Answer to Question #248599 in Molecular Physics | Thermodynamics for Jordi

Question

For each pair of molecules, identify the one with the higher boiling point (BP), melting
point (MP), viscosity (V), surface tension (ST) and vapor pressure (VP). Briefly explain
your choice.
BP MP V ST VP
Methane, CH4 carbon tetrachloride, CCl4
Dihydrogen sulfide, H2S water, H2O
Methanol, CH3OH ethanol, CH3CH2OH
Acetic acid, CH3COOH acetone, CH3OCH3

Answer 1

In resume, the boiling point, the melting point, the viscosity, and the surface tension are directly related to the intermolecular forces that are present, while the vapor pressure works the way around (the lower the intermolecular attractions, the higher the VP):

1) Methane, CH₄ / carbon tetrachloride, CCl₄

• Both molecules are nonpolar, thus the heavier molecule will have the highest intermolecular forces and then CCl₄ will have the highest boiling point (BP), melting point (MP), viscosity (V), and surface tension (ST), and it will have the lowest vapor pressure (VP) because when the intermolecular forces are strong the rate of evaporation for the liquid-vapor equilibrium will be diminished resulting in low vapor pressure.

2) Dihydrogen sulfide, H₂S / water, H₂O

• Both molecules are polar but water will have the highest intermolecular forces (because of the hydrogen interaction) and then H₂O will have the highest boiling point (BP), melting point (MP), viscosity (V), and surface tension (ST), and in consequence, H₂S will have the highest vapor pressure (VP).

3) Methanol, CH₃OH / ethanol, CH₃CH₂OH

• Both molecules are polar but the heavier molecule will have the highest intermolecular forces and then CH₃CH₂OH will have the highest boiling point (BP), melting point (MP), viscosity (V), and surface tension (ST). On the other hand, since the attractions are lower for CH₃OH then methanol will have the highest vapor pressure (VP) because it has the lowest boiling point.

4) Acetic acid, CH₃COOH / acetone, CH₃COCH₃

• Both molecules are polar but acetone has a higher dipole resulting in stronger interactions between molecules thus CH₃COCH₃ will have the highest intermolecular forces and the highest boiling point (BP), melting point (MP), viscosity (V), and surface tension (ST). On the other hand, since the attractions are lower for CH₃OH then acetic acid will have the highest vapor pressure (VP) because it is a polar molecule but the dipole is weaker than the one present in acetone.

In conclusion,

$\begin{matrix} Molecule & \text{Highest BP} & \text{Highest MP}& \text{Highest V} & \text{Highest ST} & \text{Highest VP} \\ 1)\, C{Cl}_4 & X & X & X & X & \\ 1)\, C{H}_4 & & & & & X \\ 2)\, {H}_2O & X & X & X & X & \\ 2)\, {H}_2S & & & & & X \\ 3)\, C{H}_3CH_2OH & X & X & X & X & \\ 3)\, C{H}_3OH & & & & & X \\ 4)\, {CH_3}CO{CH_3} & X & X & X & X & \\ 4)\, C{H}_3COOH & & & & & X \\ \end{matrix}$

Reference

• Chang, R., & Goldsby, K. A. (2010). Chemistry. Chemistry, 10th ed.; McGraw-Hill Education: New York, NY, USA.