Assessment 2 Flashcards
Physical quantities
Grams, volume, etc
Chemical quantities
Moles and millimoles
Chemical calculation
A mathematical process of converting a given physical quantity to the required chemical quantity
Theoretical yield
Maximum quantity of product that can be obtained from a reaction. Theoretical yield is dependent on the (amount of) limiting reagent in the reaction flask
What is reaction (experimental) percent yield?
The percentage of product obtained after a reaction
How is experimental percent yield calculated?
Calculated by dividing the amount obtained after experiment by the theoretical yield and multiplying everything by 100%. Experimental moles are divided by theoretical moles
How are mole fractions calculated?
Calculated by dividing the number of moles of a component by the total number of moles in the entire mixture
Mass percentage
Calculated by dividing the mass of a component by the total mass of the mixture and multiplying everything by 100%
Percent recovery
Calculated by dividing the actual amount of compound obtained after an experiment by the expected amount of the compound. Everything is multiplied by 100.
Two variables are considered to be proportional if
They are related by a
constant (proportionality constant)
Direct proportionality
When ratio of two variables equals a constant
Indirect proportionality
When product of two variables equals a
constant
Chromatography
Separation technique where components are partitioned between a mobile phase (solvent) and a stationary phase (TLC plate) based on their partition (distribution) coefficients. The sample has to interact differently with the mobile and stationary phase in order for this technique to be effective.
TLC plates
Coated in a silica layer. A line is drawn on the bottom of the plate, and the sample is spotted using a capillary tube. Silicon dioxide contains hydroxyl groups on its surface and is therefore polar. They can interact with polar substances. A polar substances will interact with the silica (stationary phase). We say the component is strongly adsorbed to the stationary phase and is more polar. A nonpolar substance will be close to the solvent front.
Partition coefficient (Kd)
A property that describes how a chemical substance distributes itself between two
phases
Adsorption chromatography
Liquid-solid chromatography. Separation results from repeated sorption/desorption events as the components move through the stationary phase carried by the mobile phase
Partition chromatography
Gas-liquid and liquid-liquid chromatography
Stationary phase (liquid-solid chromatography)
A solid adsorbent, located on a solid support- the TLC plate in this case. Interacts with polar compounds
Mobile phase (liquid-solid chromatography)
A solvent or mixture of solvents. The compound/mixture is spotted at the bottom of the plate and mobile phase
carries the components through the adsorbent by capillary action
Retention factor (Rf)
Solute distance/solvent distance. Rf values are a characteristic of a compound in a solvent and can therefore be used to identify unknown compounds.
Which groups have a greater affinity for the adsorbent?
Polar functional groups- carboxylic acids have the strongest interactions and therefore travel a shorter distance and have smaller Rf values
Factors that influence Rf values (3)
- Polarity of organic compounds
- Polarity of developing solvents
- Nature of the adsorbent
How does the polarity of the solvent influence the Rf values?
A polar solvent will overcome the compound’s affinity for the adsorbent. The polar compound will be dislodged from the adsorbent and will be able to travel farther, resulting in a greater Rf value. Ethanol is the most polar solvent
How does the adsorbent influence Rf values?
Rf values depend on how strongly the adsorbent can hold on to compound spots. Silica and alumina are the most common adsorbents, with alumina being the strongest
Destructive TLC visualization methods
Involve a chemical reaction between the aid reagent and components spotted on the TLC-plate. Chemical stains are an example.
Non-destructive TLC visualization methods
Don’t involve a chemical reaction with the
components spotted on the TLC-plate- UV light is an example
Applications of TLC (4)
- Identification of unknown components in a mixture.
- Monitoring reaction progress – product(s) formation and reagent(s) depletion.
- Determination of purity of compounds and reagents in the lab.
- Test for appropriate solvent system for column chromatography separation
Stationary phase (gas chromatography)
Consist of a high-boiling point (non-volatile) liquid coated on a solid support contained within a column
Mobile phase (gas chromatography)
Unreactive (inert) carrier gas [He, N 2, Ar, etc.
Applications of gas chromatography (3)
- Determine number of unique components are in a mixture
- Identify of components in a mixture
- Separation of components – Microscale
Gas chromatography process
Use 1-2 microliters of a sample drawn up into a syringe. The syringe is injected into the injector port of a gas chromatograph, and the sample vaporizes. A stream of carrier gas sweeps the vaporized components through a heated column. As the components travel through the column, they are partitioned back and forth at different rates between the liquid and gas phases. A signal is transmitted to the recorder, and the computer generates a graph (chromatogram) with 2 peaks (one for each compound in the mixture). You can use the area under each peak to determine the percentage of each compound.
Retention time (tR)
The time a component spends in the column from injection point to the time it’s concentration at the detector reaches maximum. If instrumental parameters are kept constant, tR can be used for identification and characterization of compounds
How do the mobile and stationary phases determine the length of the retention time?
The more time a component spends in the gas (mobile) phase, the shorter its retention time. The more time a component spends in the liquid (stationary)
phase, the longer its retention time
Sample parameters that affect retention time (2)
- Volatility of component (boiling points) – determines the time a component spends in the gas phase
- Attractive forces between the component and the stationary phase – determines the time that a component spends in the liquid phase
Volatility of a compound
The lower a compound’s boiling point, the more volatile it is
Experimental (instrumental) parameters that affect retention time (4)
- Column Temperature
- Flow Rate of carrier gas
- Column’s length
- Polarity of stationary phase
Thermal conductivity detector (TCD)
GC detector- response depends on thermal conductivity of a given compound
Flame ionization detector (FID)
GC detector- degree and type of ionization varies with class of compound
Detector response factor
The ratio of signal produced by a component, and the quantity of component causing it. Used to obtain corrected peak areas that are proportional to the quantity of the component(s) in mass or moles
Distillation
Utilizes differences in boiling point to separate compounds. The boiling points should be different enough that the mixture can be heated to a point above the low boiling point and below the high boiling point. The lower boiling point compound becomes a vapor that can be collected. The higher boiling point compound stays in the flask. Involves a single vaporization/condensation cycle
Limiting reagent
The reactant producing the least product- lower amount of moles
How is theoretical yield calculated?
Convert the grams of the reactants/product s to moles. Convert the moles of the limiting reagent to grams using the molecular weight of the product (not the limiting reagent weight). Use this number for theoretical mass in calculating percent yield.
Dynamic equilibrium
The concentration of molecules in the vapor phase increase until the rate at which they reenter the liquid phase equals the rate of escape from the liquid phase. Occurs in a closed beaker
Equilibrium vapor pressure
The molecules in the vapor phase exert pressure on the walls of the flask and on the liquid’s surface
How is equilibrium vapor pressure related to temperature in a closed beaker?
Equilibrium vapor pressure increases as temperature increases
Dalton’s law
states that the total pressure in a closed system is found by adding the partial pressures of each component of the gas. In an open system (beaker), total pressure equals air pressure plus equilibrium vapor pressure at a specific temperature
Boiling point
Liquid boils when its vapor pressure equals the external (total) pressure. For an open system, total pressure equals barometric pressure
A pure liquid will boil at a constant temperature if
The external pressure remains constant
As a liquid gets closer to its boiling point, equilibrium vapor pressure
Increases
In a closed system, how is pressure calculated when there is an equal volume of compound A and compound B?
Pressure= PA+PB
Raoult’s law
States that a compound’s vapor pressure is proportional to its molar composition, and that the vapor pressure of a compound can be decreased when it’s in a solution. This applies to a liquid mixture
Distilland
The liquid mixture to be distilled
Distillate
The liquid from condensed vapors in the second flask
In a condensor, where does water enter?
In the sidearm closest to the collection flask
What do the 2 lines in the graph of simple distillation represent?
The top curve represents vapor composition, the bottom represents liquid composition
Fractional distillation
Involves multiple vaporization/condensation cycles. Used a packed column (contains the thermometer) to allow for effective fractionation of components
Column efficiency
The efficiency of a particular kind of column is given by its height equivalent to a theoretical plate (HETP) value. The lower the HETP the more efficient is the fractionating column. Height of column divided by number of plates
HETP formula
Height of column/number of theoretical plates
Fractional distillation graph
Contains 2 more vertical curves- the fractional distillation curve is above the simple distillation curve at the end of the graph.
In the polarity lab, how was polarity determined?
The red and blue areas on the electrostatic potential maps indicated areas of partial negative or partial positive charge. Compounds that contained these partial charges had some degree of polarity due to their separation of charge. The colors on the electrostatic potential diagrams indicated the electron density in each region of the molecule, with red regions indicating a partial negative charge and blue regions indicating a partial positive charge. The electrical energy, dipole moments, and direction of the dipole of the molecules were also measured.
Which molecules made a compound more polar?
The sulfur atom was generally more positive, and the carbon atoms were more negative. The oxygen atoms were also negative, as they were red in color on the electrostatic potential diagrams. Oxidation appears to make sulfur more positively charged.
How does symmetry impact polarity?
Symmetry also impacts the polarity of a molecule. In a symmetrical molecule the dipole moments will cancel out, making the molecule nonpolar
Why is determining the polarity of a compound important?
Polarity can be used to determine how a compound will interact with another compound, or if a compound will dissolve in a specific solvent. Partial positive regions of one molecule would be attracted to partial negative regions of another molecule, which would influence the reaction mechanism.
Thin layer chromatography
Rapid analytical technique used for identification of compounds and/or monitoring reaction progress, like with liquid-solid chromatography. Used for qualitative analysis
With gas chromatography, how percent composition of a sample determined?
The area under the peak is used to determine percent composition of the sample. Multiply height and width of the peak, then divide that area by the total area of all the peaks
For TLC, which compounds elute first?
Less polar compounds
Distillation principle
All compounds have a characteristic vapor pressure, which is the pressure a compound exerts against the external, or atmospheric, pressure. A larger vapor pressure indicates a greater tendency of the compound to exit the liquid phase and enter the gas phase, and the liquid will begin to boil once the vapor pressure is equal to the external pressure. When a solution is in dynamic equilibrium, the amount of condensation that is occurring takes place at an equal rate to the vaporization process. The vapor that is produced in distillation depends on the vapor pressure and quantity of the compounds.
Why does vapor pressure increase as temperature increases?
When the temperature of the compound increases during heating, the molecules gain energy and are able to overcome intermolecular forces to enter the gas phase. Therefore, vapor pressure increases with temperature. The solution will begin to boil once the vapor pressure has increased to equal the atmospheric pressure
Why does the boiling temperature of an impure liquid change during the distillation experiment?
Due to the changing composition of the liquid. During distillation, the compound with the lower boiling point is the first to be removed from the mixture, resulting in an increasing proportion of the compound with the higher boiling point that is left in the flask. Therefore, the first fraction of the distillate will contain the greatest quantity of the compound with the lower boiling point
