p.4
Intermolecular Forces and Their Effects
What are dipole-dipole interactions?
Attractive forces between the permanent dipoles of two polar molecules.
p.1
Definition of Melting Point
What is the melting point?
The temperature at which a solid melts or the energy needed to break apart solid molecules.
p.3
Melting Point Ranges for Pure vs. Impure Compounds
How do impurities affect the melting point of organic compounds?
Impurities lower the melting point.
p.3
Factors Influencing Melting Point
What type of bonding typically leads to higher melting points?
Ionic bonds typically lead to higher melting points compared to covalent bonds.
p.5
Hydrogen Bonding and Its Importance
What is hydrogen bonding?
An extra strong type of dipole-dipole interaction that occurs when a hydrogen atom bonded to O, N, or F is electrostatically attracted to a lone pair of electrons on an O, N, or F atom in another molecule.
p.10
Applications of Melting Point Determination
Is melting point determination the only way to identify a compound?
No, it is not the only way to identify a compound.
p.4
Intermolecular Forces and Their Effects
How do atoms with high electronegativity affect electron density in Van der Waals interactions?
They 'hog' electron density, creating partially negative and partially positive areas of the molecule.
p.5
Hydrogen Bonding and Its Importance
Why does ice float on water?
Because hydrogen bonding makes ice less dense than water.
p.10
Applications of Melting Point Determination
What is one method to check the purity of a sample?
Run a melting point and compare it to literature values.
p.7
Factors Influencing Melting Point
How does the size of a molecule influence its melting point?
Larger molecules generally have higher melting points due to increased van der Waals forces.
p.7
Melting Point Ranges for Pure vs. Impure Compounds
How does purity affect the melting point of a substance?
Pure substances have a sharp melting point, while impurities lower and broaden the melting point range.
p.10
Applications of Melting Point Determination
What are some good sources to find literature values for melting points?
Sigma-Aldrich catalogue, ChemSpider, or Wikipedia.
p.8
Applications of Melting Point Determination
Why do we take melting point ranges?
To establish an unknown solid’s identity, determine the purity of a solid, and report the physical properties of a new compound.
p.8
Co-crystals and Their Melting Points
Why are melting points still critical for pharmaceutical co-crystals?
Because co-crystals are designed to improve solubility, tableting, and thermal stability.
p.5
Hydrogen Bonding and Its Importance
Which atoms must hydrogen be bonded to for hydrogen bonding to occur?
Oxygen (O), Nitrogen (N), or Fluorine (F).
p.8
Co-crystals and Their Melting Points
What are co-crystals designed to improve?
Solubility, tableting, and thermal stability.
p.3
Factors Influencing Melting Point
How does the size of a molecule influence its melting point?
Larger molecules tend to have higher melting points.
p.2
Factors Influencing Melting Point
Why does ice melt easier when salts like calcium chloride are added?
Salts interfere with water’s crystal lattice, requiring less heat to melt the ice.
p.3
Factors Influencing Melting Point
What is the effect of supramolecular assembly on melting point?
Supramolecular assembly, such as crystal packing with intramolecular interactions like pi-stacking and van der Waals interactions, can lead to higher melting points.
p.13
Factors Influencing Melting Point
What could be causing the incorrect melting point of m-Phenylenediamine?
The presence of impurities.
p.9
Co-crystals and Their Melting Points
What is a 1:1 co-crystal?
A co-crystal formed in a 1:1 ratio of two compounds.
p.7
Factors Influencing Melting Point
How does pressure influence the melting point of a substance?
Increased pressure can raise the melting point of a substance.
p.4
Intermolecular Forces and Their Effects
What does it mean for a molecule to be 'polarizable'?
The more a molecule can create partially negative and positive areas due to changes in electron density, the more 'polarizable' it is.
p.14
Applications of Melting Point Determination
Why do chemists take melting points?
To identify and characterize compounds, and to assess their purity.
p.14
Mixed Melting Point Experiment
What is the basis of a mixed melting point experiment?
To determine the identity of an unknown compound by mixing it with a known compound and observing the melting point.
p.7
Hydrogen Bonding and Its Importance
What role does hydrogen bonding play in determining melting points?
Hydrogen bonding increases the melting point due to stronger intermolecular forces.
p.1
Melting Point Ranges for Pure vs. Impure Compounds
Do organic compounds melt at a single temperature or over a range?
Organic compounds melt over a range of temperatures.
p.14
Melting Point Ranges for Pure vs. Impure Compounds
What affects melting point ranges?
Purity of the compound, presence of impurities, and intermolecular forces.
p.12
Mixed Melting Point Experiment
What does it mean if the melting point of the mixture is the same as the known compound's melting point?
It means the unknown compound is likely the same as the known compound.
p.9
Co-crystals and Their Melting Points
How does the melting point of a co-crystal compare to its pure compounds?
The melting point of a co-crystal will always be lower than at least one of the pure compounds.
p.10
Applications of Melting Point Determination
Why might you find different experimental values for the melting point of the same compound?
Because many compounds have similar melting points and experimental values can vary.
p.4
Intermolecular Forces and Their Effects
What causes Van der Waals interactions?
Momentary changes in electron density in a molecule.
p.2
Melting Point Ranges for Pure vs. Impure Compounds
How do the melting point ranges of pure and impure compounds differ?
Pure compounds have sharp melting point ranges of only ~1 - 2 degrees, while impure compounds have a much larger and lower (depressed) range.
p.8
Applications of Melting Point Determination
Why are melting points considered a bit obsolete?
New spectroscopy techniques have made melting points a bit obsolete.
p.8
Applications of Melting Point Determination
What are the primary uses of melting point ranges?
Establishing an unknown solid’s identity, determining purity, and reporting physical properties of new compounds.
p.12
Mixed Melting Point Experiment
Why can't you just compare your compound's melting point to literature values?
Because you might have a different compound with similar melting points.
p.12
Mixed Melting Point Experiment
What does a melting point range of 65-76 °C indicate in a mixed-melting point experiment?
It indicates that the unknown compound is not the same as the known compound.
p.1
Melting Point Ranges for Pure vs. Impure Compounds
What is a melting point range?
The temperature range that starts when you see the first drop of liquid and ends when no solids remain.
p.13
Melting Point Ranges for Pure vs. Impure Compounds
Which compound is impure based on the given melting points?
m-Phenylenediamine, with an experimental melting point range of 51-60 °C.
p.3
Factors Influencing Melting Point
How does external pressure influence the melting point?
Higher pressures change the ease of expansion/contraction upon melting, affecting the melting point.
p.13
Factors Influencing Melting Point
Why do impurities affect the melting point of a compound?
Impurities disrupt the crystal lattice, causing a broader and lower melting point range.
p.3
Hydrogen Bonding and Its Importance
How does hydrogen bonding affect the melting point?
Hydrogen bonding increases the melting point.
p.12
Mixed Melting Point Experiment
What should you do if you're not sure what your compound is?
Run a mixed-melting point experiment.
p.12
Identifying Unknown Compounds
What are the melting points of compounds A and B in the example?
Compound A: 83-84 °C, Compound B: 81-82 °C
p.14
Intermolecular Forces and Their Effects
How can you identify molecules with higher melting points?
By the presence of dipole-dipole interactions or hydrogen bonding.
p.12
Mixed Melting Point Experiment
How do you perform a mixed-melting point experiment?
Mix some unknown compound in equal parts with some known compound and take the melting point of the mixture.
