Distillation & Refractive Index

Chem 51LA

Boiling Point & Distillation

•  Distillation: Separation method for liquid compounds based on differences in _____ _____

•  Boiling point: the temperature at which the _____________ of a liquid is equal to the _____________.

Vapor Pressure & Boiling Point

•  high vapor pressure = low boiling point

vapor pressure

applied pressure

Factors That Affect Boiling Point of a Substance

•  Molecular weight: higher molecular weight = higher boiling point

methane propane hexane

MW 16 44 86

BP (°C) -164 -42 69

Factors That Affect Boiling Point of a Substance

•  Hydrogen Bonding CH3CH2CH3 CH3CH2OH

MW 44 46

BP (°C) -42 78

•  Dipole-Dipole Interactions CH3CH2CH3 CH3OCH3 CH3CH=O

MW 44 44 44

BP (°C) -42 -24 21

Temperature/Composition Diagram

•  Composition of vapor above a mixture depends on mixture composition.

•  Raoult’s Law

•  Dalton’s Law

Simple Distillation Setup

Setting Up Water Circulation

•  Study the Recirculating Pump SOP on class website BEFORE your lab section!

Simple Distillation Curve

•  Initial distillate collected above the boiling point of pentane.

•  Final distillate never reaches boiling point of hexane.

Simple Distillation Curve

30

35

40

45

50

55

60

65

70

0 10 20 30 40 50

Volume of Distillate

Dis

till

ati

on

Tem

peratu

re

Fractional Distillation Setup

•  Fractionating column allows vapor to condense and vaporize multiple times.

Fractional Distillation Curve

•  Initial distillate will be very close to boiling point of pentane.

•  Final distillate will be very close to boiling point of hexane.

Refractive Index

•  The refractive index is the ratio of refraction of _______ over refraction of _________ _______.

•  Each liquid has a different refractive index.

• Light is bent (refracted) when it passes through a liquid.

Measuring Refractive Index

•  Be VERY careful with the prism surface!   Blot gently with kimwipe to

clean.   Don’t touch the surface with a

pipette! •  Align the fields with the

crosshairs. •  Make sure the fields have

no color.

Experimental Details

Experiment Notes

•  Experiment is done in groups of 4.   One pair will do simple distillation.   One pair will do fractional distillation.   There is only ONE unknown per group of 4. You will need to

split it. •  Placement of thermometer is CRUCIAL! •  Collect temp. data every .5 mL. •  Plot your graph while collecting data. •  Do RI of sample compounds and distillate from fractional

distillation.