Melting Point Flashcards
Melting Point (2)
- temperature at which the solid phase is in equilibrium with the liquid phase
- range of ≤2°C
Why is melting point depicted in a range?
heat of transfer is uneven the whole sample will not be the same temperature at the same time
What does melting point depend upon? (2)
- intermolecular forces of attraction
- increased strength of forces means increased melting point
What is melting point used for?
identification
Is melting point an intrinsic property?
yes
Intermolecular Forces of Attraction
nonbonding interactions between neighboring molecules
What does the strength of intermolecular forces depend on?
the charge of separation within each molecule
Dipole Moment
measure of polarity of the molecule, produced by an asymmetrical charge distribution in a covalent bond
What is the polarity of a molecule dependent on?
the combined effect of individual polar bonds
What are intermolecular forces between nonpolar molecules?
Van der Waals forces
Why do increased Van der Waals forces lead to increased melting point?
the molecules are more tightly held together, so more energy is needed for separation
Why do larger nonpolar molecules have increased Van der Waals forces? (2)
- they have more electrons
- they are symmetrical and therefore have better packing
How do pure compounds melt?
sharply
Eutectic Point
the lower limit of the melting range of all possibilities
What happens when you take the melting point of a sample containing impurities?
it will have a wider and lower melting point range
Mixed Melting Point Test
a test that is used to confirm the identity of a compound and assess its purity
What happens if the melting point is sharp and unchanged in a mixed melting point test? (2)
- the two compounds are likely the same
- identical compounds don’t disrupt each other’s lattice
What happens if the melting point is broader and lower in a mixed melting point test? (2)
- the compounds are different
- even small differences in structure or impurities disrupt the crystal lattice
