Module 3: BOILING POINT AND MELTING POINT OF ORGANIC COMPOUNDS Flashcards
Boiling and Melting Points
* physical properties that can help:
✓__________ unknown compounds
✓functions as an ___________ of substances
- identify and characterize
- index of purity
BOILING POINT
- the temperature at which the ____________ of the substance is ___________ to the _______________ above it
vapor pressure equal atmospheric pressure
BOILING POINT
- the boiling point of a liquid varies with the atmospheric pressure:
- it is also an indicator of the volatility of the compound:
Note!
BOILING POINT
: a liquid at a higher pressure has a higher boiling point than a liquid at a lower atmospheric pressure (varies)
(True or False)
True
BOILING POINT
: the lower the boiling point, the more highly volatile the substance is, and vice versa (volatility)
(True or False)
True
- other factors may also affect the boiling point:
intermolecular forces of attraction
- at a given temperature, a compound will exist as:
✓ “gas” if it has a _________________________
✓ “liquid or solid” if it has _________________
- low normal boiling point
- higher normal boiling point
Intermolecular forces of attraction
- it occurs between ________________________, “influences” __________ the boiling point and are dependent on the __________________ present on the organic compound
- one molecule and another
- directly
- functional groups
- “weak” intermolecular attraction formed by "induced polarization”\ of molecules which is caused by fluctuating charges due to the nearness of the molecules as they vibrate
Van Der Waals Dispersion Forces
- the intermolecular forces go in the order
ionic bonding > hydrogen bonding > dipole-dipole interaction > Van der Waals dispersion forces
- the functional groups which most easily ionize are “amines and carboxylic acids”, such as the reaction of ammonia (NH3) with a carboxylic acid (carboxylate ion)
Ionic Bonding
- boiling point increases as the number of _______________
✓due to more energy that is required to overcome the increased forces of attraction caused by long carbon chains
carbon atoms increase
- commonly observed among [ “hydrocarbon groups” ] and occurs only within [ “short distances” ] in between molecules
Van Der Waals Dispersion Forces
- these molecules contain an [ “asymmetrical charge” ] due to the two poles of the molecule: one end is partially negative while the other end is partially positive
Dipole - Dipole Interaction
- occurs among [ “partially ionic or polar molecules” ]
Dipole - Dipole Interaction
- formed between [ “partially positive
hydrogen (H+)” ] and an [ “adjacent electronegative atom” ] which bears a partial negative charge
Hydrogen Bonding
- intermolecular ______________ tend to “rise boiling point” of an organic compound
Hydrogen Bonding
- takes place between molecules having “opposite charges” and involves an “electrostatic interaction” between the two opposite charges
Ionic Bonding
- another factor that can affect boiling point is the
length of the carbon chain
- boiling point increase in the presence of ________________ impurities
✓due to decrease in the vapour pressure of the liquid
non-volatile impurities
- boiling point decrease in the presence of __________
✓it causes the surface area of the molecules to decrease thereby decreasing the forces of attraction between individual molecules
branching
- boiling point increase as the polarity of the substance ________________
✓due to the attraction of the positive end of one molecule to the negative end of another molecule
increase
- boiling point ____________ in the presence of volatile impurities
✓due to the increase in the vapour pressure of the liquid
decrease
- the temperature at which a solid compound is directly converted to its liquid state
Melting Point
a “pure” crystalline compound has a “sharp melting point” and it completely melts within a narrow range of temperature ____________
! a solid usually melts over a range of temperature
NMT 0.5 - 1°C
- if a compound melts over a “very narrow range”, it is assumed that the compound tested is ________________
relatively pure
- the presence of “impurities” usually ___________ the melting point and may “increase the melting point range”
depresses
estimation of the purity of the substance can be obtained
theoretical melting point value
- several factors can also affect melting point:
✓ difference in the configuration of a compound (eg. isomers)
✓ intermolecular forces of attraction
✓polarity
= (difference in the arrangement of their structures even both are pure)
= (ionic bonding results to higher melting point)
= hydrogen bonding, leads to a higher melting point of compounds
BOILING POINT
= tert-butyl alcohol
82.3 °C
BOILING POINT
= chloroform
61.2 °C
BOILING POINT
= n-hexane
68.7 °C
BOILING POINT
= methanol
64.7 °C
MELTING POINT
= salicylic acid
211.0 °C
MELTING POINT
= benzoic acid
122.4 °C
MELTING POINT
= glucose
146.0 °C
MELTING POINT
= fructose
103.0 °C
- boiling point increase as the number of carbon atoms increase
(True or False)
True
- boiling point increase in the presence of branching
(True or False)
False
Correction:
* boiling point “decrease” in the presence of branching
- boiling point increase as the polarity of the substance increase
(True or False)
True
- boiling point decrease in the presence of non-volatile impurities
(True or False)
False
Correction:
* boiling point “increase” in the presence of non-volatile impurities
- boiling point decrease in the presence of volatile impurities
(True or False)
True
vapor pressure is qual to?
atmospheric pressure
