Question

What is happening between the IMFS and electrons in molecules during these phase changes: boiling, vaporization, and condesenation? Do the electrons go from ground to excited states? Please give a detailed answer, thanks.

Answer 1

I believe the student meant Intermolecular forces by the abbreviation IMFS.

IMFS generally describes the forces weaker than covalent or ionic bonds that exists between two molecules. Generally there are several types of IMFS. These are Ion-dipole interactions, dipole-dipole interactions, dipole-induced dipole interactions and induced dipole-induced dipole interactions, pi-pi interactions etc. These are forces that hold molecule together in liquids and solids. It can be generalized that as the strength of the IMFS increases the tendency of a material to go from gaseous sate to liquid state then to solid state. In solids the IMFS strength was observed maximum.

When a molecules undergoes phase change, there will not be any change in the distribution of electrons in the electronic levels ( that is the molecular orbitals) as the difference between molecular orbitals are too high. Hence no excitation or de-excitation will be observed during phase transformation. But since heat is absorbed or liberated during phase changes may cause excitation and de-excitation in the vibrational and rotational levels of the molecules. If sufficient amount heat is absorbed there will be excitation in the vibrational and rotational levels and viz. It all depends on the amount of heat supplied or liberated during the process and its closeness between the difference in vibrational and rotational levels.

Consider the phase changes

1. Boiling: When a liquid boils all the intermolecular forces must be broken down by the heat supplied and the molecules set free. As the strength of intermolecular force increases, more energy will be required and as a result boling point increases. Since its a heat absorbing process, there may be vibrational excitation. No electronic excitation was expected generally.
2. Vaporization: Molecules absorbs heat and transforms to vapour phase. Here also intermolecular forces will be broken. As the strength of intermolecular forces increases, volatility of a compound decreases.
3. Condensation: Its the reverse process of vaporization. During this process, vapour undergoes transformation to liquid or solid phase. Its a heat liberation process and IMFS undergoes formation and density of the system increases and molecules are held together as close as possible.

Note: If the molecular orbitals are too close, the possibility of electronic excitation is there but it is very uncommon to observe. It all based on the amount of heat and energy gap.