Dotsha J. RaheemDepartment of Chemistry

College of Science University of Salahaddin

The boiling point of a liquid is the temperature at which its vapor pressure is equal to the pressure of the gas above it

Definition of b.p.

In terms of intermolecular interactions the b.p. represents the energy required to overcome the various intermolecular attractions binding the molecules as a liquid (e.g. dipole-dipole attraction and hydrogen bonding) and therefore undergo a phase change into the gaseous phase . Therefore the boiling point of a liquid is also an indicator of the strength of the attractive forces between the liquid’s molecules.

The normal boiling point of a liquid is the temperature at which its vapor pressure is equal to 1 atm

Importance of b.p. B.p. is a physical constant used for:

1. Identification of unknown liquid compound

2. Purification

factors affecting b.p. Pressure

Molecular weight

Structure of the molecule & intermolecular interactions

Branching

Polarity

Van der Waals interactions

H-bonding

Impurities

a. When the pressure is less than 1 atm , the boiling point of the liquid is less than its normal b.p.

b. P = 1 atm, the b.p. of the liquid is called normal boiling point

c. When P is greater than 1 atm, the b.p. of the liquid is greater than its normal b.p.

The higher the altitude, the lower the temp at which water boils

Factors affecting B.P.

CH4 and CH3Cl

Factors affecting B.P.

H

C

HH

H

H

C

HHCl

Formula IUPAC Name Boiling Point

HCO2H methanoic acid 101 oC

CH3CO2H ethanoic acid 118 oC

CH3CH2CO2H propanoic acid 141 oC

CH3(CH2)2CO2H butanoic acid 164 oC

CH3(CH2)3CO2H pentanoic acid 186 oC

CH3(CH2)4CO2H hexanoic acid 205 oC

CH3(CH2)5CO2H heptanoic acid 223 oC

Factors affecting B.P.

Within the branched series, increased symmetry leads to higher melting point and lower boiling point

CH4 (b.p. -161.5) , CH3-O-CH3 (b.p. -24)

Butane (-0.6 oC) , acetone (56 oC)

Factors affecting B.P.

pentane (36), isopentane (31) and neopentane (9)

Pentane bp 36 oC 2,2-dimethylpropane bp 9.5 oC

Factors affecting B.P.

OH OH OH

1-butanol (118), 2-butanol (100) 2-mehtyl-2-propanol (83)

isobutanol tert-butanoln-butanol

HF higher than HCl by 100 oC

H2O higher than H2S by 160 oC

H2O ( M.wt.=18): boiling point 100 oC

H2S ( M.wt.=34): boiling point -60 oC

CH3OH (66), CH3SH (6)

CH3-O-CH3 (-24), CH3-S-CH3 (38)

Factors affecting B.P.

b.p. is supposed to increase directly with m.wt., however, this is not the case for H2O, NH3 and HF

Compound Structure Boiling point

Acetamide CH3C(O)NH2 222 oC

Acetic acid CH3CO2H 118 oC

Ethanol CH3CH2-OH 78 oC

Ethylamine CH3CH2-NH2 17 oC

Ethane CH3CH3 -89 oC

Q/ Explain the reason for the difference in b.p. of the above compounds.

A solution has a higher boiling point than a pure solvent .

The b.p. of pure H2O is 100 oC, but that b.p. can be elevated by adding a solute such as a salt.

Δ Tb = Kb Cm

where Kb is called the boiling-point elevation constant

Factors affecting B.P.

The reason for elevating b.p. is: the number of solvent molecules at the surface of the

solution is less than for pure solvent. The surface molecules can be considered “diluted” by the less volatile particles of the solute

The rate of exchange between solvent in the solution and the air above it is lower (vapor pressure of the solvent is reduced)

A lower pressure means that a higher temperature is necessary to boil the water in the solution , hence boiling point elevation

Conversely, adding common salt to water will lower its freezing point.

Ionic vs. covalent compounds

Na+

Na+

Na+

Na+

Na+Na+

Cl-

Cl-

Cl- Cl-

Cl-

1413 oC

Boiling occurs when a temp. is reached at which the thermal energy of the particles ix great enough to overcome the attractive forces that hold them in liquid

In the liquid state of a covalent compound, the weak intermolecular interactions are more are more easily overcome and boiling occurs at much lower temp.

-161.5 oC

Apparatus

A small-scale simple distillation apparatus(sample size ~ 10ml)

Miniscale boiling point determinationsample size ~0.5 – 1 ml

Procedure