Reading...
Play button
Play button
Use LEFT and RIGHT arrow keys to navigate between flashcards;
Use UP and DOWN arrow keys to flip the card;
H to show hint;
A reads text to speech;
69 Cards in this Set
- Front
- Back
|
Give an example of an extensive property in chemistry... Do they depend upon the amount of the substance being examined?
|
mass and volume... YES, extensive properties can change based on the amount examined
|
|
|
Give an example of an intensive property in chemistry... Do they depend upon the amount of the substance being examined?
|
Physical properties are independent of the amount of the substance being examined. Intensive properties include things such as melting point, boiling point, and density.
|
|
|
Define mass
|
The measurement of the amount of matter in an object
|
|
|
Define volume
|
The amount of space that an object occupies
|
|
|
How is the volume of a cube calculated?
|
Calculated by determining the product of length, width, and height (l x w x h)
|
|
|
How is the volume of a cylinder calculated?
|
Calculated by multiplying pi () times the radius squared times the height
|
|
|
How would one determine the volume of an irregularly shaped item?
|
Displacement - the irregularly shaped solid is
submerged in a known volume of liquid, and the change in liquid’s volume is equal to the volume of the solid. |
|
|
Define density
|
The relationship between mass and volume (d = mass/volume).
|
|
|
How can temperature affect density?
|
Typically as something is warmed its density decreases because the volume gets larger. Conversely, as something is cooled its density typically increases because the volume gets smaller.
|
|
|
Define melting or freezing point?
|
The temperature at which equilibrium is
reached between the solid phase and the liquid phase. In practice melting begins at the first sign of liquid formation and ends when the entire sample turns into a clear liquid. |
|
|
What is melting point used for?
|
Melting points are often used to determine the purity and/or the identity of a compound in the laboratory.
|
|
|
Define the characteristics of the melting point of a pure substance
|
Pure substances will have sharp melting point ranges, meaning they will have melting point ranges of 1-2 oC.
|
|
|
Define the characteristics of the melting point of an impure substance
|
Impurities will interfere with the intermolecular forces and cause the melting point to be lower than that of the pure substance. This will also cause it to have a range instead of one fixed melting point.
"Lower and wider range" |
|
|
Define eutectic point
|
The lowest possible melting point temperature that any combination of A and B can have is known
as the eutectic point. |
|
|
What does it mean if the melting point of a mixture is lower than the reference melting point and it melts
over a broad range? |
This tells you that your unknown is not the same as the reference
compound. |
|
|
What does it mean if the melting point of the mixture is the same temperature, and the same range, as that of your unknown.
|
This strongly indicates that the unknown sample is the same as the authentic sample.
|
|
|
What does it mean if the melting point is lower but has a sharp range?
|
This tells you that you may have inadvertently made your mixture at the eutectic point.
|
|
|
An unknown crystalline substance has a sharp melting point. Is this a pure substance?
|
Based on the lab, pure substances have a sharp melting point with a small range of temperatures. The unknown substance is a pure substance.
|
|
|
What is a possible complication of the use of mixed melting points for checking the identity of compounds with the same melting point?
|
Using a compound that shares the same melting point with many other compounds would make it difficult to narrow the identity down.
|
|
|
List at least three sources of experimental error when a melting point is determined by a capillary-tube technique. Suggest how you might avoid or minimize these errors.
|
-If solids used are wet, the melting point can be affected
-Melting points can be wrong if the compound was not mixed enough -If the mixture isn't mixed enough, air bubbles could prevent heat transfer |
|
|
You suspect an unknown sample is one of the followig compounds based on melting point
determination: urea (mp 133°C), 2,4-dimethylacetanilide (mp 133°C), phenacetin (134°C) or 3-chlorobenzamide (mp 134°C). A list of the mixture melting points of the unknown sample with each of the possible compounds is in the following table. Identify the unknown and briefly explain your reasoning. Unknown compound + m.p. range (°C) Urea 120-130 2,4-dimethylacetanilide 105 – 120 Phenacetin 133 – 134 3-chlorobenzamide 115 – 128 |
The unknown would most likely be pheacetin since when it was mixed with the known Phenacetin it had the same melting point as the published value. Since the melting point of the mixture was lower then all of the other values I know that the sample is not the same as the pure substance used in each test.
|
|
|
Three samples named X, Y and Z have identical melting points, 148-149°C. When a 2:1
mixture of X and Y is prepared, its melting point is 105-130°C. The melting point of a 1:1 mixture of Y and Z is 148-149°C. Discuss the identity of X, Y and Z and predict the melting point range of a 2:1 mixture of X and Z. |
Since Y & Z shared a melting point identical to the published values they are most likely the same substance. If this were the case, if X and Z were also mixed in a 2:1 ration the result should also be in the same range.
|
|
|
Define recrystallization
|
Recrystallization is a technique used to remove impurities from a compound.
|
|
|
Define solubility
|
Solubility is defined as the amount of a solute that will dissolve in a solvent and is most commonly represented as grams of solute per 100 grams of solvent. the solubility of most non-volitile
solids and liquids increases with increasing temperature. Also, impurities typically have different solubilities than that of the compound being purified. |
|
|
Define the process of recrystallization
|
1st the compound must be dissolved in a near boiling, solvent, followed by a rapid filtration to remove any insoluble impurities. Once the filtration is complete the solution is allowed to cool slowly. Vacuum filtration, is then used to separate the crystals from the mother liquor (the remaining solution after recrystallization). After separation the crystals should be washed with a small amount of cold solvent.
|
|
|
What are some characteristics of a good solvent used in recrystallization
|
-non-toxic and readily obtainable at low cost
-the solubility of the compound should be low at low temperatures and higher at high temperatures -The impurities should either be insoluble in the solvent at all temperatures or highly soluble at low temperatures -somewhat volatile so that it can be easily removed once recrystallization is complete |
|
|
How can you ensure a good recovery of the pure substance via recrystallization?
|
Avoid using too large a volume of solvent. Dissolving the sample to be purified in the smallest possible amount of hot solvent minimizes the amount of material lost by retention in the mother liquor
|
|
|
How can colored impurities be removed during recrystallization
|
Colored impurities can be removed by adding an adsorbent such as Norit® charcoal.
|
|
|
Once the filtration is complete, why is it important for the solution to be allowed to cool slowly?
|
If the solution is cooled too fast the crystals will form too rapidly. This can lead to entrapment of the soluble impurities in the crystals and/or the desired product can “oil out”.
|
|
|
If crystals do not firm on their own, how can you get them to initiate?
|
Crystallization can be initiated by scratching the walls of the vessel beneath the surface of the solution with a stirring rod or spatula.
|
|
|
Why is vacuum filtration not appropriate for compounds with low boiling points such as ethers?
|
The solvent evaporates under the vacuum and
the dissolved solids precipitate, clogging up the pores of the filter paper and sticking to the insides of the funnel stem. |
|
|
Sample impurities can come in two different types: soluble and insoluble impurities.
Recrytallization can be used to separate both types. In your own words explain how each can be removed using this technique. a. Soluble Impurities – b. Insoluble Impurities – |
Recrystallization can be used to separate soluble impurities by choosing a solvent that has high solubility in higher temps. By creating a mixture that includes the solvent it is brought to a near boiling temp and allowed to cool slowly. As it cools the solubility of the compound decreases and becomes saturated & crystals form. Since the impurities are in smaller amounts they are not saturated and remain in solution even after being cooled.
Insoluble - By using filtration after the compound is dissolved in a solvent. The insoluble purities will be left behind on filter paper. Norit charcoal can be used to get rid of colored impurities. |
|
|
During recrystallization it is sometimes necessary to filter the mother liquor to remove
insoluble impurities. Explain why the filter setup should be preheated. |
The filter set up should be preheated to avoid premature crystallization. If the crystals form too quickly they may trap impurities within them which would produce a contaminated product.
|
|
|
If it is necessary to filter the mother liquor to remove impurities gravity filtration is utilized.
Why would it be a potential problem to filter the hot saturated solution using vacuum filtration? |
Attempting to filter the hot saturated solution using vacuum filtration could cause it too evaporate quickly resulting in unintentional evaporation and instant crystals that contain impurities and could clog the funnel.
|
|
|
A reaction performed in the lab yields a crystalline solid that is red in color. After verifying
that the compound should indeed be red in color you proceed to recrystallize the crude sample. When you dissolve the compound in the appropriate solvent the solution turns red in color. Do you need to use Norit® to decolorize the solution? Explain your answer. |
Since I verified that the crystalline solid was supposed to be red and I have take the steps to purify it, Norit would not be required. The Norit is only used to remove impurities. If my sample is pure, the color would remain even after being dissolved in a solvent.
|
|
|
Define Extraction
|
A common technique used to separate a mixture into its individual components based on solubility.
|
|
|
Name two types of extractions
|
-solid – liquid
-liquid – liquid |
|
|
Give an example of a solid - liquid extraction
|
Making coffee or tea is a solid – liquid extraction where the caffeine and other water soluble components are removed with hot water. The insoluble components are left behind on the filter.
|
|
|
Give an example of how a liquid - liquid extraction is used
|
Liquid – liquid extractions are used to transfer a solute from one solvent into another.
|
|
|
Name some important characteristics of a liquid - liquid extraction
|
In order to perform this technique the solute must be more soluble in one of the solvents than the other, and the two solvents must not be miscible, or soluble, with one another.
|
|
|
Define distribution coefficient, KD.
|
The distribution coefficient is usually expressed as the ratio of the solubility of the compound in the organic phase (g/100 mL) to the solubility of the compound in water (g/100 mL). A larger value for KD indicates that the extraction is more efficient.
|
|
|
How can Extraction be used to separate acidic, basic, and neutral organic compounds?
|
A neutral organic acid (-CO2H) or base (-NH2) will usually be more soluble in the organic solvent.
However, the salt of either (-CO2- or -NH3+) will usually be more soluble in water because of their ionic nature (like dissolves like). Therefore, an acid can be separated from a neutral compound by extracting the organic solution containing both compounds with an aqueous solution of base, typically sodium or potassium hydroxide or sodium bicarbonate. This will convert the acid in the solution into the salt of its conjugate base. Because these salts are extremely polar they will be transferred into the aqueous layer while the neutral compound will remain in the organic layer. The organic and aqueous layers can be separated and the aqueous layer acidified to convert the salt back into the original neutral organic compound, making it relatively insoluble in water. This will cause the compound to precipitate or oil out of solution. |
|
|
Explain why acetic acid can be extracted from an organic solvent using aqueous sodium
hydroxide. Write chemical equations to describe the process? |
As the acetic acid goes from being non-polar and is depronated it turns into its conjugate base. The molecule is no longer neutral, the charge makes it very polar. This converts the solution into the salt of the conjugate base. The polar nature will then cause the salts to transfer to the aqueous layer while the neutral compound will remain in the organic layer. Having two distinct layers makes it possible to perform an extraction.
|
|
|
Define chromatography?
|
chromatography is a versatile separation technique.
|
|
|
List the different types of chromatography
|
There are several different types of
chromatography including thin layer, column, gas-liquid, ion exchange, paper, and high pressure liquid chromatography. |
|
|
Name the two types of chromatography
|
In any type of chromatographic separation, there are two phases, the mobile phase which is a liquid or gas and the stationary phase which is a solid. The components that have a higher affinity for the mobile phase will move along the stationary phase at a faster rate.
|
|
|
What affects the affinity of the components in any mixture for either phase (mobile or stationary)?
|
The affinity of the components in any mixture for either phase is determined by the structural
features, intermolecular forces, and physical properties of each component. |
|
|
What is the retention factor (used in chromatography)?
|
Typical quantitative comparisons are made with a value known as the retention factor, or Rf. The Rf is simply a ratio of how far a component in a mixture travels compared to how far the solvent travels (often referred to as the solvent front). The general equation for Rf is:
Rf = distance traveled by compound of interest distance traveled by the solvent |
|
|
Define mobile phase
|
The portion that carries the compounds through the stationary phase. (Liquid)
|
|
|
Define stationary phase
|
A solid which interacts with molecules that are passing through it.
|
|
|
Define Solvent front
|
The wet moving edge of the solvent that progresses along the surface where the separation of the mixture is occuring.
|
|
|
Why was a pencil, and not a pen, used to make the original line on the chromatography
paper? Explain your answer. |
Pencil lead is made of graphite and will not separate with the solvent since its inorganic.
|
|
|
When the chromatography paper is placed in the developing chamber it is imperative that the
solvent does not submerge the spots. Explain why this is important. |
It is important that the solvent does not submerge the spots because if it does, the pigments may be absorbed into the solvent instead of traveling the stationary phase.
|
|
|
A paper TLC experiment is performed on a generic analgesic (pain reliever) purchased from
a local pharmacy. Five distinct Rf values can be measured after the separation is complete. How many different compounds are present in the tablet? Explain your answer. |
Five distinct Rf values would indicate the presence of at least five different compounds.
|
|
|
Two components of a mixture have the same Rf value. The spectrographs for each are shown
below (wavelength vs absorbance). Are the compounds the same? Explain your answer. |
Since each compound has a different absorbency and wavelength this would indicate that the compounds are not the same. However, the two compounds are most likely similar i polarity which would result in similar Rf values.
|
|
|
Name the principle method for separating and purifying liquids?
|
Distillation
|
|
|
Describe the process of distillation
|
This process involves heating a liquid until it vaporizes and then cooling the vapor in such a way that it condenses back into a liquid to be collected in a separate container.
|
|
|
Name the four types of distillation processes
|
Simple, fractional, steam and vacuum.
|
|
|
Describe the process of simple distillation
|
-Liquid is placed in a distillation flask
-It is heated until it approaches boiling point -As the liquid heats, vapors begin to collect inside the flask. -The vapors continue to rise through the distillation head past the thermometer and then into the condenser. -The condenser has cool water passing through the outer jacket which causes the vapor to condense and then be collected in a receiving flask. |
|
|
Define a forerun
|
Any liquids present that boil at a temperature below that of the desired liquid, can be distilled
off and collected and then discarded. |
|
|
What is the purpose of greasing the ground glass joints in distillation
|
To prevent sticking and so that you can take the pieces apart easily.
|
|
|
What is the purpose of using the rubber bands in distillation
|
The rubber bands hold every thing in place.
|
|
|
What is the purpose of properly placing the thermometer in distillation
|
If the thermometer is not properly placed the actual temperature of the reading could be inaccurate.
|
|
|
What is the purpose of using a stirring bar in distillation
|
The stirring bar ensures that the mixture is evenly stirred and that heat pockets don't form.
|
|
|
What is the purpose of using cool water in the condensor in distillation
|
The cool water cools the hot vapors down so that condensation can occur. This turns the gas into a liquid agin.
|
|
|
What is the purpose of running water against gravity in distillation
|
Running the water against gravity ensures that the tubing is completely covered so that condensation can occur.
|
|
|
Why is it dangerous to heat an organic compound in a distillation apparatus that is tightly closed at every joint and has no vent or opening to the atmosphere?
|
No venting would allow vapors to build up in pressure which could cause the glass apparatus to explode.
|
|
|
What would you predict would happen if you leave gaps at the joints in your apparatus?
|
Gaps left at the joints would result in vapors escaping which would ultimately lead to a smaller amount of the pure substance being collected.
|
|
|
A student wanted to separate a mixture of hexane (b.p. = 69 oC), 1-pentanol (b.p. = 137.5
o C), and acetic acid (b.p. = 118 oC).* Explain the process a student would have to perform to separate these three compounds. In your explanation make sure you discuss the order that the compounds would be collected. |
The student would need to do a simple distillation. She should expect that hexane would boil off first then acetic acid and 1-pentanol. Each time a liquid is collected the student would need to increase the temp in order to reach the boiling point of the next item.
|
