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.