We are asked to explain why propanol and ethylene glycol have significantly different viscosity despite they have similar molecular weights.
Recall that there are several types of intermolecular forces (IMF):
1. Ion-dipole interaction – occurs between an ion and a polar covalent compound; strongest IMF
2. Hydrogen bonding – occurs in compounds where hydrogen is directly connected to an electronegative element such as N, O, or F; 2nd strongest IMF
3. Dipole-dipole interaction – occurs between two polar covalent compounds; 3rd strongest IMF
4. Dispersion forces – occurs in all compounds and is the primary IMF exhibited by nonpolar compounds; weakest IMF
Viscosity is resistance to flow. For liquids, typically the larger the intermolecular forces (IMF) the higher the viscosity due to cohesion (i.e. the attraction in a molecule of liquid to each other due to intermolecular forces). The other factors that affect viscosity are temperature and the shape of the molecule. Higher temperatures will correspond to higher average kinetic energies and faster-moving molecules.
So, we need to identify which among the given compounds has the strongest intermolecular forces.
Now, let’s analyze each compound given and determine the intermolecular forces present in each.
• Propanol (CH3CH3CH2OH)
▪ CH3CH2CH2OH → the -OH group in the compound can exhibit hydrogen bonding
▪ has the 2nd strongest IMF
|Boiling Point (oC)||Surface Tension (J/m2)||Viscosity (kg/m s)|
|Water, H2O||100||7.3 10 - 2||0.9 10 - 3|
|Ethanol, CH3CH2OH||78||2.3 10 - 2||1.1 10 - 3|
|Propanol, CH3CH2CH2OH||97||2.4 10 - 2||2.2 10 - 3|
|n-Butanol, CH3CH2CH2CH2OH||117||2.6 10 - 2||2.6 10 - 3|
|Ethylene glycol, HOCH2CH2OH||197||4.8 10 - 2||26 10 - 3|
How do you explain the fact that propanol and ethylene glycol have similar molecular weights (60 versus 62 amu), yet the viscosity of ethylene glycol is more than 10 times larger than propanol?