anhydrous magnesium sulfate from an organic solution that it has dried.
Pour mixture through quadrant folded filter paper and allow liquid to filter using only the force of gravity; rinse solid on filter paper with fresh solvent to remove residual compound adhering to solid
If unsure which layer is aqueous and which is organic?
Add a bit of water from a bottle to the separatory funnel and watch where the water goes (mix with aqueous layer)
Consider relative volumes of aqueous and organic solvents, based on quantities used in the experiment (10mL hexane and 100 mL water)
Partition coefficient: components are said to “partition” between the two layers
K= Molarity in organic phase/Molarity in aqueous phase
K is less than one= compound partitions into aqueous more than organic layer
What does the partition coefficient reflect?
solubility of the compound in the organic and aqueous layers, and so is dependent on the solvent system used.
What is multiple extraction?
quantity of solvent is used to extract one layer (aqueous) multiple times in succession. The extraction is repeated two to three times, or more if the compound has a low partition coefficient in the organic solvent. Organic layers are combined together, as the goal is to extract the compound into the organic solvent
First extraction: procedurally identical to a single extraction
Second extraction: aqueous layer from the first extraction is returned to the separatory funnel with the goal of extracting additional compound.
What is chromatography?
technique used to separate the components of a mixture. Can be used as an analytical technique to gain information about what is present in a mixture or as a purification technique to separate and collect the components of a mixture
In all chromatographic methods, a sample is first applied onto a stationary material that either __ or ___ the sample
absorbs or adsorbs
What is adsorption and absorption?
adsorption: when molecules or ions in a sample adhere to a surface
absorption: when the sample particles penetrate into the interior of another material
a sample is typically adsorbed onto a surface and can form a variety of intermolecular forces with this surface
What is the stationary phase?
the material the sample adsorbs onto and it retains the sample’s position
What is the mobile phase (TLC)?
moving material as it causes the sample to move from its original position
If a compound has strong intermolecular forces with the stationary phase it will?
remain adsorbed for a longer amount of time than a compound that has weaker intermolecular forces
What are the two commonly used stationary phases in chromatography?
silica and alumina (fine white powders)
TLC plates often have to be ____?
visualized- meaning something has to be done to the plate in order to temporarily see the compounds. Common methods are UV light or chemical stain
What is the retention factor?
report the results of a TLC plate, which quantitates a compounds movement
Rf= distance traveled by the compound/distance traveled by the solvent front
Ruler values should be recorded to nearest millimeter; let solvent run to approximately 0.5 cm of the top of the plate
Recording Rf
If spot changes appearance over time, initial appearance should be recorded
Solvent system and identity of what was spotted in each lane must be recorded
The equilibrium distribution between the two phases depends on?
strength of intermolecular forces between the sample and the stationary phase
Compounds that have oxygen or nitrogen atoms should be able to hydrogen bond with the stationary phase and have lower Rf
Depends on strength of interaction between sample and mobile phase (always less polar than stationary phase); polar (low Rf) compounds will tend to have a lesser affinity for the mobile phase than nonpolar compounds
Rf and functional groups TLC
a compound with lower Rf tends to have more polar functional groups than a compound with a higher Rf
An increase in solvent polarity increases Rf values for two reasons (TLC)
Moderately polar compounds have a greater attraction to the mobile phase
The polar solvent may occupy binding sites on the silica or alumina surface, such that they displace the sample from the stationary phase
Chemicals in spinach leaves in aqueous layer
vitamin C, oxalic acid, folic acid, potassium chloride, calcium, fructose
Which TLC procedure used the more polar solvent mixture?
The more polar solvent mixture was from the pen ink TLC
What is pipette filtration?
pipette filters are ideal as filter papers adsorb a significant amount of material; may be used if small amounts of solid are noticed in NMR or GC samples, as both instruments require analysis of liquids without suspended solids
Sodium salicylate is roughly 350 times more soluble in water than salicyclic acid due to its ___?
ionic character and it is rather insoluble in organic solvents such as diethyl ether
What would convert benzoic acid into its ionic carboxylate form?
wash with NaOH- would make it more soluble in the aqueous layer allowing for the sodium benzoate to be extracted into the aqueous layer.
What is suction filtration?
technique used for separating a solid-liquid mixture when the goal is to retain the solid.
Advantages: much faster than gravity filtration, more efficient at removing residual liquid, leading to purer solid
Disadvantages: force of suction may draw fine crystals through the filter paper pores
How should you rinse a suction-filtered solid?
the vacuum is removed and a small portion of cold solvent is poured over the solid
percent recovery
(100) (acquired/initial)
What is gas chromatography?
instrumental technique used to separate and analyze mixtures
A sample is diluted then injected into the instrument where its vaporized. Gaseous sample is pushed through a long, capillary column by inert carrier gas. Column separates components of a mixture, detects them, and a spectrum is generated displaying peaks that correspond to material that has exited column at certain times.
What is retention time?
quantity of time a compound spends inside a GC column before it is detected.
What is commonly used as a solvent for gas chromatography?
hexanes
What can be used to predict which peak represents which isomer?
boiling points
What is stationary phase in GC?
polymeric inner coating of column (high boiling point) and compounds can interact through types of IMFs
Mobile phase in GC?
carrier gas (helium) continually pushed through column; samples enter gas phase, adhere to column coating and establish an equilibrium between stationary and mobile phases. Compounds move into mobile phase if thermal energy from oven provides sufficient energy to overcome IMFs between compound and column coating
weak IMFs in GC
spend little time in stationary phase- shorter retention time
strong IMFs in GC
long time in stationary phase-long retention time
strength of interaction with column coating correlates?
closely with compounds boiling point- can be used to predict order of elution in GC
As carbon chain length increases…?
so does boiling point and retention times increase as well
