Assessment 1 Flashcards
Pure substance
A homogeneous sample consisting of molecules of having the same structure.
Recrystallization
Method used to purify solids compounds in the lab. Recrystallization is based on temperature and amount solubility characteristics.
Effects of contaminants in a chemical sample (3)
- Inhibit chemical reactions
- Catalyze undesirable side reactions
- Invalidate chemical test results
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
Reaction percent yield
The percentage of product obtained after a reaction. Calculated by dividing experimental moles or grams by theoretical moles or grams and then multiplying by 100.
Mole fraction
Calculate by dividing the number of moles of one component of the mixture by the sum of the number of moles of all components in the mixture.
Mass percentage
Calculate by dividing the mass of a component by the total mass of the mixture and multiplying everything by 100.
Percent recovery
Divide the actual amount of the compound obtained after a process by the expected amount of the same compound. Multiply everything by 100.
Direct proportionality
when ratio of two variables equals a constant
Indirect proportionality
When product of two variables equals a
constant
How is the solubility of a chemical compound enhanced? (3)
- Mechanical action, like stirring
- Increasing the amount of solvent
- Increasing the temperature of the solvent- most solids are more soluble in hot than cold solvents
Types of impurities (3)
- Insoluble
- Soluble
- Color
Hot gravity filtration
Isolates insoluble impurities
Vacuum filtration
Isolates soluble impurities
Which characteristics make a solvent appropriate for recrystallization?
- Low solubility at room temperature- will recrystallize as it cools
- High solubility for the impurity at room temperature, so that it will stay dissolved as it’s heated. Or, the impurity is insoluble at a higher temperature so it never dissolves
- High solubility at high temperatures- dissolves upon heating
Removing a color impurity
Removed using decolorizing activated charcoal, which is an adsorbent- it absorbs chemical compounds present in the solution. Charcoal is added to a solution until it clears, but adding too much charcoal leads to absorption of the compound of interest.
During recrystallization, how are purified crystals recovered?
Purified crystals are filtered out by vacuum filtration. Expected (ideal) recovery and experimental recovery are calculated
Causes of deviations from ideal recovery in recrystallization (3)
- Incomplete recrystallization
- Loss during transfers
- Errors in quantifications
During recrystallization, how is crystallization induced?
The purified solution is chilled in an ice bath after hot gravity filtration. The volume of the solvent is reduced by evaporation
Physical properties
Characteristics of a compound that can be observed or measured without changing its composition.
Physical constant
Numerical values associated with measurable physical properties of substances. They can be used to identify and characterize organic compounds. Includes melting point (solid compounds), boiling point and refractive index (liquid compounds), and specific optical rotations (for optically active compounds)
Melting point
The temperature at which the solid and liquid phase are in equilibrium at standard atmospheric pressure. It’s expressed as a range, over which a compound transitions from crystals to liquid. Pure substances melt within a range of 1-2 degrees C.
How do impurities affect the melting point?
Impurities lower the melting point of a compound and broaden the melting point range.
Eutectic point
Lowest possible melting point observed over all of the mixing ratios for the components involved.
Extraction
A separation technique that involves partitioning of chemical substances between two immiscible phases- liquids will not form a homogenous mixture when added together. Makes use of properties like solubility and chemical properties like reactivity that differ between the components we want to isolate. The layers of each component will separate out in the funnel, with the more dense aqueous layer being at the bottom and the less dense organic layer being at the top. Each layer has dissolved a specific component that we want to isolate.
Acid base extractions
To separate the acidic, basic, and neutral components, we take advantage of their differences in reactivity. A base will react with a strong (aqueous) acid like HCl, while the other components will not. The reaction creates a salt that is now water soluble. For the acidic compound, a base (aqueous sodium bicarbonate) is added, also creating a water soluble compound. The neutral compound, dissolved in ether, is the only thing remaining in the funnel. To get the neutral compound, we evaporate the ether. The other compounds are obtained using vacuum filtration.
Phase distribution
Based on difference in solubility of a solute
(compound being extracted) in the two immiscible phases in contact
Applications of extraction (2)
- To separate a desired component from crude reaction mixture.
- To remove impurities from a desired organic component
Types of extractions (2)
- Solid liquid extraction- like coffee extracted into water
- Liquid liquid extraction- caffeine extracted from water into methylene chloride solvent
Liquid-liquid extraction
Distribution of a solute between two immiscible liquids in contact. Most organic solvents are immiscible in water, and the top/bottom layer are determined by the densities of the two solvents. S1 is the original phase (solvent) that the solute is dissolved in and S2 is the extracting phase (solvent).
Partition coefficient (Kd)
Property that describes the distribution of a chemical component between 2 immiscible phases at equilibrium. Defines the ability of solvent S2 to remove solute A from solvent S1. Concentration of A in S2/concentration of A in S1. Equation- concentration (x)/volume, over concentration (g-x)/volume
Limiting reagent
The reactant producing the least product- lower amount of moles
Separatory funnel extraction technique
The funnel is shaken and vented to mix the compounds together and then release the pressure. The most dense component of the solution (the aqueous component) is found at the bottom of the funnel and can be drained
What do Kd values indicate about the solubility of a solute in a solution?
If Kd is greater than 1, solute A is more soluble in S2 than in S1. If Kd is less than 1, solute is more soluble in S1 than in S2.
S1 and S2
S1 is the original phase (solvent) that the solute is dissolved in and S2 is the extracting phase (solvent).
How does a small Kd value impact the extraction process?
A solvent system with a small Kd value requires multiple extractions to achieve quantitative recovery
Principle of acid-base extraction
Extraction takes advantage of the differences in solubility and reactivity between the compounds. Water and diethyl are added to the unknown sample. Although the neutral compound is soluble in diethyl ether, acids and bases aren’t soluble in either solvent. However, the basic and acidic compounds can be converted to ionic compounds so they will be soluble in water. Water is more dense than diethyl ether, so the two solvents will separate, and the aqueous solution will be located at the bottom of the separatory funnel. The aqueous solution, which contains either the acid or the base, can then be drained into a separate flask.
How are the acidic and basic compounds converted to an ionic form?
Hydrochloric acid reacts with the basic compound so that the base will gain a proton and therefore become ionic, while the acidic compound reacts with sodium bicarbonate and loses a proton.
Once the acidic and basic compounds become ionic so they can be isolated, how are they converted back to their original form?
The basic compound reacts with another base, sodium hydroxide. Sodium hydroxide removes the extra proton, creating the purified basic compound that is no longer ionic. Similarly, the acidic compound reacts with hydrochloric acid in order to gain the proton that it had originally lost.
